Storage Group

1. Bandilla, K. W., Benjamin Court, Thomas R. Elliot, and Michael Celia, February 2012: Comparison of Brine Production Scenarios for Geologic Carbon Sequestration Operations. Carbon Management Technology Conference, doi:10.7122/151250-MS
   [ Abstract ]

   Large volumes of CO₂ will have to be stored in the subsurface for carbon capture and geological sequestration to have a significant impact on the reduction of carbon emissions. Injection of large volumes of CO₂ into deep saline formations can lead to significant pressure increases within that formation. The increased pressure can be a limiting factor for injection rates; it can also drive vertical brine migration through leakage pathways (e.g., abandoned wells) that could contaminate sources of drinking water. Production of brine from the injection formation can reduce the pressure increase while also limiting the spatial extent of the pressure increase. The impact of brine extraction is investigated using a hypothetical injection domain conditioned by parameters from the Illinois Basin. The domain contains one injection well and encompasses several aquifers connected through diffusive brine leakage. A vertically-integrated approach is used to model the injection formation and overlying aquifers. A set of production scenarios illustrates the impact of brine production on injection rates and vertical brine movement. The scenarios include production with surface disposal and production with reinjection into overlying formations (with and without desalinization). The results show that brine production can reduce the pressure buildup in the injection formation, leading to an increase in injectivity and a concomitant reduction in fresh water contamination risk by reducing the area of potential impact. While reinjection of brine into an overlying aquifer solves the disposal problem, it also reduces the effectiveness of brine production by increasing the pressure. Injection of a smaller amount of more concentrated brine resulting from desalinization reduces the impact of reinjection and acts as an additional source of fresh water, but increases the cost of the injection operation. Based on the results from these numerical experiments pressure management through brine production should be considered for industrial-scale CO₂ injection operations, as it increases injectivity and reduces the size of the area of potential impact. However, the brine disposal problem needs to be solved for brine production to be a useful endeavor.

2. Buscheck, Thomas A., Yunwei Sun, Mingjie Chen, Yue Hao, Thomas J. Wolery, William L. Bourcier, Benjamin Court, Michael Celia, S. Julio Friedmann, and Roger D. Aines, 2012: Active CO₂ reservoir management for carbon storage: Analysis of operational strategies to relieve pressure buildup and improve injectivity. International Journal of Greenhouse Gas Control, Elsevier, 6, doi:10.1016/j.ijggc.2011.11.007 230-245
   [ Abstract ]

   For industrial-scale CO₂ injection in saline formations, pressure buildup can limit storage capacity and security. Active CO₂ Reservoir Management (ACRM) combines brine production with CO₂ injection to relieve pressure buildup, increase injectivity, manipulate CO₂ migration, and constrain brine leakage. By limiting pressure buildup, in magnitude, spatial extent, and duration, ACRM can reduce CO₂ and brine leakage, minimize interactions with neighboring subsurface activities, allowing independent assessment and permitting, reduce the Area of Review and required duration of post-injection monitoring, and reduce cost and risk. ACRM provides benefits to reservoir management at the cost of extracting brine. The added cost must be offset by the added benefits to the storage operation and/or by creating new, valuable uses that can reduce the total added cost. Actual net cost is expected to be site specific, requiring detailed analysis that is beyond the scope of this paper, which focuses on the benefits to reservoir management. We investigate operational strategies for achieving an effective tradeoff between pressure relief/improvedinjectivity and delayed CO₂ breakthrough at brine producers. For vertical wells, an injection-only strategy is compared to a pressure-management strategy with brine production from a double-ring 9-spot pattern. Brine production allows injection to be steadily ramped up while staying within the pressure-buildup target, while injection-only requires a gradual ramp-down. Injector/producer horizontal/well pairs were analyzed for a range of well spacings, storage-formation thickness and area, level and dipping formations, and for homogeneous and heterogeneous permeability. When the producer is downdip of the injector, the combined influence of buoyancy and heterogeneity can delay CO₂ breakthrough. Both vertical and horizontal wells can achieve pressure relief and improved CO₂ injectivity, while delaying CO₂ breakthrough. Pressure buildup and CO₂ breakthrough are sensitive to storage-formation permeability and insensitive to all other hydrologic parameters except caprock-seal permeability, which only affects pressure buildup for injection-only cases.

3. Celia, Michael, and Jan M. Nordbotten, 2012: Geological Storage of CO₂: Modeling Approaches for Large-Scale Simulation , John Wiley & Sons, Inc., 264pp.
4. Court, Benjamin, K. W. Bandilla, Michael Celia, Thomas A. Buscheck, Jan M. Nordbotten, M. Dobossy, and Adam Janzen, 2012: Initial evaluation of advantageous synergies associated with simultaneous brine production and CO₂ geological sequestration. International Journal of Greenhouse Gas Control, Elsevier, 8, doi:10.1016/j.ijggc.2011.12.009 90-100
   [ Abstract ]

   Mitigation of global atmospheric carbon emissions requires a worldwide ramping up of CO₂ capture and sequestration (CCS) implementation in the next decades. While CCS could be deployed in isolation, there is also the possibility to consider CO₂ injection within a much broader framework of reservoir and resource management including active water (brine) management. The goal of this study is to provide an initial analysis of three identified synergies related to active brine management in CCS operations. The potential advantages of coupling simultaneous brine production to a large-scale CO₂ geological sequestration operation are explored through three separate modeling studies. Our results demonstrate that brine production can provide important pressure-control benefits, including increased injectivity potential through reduction of the injection well pressure, significant reduction of the extent of the Area of Review, within which operators must procure property rights and monitor and remediate potential leakage pathways, and reduction in the risk of CO₂ and brine leakage. The latter is especially important in reservoirs, like many in North America, where a significant number of potential leakage pathways, particularly abandoned wells, may exist within the Area of Review. We also observe that brine production has minimal impact on the overall shape of the CO₂ plume, with plume shape and extent strongly governed by formation parameters.

5. Elliot, Thomas R., and Michael Celia, 2012: Potential restrictions for CO₂ sequestration sites due to shale and tight gas production. Environmental Science and Technology, Washington, DC, American Chemical Society, (February 21, 2012), doi:10.1021/es2040015 1-16
   [ Abstract ]

   Carbon Capture and Geological Sequestration is the only available technology that both allows continued use of fossil fuels in the power sector and reduces significantly the associated CO₂ emissions. Geological sequestration requires a deep permeable geological formation into which captured CO₂ can be injected, and an overlying impermeable formation, called a caprock, that keeps the buoyant CO₂ within the injection formation. Shale formations typically have very low permeability and are considered to be good caprock formations. Production of natural gas from shale and other tight formations involves fracturing the shale with the explicit objective to greatly increase the permeability of the shale. As such, shale gas production is in direct conflict with the use of shale formations as a caprock barrier to CO₂ migration. We have examined the locations in the United States where deep saline aquifers, suitable for CO₂ sequestration, exist, as well as the locations of gas production from shale and other tight formations. While estimated sequestration capacity for CO₂ sequestration in deep saline aquifers is large, up to 80% of that capacity has areal overlap with potential shale-gas production regions and, therefore, could be adversely affected by shale and tight gas production. Analysis of stationary sources of CO₂ shows a similar effect: about two-thirds of the total emissions from these sources are located within 20 miles of a deep saline aquifer, but shale and tight gas production could affect up to 85% of these sources. These analyses indicate that co-location of deep saline aquifers with shale and tight gas production could significantly affect the sequestration capacity for CCS operations. This suggests that a more comprehensive management strategy for subsurface resource utilization should be developed.

6. Gasda, S., Jan M. Nordbotten, and Michael Celia, 2012: Application of simplified models to CO₂ migration and immobilization in large-scale geological systems. International Journal of Greenhouse Gas Control, Elsevier, 9, doi:10.1016/j.ijggc.2012.03.001 72-84
   [ Abstract ]

   Long-term stabilization of injected carbon dioxide (CO₂) is an essential component of risk management for geological carbon sequestration operations. However, migration and trapping phenomena are inherently complex, involving processes that act over multiple spatial and temporal scales. One example involves centimeter-scale density instabilities in the dissolved CO₂ region leading to large-scale convective mixing that can be a significant driver for CO₂ dissolution. Another example is the potentially important effect of capillary forces, in addition to buoyancy and viscous forces, on the evolution of mobile CO₂. Local capillary effects lead to a capillary transition zone, or capillary fringe, where both fluids are present in the mobile state. This small-scale effect may have a significant impact on large-scale plume migration as well as long-term residual and dissolution trapping. Computational models that can capture both large and small-scale effects are essential to predict the role of these processes on the long-term storage security of CO₂ sequestration operations. Conventional modeling tools are unable to resolve sufficiently all of these relevant processes when modeling CO₂ migration in large-scale geological systems. Herein, we present a vertically-integrated approach to CO₂ modeling that employs upscaled representations of these subgrid processes. We apply the model to the Johansen formation, a prospective site for sequestration of Norwegian CO₂ emissions, and explore the sensitivity of CO₂ migration and trapping to subscale physics. Model results show the relative importance of different physical processes in large-scale simulations. The ability of models such as this to capture the relevant physical processes at large spatial and temporal scales is important for prediction and analysis of CO₂ storage sites.

7. Gor, Gennady Y., Thomas R. Elliot, and Jean Hervé Prévost, 2012: Effects of thermal stresses on caprock integrity during CO₂ storage. International Journal of Greenhouse Gas Control, Elsevier Ltd., 12, doi:10.1016/j.ijggc.2012.11.020 300-309
   [ Abstract ]

   Subsurface fluid injection results in a pore pressure increase, which induces geomechanical stresses. Additionally, if there exists a difference between the ambient formation temperature and the temperature of injected fluid, thermal stresses can develop. Herein we study the effect of CO₂ injection temperature on caprock integrity using coupled thermo-poromechanical multi-phase simulations. Calculations show that when CO₂ is injected within several years at a temperature below the ambient value in the formation, the stresses above the horizontal injection well lead to tensile or shear failure of the caprock. We study the sensitivity of resulting stresses to the injection temperature, caprock density and initial in situ stresses. We also show that the caprock failure can lead to propagating fractures, which may serve as pathways for CO₂ leakage. Based on the results of our simulations we estimate the rate of fracture propagation and study the effect of caprock permeability on this rate. Our results show that injection of CO₂ at temperature close to the ambient value in the aquifer significantly reduces the risk of caprock fracturing and, therefore, of possible leakage.

8. Nogues, J. P., Benjamin Court, M. Dobossy, Jan M. Nordbotten, and Michael Celia, 2012: A methodology to estimate maximum probable leakage along old wells in a geological sequestration operation. International Journal of Greenhouse Gas Control, Elsevier, 7, doi:10.1016/j.ijggc.2011.12.003 39-47
   [ Abstract ]

   This study presents a computational methodology to estimate the maximum probable leakage of CO₂ along old wells in a geological sequestration operation. The methodology quantifies the maximum probable CO₂ leakage as a function of the statistical characterization of existing wells. We use a Monte Carlo approach based on a computationally efficient simulator to run many thousands of realizations. Results from the Monte Carlo simulations are used to determine maximum leakage rates at 95% confidence. Uncertainty in the analysis is due to leaky well parameters, which are known to be highly uncertain. We consider a wide range of parameter values, with our focus on assignment of effective well permeability values and the correlation of those values along individual wells. We use a specific location in Alberta, Canada, to demonstrate the methodology using a hypothetical injection and an assumed probability structure for the well permeabilities. We show that for a wide range of parameter values, the amount of leakage is within the bounds suggested as acceptable for climate change mitigation.

9. Al-Housseiny, Talal, Peichun Tsai, Zhong Zheng, and Howard Stone, 2011: The effect of permeability gradients on immiscible displacement in Hele-Shaw flows. American Physical Society,
   [ Abstract ]

   In heterogeneous media, it is well known that when a fluid of high viscosity displaces a less viscous fluid, the interface can still be unstable and exhibit finger-like patterns due to capillary fingering. Motivated by porous media flows in natural geological formations, we consider homogeneous displacement in a Hele-Shaw cell subjected to a permeability gradient. The permeability gradient is introduced by linearly varying the Hele-Shaw cell depth. We study how capillary forces can affect interfacial stability in the presence of the gradient via linear stability analysis. Depending on the system, we find that surface tension can either have a stabilizing or a destabilizing role. We report the emergence of an important dimensionless parameter--the ratio of the permeability gradient to the capillary number--that determines the stability of the interface along with the well-studied viscosity ratio. Experiments testing the theoretical findings will also be presented.

10. Celia, Michael, and Jan M. Nordbotten, 2011: How Simple Can We Make Models for CO₂ Injection, Migration, and Leakage? Energy Procedia, Elsevier, 4, doi:10.1016/j.egypro.2011.02.322 3857-3864
    [ Abstract ]

    Analysis of geological storage of CO₂ almost always involves some set of computational models that provide a mathematical description of the problem. These models can have many purposes, but ultimately they should be able to answer practical questions about the system. These questions usually involve the spatial extent of the CO₂ plume, the spatial extent of pressure perturbations, the spatial and temporal dynamics of leakage out of the injection formation, and the spatial temporal evolution of different trapping mechanisms. Answers to these questions require models that apply to large spatial and temporal scales while including certain small-scale features like leakage pathways. Development of computationally efficient models that can span the appropriate scales may be achieved by analyzing the length and time scales associated with the important processes in the system, and incorporating those scales into a systematic model development. Such a procedure can be described as multi-part on scaling arguments for the physical processes involved, to produce a sequence of successively simpler models. Through this approach, the assumptions in all of the simplified models are made transparent, and the length and time scales appropriate for different models can be identified. In addition, by associating length and time scales to the questions being asked, models can be developed that are consistent with those scales and therefore are appropriate to answer the questions.

11. Celia, Michael, Jan M. Nordbotten, Benjamin Court, M. Dobossy, and S. Bachu, 2011: Field-scale application of a semi-analytical model for estimation of CO₂ and brine leakage along old wells. International Journal of Greenhouse Gas Control, Elsevier, (5), doi:10.1016/j.ijggc.2010.10.005
    [ Abstract ]

    Carbon capture and geological storage (CCS) operations will require an environmental risk analysis to determine, among other things, the risk that injected CO₂ or displaced brine will leak from the injection formation into other parts of the subsurface or surface environments. Such an analysis requires site characterization including identification of potential leakage pathways. In North America, the century-long legacy of oil and gas exploration and production has left millions of oil and gas wells, many of which are co-located with otherwise good geological storage sites. Potential leakage along existing wells, coupled with layered stratigraphic sequences and highly uncertain parameters, makes quantitative analysis of leakage risk a significant computational challenge. However, new approaches to modeling CO₂ injection, migration, and leakage allow for realistic scenarios to be simulated within a probabilistic framework. Using a specific field site in Alberta, Canada, we perform a range of computational studies aimed at risk analysis with a focus on CO₂ and brine leakage along old wells. The specific calculations focus on the injection period, when risk of leakage is expected to be largest. Specifically, we simulate 50 years of injection of supercritical CO₂ and use a Monte Carlo framework to analyze the overall system behavior. The simulations involve injection, migration, and leakage over the 50-year time horizon for domains of several thousand square kilometers having multiple layers in the sedimentary succession and several thousand old wells within the domain. Because we can perform each simulation in a few minutes of computer time, we can run tens of thousands of simulations and analyze the outputs in a probabilistic framework. We use these kinds of simulations to demonstrate the importance of residual brine saturations, the range of current options to quantify leaky well properties, and the impact of depth of injection and how it relates to leakage risk.

12. Court, Benjamin, Thomas R. Elliot, J.A. Dammel, Thomas A. Buscheck, J. Rohmer, and Michael Celia, 2011: Promising synergies to address water, sequestration, legal, and public acceptance issues associated with large-scale implementation of CO₂ sequestration. The Journal for Mitigation and Adaption Strategies for Global Change, Springer, doi:10.1007/s11027-011-9314-x
    [ Abstract ]

    Stabilization of CO₂ atmospheric concentrations requires practical strategies to address the challenges posed by the continued use of coal for baseload-electricity production. Over the next two decades, CO₂ capture and sequestration (CCS) demonstration projects would need to increase several orders of magnitude across the globe in both size and scale. This task has several potential barriers which will have to be accounted for. These barriers include those that have been known for a number of years including safety of subsurface sequestration, pore-space competition with emerging activities like shale gas production, legal and regulatory frameworks, and public acceptance and technical communication. In addition water management is a new challenge that should be actively and carefully considered across all CCS operations. A review of the new insights gained on these previously and newly identified challenges, since the IPCC special report on CCS, is presented in this paper. While somewhat daunting in scope, some of these challenges can be addressed more easily by recognizing the potential advantageous synergies that can be exploited when these challenges are dealt with in combination. For example, active management of water resources, including brine in deep subsurface formations, can provide the additional cooling-water required by the CO₂ capture retrofitting process while simultaneously reducing sequestration leakage risk and furthering efforts toward public acceptance. This comprehensive assessment indicates that water, sequestration, legal, and public acceptance challenges ought to be researched individually, but must also be examined collectively to exploit the promising synergies identified herein. Exploitation of these synergies provides the best possibilities for successful large-scale implementation of CCS.

13. Dobossy, M., Michael Celia, and Jan M. Nordbotten, 2011: An Efficient Software Framework for Performing Industrial Risk Assessment of Leakage for Geological Storage of CO₂. International Conference on Greenhouse Gas Technologies (GHGT 10), Elsevier/Energy Procedia, doi:10.1016/j.egypro.2011.02.368 4207-4214
    [ Abstract ]

    In response to anthropogenic CO₂ emissions, geological storage has emerged as a practical and scalable bridge technology while renewables and other environmentally friendly energy production methods mature. While an attractive solution, geological storage of CO₂ has inherent risk. Two primary concerns are recognized: 1) leakage of CO₂through caprock imperfections, and 2) brine displacement resulting in contamination of drinking water sources. Three mechanisms for both CO₂ and brine leakage have been identified: diffuse leakage through the caprock, leakage through faults and fractures in the caprock, and finally, leakage through man-made pathways such as abandoned wells from oil and gas exploration. While the first two leakage mechanisms are important, we emphasize the risks associated with the presence of abandoned wells. This is due to the large number and density of wells from a history of oil and gas exploration around the world, and the high degree of uncertainty surrounding the properties of these abandoned wells. With current proposed legislation in both the United States and Europe, a need is emerging for practical assessment of leakage risk. In order to accurately predict leakage of brine and CO₂ from the injection layer, the geological information for the injection site and the location and makeup of the man-made leakage pathways previously alluded to must be taken into account. Unfortunately, both the geology and abandoned well metadata are typically high in uncertainty, which must be accounted for. With such a high number of random variables, the current state of the art is running many realizations of a system, using a Monte Carlo approach. This requires that the underlying solution algorithms be accurate, and efficient. In the past, many researchers in both academia and industry have turned to robust numerical analysis packages used in the oil industry. However, due to the large range of scales important to this problem (domains of tens of kilometers on a side affected by leakage pathways with diameters of tens of centimeters) such modeling techniques become computationally expensive for all but the most basic analysis. A computational model developed at Princeton University, and currently being commercialized by Geological Storage Consultants, LLC has been shown to be efficient with sufficient accuracy to allow for comprehensive risk assessment of CO₂ injection projects. The model allows for mixing solution methods- using computationally expensive algorithms for formations of greater importance (e.g.- the injection formation) and more efficient, simplified algorithms in other areas of the domain. This ability to arbitrarily mix solution methods offers significant flexibility in the design and execution of models. This paper addresses the framework and algorithms used, and illustrates the importance of efficiency and parallelism using the case study of an injection site in Alberta, Canada. We show how the framework can be used for project planning, for risk mitigation (insurance), and for regulatory groups. Finally, the importance of flexible analysis tools that allow for efficient and effective management of computational resources is discussed.

14. Ellis, B. R., Grant S. Bromhal, Dustin L. McIntyre, and Catherine A. Peters, 2011: Changes in caprock integrity due to vertical migration of CO₂-enriched brine. Energy Procedia, 4, doi:10.1016/j.egypro.2011.02.514 5327-5334
    [ Abstract ]

    In geologic carbon sequestration, caprock fractures may act as leakage pathways, threatening the long term sealing ability of the formation. A flow-through experiment was performed to investigate fracture evolution of a fractured carbonate caprock during simulated leakage of CO₂-acidified brine. The initial brine composition represented that of a CO₂-saturated brine having previously reacted with the injection formation minerals resulting in a starting pH of 4.9. Experimental temperature and pressure conditions were 40°C and 10 MPa, corresponding to injection at a depth of 1 km. A combination of X-ray computed tomography and scanning electron microscopy was used to observe fracture evolution and investigate the mineralogical changes that occurred along the fracture wall. After one week of brine flow, the cross-sectional fracture area increased by an average of 2.7 times that of the initial fracture. The fracture surface was not eroded uniformly, with the largest areas of aperture growth corresponding to direct contact between the acidified brine and calcite. This preferential dissolution of calcite led to a large increase in fracture surface roughness and in some instances, created a silicate mineral-rich microporous coating along the fracture wall. Results from this study suggest that the clay content of low permeability carbonate formations may be an important factor in controlling their long term integrity while in contact with acidified brine and should be considered when selecting appropriate injection sites for geologic CO₂ sequestration.

15. Gasda, S., Jan M. Nordbotten, and Michael Celia, 2011: The impact of local-scale processes on large-scale CO₂ migration and immobilization. International Conference on Greenhouse Gas Technologies (GHGT 10), Elsevier/Energy Procedia, doi:10.1016/j.egypro.2011.02.327 3896-3903
    [ Abstract ]

    Storage security of injected carbon dioxide (CO₂) is an essential component of risk management for geological carbon sequestration operations. During the injection and early post-injection periods, CO₂ leakage may occur along faults and leaky wells, but this risk may be partly managed by proper site selection and sensible deployment of monitoring and remediation technologies. On the other hand, long-term storage security is an entirely different risk management problem—one that is dominated by a mobile CO₂ plume that may travel over very large spatial distances, over long time periods, before it is trapped by a variety of different physical and chemical processes. In the post-injection phase, the mobile CO₂ plume migrates in large part due to buoyancy forces, following the natural topography of the geological formation. The primary trapping mechanisms are capillary and solubility trapping, which evolve over thousands to tens of thousands of years and can immobilize a significant portion of the mobile, free-phase CO₂ plume. However, both the migration and trapping processes are inherently complex, involving a combination of small and large spatial scales and acting over a range of time scales. Solubility trapping is a prime example of this complexity, where small-scale density instabilities in the dissolved CO₂ region leads to convective mixing that has that has a significant effect on the large-scale dissolution process over very long time scales. Another example is the effect of capillary forces on the evolution of mobile CO₂, an often-neglected process except with regard to residual trapping. As the plume migrates due to buoyancy and viscous forces, local capillary effects acting at the CO₂-brine interface lead to a transition zone where both fluids are present in the mobile state. This small-scale effect may have a significant impact on large-scale plume migration as well as long-term residual and dissolution trapping. Using appropriate models that can capture both large and small-scale effects is essential for understanding the role of these processes on the long-term storage security of CO₂ sequestration operations. There are several approaches to modeling long-term CO₂ trapping mechanisms. One modeling option is the use of traditional numerical methods, which are often highly sophisticated models that can handle multiple complex phenomena with high levels of accuracy. However, these complex models quickly become prohibitively expensive for the type of large-scale, long-term modeling that is necessary for risk assessment applications such as the late post-injection period. We present an alternative modeling option that combines vertically-averaged governing equations with an upscaled representation of the dissolutionconvective mixing process and the local capillary transition zone at the CO₂-brine interface. CO₂ injection is solved numerically on a coarse grid, capturing the large-scale injection problem and the post-injection capillary trapping, while the upscaled dissolution and capillary fringe models capture these subscale effects and eliminate the need for expensive grid refinement to capture the subscale instabilities associated with convective mixing or the details of the capillary transition zone. With thismodeling approach, we demonstrate the effect of different modeling choices associated with dissolution and capillary processes for typical large-scale geological systems.

16. Gasda, S., Jan M. Nordbotten, and Michael Celia, 2011: Vertically averaged approaches for CO₂ migration with solubility trapping. Water Resources Research, American Geophysical Union, 47(W05528), doi:10.1029/2010WR009075 1-14
    [ Abstract ]

    The long-term storage security of injected carbon dioxide (CO₂) is an essential component of geological carbon sequestration operations. In the postinjection phase, the mobile CO₂ plume migrates in large part because of buoyancy forces, following the natural topography of the geological formation. The primary trapping mechanisms are capillary and solubility trapping, which evolve over hundreds to thousands of years and can immobilize a significant portion of the mobile CO₂ plume. However, both the migration and trapping processes are inherently complex, spanning multiple spatial and temporal scales. Using an appropriate model that can capture both large- and small-scale effects is essential for understanding the role of these processes on the long-term storage security of CO₂ sequestration operations. Traditional numerical models quickly become prohibitively expensive for the type of large-scale, long-term modeling that is necessary for characterizing the migration and immobilization of CO₂ during the postinjection period. We present an alternative modeling option that combines vertically integrated governing equations with an upscaled representation of the dissolution-convection process. With this approach, we demonstrate the effect of different modeling choices for typical large-scale geological systems and show that practical calculations can be performed at the temporal and spatial scales of interest.

17. Gasda, S., James Z. Wang, and Michael Celia, 2011: Analysis of in-situ wellbore integrity data for existing wells with long-term exposure to CO₂. Energy Procedia, Elsevier, 4, doi:10.1016/j.egypro.2011.02.525 5406-5413
    [ Abstract ]

    An important aspect of the risk associated with geological carbon dioxide sequestration is the integrity of existing wellbores that penetrate geological layers targeted for CO₂ injection. CO₂ leakage may occur through multiple pathways along a wellbore within the 'disturbed zone' surrounding the well casing. The disturbed zone is defined as the annular region along the exterior of the steel wellbore casing that includes the Portland cement sheath, the damage zone of the host rock and the casing-cement-rock interfaces. The effective permeability of this zone is a key parameter of wellbore integrity required for validation of numerical models. Effective permeability may depend on a number of complex factors, including long-term attack by aggressive fluids, poor well completion or actions related to production of fluids through the wellbore. Field tests are essential to understanding the in situ leakage properties of the millions of wells that exist in mature sedimentary basins in North America. We present results from recent field studies of different CO₂ producing wells from both natural CO₂ reservoirs and enhanced oil recovery (EOR) operations. These surveys have included a particular downhole pressure test, the vertical interference test (VIT), designed to determine the extent of hydraulic communication along the exterior of the well casing. The VIT test involves perforating the well casing in two separate intervals, both of which are located within the shale caprock and bracket a zone of cement identified to have a lower quality bond. Once the intervals are isolated with an inflatable packer, the system is pressurized from surface and held at a constant pressure, while simultaneously, the transient pressure response is measured in the lower isolated interval. The pressure transient data is an indicator of the extent of hydraulic communication and is the focus of subsequent analysis. The effective wellbore permeability can be determined through numerical analysis of the VIT data. Our objective is to identify to most effective method of analysis for estimating wellbore permeability. We evaluate two different automated parameter estimation methods, nonlinear regression and shuffled complex evolution metropolis methods. Within this study, we also estimated parameters such as permeability and compressibility of the low permeability shale zone to determine their effect on the resulting estimate of wellbore permeability. The results of this work demonstrate that parameter estimation can be effective at identifying the key parameters associated with wellbore integrity from VIT field tests, and ultimately reducing the uncertainty regarding the integrity of existing wellbores.

18. Goumiri, I., Jean Hervé Prévost, and M. Preisig, 2011: The effect of capillary pressure on the saturation equation of two-phase flow in porous media. International Journal for Numerical and Analytical Methods in Geomechanics, John Wiley & Sons, Ltd., doi:10.1002/nag.1022
    [ Abstract ]

    A complete and accurate simulation of two-phase flow in porous media requires knowledge of all the controlling physics (and values of physical parameters) that play a relevant role and an understanding of the effects of each one on the solution. Of particular concern here is the effect of capillary pressure and the length scale over which it is relevant. The goal of this paper is to provide guidance onto when to include the effects of capillary pressure in the model, and onto what are the resulting length scale restrictions if those effects are to be included.

19. Kim, D., Catherine A. Peters, and W. B. Lindquist, 2011: Upscaling geochemical reaction rates accompanying acidic CO₂-saturated brine flow in sandstone aquifers. Water Resources Research, American Geophysical Union, 47, W01505, doi:10.1029/2010WR009472 1-16
    [ Abstract ]

    Network flow models were used to simulate the flow of CO₂-saturated brine in the pore networks corresponding to three different sandstones. The simulations were used to study upscaling of anorthite and kaolinite reaction rates from pore to core scales. Unique to our simulations is the use of computed tomography to capture the mineral distribution in the samples as well as the sample pore network. The upscaled reaction rates determined from these simulations incorporate mass balance principles and microscale reaction rate laws and capture the physical, mineral, and flow heterogeneities in the network. These upscaled rates were compared with upscaled rates predicted by a continuum model and by a volumeaveraged- concentration method. For the anorthite reaction, which remains far from equilibrium, the volume-averaged reaction rate exceeded the reaction rate of the network model by 18% to 46%. While the continuum model rate also exceeded the network model rate by −1% to 53%, its predicted values were generally better than the volume-averaged method. The kaolinite reaction is near equilibrium and is heavily influenced by the form of the microscale rate law in the precipitation regime. Three alternate rate laws were tested, which produced significantly different predictions for the bulk reaction rates. For all three rate laws, continuum and volume-averaged reaction rates incorrectly predicted the magnitude of the kaolinite reaction rate (disagreements of −700% to 55%), and the predicted reaction type, dissolution versus precipitation, was also often opposite to that of the network model. Finally, for both anorthite and kaolinite, all upscaled reaction rates showed significant flow rate dependence.

20. Liu, Yang, Athanassios Z. Panagiotopoulos, and Pablo Debenedetti, 2011: Monte Carlo Simulations of High-Pressure Phase Equilibria of CO₂-H₂O Mixtures. Journal of Physical Chemistry, American Chemical Society, B(115 (20)), doi:10.1021/jp201520u 6629-6635
    [ Abstract ]

    Histogram-reweighting grand canonical Monte Carlo simulations were used to obtain the phase behavior of CO₂—H₂O mixtures over a broad temperature and pressure range (50 °C ≤ T ≤ 350 °C, 0 ≤ P ≤ 1000 bar). We performed a comprehensive test of several existing water (SPC, TIP4P, TIP4P2005, and exponential-6) and carbon dioxide (EPM2, TraPPE, and exponential-6) models using conventional Lorentz—Berthelot combining rules for the unlike-pair parameters. None of the models we studied reproduce adequately experimental data over the entire temperature and pressure range, but critical assessments were made on the range of T and P where particular model pairs perform better. Away from the critical region (T ≤ 250 °C), the exponential-6 model combination yields the best predictions for the CO₂-rich phase, whereas the TraPPE/TIP4P2005 model combination provides the most accurate coexistence composition and pressure for the H₂O-rich phase. Near the critical region (250 °C ≤ T ≤ 350 °C), the critical points are not accurately estimated by any of the models studied, but the exponential-6 models are able to qualitatively capture the critical loci and the shape of the phase envelopes. Local improvements can be achieved at specific temperatures by introducing modification factors to the Lorentz—Berthelot combining rules, but the modified combining rule is still not able to achieve global improvements over the entire temperature and pressure range. Our work points to the challenge and importance of improving current atomistic models so as to accurately predict the phase behavior of this important binary mixture.

21. Matteo, Edward, George Scherer, Bruno M. Huet, and Leo Pel, 2011: Understanding Boundary Condition Effects on the Corrosion Kinetics of Class H Well Cement. Energy Procedia, Elsevier, 4, doi:10.1016/j.egypro.2011.02.520 5370-5376
    [ Abstract ]

    Storing carbon dioxide in depleted petroleum reservoirs is a viable strategy for carbon mitigation, but ensuring that the sequestered CO₂ remains in the formation is vital to the success of such projects. There is great concern for the development of leakage pathways through annuli between the well cement and the formation or the casing. Predicting the behavior of such potential leakage pathways is critical. Numerical simulations conducted using a reactive transport module match well with experimental studies [1], but also show the necessity of quantifying the transport and mechanical properties of the leached solid cementitious solids--predominantly silica gel-- produced by carbonic acid corrosion of well cement. Bench-top experiments have been performed with the following goals in mind: 1) to investigate the parameter space of relevant corrosion boundary conditions, e.g. pH, CO₂ concentration, and calcium ion concentration, 2) to produce samples that can be used to quantify the transport and mechanical properties of acid corroded Class H well cement, and 3) to validate and improve the accuracy of numerical simulations of the reaction of well cement with carbonic acid. Class H cement samples were uniaxially corroded via exposure to a brine of constant composition. Constant composition is ensured by constant renewal of the brine at a rate larger than cement reaction rate. H⁺, Ca²⁺ and CO₂ total aqueous concentration in the NaCl brine are controlled independently by adding known amounts of NaCl, HCl, CaCl₂ and NaHCO₃ and by controlling CO₂ partial pressure. Microscopic (30X) time-lapse videos were taken of each sample so that corrosion front movements could be accurately measured. These experiments have yielded corrosion front measurements that clearly show that corrosion front advancement is diffusion controlled (i.e., linear as a function of the square root of time). The uniaxial corrosion of these samples has not only allowed for detailed measurements of the corrosion front, but also affords the opportunity to measure the mechanical properties of the corroded samples as a function of depth. The one-dimensional corrosion also allows for measuring the diffusion coefficient of the outer layer of silica gel by low field Nuclear Magnetic Resonance (NMR). Measuring the kinetics under various boundary conditions has validated the modeling results reported by Huet et al. [1]. The measurements of mechanical and transport properties can now be used to improve the predictive power of these simulations by providing much needed information on the exterior layer of corroded Class H well cement. Additionally, these experiments offer experimental validation that the corrosion kinetics are enhanced by the presence of CO₂ and open the door to better understanding of the mechanism of, and boundary conditions that might lead to, "pore-plugging" by the corrosion products, which in turn leads to a drastic retardation of the corrosion reaction.

22. Nogues, J. P., Jan M. Nordbotten, and Michael Celia, 2011: Detecting leakage of brine or CO₂ through abandoned wells in a geological sequestration operation using pressure monitoring wells. Energy Procedia, 4, doi:10.1016/j.egypro.2011.02.292 3620-3627
    [ Abstract ]

    For risk assessment, policy design and GHG emission accounting it is extremely important to know if any CO₂ or brine has leaked from a geological sequestration (GS) operation. As such, it is important to understand if it is possible to use certain technologies to detect it. This detection of leakage is one of the most challenging problems associated with GS due to the high uncertainty in the nature and location of leakage pathways. In North America for example millions of legacy oil and gas wells present the possibility of CO₂ and brine to leak out of the injection formation. The available information for these potential leaky wells is very limited and the main parameters that control leakage, like permeability of the sealing material are not known. Here we propose to explore the possibility of detecting such leakage by the use of pressure-monitoring wells located in a formation overlying the injection formation. The detection analysis is based on a system of equations that solve for the propagation of a pressure pulse using the superposition principle and an approximation to the well function. We explore the questions of what can be gained by using pressure-monitoring wells and what are the limitations given a specific accuracy threshold of the measuring device. We also try to answer the question of where these monitoring wells should be placed to optimize the objective of a monitoring scheme. We believe these results can ultimately lead to practical design strategies for monitoring schemes, including quantitative estimation of increased probability of leak detection per added observation well.

23. Nordbotten, Jan M., and H. Dahle, 2011: Impact of the capillary fringe in vertically integrated models for CO₂ storage. Water Resources Research, American Geophysical Union, 47, W02537, doi:10.1029/2009WR008958 1-11
    [ Abstract ]

    This paper investigates vertically integrated equilibrium models for CO₂ storage. We pay particular attention to the importance of including the effect of fine-scale capillary forces in the integrated equations. This aspect has been neglected in previous work, where the fluids are segregated by a sharp interface. Our results show that the fine-scale capillary forces lead to qualitative and quantitative alterations of the integrated equations. Interestingly, while such forces are dispersive on the fine scale, they lead to self-sharpening of the solution on the integrated scale. We discuss these aspects for injection, leakage, and long-term migration through the application by comparison to common sharp interface models proposed in the literature.

24. Peters, Catherine A., P. F. Dobson, C. M Oldenburg, Joseph S.Y. Wang, T. C. Onstott, George Scherer, B. Freifeld, T. S. Ramakrishnan, Eric L. Stabinski, Kenneth Liang, and Sandeep Verma, 2011: LUCI: A facility at DUSEL for large-scale experimental study of geologic carbon sequestration. Energy Procedia, Elsevier, 4, doi:10.1016/j.egypro.2011.02.478 5050-5057
    [ Abstract ]

    LUCI, the Laboratory for Underground CO₂ Investigations, is an experimental facility being planned for the DUSEL underground laboratory in South Dakota, USA. It is designed to study vertical flow of CO₂ in porous media over length scales representative of leakage scenarios in geologic carbon sequestration. The plan for LUCI is a set of three vertical column pressure vessels, each of which is ∼500 m long and ∼1 m in diameter. The vessels will be filled with brine and sand or sedimentary rock. Each vessel will have an inner column to simulate a well for deployment of down-hole logging tools. The experiments are configured to simulate CO₂ leakage by releasing CO₂ into the bottoms of the columns. The scale of the LUCI facility will permit measurements to study CO₂ flow over pressure and temperature variations that span supercritical to subcritical gas conditions. It will enable observation or inference of a variety of relevant processes such as buoyancy-driven flow in porous media, Joule-Thomson cooling, thermal exchange, viscous fingering, residual trapping, and CO₂ dissolution. Experiments are also planned for reactive flow of CO₂ and acidified brines in caprock sediments and well cements, and for CO₂-enhanced methanogenesis in organic-rich shales. A comprehensive suite of geophysical logging instruments will be deployed to monitor experimental conditions as well as provide data to quantify vertical resolution of sensor technologies. The experimental observations from LUCI will generate fundamental new understanding of the processes governing CO₂ trapping and vertical migration, and will provide valuable data to calibrate and validate large-scale model simulations.

25. Preisig, M., and Jean Hervé Prévost, 2011: Coupled multi-phase thermo-poromechanical effects. Case study: CO₂ injection at In Salah, Algeria. International Journal of Greenhouse Gas Control, doi:10.1016/j.ijggc.2010.12.006
    [ Abstract ]

    Coupled simulations of fluid injection and extraction in porous media are an important tool for assessing feasibility, safety and productivity of such operations. Different methods for coupling the fluid flow with geomechanics are currently used. In this paper we compare one-way and iterative coupling with full coupling. With the fully coupled two-phase thermo-poromechanical model we simulate the CO₂ injection operation, which is ongoing at In Salah, Algeria. The results suggest that pressure increase in the well and ground uplift for a given data set can accurately be modeled using our method. Finally, we illustrate the crucial effect of the temperature difference between injected fluid and reservoir on the possibility of creating and/or re-opening fractures perpendicular to the well in the cap rock that were observed in the field.

26. Preisig, M., and Jean Hervé Prévost, 2011: Fully coupled simulation of fluid injection into geomaterials with focus on nonlinear near-well behavior. International Journal for Numerical and Analytical Methods in Geomechanics, Wiley Online Library, doi:10.1002/nag.1039
    [ Abstract ]

    An important part of our global wealth depends on the extraction of fluids from porous media. More recently, sequestration of carbon dioxide (CO₂) into deep geological layers as a possible measure to mitigate climate change has increased interest in fluid injection into porous media. Sophisticated numerical models play an important role in managing the uncertainties related to the subsurface, and finite element methods are the most versatile tool allowing the coupling of fluid flow, geomechanics and other physical processes. This paper gives insight into two important aspects of fluid injection/extraction in porous media: the correct modeling of the bore hole through specification of initial stresses, which together with a fully coupled strategy allows simulation of nonlinear poromechanics, and the imposition of appropriate boundary conditions that allow the controlled injection/extraction of a total specified amount of fluid in an anisotropic porous medium, without exceeding a safe operating pressure.

27. Sarupria, S., and Pablo Debenedetti, 2011: Molecular Dynamics Study of Carbon Dioxide Hydrate Dissociation. Journal of Physical Chemistry, American Chemical Society, A(115 (23)), doi:10.1021/jp110868t 6102-6111
    [ Abstract ]

    We present results from a molecular dynamics study of the dissociation behavior of carbon dioxide (CO₂) hydrates. We explore the effects of hydrate occupancy and temperature on the rate of hydrate dissociation. We quantify the rate of dissociation by tracking CO₂ release into the liquid water phase as well as the velocity of the hydrate
    liquid water interface. Our results show that the rate of dissociation is dependent on the fractional occupancy of each cage type and cannot be described simply in terms of overall hydrate occupancy. Specifically, we find that hydrates with similar overall occupancy differ in their dissociation behavior depending on whether the small or large cages are empty. In addition, individual cages behave differently depending on their surrounding environment. For the same overall occupancy, filled small and large cages dissociate faster in the presence of empty large cages than when empty small cages are present. Therefore, hydrate dissociation is a collective phenomenon that cannot be described by focusing solely on individual cage behavior.

28. Scherer, George, B. Kutchko, N. Thaulow, A. Duguid, and Bryant Mook, 2011: Characterization of cement from a well at Teapot Dome Oil Field: Implications for geological sequestration. International Journal of Greenhouse Gas Control, Elsevier, (5), doi:10.1016/j.ijggc.2010.06.010 115-124
    [ Abstract ]

    Wellbores represent the weakest link in terms of CO₂ storage permanence. As a result, special attention to the numerous existing wells that perforate storage formations is needed. The pre-injection condition of the cement can influence the rate (and type) of alteration by the injected CO₂ plume. The condition of the existing well cement depends on a variety of factors including wellbore/formation and wellbore/brine interactions as well as the composition and type of cement placed in the well (i.e. type of admixtures used, water/solids ratio, sulfate resistant mixes, etc.). In this paper, the details of recovering wellbore cement from an older well to determine pre-injection seal integrity are described. Petrographical and chemical analyses are presented for samples of cement that were retrieved from a 19-year-old well at Teapot Dome in Wyoming. Examination revealed that the retrieved cement had altered as a result of original slurry composition and with respect to the local downhole wellbore environment. Although samples were obtained from a single well, significant differences were observed in their alteration and condition. Sulfate attack resulted in abundant ettringite formation in a cement sample taken adjacent to the Wall Creek sandstone (3060 ft), while cement taken adjacent to the Tensleep formation (5478 ft) was decalcified and enriched in magnesium, owing to reaction of calcium hydroxide in the cement with the dolomitic formation.

29. Thomas, Jeffrey J., Joseph J. Biernacki, J.W. Bullard, Shashank Bishnoi, Jorge S. Dolado, George Scherer, and Andreas Luttge, 2011: Modeling and simulation of cement hydration kinetics and microstructure development. Cement and Concrete Research, doi:10.1016/j.cemconres.2010.10.004
    [ Abstract ]

    Efforts to model and simulate the highly complex cement hydration process over the past 40 years are reviewed, covering different modeling approaches such as single particle models, mathematical nucleation and growth models, and vector and lattice-based approaches to simulating microstructure development. Particular attention is given to promising developments that have taken place in the past few years. Recent applications of molecular-scale simulation methods to understanding the structure and formation of calcium-silicate-hydrate phases, and to understanding the process of dissolution of cement minerals in water are also discussed, as these topics are highly relevant to the future development of more complete and fundamental hydration models.

30. Bullard, J.W., Hamlin M. Jennings, Richard A. Livingston, Andre Nonat, George Scherer, Jeffrey S. Schweitzer, Karen L. Scrivener, and Jeffrey J. Thomas, 2010: Mechanisms of Cement Hydration. Cement and Concrete Research, Elsevier, doi:10.1016/j.cemconres.2010.09.011
    [ Abstract PDF ]

    The current state of knowledge of cement hydration mechanisms is reviewed, including the origin of the period of slow reaction in alite and cement, the nature of the acceleration period, the role of calcium sulfate in modifying the reaction rate of tricalcium aluminate, the interactions of silicates and aluminates, and the kinetics of the deceleration period. In addition, several remaining controversies or gaps in understanding are identified, such as the nature and influence on kinetics of an early surface hydrate, the mechanistic origin of the beginning of the acceleration period, the manner in which microscopic growth processes lead to the characteristic morphologies of hydration products at larger length scales, and the role played by diffusion in the deceleration period. The review concludes with some perspectives on research needs for the future.

31. Court, Benjamin, Michael Celia, Jan M. Nordbotten, and Thomas R. Elliot, 2010: Active and Integrated Management of Water Resources Throughout CO₂ Capture and Sequestration Operations. International Conference on Greenhouse Gas Technologies (GHGT 10), Elsevier/Energy Procedia,
    [ Abstract ]

    Most projected climate change mitigation strategies will require a significant expansion of CO₂ Capture and Sequestration (CCS) in the next two decades. Four major categories of challenges are being actively researched: CO₂ capture cost, geological sequestration safety, legal and regulatory barriers, and public acceptance. Herein we propose an additional major challenge category across all CCS operations: water management. For example a coal-fired power plant retrofitted for CCS requires twice as much cooling water as the original plant. This increased demand may be accommodated by brine extraction and treatment, which would concurrently function as large-scale pressure management and a potential source of freshwater. At present the interactions among freshwater extraction, CO₂ injection, and brine management are being considered too narrowly -in the case of freshwater almost completely overlooked- in the technical and regulatory CCS community. This paper presents an overview of each of these challenges and potential integration opportunities. Active management of CCS operations through an integrated approach -including brine production, treatment, use for cooling, and partial reinjection- can address challenges simultaneously with several synergistic advantages. The paper also considers the related potential impacts of pore space competition (with future groundwater use, gas storage and shale gas) on CCS expansion. Freshwater and brine must become key decision making inputs throughout CCS operations, building on existing successful industrial-scale integrations.

32. Crow, W., J. W. Carey, S. Gasda, D. B. Williams, and Michael Celia, 2010: Wellbore integrity analysis of a natural CO₂ producer. International Journal of Greenhouse Gas Control, Elsevier, (4), doi:10.1016/j.ijggc.2009.10.010 186-197
    [ Abstract ]

    Long-term integrity of existing wells in a CO₂-rich environment is essential for ensuring that geological sequestration of CO₂ will be an effective technology for mitigating greenhouse gas-induced climate change. The potential for wellbore leakage depends in part on the quality of the original construction as well as geochemical and geomechanical stresses that occur over its life-cycle. Field data are essential for assessing the integrated effect of these factors and their impact on wellbore integrity, defined as the maintenance of isolation between subsurface intervals. In this report, we investigate a 30-year-old well from a natural CO₂ production reservoir using a suite of downhole and laboratory tests to characterize isolation performance. These tests included mineralogical and hydrological characterization of 10 core samples of casing/cement/formation, wireline surveys to evaluate well conditions, fluid samples and an in situ permeability test. We find evidence for CO₂ migration in the occurrence of carbonated cement and calculate that the effective permeability of an 11'-region of the wellbore barrier system was between 0.5 and 1 milliDarcy. Despite these observations, we find that the amount of fluid migration along the wellbore was probably small because of several factors: the amount of carbonation decreased with distance from the reservoir, cement permeability was low (0.3-30 microDarcy), the cement-casing and cement-formation interfaces were tight, the casing was not corroded, fluid samples lacked CO₂, and the pressure gradient between reservoir and caprock was maintained. We conclude that the barrier system has ultimately performed well over the last 3 decades. These results will be used as part of a broader effort to develop a long-term predictive simulation tool to assess wellbore integrity performance in CO₂ storage sites.

33. Duguid, A., and George Scherer, 2010: Degradation of oilwell cement due to exposure to carbonated brine. International Journal of Greenhouse Gas Control, 4, doi:10.1016/j.ijggc.2009.11.001 546–560
    [ Abstract ]

    The growing interest in geologic carbon sequestration has highlighted the need for more data on how well cements react to CO₂ exposure. This paper describes a series of experiments that was conducted to examine the effects of flowing carbonated brine on well cements. Class H cement pastes were exposed to the ranges of temperature (20–50 °C) and pH (2.4–5) characteristic of geosequestration conditions at a depth of about 1 km. The exposed cements and the reactor effluents were analyzed using multiple techniques including optical microscopy, X-ray diffraction, EPMA, and ICP-OES. The results showed that if the solution was pre-equilibrated with calcium carbonate, as would be expected in a limestone formation, there was no detectable attack. However, under the pH and temperature conditions to be expected in a sandstone formation, the initial rate of attack was on the order of millimeters per month. The outer layers of the cements reacted under sandstone-like conditions were fully degraded based on the results of the XRD and EPMA analyses. Inside the degraded layers there was a calcium carbonate-rich layer, a layer depleted of calcium hydroxide, and an unreacted cement core. The rate of degradation of the cement in these experiments was controlled by the rate of dissolution of the calcium carbonate-rich layer, after its formation, and diffusion through the fully degraded layers.

34. Ellis, B. R., Lauren E. Crandell, and Catherine A. Peters, 2010: Limitations for brine acidification due to SO₂ co-injection in geologic carbon sequestration. International Journal of Greenhouse Gas Control, Elsevier, (4), doi:10.1016/j.ijggc.2009.11.006 575-582
    [ Abstract ]

    Co-injection of sulfur dioxide during geologic carbon sequestration can cause enhanced brine acidification. The magnitude and timescale of this acidification will depend, in part, on the reactions that control acid production and on the extent and rate of SO₂ dissolution from the injected CO₂ phase. Here, brine pH changes were predicted for three possible SO₂ reactions: hydrolysis, oxidation, or disproportionation. Also, three different model scenarios were considered, including models that account for diffusion-limited release of SO₂ from the CO₂ phase. In order to predict the most extreme acidification potential, mineral buffering reactions were not modeled. Predictions were compared to the case of CO₂ alone which would cause a brine pH of 4.6 under typical pressure, temperature, and alkalinity conditions in an injection formation. In the unrealisticmodel scenario of SO₂ phase equilibrium between the CO₂ and brine phases, co-injection of 1% SO₂ is predicted to lead to a pH close to 1 with SO₂ oxidation or disproportionation, and close to 2 with SO₂ hydrolysis. For a scenario in which SO₂ dissolution is diffusion-limited and SO₂ is uniformly distributed in a slowly advecting brine phase, SO₂ oxidation would lead to pH values near 2.5 but not until almost 400 years after injection. In this scenario, SO₂ hydrolysis would lead to pH values only slightly less than those due to CO₂ alone. When SO₂ transport is limited by diffusion in both phases, enhanced brine acidification occurs in a zone extending only 5 m proximal to the CO₂ plume, and the effect is even less if the only possible reaction is SO₂ hydrolysis. In conclusion, the extent to which co-injected SO₂ can impact brine acidity is limited by diffusion-limited dissolution from the CO₂ phase, and may also be limited by the availability of oxidants to produce sulfuric acid.

35. Goumiri, I., and Jean Hervé Prévost, 2010: Cell to Node Projections: An Assessment of Error. International Journal for Numerical and Analytical Methods in Geomechanics, doi:10.1002/nag.927 1-10
    [ Abstract ]

    Reservoir simulators typically use cell-centered finite volume schemes and do not model directly the coupling of the flow processes with the geomechanics. Coupling of geomechanics with fluid flow can be important in many cases, but introducing fully coupled geomechanical effects in those simulators is not a trivial issue, because the geomechanics is better done by using the Galerkin vertex-centered finite element methods by which the solid displacements are computed at the vertices of the cells. This creates difficulties in interfacing cell variables with nodal variables. Uncoupled or loosely coupled models are used by many researchers/practitioners by which a reservoir model is coupled to a geomechanical model by staggering in-time flow and deformation via a sophisticated interface that repeatedly calls first flow and then mechanics. The method therefore requires projection of the reservoir cell variables onto the nodes of the geomechanics Galerkin finite element mesh. In this note, we attempt to quantify the errors associated with cell to node projection operations. For that purpose, we use a simple model of the pressure equation for a heterogeneous medium in one dimension. We are able to derive the exact analytical solution for this problem for both nodal and cell pressures. This allows us to compute the errors due to projection analytically, function of meshing refinement and permeability field variations. We compute upper and lower bounds for the errors, and analyze their magnitude for a variety of cases. We conclude that, in general, cell to node projection operations lead to substantial errors.

36. Huet, Bruno M., Jean Hervé Prévost, and George Scherer, 2010: Quantitative reactive transport modeling of Portland cement in CO₂-saturated. International Journal of Greenhouse Gas Control, 4, doi:10.1016/j.ijggc.2009.11.003 561-574
    [ Abstract ]

    A modular reactive transport model, Dynaflow^TM, is used to simulate the reactivity of cement in CO₂- saturated water of intermediate salinity (0.5 M). Methodology for coupling transport and geochemical modules is derived and its assumptions are discussed. The modules are coupled in a sequential iterative approach to accurately model: (1) mineral dissolution/precipitation (2) aqueous phase speciation and (3) porosity-dependent transport properties. Simulation results reproduce qualitatively the dissolution of cement hydrates (CH, C-S-H, AFm, AFt) and intermediate products (CaCO₃) that have been observed experimentally. However, when using a standard power law to relate effective transport properties to porosity, modeling and experimental results do not coincide; here, agreement between simulations and observations is obtained by modifying the functional dependence of effective diffusivity on mineralogy. Furthermore, for this particular system for which concentration gradients are the only driving force, the assumption of neglecting the mass balance of water or density changes might show its limits. Therefore, future work should investigate the likely need to account for reaction-driven advection.

37. Liu, Yang, Athanassios Z. Panagiotopoulos, and Pablo Debenedetti, 2010: Finite-size scaling study of the vapor-liquid critical properties of confined fluids: crossover from three dimensions to two dimensions. Journal of Chemical Physics, 132, doi:10.1063/1.3377089 144107-144107-10
    [ Abstract ]

    We perform histogram-reweighting grand canonical Monte Carlo simulations of the Lennard-Jones fluid confined between two parallel hard walls and determine the vapor-liquid critical and coexistence properties in the range of
    σ≤H≤6σ
    and 10σ≤L_x ,L_y
    ≤28σ, where H is the wall separation, L_x=L_y is the system size and
    is the characteristic length. By matching the probability distribution of the ordering operator, P(M), to the three-dimensional 3(D) and two-dimensional 2(D) Ising universality classes according to the mixed-field finite-size scaling approach, we establish a “phase diagram” in the (H,L) plane, showing the boundary between four types of behavior: 3D, quasi-3D, quasi-2D, and 2D. In order to facilitate 2D critical point calculation, we present a four-parameter analytical expression for the 2D Ising universal distribution. We show that the infinite-system-size critical points obtained by extrapolation from the apparent 3D and 2D critical points have only minor differences with each other. In agreement with recent reports in the literature [Jana et al., J. Chem. Phys. 130, 214707
    2009], we find departure from linearity in the relationship between critical temperature and inverse wall separation, as well as nonmonotonic dependence of the critical density and the liquid density at coexistence upon wall separation. Additional studies of the ST2 model of water show similar behavior, which suggests that these are quite general properties of confined fluids. © 2010 American Institute of Physics.

38. Matteo, Edward, George Scherer, Bruno M. Huet, and Leo Pel, 2010: Understanding Boundary Condition Effects on the Corrosion Kinetics of Class H Well Cement. International Conference on Greenhouse Gas Technologies (GHGT 10), Elsevier/Energy Procedia,
    [ Abstract ]

    Storing carbon dioxide in depleted petroleum reservoirs is a viable strategy for carbon mitigation, but ensuring that the sequestered CO₂ remains in the formation is vital to the success of such projects. There is great concern for the development of leakage pathways through annuli between the well cement and the formation or the casing. Predicting the behavior of such potential leakage pathways is critical. Numerical simulations conducted using a reactive transport module match well with experimental studies [1], but also show the necessity of quantifying the transport and mechanical properties of the leached solid cementitious solids -- predominantly silica gel -- produced by carbonic acid corrosion of well cement. Bench-top experiments have been performed with the following goals in mind: 1) to investigate the parameter space of relevant corrosion boundary conditions, e.g. pH, CO₂ concentration, and calcium ion concentration, 2) to produce samples that can be used to quantify the transport and mechanical properties of acid corroded Class H well cement, and 3) to validate and improve the accuracy of numerical simulations of the reaction of well cement with carbonic acid. Class H cement samples were uniaxially corroded via exposure to a brine of constant composition. Constant composition is ensured by constant renewal of the brine at a rate larger than cement reaction rate. H⁺, Ca2+ and CO₂ total aqueous concentration in the NaCl brine are controlled independently by adding known amounts of NaCl, HCl, CaCl₂ and NaHCO₃ and by controlling CO₂ partial pressure. Microscopic (30X) time-lapse videos were taken of each sample so that corrosion front movements could be accurately measured. These experiments have yielded corrosion front measurements that clearly show that corrosion front advancement is diffusion controlled (i.e., linear as a function of the square root of time). The uniaxial corrosion of these samples has not only allowed for detailed measurements of the corrosion front, but also affords the opportunity to measure the mechanical properties of the corroded samples as a function of depth. The one-dimensional corrosion also allows for measuring the diffusion coefficient of the outer layer of silica gel by low field Nuclear Magnetic Resonance (NMR). Measuring the kinetics under various boundary conditions has validated the modeling results reported by Huet et al. [1]. The measurements of mechanical and transport properties can now be used to improve the predictive power of these simulations by providing much needed information on the exterior layer of corroded Class H well cement. Additionally, these experiments offer experimental validation that the corrosion kinetics are enhanced by the presence of CO₂ and open the door to better understanding of the mechanism of, and boundary conditions that might lead to, “pore-plugging” by the corrosion products, which in turn leads to a drastic retardation of the corrosion reaction.

39. Motley, M. R., and Jean Hervé Prévost, 2010: Simulation of transient heat conduction using one-dimensional mapped infinite elements. International Journal for Numerical Methods in Engineering, John Wiley & Sons, Ltd., doi:10.1002/nme.2847
    [ Abstract ]

    Many engineering problems exist in physical domains that can be said to be infinitely large. A common problem in the simulation of these unbounded domains is that a balance must be met between a practically sized mesh and the accuracy of the solution. In transient applications, developing an appropriate mesh size becomes increasingly difficult as time marches forward. The concept of the infinite element was introduced and implemented for elliptic and for parabolic problems using exponential decay functions. This paper presents a different methodology for modeling transient heat conduction using a simplified mesh consisting of only two-node, one-dimensional infinite elements for diffusion into an unbounded domain and is shown to be applicable for multi-dimensional problems. A brief review of infinite elements applied to static and transient problems is presented. A transient infinite element is presented in which the element length is time-dependent such that it provides the optimal solution at each time step. The element is validated against the exact solution for constant surface heat flux into an infinite half-space and then applied to the problem of heat loss in thermal reservoirs. The methodology presented accurately models these phenomena and presents an alternative methodology for modeling heat loss in thermal reservoirs.

40. Nordbotten, Jan M., J. P. Nogues, and Michael Celia, March 2010: Appropriate Choice of Average Pressure for Upscaling Relative Permeability in Dynamic Flow Conditions. SPE Journal, 15(1), doi:10.2118/113558-PA 228-237
    [ Abstract ]

    A new macroscale pressure definition is investigated through a series of upscaling calculations for two-phase flow in porous media. This definition is taken from previous theoretical work by the authors and aims at correcting for systematic subscale heterogeneities including those generated by nonlinear dependencies on (heterogeneous) saturation distributions. Traditional intrinsic phase-averaged pressure leads to nonmonotone and multivalued upscaled constitutive functions (e.g., discontinuous upscaled-relative-permeabilities exceeding unity). Using both analytical and numerical calculations, the new macroscale pressure definition is shown to lead to better-behaved upscaled functions.

41. Peethamparan, S., E. Weissinger, J. Vocaturo, J. Zhang, and George Scherer, 2010: Monitoring Chemical Shrinkage Using Pressure Sensors. American Concrete Institute, http://www.concrete.org/PUBS/JOURNALS/OLJDetails.asp?Home=SP&ID=51663740, SP-270, 77-88
    [ Abstract ]

    This paper discusses the design and development of an automated test device for measuring the chemical shrinkage of hydrating cements. The device uses a very sensitive pressure sensor to determine water level drop in an open capillary tube, from which the volume change due to chemical shrinkage can be calculated continuously. The repeatability of the measurements and the stability of the measuring device have been demonstrated. The comparison of the result produced by the new device with that of a standard ASTM test showed excellent matching. The range of applicability of this device was demonstrated by obtaining shrinkage measurements for different types of cements, such as ordinary portland cement, Class H oil well cement and white portland cements, and for different temperatures.

42. Person, M., A. Banerjee, J. Rupp, Cristian Medina, P. Lichtner, Carl Gable, R. Pawar, Michael Celia, Jennifer McIntosh, and Victor Bense, 2010: Assessment of Basin-scale Hydrologic Impacts of CO₂ Sequestration, Illinois Basin. International Journal of Greenhouse Gas Control, (4), doi:10.1016/j.ijggc.2010.04.004 840-854
    [ Abstract ]

    Idealized, basin-scale sharp-interface models of CO₂ injection were constructed for the Illinois basin. Porosity and permeability were decreased with depth within the Mount Simon Formation. Eau Claire confining unit porosity and permeability were kept fixed. We used 726 injection wells located near 42 power plants to deliver 80 million metric tons of CO₂/year. After 100 years of continuous injection, deviatoric fluid pressures varied between 5.6 and 18 MPa across central and southern part of the Illinois basin. Maximum deviatoric pressure reached about 50% of lithostatic levels to the south. The pressure disturbance (>0.03 MPa) propagated 10–25 km away from the injection wells resulting in significant well–well pressure interference. These findings are consistent with single-phase analytical solutions of injection. The radial footprint of the CO₂ plume at each well was only 0.5–2 km after 100 years of injection. Net lateral brine displacement was insignificant due to increasing radial distance from injection well and leakage across the Eau Claire confining unit. On geologic time scales CO2 would migrate northward at a rate of about 6 m/1000 years. Because of paleo-seismic events in this region (M5.5–M7.5), care should be taken to avoid high pore pressures in the southern Illinois basin.

43. Peters, Catherine A., P. F. Dobson, C. M Oldenburg, Joseph S.Y. Wang, and George Scherer, 2010: LUCI: A Facility at DUSEL for Large-Scale Experimental Study of Geologic Carbon Sequestration. International Conference on Greenhouse Gas Technologies (GHGT 10), Elsevier/Energy Procedia,
    [ Abstract ]

    LUCI, the Laboratory for Underground CO₂ Investigations, is an experimental facility being planned for the DUSEL underground laboratory in South Dakota, USA. It is designed to study vertical flow of CO₂ in porous media over length scales representative of leakage scenarios in geologic carbon sequestration. The plan for LUCI is a set of three vertical column pressure vessels, each of which is ~500 m long and ~1 m in diameter. The vessels will be filled with brine and sand or sedimentary rock. Each vessel will have an inner column to simulate a well for deployment of down-hole logging tools. The experiments are configured to simulate CO₂ leakage by releasing CO₂ into the bottoms of the columns. The scale of the LUCI facility will permit measurements to study CO₂ flow over pressure and temperature variations that span supercritical to subcritical gas conditions. It will enable observation or inference of a variety of relevant processes such as buoyancy-driven flow in porous media, Joule- Thomson cooling, thermal exchange, viscous fingering, residual trapping, and CO₂ dissolution. Experiments are also planned for reactive flow of CO₂ and acidified brines in caprock sediments and well cements, and for CO₂ -enhanced methanogenesis in organic-rich shales. A comprehensive suite of geophysical logging instruments will be deployed to monitor experimental conditions as well as provide data to quantify vertical resolution of sensor technologies. The experimental observations from LUCI will generate fundamental new understanding of the processes governing CO₂ trapping and vertical migration, and will provide valuable data to calibrate and validate large-scale model simulations.

44. Prévost, Jean H., and M. Preisig, 2010: Numerical simulation of CO2 injection into an aquifer and the importance of two-way coupling between fluid pressure and geomechanics. Conference of the Engineering Mechanics Institute, 8-11
45. Preisig, M., and Jean Hervé Prévost, 2010: Stabilization procedures in coupled poromechanics problems: A critical assessment. International Journal for Numerical and Analytical Methods in Geomechanics, doi:10.1002/nag.951 1-19
    [ Abstract ]

    Numerical solutions for problems in coupled poromechanics suffer from spurious pressure oscillations when small time increments are used. This has prompted many researchers to develop methods to overcome these oscillations. In this paper, we present an overview of the methods that in our view are most promising. In particular we investigate several stabilized procedures, namely the fluid pressure Laplacian stabilization (FPL), a stabilization that uses bubble functions to resolve the fine-scale solution within elements, and a method derived by using finite increment calculus (FIC). On a simple one-dimensional test problem, we investigate stability of the three methods and show that the approach using bubble functions does not remove oscillations for all time step sizes. On the other hand, the analysis reveals that FIC stabilizes the pressure for all time step sizes, and it leads to a definition of the stabilization parameter in the case of the FPL-stabilization. Numerical tests in one and two dimensions on 4-noded bilinear and linear triangular elements confirm the effectiveness of both the FPL- and the FIC-stabilizations schemes for linear and nonlinear problems.

46. Scherer, George, Gary P. Funkhouser, and S. Peethamparan, 2010: Effect of pressure on early hydration of class H and white cement. Cement and Concrete Research, 40, doi:10.1016/j.cemconres.2010.01.013 845-850
    [ Abstract ]

    The change in viscosity of cement slurry with temperature and pressure can be predicted by assuming that hydration can be treated as an activated process and that a given viscosity corresponds to a fixed degree of reaction. For Class H and White cements, chemical shrinkage experiments indicate that the activation energy is 33.8 kJ/mole and rheological measurements yield an activation volume of -30 cm³/mole. With these parameters, it is possible to predict the limit of pumpability of the slurry (which corresponds to a viscosity of about 2.5 Pa s) for arbitrary temperature and pressure cycles. This method of prediction requires that the physics of the process remain the same, but simply change in rate; therefore, the range of applicability is expected to be limited to temperatures below about 100 °C, since new phases occur at higher temperatures.

47. Sun, Zhenhua, and George Scherer, 2010: Pore size and shape in mortar by thermoporometry. Cement and Concrete Research, 40, doi:10.1016/j.cemconres.2009.11.011 740-751
    [ Abstract ]

    The pore structure of mortar (w/c = 0.55) was examined using thermoporometry (TPM), nitrogen adsorption/desorption (NAD), and mercury intrusion porosimetry (MIP). The TPM measurements were calibrated by comparison to NAD and MIP measurements on porous glass; similar comparisons were made on dried and resaturated mortars. For undried mortars, TPM provides the size of pore entries (from the freezing cycle) and interiors (from the melting cycle). In keeping with previous studies, we find that there is an unfrozen layer of water between the ice and the pore wall in porous glass that is about 0.8 nm thick; when lime-saturated water is used, the thickness of that layer increases by about 10%. In mortar, the unfrozen layer is about 1.0–1.2 nm thick, so no freezing occurs in pores with diameters ≤ 4.5 nm, at least down to − 40 °C (where the radius of the crystal/liquid interface is ~1.5 nm). Based on the hysteresis in the freezing and melting curves, the larger mesopores in mortar were found to be rather spheroidal, while the smaller ones were more cylindrical.

48. Zhang, J., E. Weissinger, S. Peethamparan, and George Scherer, 2010: Early hydration and setting of oil well cement. Cement and Concrete Research, 40, doi:10.1016/j.cemconres.2010.03.014
    [ Abstract ]

    A broad experimental study has been performed to characterize the early hydration and setting of cement pastes prepared with Class H oil well cement at water-to-cement ratios (w/c) from 0.25 to 0.40, cured at temperatures from 10 to 60 °C, and mixed with chemical additives. Chemical shrinkage during hydration was measured by a newly developed system, degree of hydration was determined by thermogravimetric analysis, and setting time was tested by Vicat and ultrasonic velocitymeasurements. A Boundary Nucleation and Growth model provides a good fit to the chemical shrinkage data. Temperature increase and accelerator additions expedite the rate of cement hydration by causing more rapid nucleation of hydration products, leading to earlier setting; conversely, retarder and viscosity modifying agents delay cement nucleation, causing later setting times. Lower w/c paste needs less hydration product to form a percolating solid network (i.e., to reach the initial setting point).However, for the systems evaluated, at a givenw/c, the degree of hydration at setting is a constant, regardless of the effects of ambient temperature or the presence of additives.

49. Bachu, S., and Michael Celia, 2009: Assessing the Potential for CO2 Leakage, Particularly through Wells, from CO2 Storage Sites. The Science and Technology of Carbon Sequestration, http://cat.inist.fr/?aModele=afficheN&cpsidt=22364988, 183, 203-216
    [ Abstract ]

    Assessment of the potential for CO2 leakage from geological storage sites is essential for the implementation of CO2 capture and storage in geological media. Possible pathways for CO2 leakage from a storage site include natural interruptions and breaches through the confining strata, faults and fractures, and degraded wells. Knowledge of the geology and stress regime is essential in assessing the potential for CO2 leakage through natural features and induced fractures. Assessment of the potential for leakage through degraded wells is much more difficult because of the large number of wells, the lack of knowledge about their condition, and the computational difficulties relating to the simulation of CO2 leakage through many wells across a multilayered succession of aquifers and aquitards. The large number of wells and the variability in their present and future conditions require a stochastic approach by which a large number of statistical realizations provides a probability distribution for CO2 leakage. The large disparity between the length scales associated with injected plumes and those associated with leakage pathways along wells leads to numerical intractability for statistical simulations. Semianalytical models, although constrained by assumptions needed to solve the mathematical system of equations, provide a framework for estimating the potential for and rates of CO2 leakage through degraded wells. An example from the Alberta Basin in Canada provides an illustration of the types of information these models can generate. The models must be coupled to specific field observational and measurement programs to support full implementation of CO2 geological storage.

50. Celia, Michael, Jan M. Nordbotten, , M. Dobossy, and Benjamin Court, 2009: Risk of Leakage versus Depth of Injection in Geological Storage. Energy Procedia, 1(1), doi:10.1016/j.egypro.2009.02.022 2573-2580
    [ Abstract ]

    One of the outstanding challenges for large-scale CCS operations is to develop reliable quantitative risk assessments with a focus on leakage of both injected CO₂ and displaced brine. A critical leakage pathway is associated with the century-long legacy of oil and gas exploration and production, which has led to many millions of wells being drilled. Many of those wells are in locations that would otherwise be excellent candidates for CCS operations, especially across many parts of North America. Quantitative analysis of the problem requires special computational techniques because of the unique challenges associated with simulation of injection and leakage in systems that include hundreds or thousands of existing wells over domains characterized by layered structures in the vertical direction and very large horizontal extent. An important feature of these kinds of systems is the depth of each well, and the fact that the number of wells penetrating different formations decreases as a function of depth. As such, one might reasonably expect the risk of leakage to decrease with depth of injection. With the special computational models developed to simulate injection and leakage along multiple wells, in layered systems with multiple formations, quantitative assessment of risk reduction as a function of injection depth can be made. An example of such a system corresponds to the Wabamun Lake area southwest of Edmonton, Alberta, Canada, where several large coal-fired power plants are located. Use of information about both the existing wells and the local stratigraphy allows a realistic model to be constructed. Leakage along existing wells is assumed to follow Darcy’s Law, and is characterized by a set of effective permeability values. These values are assigned stochastically, using several different methods, within a Monte Carlo simulation framework. Computational results show the clear trade-off between depth of injection and risk of leakage. The results also show how properties within the different formations affect the risk profiles. In the Wabamun Lake area, one of the formations has the highest injectivity, by far, while having a moderate number of existing wells. Its moderate risk of leakage, as compared to injections in formations above and below, shows some of the key factors that are likely to influence injection design for large-scale CCS operations.

51. Celia, Michael, and Jan M. Nordbotten, 2009: Practical Modeling Approaches for Geological Storage of Carbon Dioxide. Ground Water, 47(5), doi:10.1111/j.1745-6584.2009.00590.x 627–638
    [ Abstract ]

    The relentless increase of anthropogenic carbon dioxide emissions and the associated concerns about climate change have motivated new ideas about carbon-constrained energy production. One technological approach to control carbon dioxide emissions is carbon capture and storage, or CCS. The underlying idea of CCS is to capture the carbon before it emitted to the atmosphere and store it somewhere other than the atmosphere. Currently, the most attractive option for large-scale storage is in deep geological formations, including deep saline aquifers. Many physical and chemical processes can affect the fate of the injected CO2, with the overall mathematical description of the complete system becoming very complex. Our approach to the problem has been to reduce complexity as much as possible, so that we can focus on the few truly important questions about the injected CO2, most of which involve leakage out of the injection formation. Toward this end, we have established a set of simplifying assumptions that allow us to derive simplified models, which can be solved numerically or, for the most simplified cases, analytically. These simplified models allow calculation of solutions to large-scale injection and leakage problems in ways that traditional multicomponent multiphase simulators cannot. Such simplified models provide important tools for system analysis, screening calculations, and overall risk-assessment calculations. We believe this is a practical and important approach to model geological storage of carbon dioxide. It also serves as an example of how complex systems can be simplified while retaining the essential physics of the problem.

52. Class, H., R. Ebigbo, R. Helmig, H. Dahle, Jan M. Nordbotten, Michael Celia, P. Audigane, M. Darcis, J. Ennis-King, Y. Fan, B. Flemisch, and S. Gasda, et al., 2009: A Benchmark Study on Problems Related to CO₂ Storage in Geological Formations: Summary and Discussion of the Results. Computational Geosciences, doi:10.1007/s10596-009-9146-x
    [ Abstract ]

    This paper summarises the results of a benchmark study that compares a number of mathematical and numerical models applied to specific problems in the context of carbon dioxide (CO₂) storage in geologic formations. The processes modeled comprise advective multiphase flow, compositional effects due to dissolution of CO₂ into the ambient brine, and non-isothermal effects due to temperature gradients and the Joule-Thompson effect. The problems deal with leakage through a leaky well, methane recovery enhanced by CO₂ injection, and a reservoir-scale injection scenario into a heterogeneous formation. We give a description of the benchmark problems, then briefly introduce the participating codes, and finally present and discuss the results of the benchmark study.

53. Crandell, Lauren E., B. R. Ellis, and Catherine A. Peters, December 2009: Dissolution Potential of SO₂ Co-Injected with CO₂ in Geologic Sequestration. Environmental Science and Technology, University of Iowa, Iowa City, doi:10.1021/es902612m
    [ Abstract ]

    Sulfur dioxide is a possible co-injectant with carbon dioxide in the context of geologic sequestration. Because of the potential of SO₂ to acidify formation brines, the extent of SO₂ dissolution from the CO₂ phase will determine the viability of co-injection. Pressure-, temperature-, and salinity-adjusted values of the SO₂ Henry's Law constant and fugacity coefficient were determined. They are predicted to decrease with depth, such that the solubility of SO₂ is a factor of 0.04 smaller than would be predicted without these adjustments. To explore the potential effects of transport limitations, a nonsteady-state model of SO₂ diffusion through a stationary cone-shaped plume of supercritical CO₂ was developed. This model represents an end-member scenario of diffusion-controlled dissolution of SO₂ , to contrast with models of complete phase equilibrium. Simulations for conditions corresponding to storage depths of 0.8−2.4 km revealed that after 1000 years, 65−75% of the SO₂ remains in the CO₂ phase. This slow release of SO₂ would largely mitigate its impact on brine pH. Furthermore, small amounts of SO₂ are predicted to have a negligible effect on the critical point of CO₂ but will increase phase density by as much as 12% for mixtures containing 5% SO₂ .

54. Crow, W., D. B. Williams, J. W. Carey, Michael Celia, and S. Gasda, 2009: Solubility and Diffusivity of SO₂ for Co-injection with CO₂ in Geological Sequestration. EOS Trans. AGU, 89(53), Fall Meet. S,
    [ Abstract ]

    There are potential economic benefits to the co-injection of SO₂ with CO₂ in the context of geological sequestration, but the impact of this co-injection on the fate and migration of SO₂ and CO₂ is poorly understood. Previous modeling studies have shown that injection of SO₂ with CO₂ would create highly acidic conditions due to formation of sulfuric acid. However, little is known regarding the solubility of SO₂ under high pressure, high salinity conditions, and the kinetic limitations of SO₂ diffusion in a CO₂ phase. A method to estimate the phase partitioning of SO₂ under geological storage conditions was developed in this study. The method uses the Krichevsky-Ilinskaya equation to correct for high pressures and the Schumpe model for mixed electrolyte solutions. Henry's constants for a broad range of brine solutions were calculated at storage conditions of 100 bar pressure. The Henry's constant for SO₂ is 1.5 M/atm at 40°C and is 0.86 M/atm at 60°C. Under these same conditions, the Henry's constant for CO₂ is much smaller, roughly 0.01 M/atm (40°C to 60°C). Henry's constants increase with increasing pressure but decrease with increasing temperature. These effects can be observed by comparing the SO₂ Henry's constants under storage conditions with the value under ambient temperature and pressure conditions in pure water, 1.2 M/atm. To simulate diffusion through stationary CO₂, a nonsteady state two-dimensional model of SO2 diffusion through supercritical CO₂ was also created. A binary diffusion coefficient of 5×10-8 m²/sec was estimated based on the Takahashi correlation to account for high pressures, where a low pressure coefficient was determined using the Fuller estimation. Binary diffusion coefficients for polar compounds in supercritical CO₂ have been previously studied and are on the same order of magnitude as the binary diffusion coefficient estimated in this study. The system that was modeled is a cone-shaped system representing separate-phase CO₂ confined in a formation after injection. Boundary conditions consisted of a no-flux boundary at the top of the cone to account for the impermeable confining caprock, and a zero concentration boundary at the cone edge to simulate a worst case scenario for dissolution. The initial conditions considered a uniform concentration of one percent SO₂ everywhere in the cone. To numerically simulate the concentration profile throughout the cone, a time-split explicit difference method was applied. The diffusion modeling results show that contact between SO₂ and formation brine will be diffusion limited; after 3000 years pproximately 75% of sulfur remains in the cone. In summary, while SO₂ is highly soluble in water, its slow diffusion through a supercritical CO2 phase will likely inhibit its mass transfer.

55. Crow, W., D. B. Williams, J. W. Carey, Michael Celia, and S. Gasda, 2009: Wellbore Integrity Analysis of a Natural CO₂ Producer. Energy Procedia, 1, doi:10.1016/j.egypro.2009.02.150 3561-3569
    [ Abstract ]

    The long-term integrity of wellbores in a CO₂-rich environment is a complex function of material properties and reservoir conditions including brine and rock compositions, CO₂- pressure, and formation pressure and temperature gradients. Laboratory experiments can provide essential information on rates of material reaction with CO₂-. However, field data are essential for assessing the integrated effect of these factors in subsurface conditions to provide a basis for validation of numerical models of wellbore behavior. We present a comprehensive study and conclusions from an investigation of a 30-year old well from a natural CO₂- production reservoir. The wellbore was exposed to a 96% CO₂- fluid from the time of cement placement. This site is unique for two reasons: it represents a higher, sustained concentration of CO₂- compared to enhanced oil recovery fields and both the reservoir and caprock are clastic rocks that may possess less buffering capacity than carbonate reservoirs. A sampling program resulted in the recovery of 10 side-wall cement cores extending from the reservoir through the caprock. The hydrologic, mineralogical and mechanical properties of these samples were measured and those results were combined with an in-situ pressure-response test to investigate cement integrity over a range of length scales. Fluid sampling was conducted with pressure and temperature measurements for geochemical analysis of the cemented annulus and the adjacent formation. These combined data sets provide an assessment of well integrity including original cement seal and the impacts of CO₂-. Cement evaluation wireline surveys indicate good coverage and bonding, consistent with observations from sidewall cement core samples that have tight interfaces with the casing and formation. Although alteration of the cement samples is present in all cores in varying degrees, hydraulic isolation has prevented leakage based on the pressure gradient measured between the caprock and CO₂- formation as well as lack of corrosion and no casing pressure history. Simulation of a hydraulic isolation test (Vertical Interference Test) indicates the best match for effective permeability of the wellbore system is approximately 1-10 millidarcies which suggests cement interfaces are a more significant potential migration pathway as compared with the cement matrix. Effective placement of the Portland-fly ash cement system was a key element in the observed performance of the barrier system that provides hydraulic isolation. The types of information collected in this survey permit analysis of individual components (casing, cement and reservoir fluid and pressure measurements) for comparison to the larger scale system including the interfaces. The results will be used as part of the CO₂- Capture Project’s effort to develop a long-term predictive simulation tool to assess wellbore integrity performance in CO₂- storage sites.

56. Debenedetti, Pablo, and S. Sarupria, 2009: Hydrate Molecular Ballet. Science, 326(5956), doi:10.1126/science.1183027 1070-1071
    [ Abstract ]

    Hydrates are crystalline solids in which guest molecules are trapped within polyhedral water cages (1). They are important in hydrocarbon processing (2) and could play a major role in sustainable energy production (3, 4). Methane hydrate occurs naturally and in vast quantities on ocean floors and in permafrost, with implications for climate change and energy recovery (2). However, the molecular mechanisms leading to hydrate formation are poorly understood; this knowledge gap affects not just the science and technology of these materials, but our comprehension of hydrophobicity (5) and of disorder-order phase transitions. On page 1095 of this issue, Walsh et al. report a computational tour de force that offers a fascinating glimpse of the molecular events leading to methane hydrate formation (6).

57. Gasda, S., Michael Celia, and Jan M. Nordbotten, 2009: Vertical Equilibrium with Subscale Analytical Methods for Geological CO₂ Sequestration. Computational Geosciences, doi:10.1007/S10596-009-9138-X
    [ Abstract ]

    Large-scale implementation of geological CO₂ sequestration requires quantification of risk and leakage potential. One potentially important leakage pathway for the injected CO₂ involves existing oil and gas wells. Wells are particularly important in North America, where more than a century of drilling has created millions of oil and gas wells. Models of CO₂ injection and leakage will involve large uncertainties in parameters associated with wells, and therefore a probabilistic framework is required. These models must be able to capture both the largescale CO₂ plume associated with the injection and the small-scale leakage problem associated with localized flow along wells. Within a typical simulation domain, many hundreds of wells may exist. One effective modeling strategy combines both numerical and analytical models with a specific set of simplifying assumptions to produce an efficient numerical–analytical hybrid model. The model solves a set of governing equations derived by vertical averaging with assumptions of a macroscopic sharp interface and vertical equilibrium. These equations are solved numerically on a relatively coarse grid, with an analytical model embedded to solve for wellbore flow occurring at the sub-gridblock scale. This vertical equilibrium with sub-scale analytical method (VESA) combines the flexibility of a numerical method, allowing for heterogeneous and geologically complex systems, with the efficiency and accuracy of an analytical method, thereby eliminating expensive grid refinement for sub-scale features. Through a series of benchmark problems, we show that VESA compares well with traditional numerical simulations and to a semi-analytical model which applies to appropriately simple systems. We believe that the VESA model provides the necessary accuracy and efficiency for applications of risk analysis in many CO₂ sequestration problems.

58. Nordbotten, Jan M., D. Kavetski, Michael Celia, and , 2009: Model for CO₂ Leakage Including Multiple Geological Layers and Multiple Leaky Wells. Environmental Science and Technology, 43(3), doi:10.1021/es801135v 743-749
    [ Abstract ]

    Geological storage of carbon dioxide (CO₂) is likely to be an integral component of any realistic plan to reduce anthropogenic greenhouse gas emissions. In conjunction with large-scale deployment of carbon storage as a technology, there is an urgent need for tools which provide reliable and quick assessments of aquifer storage performance. Previously, abandoned wells from over a century of oil and gas exploration and production have been identified as critical potential leakage paths. The practical importance of abandoned wells is emphasized by the correlation of heavy CO₂) emitters (typically associated with industrialized areas) to oil and gas producing regions in North America. Herein, we describe a novel framework for predicting the leakage from large numbers of abandoned wells, forming leakage paths connecting multiple subsurface permeable formations. The framework is designed to exploit analytical solutions to various components of the problem and, ultimately, leads to a grid-free approximation to CO₂) and brine leakage rates, as well as fluid distributions. We apply our model in a comparison to an established numerical solver for the underlying governing equations. Thereafter, we demonstrate the capabilities of the model on typical field data taken from the vicinity of Edmonton, Alberta. This data set consists of over 500 wells and 7 permeable formations. Results show the flexibility and utility of the solution methods, and highlight the role that analytical and semi-analytical solutions can play in this important problem.

59. Peters, Catherine A., George Scherer, Michael Celia, Jean Hervé Prévost, T. C. Onstott, P. F. Dobson, C. M Oldenburg, B. Freifeld, J. Birkholzer, J. Wang, S. Benson, and T. J. Phelps, et al., in press: Collaborative Research: DUSEL CO₂, A Deep Underground Laboratory for Geologic CO₂ Sequestration Studies: A proposal for the conceptual design of the facility and experiments. NSF. 0/09.
    [ Abstract ]

    Princeton University and Lawrence Berkeley National Laboratory have forged a new collaboration to examine the feasibility and risks of carbon sequestration, a method of countering global warming by storing greenhouse gases deep underground. To develop a sound understanding of carbon sequestration, we will build a deep underground laboratory to study the processes of trapping and storing CO₂, including the risks of unintended leakage. It will be part of the new DUSEL facility at the Homestake mine in South Dakota. The “DUSEL CO₂, facility will make the United States the only country with a deep underground laboratory for controlled study of geologic carbon sequestration, providing a unique opportunity for global leadership. The findings from these unique experiments will advance carbon management technology worldwide and help reduce global greenhouse gas emissions. The features and capabilities of the planned facility are unprecedented. The experimental design exploits the nearly half-kilometer vertical extent of existing “sandline” borings at Homestake. Pipes will be installed within the sandlines to serve as long flow columns. These columns will contain the CO₂, and allow experimentation at the same pressure and temperature conditions as in deep subsurface reservoirs. Fill materials will mimic sedimentary layering, as well as cements in plugged wells. Instrumentation will enable detailed monitoring of flow, pressure, temperature, brine composition, geomechanics, and microbial activity. As part of the initial suite of experiments, we plan to simulate a leak in which CO₂, changes from a supercritical fluid to a subcritical gas as the pressure drops during upflow over tens to hundreds of meters. We will test for possible acceleration in CO₂, flow due to increasing buoyancy. Also, we will examine the interactions of CO₂, with cap-rocks and well cements, and determine whether CO₂, will enlarge flow pathways or cause selfsealing. Finally, we will investigate the effects of anaerobic, thermophilic bacteria on CO₂, conversion to methane and carbonate. This project is being led by researchers at Princeton and LBNL, and involves no-cost collaboration with individuals at ORNL, Stanford University, Schlumberger and the U.S. DOE NETL. During this three-year project, the team is working to (i) prioritize future experiments that will be conducted at DUSEL CO₂, (ii) build models that simulate experimental conditions and predict process dynamics, and (iii) develop a Work-Breakdown Structure (WBS) schedule for design, procurement, construction, operation and deconstruction of the facility over the facility lifetime. International awareness about DUSEL CO₂, is being fostered through international workshops and formation of an International Advisory Committee. Also, we are collaborating with other DUSEL scientists on education and outreach about “deep science,” with particular focus on climate change and energy solutions. DUSEL education and outreach activities are focused on Native American communities in South Dakota and operation of the Visitor Center at the Sanford Lab at Homestake. To inspire and educate the next generation of leaders, we are involving undergraduate and graduate students in DUSEL CO₂, research at Princeton University.

60. Scherer, George, and B. J. Huet, 2009: Carbonation of wellbore cement by CO₂ diffusion from caprock. International Journal of Hydrogen Energy, Elsevier, (G Model IJGGC-188), doi:10.1016/j.ijggc.2009.08.002
    [ Abstract ]

    To evaluate the risk of corrosion of cement by geosequestered CO₂, samples are being retrieved from wells placed in natural CO₂ deposits [e.g., Crowet al., 2009]. If the cement passing through the cap rock is carbonated, it may indicate that annular gaps or cracks have allowed carbonic acid to come into contact with the cement. However, it must be recognized that the pore water in the cap rock has become saturated with CO₂ over geological time. After the well is placed, the CO₂ will diffuse toward the cement and react with it. A simple analysis of the diffusion kinetics demonstrates that carbonation depths of millimeters to centimeters can be expected from this reaction within the lifetime of a well, in the absence of any cracks or gaps. Therefore, the occurrence of carbonation in cement sealing natural CO₂ deposits must be interpreted with caution.

61. Scherer, George, Jean Hervé Prévost, and Z. H. Wang, 2009: Bending of a Poroelastic Beam with Lateral Diffusion. International Journal of Solids and Structures, 46(18-19), doi:10.1016/j.ijsolstr.2009.05.016 3451-3462
    [ Abstract ]

    Bending an elastic beam leads to a complicated 3D stress distribution, but the shear and transverse stresses are so small in a slender beam that a good approximation is obtained by assuming purely uniaxial stress. In this paper, we demonstrate that the same is true for a saturated poroelastic beam. Previous studies of poroelastic beams have shown that, to satisfy the Beltrami–Michell compatibility conditions, it is necessary to introduce either a normal transverse stress or shear stresses in addition to the bending stress. The problem is further complicated if lateral diffusion is permitted. In this study, a fully coupled finite element analysis (FEA) incorporating the lateral diffusion effect is presented. Results predicted by the “exact” numerical solution, including load relaxation, pore pressure, stresses and strains, are compared to an approximate analytical solution that incorporates the assumptions of simple beam theory. The applicability of the approximate beam-bending solution is investigated by comparing it to FEA simulations of beams with various aspect ratios. For “beams” with large width-to-height ratios, the Poisson effect causes vertical deflections that cannot be neglected. It is suggested that a theory of plate bending is needed in the case of poroelastic media with large width-to-height ratios. Nevertheless, use of the approximate solution yields very small errors over the range of width-to-height ratios (viz., 1–4) explored with FEA.

62. Smith, D., A. C. Schuerger, M. Davidson, Stephen W. Pacala, C. Bakermans, and T. C. Onstott, 2009: Survivability of Psychrobacter Cryohalolentis K5 Under Simulated Martian Surface Conditions. Astrobiology, 9(2), doi:10.1089/ast.2007.0231 221-228
    [ Abstract ]

    Spacecraft launched to Mars can retain viable terrestrial microorganisms on board that may survive the interplanetary transit. Such biota might compromise the search for life beyond Earth if capable of propagating on Mars. The current study explored the survivability of Psychrobacter cryohalolentis K5, a psychrotolerant microorganism obtained from a Siberian permafrost cryopeg, under simulated martian surface conditions of high ultraviolet irradiation, high desiccation, low temperature, and low atmospheric pressure. First, a desiccation experiment compared the survival of P. cryohalolentis cells embedded, or not embedded, within a medium/salt matrix (MSM) maintained at 25°C for 24 h within a laminar flow hood. Results indicate that the presence of the MSM enhanced survival of the bacterial cells by 1 to 3 orders of magnitude. Second, tests were conducted in a Mars Simulation Chamber to determine the UV tolerance of the microorganism. No viable vegetative cells of P. cryohalolentis were detected after 8 h of exposure to Mars-normal conditions of 4.55 W/m2 UVC irradiation (200–280 nm), −12.5°C, 7.1 mbar, and a Mars gas mix composed of CO2 (95.3%), N2 (2.7%), Ar (1.6%), O2 (0.2%), and H2O (0.03%). Third, an experiment was conducted within the Mars chamber in which total atmospheric opacities were simulated at ô = 0.1 (dust-free CO2 atmosphere at 7.1 mbar), 0.5 (normal clear sky with 0.4 = dust opacity and 0.1 = CO2-only opacity), and 3.5 (global dust storm) to determine the survivability of P. cryohalolentis to partially shielded UVC radiation. The survivability of the bacterium increased with the level of UVC attenuation, though population levels still declined several orders of magnitude compared to UVC-absent controls over an 8 h exposure period.

63. Webb, M. B., S. H. Garofalini, and George Scherer, June 2009: Use of a Dissociative Potential to Simulate Hydration of Na+ and Cl−. Journal of Physical Chemistry, 113, doi:10.1021/jp901667c 9886–9893
    [ Abstract ]

    We have developed interatomic interaction parameters for Na+ and Cl- hydration using the dissociative water potential of Mahadevan and Garofalini [J. Phys. Chem. B 2007, 111, 8919] suitable for molecular dynamic simulations. Simulations were performed for small ion−water clusters Na(H₂O)_n + (n = 1−6) and Cl(H₂O)m − (m = 1−5), as well as dilute aqueous solutions of the ions in water, reproducing the structure and energies found in the literature. A simulation of an HCl molecule in water demonstrated the dissociation of the molecule. The Na+ and Cl- ion−ion interaction parameters also reproduce the energy and density of crystalline NaCl. A series of simulations of NaCl at progressively increasing temperatures from 300 to 1400 K produced solid densities varying by less than 1% from experiment.

64. Binning, P. J., and Michael Celia, 2008: Pseudokinetics arising from the Upscaling of Geochemical Equilibrium. Water Resources Research, 44, doi:10.1029/2007WR006147
    [ Abstract ]

    Multicomponent contaminant transport models in groundwater are typically based on assumptions of local geochemical equilibrium on the grid scale. However, in heterogenous systems there may be significant coupling between transport processes and geochemical equilibrium at smaller than grid block scale. Here various pore- and fieldscale examples are considered to illustrate the impact of transport processes on assumptions of geochemical equilibrium. In each example the flow length scales required to reach equilibrium are calculated. It is shown that these can be as large as many meters at the pore scale and kilometers at field scales. The influence of heterogeneity in the distribution of the reactive zones is assessed for the pore-scale example, and it is shown that patchiness of reactive zones within a pore increases equilibration length, with the length and density of reactive zones, pore radius, and diffusion coefficient all playing a role in the equilibration length. When constructing models of field-scale problems it may not be reasonable to apply geochemical equilibrium, and it may be necessary to explicitly couple pore-scale and field-scale models in a multiscale simulation. A field-scale example is also shown to illustrate that the upscaling of geochemical equilibrium poses a significant practical problem because we usually do not know the spatial location and distribution of geochemically active sites, and this information is essential input to geochemical transport models.

65. Deardorff, J. W., 2008: The Geologic Carbon Sequestration Potential of the Denver- Julesburg Basin of Colorado: Applied Methodologies for Basin Scale and Sitespecific Assessment of CO₂ Sequestration Potential. M.S. Thesis, Colorado School of Mines, Not in Circulation,
66. Ellis, B. R., Lauren E. Crandell, and Catherine A. Peters, 2008: Co-injection of SO₂ With CO₂ in Geological Sequestration: Potential for Acidification of Formation Brines. EOS Trans. AGU,
    [ Abstract ]

    Coal-fired power plants produce flue gas streams containing 0.02-1.4% SO₂ after traditional sulfur scrubbing techniques are employed. Due to the corrosive nature of H₂ SO₄ , it will likely be necessary to remove the residual SO₂ prior to carbon capture and transport; however, it may still be economically advantageous to reintroduce the SO₂ to the injection stream to mitigate the cost of SO₂ disposal and/or to get credits for SO₂ emissions reduction. This study examines the impact of SO₂ co-injection on the pH of formation brine. Using phase equilibrium modeling, it is shown that a CO₂ gas stream with 1% SO₂ under oxidizing conditions can create extremely acidic conditions (pH<1), but this will occur only near the CO₂ plume and over a short time frame. Nearly all of the SO₂ will be lost to the brine during this first phase equilibration, within approximately a decade, and the pH after the second is only 3.7, which is the pH that would occur from the carbonic acid alone. This suggests that although SO₂ will create low pH values due to the formation of H₂ SO₄ , the effect will have a very limited lifespan and a localized impact spatially. SO₂ is much more soluble than CO₂ and as the relative of amount of SO₂ to CO₂ is very small, the SO₂ will quickly dissolve into the formation brine. The extent of H₂ SO₄ formation is dependent on the redox conditions of the system. Several SO₂ oxidation pathways are investigated, including SO₂ disproportionation which produces both sulfate and the weaker acid, H₂ S. Further modeling considers a time varying, diffusion limited flux of SO₂ . Relative to the case of instantaneous phase equilibrium, this results in a smaller decrease in pH occurring over a longer duration. Our overall conclusion is that brine acidification due to SO₂ co-injection is not likely to be significant over relevant time and spatial scales.

67. Gasda, S., Jan M. Nordbotten, and Michael Celia, 2008: Determining Effective Wellbore Permeability from a Field Pressure Test: A Numerical Analysis of Detection Limits. Environmental Geology, 54(6), doi:10.1007/S00254-007-0903-7 1207-1215
    [ Abstract ]

    We propose a simple pressure test that can be used in the field to determine the effective permeability of existing wellbores. Such tests are motivated by the need to understand and quantify leakage risks associated with geological storage of CO₂ in mature sedimentary basins. If CO₂ is injected into a deep geological formation, and the resulting CO₂ plume encounters a wellbore, leakage may occur through various pathways in the ‘‘disturbed zone’’ surrounding the well casing. The effective permeability of this composite zone, on the outside of the well casing, is an important parameter for models of leakage. However, the data that exist on this key parameter do not exist in the open literature, and therefore specific field tests need to be done in order to reduce the uncertainty inherent in the leakage estimates. The test designed and analyzed herein is designed to measure effective wellbore permeability within a lowpermeability caprock, bounded above and below by permeable reservoirs, by pressurizing the reservoir below and measuring the response in the reservoir above. Alternatively, a modified test can be performed within the caprock without directly contacting the reservoirs above and below. We use numerical simulation to relate pressure response to effective well permeability and then evaluate the range of detection of the effective permeability based on instrument measurement error and limits on fracture pressure. These results can guide field experiments associated with site characterization and leakage analysis.

68. Gasda, S., Michael Celia, and Jan M. Nordbotten, 2008: Upslope Plume Migration and Implications for Geological CO₂ Sequestration in Deep Saline Aquifers. IES Journal A: Civil and Structural Engineering, 1(1), doi:10.1080/19373260701620154
    [ Abstract ]

    Recent investigations regarding CO₂ sequestration in deep saline aquifers have focused on characterization of the injected plume, its migration within the aquifer over time, and possible leakage out of the aquifer. To study these complex flow systems, simplified models are sometimes used to describe both plume evolution and the amount of leakage. Simplifications may include an assumption of perfectly horizontal geological formations, negligible capillary pressure, and symmetry of the injection plume. In this study, we explicitly test the limits of the assumption of a horizontal aquifer through numerical simulation of typical injection scenarios in continental sedimentary basins. Our approach is to simulate injection of CO₂ into a confined saline aquifer for an extended period (we have used 15 years) and examine the effect of different degrees of slope, as well as other system parameters, on plume asymmetry using measures such as the location of the centroid of the CO₂ plume. Dimensional analysis of this system shows that the centroid migrates upslope in proportion with buoyancy, aquifer permeability, and slope, whereas increased porosity and CO₂ viscosity mitigate upslope migration of the centroid. The results of this study show that the effect of slope can be ignored for many aquifers likely to become CO₂ sequestration sites in North America. However, slope will be more important for higher permeability aquifers, such as the site used in the Sleipner sequestration project in the North Sea.

69. Huet, B. J., Jean Hervé Prévost, and George Scherer, 2008: Reactive Transport Modeling of Cement Degradation in Brine: Effect of pH and CO₂ content. Geophysical Review, 10,
    [ Abstract ]

    As a CO₂ plume is moving into a reservoir, the chemistry of the fluid at the bottom of an abandoned well changes in successive stages. The first one consists in an increase of the CO₂ content of the brine, while brine saturation remains close to its initial value. A modular reactive transport model, Dynaflow, is used to analyze the reactivity of well cement paste during this first stage. The geochemical module accurately models aqueous speciation and mineral dissolution and/or precipitation within the porous material. Hydrated cement paste is found to dissolve in brines with various content in CO₂. Simulation of a reference case is successfully compared with experimental results, in terms of mineral zoning and dissolution rate. Between pH 2.4 and 5.0, the CO₂ content of a 0.5 M brine controls the degradation rate of cement whereas the pH does not affect it meaningfully. A minimum degradation rate is obtained when the CO₂ molality equals the total molality of aqueous calcium in equilibrium with portlandite. This minimum is related to the maximal amount of calcite precipitated and the relative decrease of the diffusivity within the calcite rich zone.

70. Nordbotten, Jan M., Michael Celia, H. Dahle, and S. M. Hassanizadeh, 2008: On the Definition of Macroscale Pressure for Multiphase Flow in Porous Media. Water Resources Research, 44(W06S02), doi:10.1029/2006WR005715
    [ Abstract ]

    We consider immiscible two-phase flow in porous media, starting with the Stokes equations. Our analysis leads to Darcy’s law but with notable differences from the usual interpretation. The most immediate difference is the interpretation of macroscale pressure, which, contrary to previous derivations, does not equal the intrinsic phase average pressure. We recover the intrinsic average only when systematic subscale heterogeneities, in material properties or fluid distribution, are absent. Examples using capillary tube and dynamic pore network models are given. These results impact our understanding of multiphase flow and have a direct effect on numerical upscaling efforts, including calculations of continuum-scale flow parameters from pore-scale network models.

71. Nordbotten, Jan M., and J. P. Nogues, et al., in press: Appropriate Choice of Average Pressure for Upscaling Relative permeability in Dynamic Flow Conditions. SPE Journal. 0/08.
72. Person, M., A. Banerjee, J. Rupp, P. Lichtner, R. Pawar, and Michael Celia, 2008: Basin-scale Hydrologic Impacts of CO₂ Sequestration within the Mt. Simojn Formation, Illinois Basin: Scaling Calculations using Sharp-Interface Theory. American Geophysical Union, Fall Meeting,
    [ Abstract ]

    The Illinois Basin hosts dozens of coal fired power plants generating more than 80 million metric tons of CO₂ annually. Here we present a suite of basin-scale, hydrologic models of the Mt Simon formation, Illinois Basin using sharp interface theory. The goal of these models is to determine what are the basin-scale hydrologic consequences of CO₂2 injection and whether some regions of the Illinois Basin would represent a better venue for carbon sequestration than others. While this approach makes some restrictive simplifying assumptions, it allows us to assess the problem at the sedimentary basin scale. Our solution domain spans the northern two thirds of the Illinois Basin (about 230,000 km²). We allowed porosity and permeability to decrease with depth from 0.2 to 0.05 and 400 to 2 mD, respectively. We injected CO₂ using 727, 10 inch diameter injection wells delivering about 210 kg/minute/well. The wells were positioned about 2 km apart in a radial pattern around known power plant locations. We ran the injection wells for 100 years. The wells were then shut in for an additional 900 years. Results indicate that after 100 years of continuous injection, deviatoric fluid pressures varied between 9.2 to 0.5 MPa between the deepest and shallowest injection wells. For the deepest portion of the basin (~ 3.1 km), deviatoric pressures reach about 22 percent of lithostatic levels. Owing to the rather subtle regional hydraulic gradient (200m/500km), long-range separate-phase CO₂-migration is driven by buoyancy at a rate of only 2 m/year. If CO₂ remained as a separate phase on time scales of 100,000 years, the injected CO₂ would migrate about 200 km to the north before charging gentle structural traps. Owing to the radial, bowl-shaped geometry of the Illinois Basin, net brine displacement to the north would be small, probably less than 100 m. Our analysis suggest that the Mt. Simon formation represents a good venue for CO₂ sequestration although shallower regions ( ~ 2 km depth) would pose less risk of catastrophic breaching due to high deviatoric fluid pressures. Fluid pressures do not return to hydrostatic conditions after 1000 years due to buoyant forces resulting from the presence of a separate CO₂ phase.

73. Peters, Catherine A., 2008: Accessibilities of reactive minerals in consolidated sedimentary rock: An imaging study of three sandstones. Chemical Geology, doi:10.1016/j.chemgeo.2008.11.014
    [ Abstract ]

    Widespread implementation of geological storage of CO₂ requires an understanding of dissolution reactions with formation minerals. This will be aided by reactive transport modeling, which relies on accurate estimates of the accessible surface areas of reactive minerals in consolidated sedimentary rocks. For three Viking sandstones (Alberta sedimentary basin, Canada), we have employed backscattered electron microscopy and energy dispersive X-ray spectroscopy to examine mineral content and to statistically characterize mineral contact with pore space. Porosities range from 20% in a lightly-cemented sandstone with grains on the order of 100 ìm, to 8% in a highly-cemented shaly sandstone with a mix of primary pore space and fractures, to 7% in a lightly-cemented conglomerate sandstone with grain sizes between 500 ìm and 1 mm. In all three specimens, kaolinite is the primary authigenic clay mineral cementing quartz grains. It accounts for only 5% to 31% of mineral content, but 65% to 86% of pore-mineral contact boundaries. The sandstone specimen has 6% minerals in the “reactive” category, which in this study includes minerals other than kaolinite and quartz, such as K-feldspar, apatite and pyrite. For this specimen, only one third of the reactive minerals are accessible to pore fluids due to clay-mineral grain coatings. For the shaly sandstone, only one fifth of its 5% reactive minerals are accessible to pore fluids due to regions of cementation of fine detrital matrix. Thus, if a mineral volume fraction is used in reactive transport modeling as a proportional measure of accessible surface area in consolidated sandstones, the reaction rates are likely to be overestimated by three to five times. The conglomerate sandstone has only 1% of its mineral matter in this category, and these are often found as inclusions rather than grains.

74. Peters, Catherine A., W. B. Lindquist, and Michael Celia, March 2008: Up-Scaling Mineral Accessibility and Pore Networks for CO₂ Reactive Transport in Sandstones. Global Change Biology,
    [ Abstract ]

    Widespread implementation of geological storage of CO₂ will require an understanding of acid-driven reactions with formation minerals. Predicting these reactions and their time scales requires rate laws that are appropriate for sedimentary rocks and estimates of accessible surface areas of reactive minerals. This project addresses these needs through a study that combines imaging of sandstone pore structure and minerals, and network-modeling of reaction rates in porous media. Rock specimens come from the Viking formation in the Alberta Sedimentary Basin. Imaging methods include X-ray computed microtomography (CT), backscatter electron microscopy (BSE) and energy dispersive X-ray (EDX) spectroscopy.
    One important goal is to characterize pore contact with individual minerals thereby quantifying meaningful surface areas for use in reactive transport models. The suite of techniques employed and the innovative means by which the images are collectively interpreted provides a wealth of information to address this goal. For example, a novel method of interpreting BSE images (which are high resolution) combined with EDX images (which can generate mineral maps) leads to 2D images that provide detailed characterization of the proximity of reactive minerals to pore space. Extension of this image processing approach to 3D, using CT images to broadly classify mineral categories, allows us to relate detailed information about pore structure with mineral accessibility, albeit with coarser resolution.
    All the specimens are sandstones of comparable porosity and grain diameter, and yet order of magnitude variation is found in pore structure and reactive mineral properties across them. In general, we have found that mineral volumetric content is a poor indicator of proportionate poreto- mineral surface area due to the means by which minerals are obscured in consolidated media. For example, kaolinite and other authigenic clay minerals that coat grains and fill primary pore space account for only 5% to 30% of mineral content, but 65% to 90% of pore-mineral contact boundaries. Minerals that would react under acidic conditions may account for 5% to 10% (vol.) of mineral matter, but if these percentages are used to apportion surface area, they would overestimate reaction rates by three to five times.
    These detailed characterizations of pore structure and mineral spatial patterning are being used to develop pore-network models that simulate reactive transport. Simulations of conditions representative of COWidespread implementation of geological storage of CO2 will require an understanding of acid-driven reactions with formation minerals. Predicting these reactions and their time scales requires rate laws that are appropriate for sedimentary rocks and estimates of accessible surface areas of reactive minerals. This project addresses these needs through a study that combines imaging of sandstone pore structure and minerals, and network-modeling of reaction rates in porous media. Rock specimens come from the Viking formation in the Alberta Sedimentary Basin. Imaging methods include X-ray computed microtomography (CT), backscatter electron microscopy (BSE) and energy dispersive X-ray (EDX) spectroscopy. One important goal is to characterize pore contact with individual minerals thereby quantifying meaningful surface areas for use in reactive transport models. The suite of techniques employed and the innovative means by which the images are collectively interpreted provides a wealth of information to address this goal. For example, a novel method of interpreting BSE images (which are high resolution) combined with EDX images (which can generate mineral maps) leads to 2D images that provide detailed characterization of the proximity of reactive minerals to pore space. Extension of this image processing approach to 3D, using CT images to broadly classify mineral categories, allows us to relate detailed information about pore structure with mineral accessibility, albeit with coarser resolution. All the specimens are sandstones of comparable porosity and grain diameter, and yet order of magnitude variation is found in pore structure and reactive mineral properties across them. In general, we have found that mineral volumetric content is a poor indicator of proportionate poreto- mineral surface area due to the means by which minerals are obscured in consolidated media. For example, kaolinite and other authigenic clay minerals that coat grains and fill primary pore space account for only 5% to 30% of mineral content, but 65% to 90% of pore-mineral contact boundaries. Minerals that would react under acidic conditions may account for 5% to 10% (vol.) of mineral matter, but if these percentages are used to apportion surface area, they would overestimate reaction rates by three to five times. These detailed characterizations of pore structure and mineral spatial patterning are being used to develop pore-network models that simulate reactive transport. Simulations of conditions representative of CO₂ injection for geological storage are being used to examine up-scaling of reaction rates from the pore-scale to the core-scale.

75. Scherer, George, 2008: Poromechanics Analysis of a Flow-Through Permeameter with Entrapped Air. Cement and Concrete Research, 38(3), doi:10.1016/j.cemconres.2007.09.028 368-378
    [ Abstract ]

    In a standard flow-through permeameter, a hydrostatic head is applied to one side of a sample and the flux of fluid through the sample is measured. In this paper, we apply poromechanics to calculate the evolution of the pressure distribution and the flux through the sample. We allow for negative capillary pressure in the pores at the start of the experiment (owing to self-desiccation during hydration of cement), as well as entrapped air. The time required to reach steady state flow can increase by an order of magnitude by the presence of 1 vol.% of air in the pore liquid; the delay increases as the applied pressure decreases.

76. Wang, Z. H., Jean Hervé Prévost, and O. Coussy, 2008: Bending of Fluid-Saturated Linear Poroelastic Beams with Compressible Constituents. International Journal for Numerical and Analytical Methods in Geomechanics, 33(4), doi:10.1002/nag.722 425-447
    [ Abstract ]

    Analytical solutions are presented for fluid-saturated linear poroelastic beams under pure bending. The stress-free boundary condition at the lateral surfaces is satisfied in the St Venant’s sense and the Beltrami– Michell compatibility conditions are resolved rigorously, rendering the flexure of the beams analytically tractable. Two sets of formulations are derived based on the coupled and uncoupled diffusion equations respectively. The analytical solutions are compared with three-dimensional finite element simulations. Both sets of analytical formulations are capable of capturing exactly both the initial (undrained) and the steady-state (fully drained) deflection of the beams. However, the analytical solutions are found to be deficient during the transient phase. The cause for the deficiency of the transient analytical solutions is discussed. The accuracy of the analytical solutions improves as Poisson’s ratio and the compressibility of the constituents of the porous beam increase, where the St Venant’s edge effect at the lateral surfaces is mitigated.

77. de Boer, A. M., Daniel Sigman, J. R. Toggweiler, and J. L. Russell, 2007: The Effect of global ocean temperature change on deep ocean ventilation. Paleoceanography, 22(PA2210), doi:10.1029/2005PA001242
    [ Abstract ]

    A growing number of paleoceanographic observations suggest that the ocean’s deep ventilation is stronger in warm climates than in cold climates. Here we use a general ocean circulation model to test the hypothesis that this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature; that is, at lower temperatures the surface cooling is not as effective at densifying fresh polar waters and initiating convection. In order to isolate this factor from other climate-related feedbacks we change the model ocean temperature only where it is used to calculate the density (to which we refer below as ‘‘dynamic’’ temperature change). We find that a dynamically cold ocean is globally less ventilated than a dynamically warm ocean. With dynamic cooling, convection decreases markedly in regions that have strong haloclines (i.e., the Southern Ocean and the North Pacific), while overturning increases in the North Atlantic, where the positive salinity buoyancy is smallest among the polar regions. We propose that this opposite behavior of the North Atlantic to the Southern Ocean and North Pacific is the result of an energy-constrained overturning.

78. DePaolo, D. J., F. M. Orr, Jr., S. Benson, and Michael Celia, et al., June 2007: Basic Research Needs for Geosciences: Facilitating 21st Century Energy Systems. Office of Basic Energy Sciences, U.S. DOE, Report from the Workshop Held February 21-23, 2007, http://www.science.doe.gov/bes/reports/files/GEO_rpt.pdf,
    [ Abstract ]

    Serious challenges must be faced in this century as the world seeks to meet global energy needs and at the same time reduce emissions of greenhouse gases to the atmosphere. Even with a growing energy supply from alternative sources, fossil carbon resources will remain in heavy use and will generate large volumes of carbon dioxide (CO₂). To reduce the atmospheric impact of this fossil energy use, it is necessary to capture and sequester a substantial fraction of the produced CO₂. Subsurface geologic formations offer a potential location for long-term storage of the requisite large volumes of CO₂. Nuclear energy resources could also reduce use of carbon based fuels and CO₂ generation, especially if nuclear energy capacity is greatly increased. Nuclear power generation results in spent nuclear fuel and other radioactive materials that also must be sequestered underground. Hence, regardless of technology choices, there will be major increases in the demand to store materials underground in large quantities, for long times, and with increasing efficiency and safety margins.
    Rock formations are composed of complex natural materials and were not designed by nature as storage vaults. If new energy technologies are to be developed in a timely fashion while ensuring public safety, fundamental improvements are needed in our understanding of how these rock formations will perform as storage systems.
    This report describes the scientific challenges associated with geologic sequestration of large volumes of carbon dioxide for hundreds of years, and also addresses the geoscientific aspects of safely storing nuclear waste materials for thousands to hundreds of thousands of years. The fundamental crosscutting challenge is to understand the properties and processes associated with complex and heterogeneous subsurface mineral assemblages comprising porous rock formations, and the equally complex fluids that may reside within and flow through those formations. The relevant physical and chemical interactions occur on spatial scales that range from those of atoms, molecules, and mineral surfaces, up to tens of kilometers, and time scales that range from picoseconds to millennia and longer. To predict with confidence the transport and fate of either CO₂ or the various components of stored nuclear materials, we need to learn to better describe fundamental atomic, molecular, and biological processes, and to translate those microscale descriptions into macroscopic properties of materials and fluids. We also need fundamental advances in the ability to simulate multiscale systems as they are perturbed during sequestration activities and for very long times afterward, and to monitor those systems in real time with increasing spatial and temporal resolution. The ultimate objective is to predict accurately the performance of the subsurface fluid-rock storage systems, and to verify enough of the predicted performance with direct observations to build confidence that the systems will meet their design targets as well as environmental protection goals.
    The report summarizes the results and conclusions of a Workshop on Basic Research Needs for Geosciences held in February 2007. Five panels met, resulting in four Panel Reports, three Grand Challenges, six Priority Research Directions, and three Crosscutting Research Issues. The Grand Challenges differ from the Priority Research Directions in that the former describe broader, longterm objectives while the latter are more focused.

79. Duguid, A., M. Radonjic, and George Scherer, 2007: The Effect of Carbonated Brine on Well Cement Used in Geologic Formations. Proceedings of the 12th ICCC, eds. J.J. Beaudoin, J.M. Makar, L. Raki (Canada), paper TH4-10.2,
    [ Abstract ]

    Carbon sequestration in abandoned petroleum fields may be a short-term solution to reducing anthropogenic emissions of CO₂. If sequestration is adopted on a large scale, it will be important to understand how CO₂ may leak out of sequestration formations via abandoned wells. Within an abandoned petroleum well there are multiple pathways that CO₂ may use to escape to the atmosphere. These include leakage through the cement that makes up the primary and/or plug cement in the well, leakage through the interface between the primary cement and the geologic formation, and leakage through the interface between the well cement and the well casing. As the plume of carbonic acid created by sequestration reaches an abandoned well, the first potential pathway with which it come in contact is the interface between the geologic formation and the primary well cement. In order to better understand what takes place when carbonic acid reaches the cement-rock interface, a series of batch experiments was conducted. This paper describes experiments that were conducted on composite samples that were made from Class H well cement and Salem limestone or Berea sandstone.

80. Huet, B. J., Jean Hervé Prévost, and George Scherer, June 2007: Cement reactivity in CO₂ saturated brines: use of a reactive transport code to highlight key degradation mechanisms. Eurotherm Seminar N 81 Reactive Heat Transfer in Porous Media, Albi, France, http://eurotherm81.enstimac.fr/papers_pdf/22-Huet.pdf,
    [ Abstract ]

    A modular reactive transport code is proposed to analyze the reactivity of cement in CO₂ saturated brine. The coupling of the transport module and the geochemical module within Dynaflow^TM is derived. Both modules are coupled in a sequential iterative approach to accurately model: (1) mineral dissolution/precipitation and (2) porosity dependent transport properties. Results of the model reproduce qualitatively the dissolution of cement hydrates (C-H, C-S-H, AFm, AFt) and intermediate products (CaCO₃) into the brine. Slight discrepancies between modeling and experimental results were found concerning the dynamics of the mineral zoning. Results suggest that the power law relationship to model effective transport properties from porosity values is not accurate for very reactive case.

81. Huet, B. J., R. Fuller, and Jean Hervé Prévost, 2007: Development of a coupled geochemical transport code to simulate cement degradation in CO₂ saturated brine. Proceedings of the 8th International Conference on Greenhouse Gas Control Technologies (GHGT-8), https://www.princeton.edu/~cmi/research/Storage/Presentations/DevelopmentCoupledGeochemical.pd,
    [ Abstract ]

    This work aims at modeling well-bore leakage of carbon dioxide (CO₂) from sequestration reservoirs. The leakage of CO₂ is a function of the geometry of the low permeability path and of the boundary conditions. Furthermore, the CO₂ flow can also be strongly influenced by the chemical reactivity of cement, leading either to the sealing or to the widening of the annulus. Thus, a multiphase transport model that aims at assessing CO₂ leakage along high permeability paths must account for the reactivity of the porous environment. Therefore a geochemical module must be integrated. This study presents preliminary results of the coupling of a geochemical module with a transport module. Governing equations of the two modules are introduced. Results of benchmark simulations of chemical degradation of cement paste (CEM I) in pure deionized water are presented. The influence of the different terms of the transport equation on mineral profile is discussed.

82. Meng, K., Robert H. Williams, and Michael Celia, 2007: Opportunities for low-cost CO₂ storage demonstration projects in China. Energy Policy, 35(4), doi:10.1016/j.enpol.2006.08.016 2368-2378
    [ Abstract ]

    Several CO₂ storage demonstration projects are needed in a variety of geological formations worldwide to prove the viability of CO₂ capture and storage as a major option for climate change mitigation. China has several low-cost CO₂ sources at sites that produce NH₃ from coal via gasification. At these plants, CO₂ generated in excess of the amount needed for other purposes (e.g., urea synthesis) is vented as a relatively pure stream. These CO₂ sources would potentially be economically interesting candidates for storage demonstration projects if there are suitable storage sites nearby. In this study a survey was conducted to estimate CO₂ availability at modern Chinese coal-fed ammonia plants. Results indicate that annual quantities of available, relatively pure CO₂ per site range from 0.6 to 1.1 million tonnes. The CO₂ source assessment was complemented by analysis of possible nearby opportunities for CO₂ storage. CO₂ sources were mapped in relation to China’s petroliferous sedimentary basins where prospective CO₂ storage reservoirs possibly exist. Four promising pairs of sources and sinks were identified. Project costs for storage in deep saline aquifers were estimated for each pairing ranging from $15–21/t of CO₂. Potential enhanced oil recovery and enhanced coal bed methane recovery opportunities near each prospective source were also considered.

83. Nordbotten, Jan M., Michael Celia, H. Dahle, and S. M. Hassanizadeh, 2007: Interpretation of Macro-Scale Variables in Darcy's Law. Water Resources Research, 43(W08430), doi:10.1029/2006WR005018
    [ Abstract ]

    The pursuit of a theoretical foundation of Darcy’s law based on volume averaging of equations at the scale of flow in pores has a long history. While theories are well established for homogeneous systems, more complex systems exhibit inconsistencies in the resulting equations. The difficulties often lie in the treatment of surface integral terms arising from the classical averaging theorems used to transform averages of derivatives into derivatives of averages. In this work we extend the intrinsic phase average as a macroscale variable to a family of more general macroscale variables, which take into account systematic dependencies of averaging volume size on the macroscale. Comparison to Darcy’s law gives new insight into the relationship between variables at the microscale and macroscale.

84. Nordbotten, Jan M., I. Rodriguez-Iturbe, and Michael Celia, 2007: Stochastic Coupling of Rainfall and Biomass Dynamics In , 43(W01408), doi:10.1029/2006WR005068
    [ Abstract ]

    A modeling scheme is presented to derive the probabilistic structure of plant biomass when subjected to stochastic precipitation conditions. Using the fact that soil moisture varies on a much shorter scale than plant biomass, analytical expressions are derived for the steady state probability distribution of the average plant biomass over a period T, which is expected to be of the order of 2 or 3 months. Analytical expressions are also given for the time-dependent mean and variance of the biomass over a period T. The analysis is based on a simple model linking the daily dynamics of plant growth and soil moisture. The derived analytical expressions reproduce the results obtained from a full simulation of the underlying model very well. The results allow the study of the impact of different climatic scenarios regarding changes in frequency and intensity of rainfall, as well as changes in the mean seasonal temperature, on the expected value and variance of plant biomass throughout time.

85. Puma, M. J., I. Rodriguez-Iturbe, Michael Celia, and A. J. Guswa, 2007: Implications of Rainfall Temporal Resolution for Soil-moisture and Transpiration Modeling. Transport in Porous Media, 68, doi:10.1007/S11242-006-9057-4 37-67
    [ Abstract ]

    Dimensionless groups of parameters characterizing an ecosystem are valuable indicators for the a priori assessment of the effect of rainfall data resolution on predictions of soil moisture and transpiration. Knowledge of these dimensionless groups enables identification of appropriate levels of rainfall data resolution, when using historical rainfall directly or when using it to derive rainfall model parameters for use in models of soil–plant–climate systems.Detailed simulation studies of the soil, plant, and climate systems in Colorado and Texas, highly resolved in time and vertical space, show that historical rainfall data resolved at the daily level allow accurate prediction of soil-moisture and transpiration dynamics for smaller time resolutions. These results support inferences based on the dimensionless groups. Furthermore, no significant improvement in the prediction of soil-moisture and transpiration dynamics is attained, when representing rainfall through a more complex Neyman–Scott model rather than the simple rectangular pulses Poisson model.

86. Scherer, George, J. J. Valenza II, and G. Simmons, 2007: New Methods to Measure Liquid Permeability in Porous Materials. Cement and Concrete Research, 37(3), doi:10.1016/j.cemconres.2006.09.020 386-397
    [ Abstract ]

    Several novel methods have recently been proposed for rapid measurement of the liquid permeability of saturated cement paste, mortar and concrete. The relative merits of the techniques are discussed, and some recent results obtained on pastes and mortars are presented. The low permeabilities seen in cement paste are inconsistent with the pore size distributions measured following drying, indicating that the pore structure is significantly changed by drying.

87. Alberta Geological, Survey, 2006: Test Case for Comparative Modelling of CO₂ Injection, Migration and Possible Leakage. Wabamun Lake Area, Alberta Canada, Alberta Geological Survey, http://www.ags.gov.ab.ca/co2_h2s/wabamun/Wabamun_base.html,
    [ Abstract ]

    Geological storage of CO₂ in deep saline aquifers is an option for significantly reducing emissions into the atmosphere. In November 2005, participants at a Workshop on Geological Storage of CO₂ at Princeton University agreed on the need for a common test problem to assess various models for simulating the fate of CO₂ injected into the subsurface. The Alberta Geological Survey offered to make available the data for the Wabamun Lake area in Alberta, Canada, that were assembled for the purpose of developing a comprehensive model for studying CO₂ geological storage.

88. Celia, Michael, , Jan M. Nordbotten, D. Kavetski, and S. Gasda, 2006: A Risk Assessment Modeling Tool to Quantify Leakage Potential through Wells in Mature Sedimentary Basins. Proceedings of the 8th International Conference on Greenhouse Gas Control Technologies (GHGT-8),
    [ Abstract ]

    The mature sedimentary basins of North America have a long history of oil and gas exploration and production. This has resulted in many wells being drilled, with a substantial number of them now abandoned. Therefore, injection and storage of CO₂ in these basins requires analysis of possible leakage along those wells. A computationally fast semianalytical model of CO₂ injection and potential leakage along wells has been developed, capturing many of the dominant physical characteristics of largescale injection systems. This paper illustrates the capabilities of the model using a case study based on a potential CO₂ sequestration site in Alberta, Canada. The selection of model inputs reflecting the uncertainty in the condition of abandoned wells is considered. The use of the semianalytical model in a probabilistic risk assessment framework is discussed, outlining avenues for systematic regulatory analysis of injection scenarios and systems.

89. Duguid, A., 2006: The Effect of Carbonic Acid on Well Cements. Ph.D. thesis for Civil and Environmental Engineering, Princeton University, http://proquest.umi.com/pqdlink?Ver=1&Exp=09-27-2014&FMT=7&DID=1192184611&RQT=309&attempt=1&,
    [ Abstract ]

    Sequestration in abandoned petroleum fields has the potential to reduce atmospheric emissions of CO₂ if adopted on a large scale. When CO₂ comes in contact with brine in the sequestration formation, it will form carbonic acid. The acid may damage cement that is used to construct and abandon wells in the sequestration formation, allowing the wells to leak. CO₂ release from a sequestration formation through abandoned wells to the vadose zone and then from the vadose zone into a residence could cause exposure to high levels of CO₂. This study presents the results of two types of experiments that were conducted in order to understand how CO₂ will affect well cements. This dissertation also presents a hypothetical risk assessment that examines the risks that a sequestration site may pose. The experiments examine how well cements react when exposed to carbonated brines at 20° and 50°C. Month-long flow-through experiments were conducted on samples made from Class H neat paste and Class H cement containing 6% bentonite under conditions that simulated sandstone (pH 2.4 and 3.7) and limestone (pH 5) sequestration formations. Year-long batch experiments were conducted on composite samples made from Class H well cement and either sandstone or limestone in order to determine how carbonated brine affects the cement-stone interface. The results of the flow-through experiments showed that calcium-containing phases in cement may be completely degraded and the formation of the calcium carbonate layer acts to slow, but not stop, degradation. The results of the batch experiments showed that carbonated brines can degrade cement and damage the integrity of the cement-stone interface. The damage to the cement at the cement-stone interface caused an order-of-magnitude increase in permeability in the sandstone-cement samples. The risk assessment identifies two potential screening levels. The assessment then uses a semianalytical wellfield model coupled with analytical models of diffusion through the vadose zone and foundation walls to establish CO₂ concentrations in residential basements. The results show CO₂ levels may not reach either screening level unless the value of the air-exchange rate in the houses is small.

90. Duguid, A., and Michael Celia, May 2006: Geologic CO₂ sequestration in abandoned oil and gas fields and human health risk assessment. Proceedings of the 5th Annual Conference on Carbon Capture and Sequestration,
    [ Abstract ]

    Sequestration in abandoned petroleum fields has the potential to reduce atmospheric emissions of CO₂ if it is adopted on a large scale. However, sequestration sites may pose risks to people who live in their vicinity. CO₂ release from the sequestration formation through abandoned wells to the vadose zone and then from the vadose zone into people’s houses could cause exposure to high levels of CO₂. CO₂ is different from many other chemicals that may be released into the environment because its effects are acute instead of chronic. Existing literature on the health effects of CO₂ in humans was surveyed to establish risk-based screening levels that could be used near a sequestration site. Two potential screening levels were identified: (1) one person in a million becomes dizzy from inhalation of CO₂ in the basement of a house (3.7780% CO₂), and (2) one person in a million loses consciousness from exposure to CO₂ (6.6744% CO₂). A hypothetical risk assessment was conducted using a semianalytical wellfield model developed at Princeton University coupled with analytical models of diffusion through the vadose zone and foundation walls. The assessment assumed that a wellfield in Alberta, Canada, was transformed into a sequestration site with an injection rate of 43,200 t-CO₂/day and that a subdivision has been built near the site. The results showed that CO₂ levels on the site will not reach either of the identified screening levels unless the value used for the exchange rate for air in the houses is very small.

91. Duguid, A., M. Radonjic, and George Scherer, June 2006: The effect of carbonated brine on the interface between well cement and geologic formations under diffusion controlled conditions. Proceedings of the 8th International Conference on Greenhouse Gas Control Technologies (GHGT-8),
    [ Abstract ]

    A series of experiments was conducted on composite samples made from Class H well cement and either sandstone or limestone in order to determine the effect on the cement-stone interface in a petroleum well from exposure to carbonated brine. The experiments were carried out over the course of a year and showed that carbonated brines can degrade cement at the cement-stone interface.

92. Fuller, R., Jean Hervé Prévost, and M. Piri, 2006: Three-Phase Equilibrium and Partitioning Calculations for CO₂ Sequestration in Saline Aquifers. Journal of Geophysical Research, 111(B06207), doi:10.1029/2005JB003618
    [ Abstract ]

    We show how the use of appropriate variables results in a flash calculation that uses only equilibrium constraints; it is thus not necessary to solve the mass balance equations self-consistently with the equilibrium equations. We use this implicit material balance in flash calculation. We show its advantages over the current approach that uses an explicit material balance. For the flash calculation for CO₂ storage in brine aquifers, use of appropriate variables also allows us to find the dew, bubble, and precipitation points where the liquid, vapor, and solid salt phases, respectively, emerge. Our calculation includes the water content of the vapor phase, which arises from evaporation of the brine. Evaporation leads to increased brine salinity, which results in a large reduction in CO₂ solubility in the salting-out effect, and eventually in precipitation of solid salt and ultimately the disappearance of the liquid phase. The flash calculation also relies on our derivation of fugacities for H₂O and CO₂ in both the brine and the vapor phase.

93. Gasda, S., Jan M. Nordbotten, and Michael Celia, June 2006: Significance of Dipping Angle on CO₂ Plume Migration in Deep Saline Aquifers. Proceedings of the XVI Intl Conf on Computational Methods in Water Resources, Copenhagen, http://proceedings.cmwr-xvi.org/getFile.py/access?contribId=63,
    [ Abstract ]

    Recent investigations regarding CO₂ sequestration in deep, saline aquifers have focused on characterization of the injected plume, its migration within the aquifer over time, and possible leakage out of the aquifer. As part of our efforts to understand and quantify leakage potential in CO₂ storage systems, a semi-analytical solution has been developed that describes the plume shape evolution as well the amount of leakage, with a focus on leakage along abandoned wells. The semi-analytical solutions require a number of simplifying assumptions, including a perfectly horizontal aquifer, negligible capillary pressure, and symmetry of the injection plume. Each of these assumptions can be tested systematically through application of more general numerical simulators. In typical sedimentary basins, it is common to have sloping aquifers with a vertical rise of up to 3-4 km over the total horizontal length of the basin (hundreds of kilometers). In this study, we use a general two-phase numerical simulator to assess the limitations of the assumptions required to derive semi-analytical solutions to these systems. In this presentation we will present results from these simulations and discuss their implications regarding the extent to which CO₂ injection systems can be simplified.

94. Haegland, H., H. Dahle, G. T. Eigestad, Jan M. Nordbotten, Michael Celia, and A. Assteerawatt, June 2006: Streamline Methods on Fault Adapted Grids for Risk Assessment of Storage of CO₂ in Geological Formations. Proceedings of the XVI Intl Conf on Computational Methods in Water Resources, Copenhagen, http://proceedings.cmwr-xvi.org/getFile.py/access?contribId=68,
    [ Abstract ]

    Geological storage of CO₂ in possibly fractured and faulted media, involves the risk of leakage. The extent of leakage may be assessed with statistical methods through analysis of simulations of multiple realizations of a stochastic model. Numerical simulation of numerous such realizations typically requires considerable computational cost, motivating the use of fast numerical methods, such as streamline simulation, for screening. Streamline methods have shown to be eective for reservoir characterization and simulation. In this work we will develop methodology which allows for tracing of streamlines in fractured or faulted media. The work is motivated in part by the need to assess potential of geological storage of CO₂ and is also highly relevant for reservoir simulation.

95. Huet, B. J., R. Fuller, and Jean Hervé Prévost, March 2006: Development of a geochemical code to assess cement reactivity in CO₂/brine mixtures. Wellbore Integrity Workshop, Princeton University, http://www.princeton.edu/~cmi/research/Storage/Papers/DevelopmentGeochemicalCodeCO2SC.pdf, 164-168
    [ Abstract ]

    This work aims at modeling well-bore leakage of carbon dioxide (CO₂) from sequestration reservoirs. The leakage of CO₂ is a function of the geometry of the low permeability path and of the boundary conditions. Furthermore, the CO₂ flow can also be strongly influenced by the chemical reactivity of cement, leading either to the sealing or to the widening of the annulus. Thus, a multiphase transport model that aims at assessing CO₂ leakage along high permeability paths must account for the reactivity of the porous environment. Therefore a geochemical module must be integrated. This study presents preliminary results of the coupling of a geochemical module with a transport module. Governing equations of the two modules are introduced. Results of benchmark simulations of chemical degradation of cement paste (CEM I) in pure deionized water are presented. The influence of the different terms of the transport equation on mineral profile is discussed.

96. Huet, B. J., R. Fuller, Jean Hervé Prévost, and George Scherer, May 2006: Numerical simulation of multiphase flow of CO₂ along low permeability path up the wells. Introduction of the interactions with cement. Proceedings of the Fifth Annual Conference on Carbon Capture & Sequestration, Alexandria, Virginia,
    [ Abstract ]

    This work aims at modeling well-bore leakage of Carbon Dioxide (CO₂) from sequestration reservoirs. The leakage of CO₂ is function of the geometry of the low permeability path and boundary conditions. However, the CO₂ flow can also be strongly influenced by the chemical reactivity of cement leading either to the sealing or to the widening of the annulus. This study presents results of the integration of a geochemical module into a robust three-phase transport module developed by Fuller et al. [1]. The numerical model assesses the chemical and physical integrity of cement in injection and/or abandoned sealed wells. The geochemistry part of the simulator assumes local chemical equilibrium. It takes into account the reactivity of cement, i.e. both mineral solid phase changes, like portlandite dissolution and calciium carbonate formation, and aqueous speciation in the interstitial solution. As a result of minerals dissolution and formation, the changes in porosity and the related changes in diffusion and permeability properties of cement are also evaluated. The simulator allows the study of the influence of the well geometry, e.g., the initial width of the annulus between hydrated cement and steel casing, or between hydrated cement and reservoir stone. Both diffusion and advection transport mechanisms are studied separately to account for the different cement pressure environments. Results indicate drastically different degradation rates for cement. Thus, the mean width of the annulus is a key parameter to assess the reliability of injection or abandoned wells.

97. Kavetski, D., Jan M. Nordbotten, and Michael Celia, June 2006: Analysis of Potential CO₂ Leakage through Abandoned Wells using a Semi-analytical Model. Proceedings of the XVI Intl Conf on Computational Methods in Water Resources, Copenhagen, http://proceedings.cmwr-xvi.org/getFile.py/access?contribId=1,
    [ Abstract ]

    Potential injection sites for geological CO₂ storage include deep formations in mature sedimentary basins. Many of these basins have a long history of oil and gas exploration and production and the vicinity of the injection site may therefore be perforated by hundreds of wells, potentially penetrating into the injection formation. Since typical injection operations may lead to CO₂ plums extending tens of kilometres from the injection site, geosequestration models must be able to simulate large spatial domains (up to and above 1,000 km²), while resolving the local dynamics in all the wells. Furthermore, many of these wells are abandoned and their locations and hydraulic properties might be uncertain or unknown. Therefore, risk assessment based on Monte Carlo simulations may be necessary to estimate the resulting uncertainty in the leakage. In this paper, we present a semi-analytical model that simulates the evolution of CO₂ plumes and leakage in multiple brine aquifers penetrated by multiple wells over decadal to century time scales. The model equations and state variables are obtained from the self-similarity of the plume shapes and are defined solely at well locations. Since the model does not require domain discretisation in the traditional numerical sense, it is highly computationally efficient, potentially thousands of times faster than existing numerical multiphase simulators. This paper demonstrates the insights gained by applying this model to a potential injection site in the Alberta Basin, Canada, with more than 500 existing wells over a domain of 900 km². Several leakage measures and statistics are presented and discussed.

98. Lin, Li-Hung, P.-L. Wang, D. Rumble, J. Lippmann-Pipke, E. Boice, L. Pratt, B. S. Lollar, E. L. Brodie, T. C. Hazen, G. L. Andersen, T. Z. DeSantis, D. Moser, D. Kershaw, and T. C. Onstott, 2006: Long term biosustainability in a high energy, low diversity crustal biome In , 314, doi:10.1126/science.1127376 479-482
    [ Abstract ]

    Geochemical, microbiological, and molecular analyses of alkaline saline groundwater at 2.8 kilometers depth in Archaean metabasalt revealed a microbial biome dominated by a single phylotype affiliated with thermophilic sulfate reducers belonging to Firmicutes. These sulfate reducers were sustained by geologically produced sulfate and hydrogen at concentrations sufficient to maintain activities for millions of years with no apparent reliance on photosynthetically derived substrates.

99. Nordbotten, Jan M., and Michael Celia, June 2006: Analysis of Plume Extent using Analytical Solutions for CO₂ Storage. Proceedings of the XVI Intl Conf on Computational Methods in Water Resources, Copenhagen, http://proceedings.cmwr-xvi.org/getFile.py/access?contribId=50,
    [ Abstract ]

    The evaluation of possible leakage pathways from CO₂ storage operations requires attention to the magnitude, concentration and timescales involved. Herein we discuss a problem related to CO₂ storage, the migration of CO₂ as it is injected. This is accomplished through the application of analytical solutions. In particular, we derive a new analytical insight into the problem of fluid injection into a confined aquifer, which gives us analytically the furthest extent of the injected fluid (CO₂), as well as the extent of the region in which the formation fluid (brine) has evaporated into the injected fluid. We apply these new analytical solutions to a hypothetical injection case based on data from Alberta, Canada, and discuss the results in terms of the surprising variability in observed system responses. We conclude this paper by emphasizing the value of analytical solutions both in semi-analytical frameworks and as benchmarks for numerical simulations.

100. Nordbotten, Jan M., and Michael Celia, 2006: An Improved Analytical Solution for Interface Upconing around a Well. Water Resources Research, 42(W08433), doi:10.1029/2005WR004738
     [ Abstract ]

     Interface models of two-phase flow to a well usually apply vertical equilibrium assumptions to capture the vertical pressure variation in the two fluids. When large gradients in interface height occur, this leads to poor approximations. To deal with largegradient interfaces, a variety of correction factors have been proposed on the basis of single-phase flow analogies. Herein we propose to include the effects of vertical flow directly through a nonlinear vertical pressure variation. This more complex formulation captures significantly more of the flow dynamics in the near-well region.

101. Nordbotten, Jan M., and Michael Celia, 2006: Similarity Solutions for Fluid Injection into Confined Aquifers. Journal of Fluid Mechanics, 561, doi:10.1017/S0022112006000802 307-327
     [ Abstract ]

     Fluid injection into the deep subsurface, such as injection of carbon dioxide (CO₂) into deep saline aquifers, often involves two-fluid flow in confined geological formations. Similarity solutions may be derived for these problems by assuming that a sharp interface separates the two fluids, by imposing a suitable no-flow condition along both the top and bottom boundaries, and by including an explicit solution for the pressure distribution in both fluids. When the injected fluid is less dense and less viscous than the resident fluid, as is the case for CO₂ injection into a resident brine, gravity override produces a fluid flow system that is captured well by the similarity solutions. The similarity solutions may be extended to include slight miscibility between the two fluids, as well as compressibility in both of the fluid phases. The solutions provide the location of the interface between the two fluids, as well as drying fronts that develop within the injected fluid. Applications to cases of supercritical CO₂ injection into deep saline aquifers demonstrate the utility of the solutions, and comparisons to solutions from full numerical simulations show the ability to predict the system behaviour.

102. Nordbotten, Jan M., I. Rodriguez-Iturbe, and Michael Celia, 2006: Non-uniqueness of Evapotranspiration due to Spatial heterogeneity of Plant Species. Proceedings of The Royal Society of London, 462(2072), doi:10.1098/rspa.2005.1641 2359-2371
     [ Abstract ]

     Spatially averaged soil moisture dynamics are studied under seasonally fixed conditions. We consider rainfall as a marked Poisson process, uniformly covering a spatial domain consisting of multiple plant types. Each plant type is considered to have different characteristics in terms of evapotranspiration functions, root-zone depth and rainfall interception. Equations for the evolution of joint probability density functions for individual soil moistures associated with different plant types are developed, and the non-uniqueness of the spatially averaged evapotranspiration function as a function of the average soil moisture is demonstrated and quantified in an example.

103. Onstott, T. C., D. McGown, J. Kessler, B. S. Lollar, K. K. Lehman, and S. M. Clifford, 2006: Martian CH₄: sources, flux and detection. Astrobiology, 6(2), doi:10.1089/ast.2006.6.377 377-395
     [ Abstract ]

     Recent observations have detected trace amounts of CH₄ heterogeneously distributed in the martian atmosphere, which indicated a subsurface CH₄ flux of ˜2 X 10⁵ to 2 X 10⁹ cm^-2 s^-1. Four different origins for this CH₄ were considered: (1) volcanogenic; (2) sublimation of hydrate- rich ice; (3) diffusive transport through hydrate-saturated cryosphere; and (4) microbial CH₄ generation above the cryosphere. A diffusive flux model of the martian crust for He, H₂, and CH₄ was developed based upon measurements of deep fracture water samples from South Africa. This model distinguishes between abiogenic and microbial CH₄ sources based upon their isotopic composition, and couples microbial CH₄ production to H₂ generation by H₂O radiolysis. For a He flux of ˜10⁵ cm^-2 s^-1 this model yields an abiogenic CH₄ flux and a microbial CH₄ flux of ˜10⁶ and ˜10⁹ cm^-2 s^-1, respectively. This flux will only reach the martian surface if CH₄ hydrate is saturated in the cryosphere; otherwise it will be captured within the cryosphere. The sublimation of a hydrate-rich cryosphere could generate the observed CH₄ flux, whereas microbial CH₄ production in a hypersaline environment above the hydrate stability zone only seems capable of supplying ˜10⁵ cm^-2 s^-1 of CH₄. The model predicts that He/H₂/CH₄/C₂H₆ abundances and the C and H isotopic values of CH₄ and the C isotopic composition of C₂H₆ could reveal the different sources. Cavity ring-down spectrometers represent the instrument type that would be most capable of performing the C and H measurements of CH₄ on near future rover missions and pinpointing the cause and source of the CH₄ emissions.

104. Onstott, T. C., Li-Hung Lin, M. Davidson, B. Mislowack, M. Borcsik, J. Hall, G. Slater, J. Ward, B. S. Lollar, J. Lippmann-Pipke, E. Boice, and L. Pratt, et al., 2006: The origin and age of biogeochemical trends in deep fracture water of the Witwatersrand Basin, South Africa. Geomicrobiology Journal, 12(6), doi:10.1080/01490450600875688 369-414
     [ Abstract ]

     Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH>sub>4 and hydrocarbon, occasionally N₂, originally formed at ∼250–300°C and during cooling isotopically exchanged O and H with minerals and accrued H₂, ⁴He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼10 Ka to >1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO₂ and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past∼100 Myr, a process which inoculates previously sterile environments at depths >2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 10⁷ times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bio available chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The ⁴He concentrations and theoretical calculations indicate that the H₂ that is sustaining the subsurface microbial communities (e.g. H₂-utilizing SRB and methanogens) is produced by water radiolysis at a rate of ∼1nMyr⁻¹. Microbial CH₄ mixes with abiogenicCH₄ to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from∼100 at < 1 kmbls, to <0.01nMyr⁻¹ at >3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ¹³C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces.

105. Peters, Catherine A., J. A. Lewandowski, M. L. Maier, Michael Celia, and , 2006: Mineral Grain Spatial Patterns and Reaction Rate Up-Scaling. Proceedings of the XVI Intl Conf on Computational Methods in Water Resources, Copenhagen, http://esd.lbl.gov/ESD_staff/li_li/lilicmwrxviCAP.pdf,
     [ Abstract ]

     Reactive transport models that describe mineral reactions in porous media rely on laboratory measurements of rate parameters that may fail to represent reactions defined at larger averaging scales. In recently completed work, we used pore-scale network models to investigate the effects of heterogeneities in pore structure and mineral distribution on geochemical reaction rates in porous media. Our findings revealed significant scaling effects from variations in reactive mineral distribution, especially for the highly acidic conditions encountered in geological sequestration of carbon dioxide. In this paper we present preliminary findings from electron scanning BSE maps, to analyze spatial patterns of minerals in sedimentary rocks. Samples include sandstones from the Viking formation in the Alberta basin in western Canada. Image analysis was used to quantify pore space and examine reactive minerals in relation to pore locations. Typically, reactive minerals occur as distinct grains and inclusions, and their percent abundance is larger than the extent of their contact with pore fluids.

106. Celia, Michael, , Jan M. Nordbotten, D. Kavetski, and S. Gasda, May 2005: Modeling Critical Leakage Pathways in a Risk Assessment Framework: Representation of Abandoned Wells. Proceedings of the 4th Annual Conference on Carbon Capture and Sequestration, http://www.netl.doe.gov/publications/proceedings/05/carbon-seq/Tech%20Session%20Paper%20115.pd,
     [ Abstract ]

     In many locations in North America, likely injection sites for CO₂ storage in deep geological formation are located in mature sedimentary basins. These basins have a century-long history of oil and gas exploration and production, which has led to hundreds of thousands of wells (the Alberta Basin) to more than a million wells (Texas) being drilled. The spatial density of these wells is on the order of 0.5 to 5 wells per square kilometer. Therefore, a typical injection will produce a CO₂ plume that intersects hundreds of existing wells, many of which are abandoned and some of which have uncertain or unknown locations. In order to analyze the leakage potential in such situations, computational models must be developed that can cover large spatial areas (of order 1,000 km²) while resolving the local flow dynamics in all of the hundreds of wells. In addition, both the layered structure of the subsurface, and possible leakage along wells and into successive overlying permeable layers in the subsurface, also need to be represented. We have developed a semi-analytical model that can simulate all of these attributes, over decadal to century time scales, while running quickly on a laptop computer. With this tool, risk assessment based on Monte Carlo analysis can be carried out, and a quantitative analysis of leakage potential can be performed.

107. Ciardullo, J. P., D. J. Sweeney, and George Scherer, 2005: Thermal expansion kinetics: Method to measure permeability of cementitious materials: IV, Effect of thermal gradients and viscoelasticity. Journal of the American Ceramic Society, 88(5), doi:10.1111/j.1551-2916.2005.00214.x 1213-1221
     [ Abstract ]

     When a porous material that is saturated with liquid is heated, the liquid expands much more than the solid phase. If the permeability is low, then the liquid may not be able to escape as it expands, so it expands within the pores and causes dilatation of the body. In that case, by analyzing the kinetics of dilatation during a change in temperature, it is possible to extract the permeability. Previous papers have examined the behavior of an elastic or viscoelastic (VE) porous solid subjected to a thermal cycle slow enough to avoid internal temperature gradients. However, for cementitious samples, the sample size must be large enough that thermal gradients are likely. In this paper, we show that the effect of the gradient can be readily incorporated into the analysis of experimental data. For cement paste, experiments reveal that VE relaxation has a greater influence on the results than the gradient in temperature.

108. Dahle, H., Michael Celia, and S. M. Hassanizadeh, 2005: Bundle-of-Tubes Model for Calculating Dynamic Effects in the Capillary Pressure – Saturation Relationship. Transport in Porous Media, 58(1-2), doi:10.1007/1-4020-3604-3 5-22
     [ Abstract ]

     Traditional two-phase flow models use an algebraic relationship between capillary pressure and saturation. This relationship is based on measurements made under static conditions. However, this static relationship is then used to model dynamic conditions, and evidence suggests that the assumption of equilibrium between capillary pressure and saturation may not be be justified. Extended capillary pressure–saturation relationships have been proposed that include an additional term accounting for dynamic effects. In the present work we study some of the underlying pore-scale physical mechanisms that give rise to this so-called dynamic effect. The study is carried out with the aid of a simple bundle-of-tubes model wherein the pore space of a porous medium is represented by a set of parallel tubes. We perform virtual two-phase flow experiments in which a wetting fluid is displaced by a non-wetting fluid. The dynamics of fluid–fluid interfaces are taken into account. From these experiments, we extract information about the overall system dynamics, and determine coefficients that are relevant to the dynamic capillary pressure description. We find dynamic coefficients in the range of 10²−10³ kgm⁻¹ s⁻¹, which is in the lower range of experimental observations. We then analyze certain behavior of the system in terms of dimensionless groups, and we observe scale dependency in the dynamic coefficient. Based on these results, we then speculate about possible scale effects and the significance of the dynamic term.

109. Duguid, A., M. Radonjic, and George Scherer, 2005: Degradation of well cements exposed to carbonated brine. Proceedings of the 4th Annual Conference on Carbon Capture and Sequestration, http://www.netl.doe.gov/publications/proceedings/05/carbon-seq/Tech%20Session%20Paper%20186.pd,
     [ Abstract ]

     With increased attention on how people affect the climate, interest has grown in the anthropogenic emissions of greenhouse gases such as carbon dioxide. Subsurface carbon sequestration in abandoned petroleum fields may represent a chance to reduce CO₂ emissions. In order to understand how CO₂ may escape from the storage formation back to the atmosphere through abandoned wells, a set of four experiments was conducted. The experiments looked at how well cements reacted to carbonated brines at 20° and 50°C at pH 2.4 and 3.7. The results showed severe degradation to samples made from Class H well cement. The degradation occurred over the course of 31 days.

110. Duguid, A., M. Radonjic, and George Scherer, April 2005: Durability of well cements: corrosion by carbonic acid. Proceedings of the 107th Annual Meeting of the American Ceramics Society,
     [ Abstract ]

     With an increased focus on how people affect the climate, interest has grown in the anthropogenic emissions of greenhouse gases such as carbon dioxide. Subsurface carbon sequestration in abandoned petroleum fields may represent a chance to reduce CO₂ emissions. In order to understand how CO₂ may escape from the storage formation back to the atmosphere through abandoned wells, a set of four experiments was conducted. The experiments looked at how well cements reacted to carbonated brines at 20° and 50°C at pH 2.4 and 3.7. The results showed severe degradation to samples made from Class H well cement, the degradation occurring over the course of 31 days.

111. Gasda, S., and Michael Celia, 2005: Upscaling Relative Permeabilities in a Structured Porous Medium. Advances in Water Resources, 28(5), doi:10.1016/j.advwatres.2004.11.009
     [ Abstract ]

     Upscaling of multi-phase flow problems for a heterogeneous porous medium requires modification of constitutive functions at the grid-block scale. A particular type of heterogeneity that has important environmental consequences involves thin, continuous streaks of high permeability through lower-permeability background rocks. These streaks, which may correspond to features like abandoned wells in mature sedimentary basins, can become preferential flow paths for an invading fluid. Quantification of flow through these types of heterogeneities in deep, geological formations is necessary for estimates of migration and possible leakage of injected fluids such as hazardous liquid wastes, municipal liquid wastes, and, possibly, carbon dioxide. One of the important constitutive functions for proper estimation of flow through these flow paths is the relative permeability function. In the simple case of a single high-permeability streak in a uniform rock matrix, with both materials having identical (local) relative permeability functions, the upscaled relative permeability must be changed significantly to capture the proper leakage. Standard petroleum reservoir pseudo-functions for relative permeability capture the general features of the upscaled function, but they still produce errors of several hundred percent in the leakage estimation. Detailed three-dimensional numerical simulations and associated upscaled calculations demonstrate the proper form for the upscaled relative permeability, and provide a modified derivation of pseudo-functions to capture the leakage behavior in upscaled models.

112. Gielen, T., S. M. Hassanizadeh, A. Leijnse, and H. Nordhaug, 2005: Dynamic Effects in Multiphase Flow: A Pore-Scale Network Approach. Proceedings of the Computational Methods in Water Resources 2004 Conference, Section II, doi:10.1007/1-4020-3604-3
     [ Abstract ]

     Current theories of multiphase flow rely on capillary pressure and saturation relationships that are commonly measured under static conditions. To incorporate transient behaviour, new multiphase flow theories have been proposed. These include an extended capillary pressure-saturation relationship that is valid under dynamic conditions. In this relationship, the difference between the two fluid pressures is called dynamic capillary pressure, and is assumed to be a function of saturation and its time rate of change. The dependency is through a so-called damping coefficient. In this work, this proportionality between capillary pressure and saturation rate of change is investigated using a porescale network model. It consist of a three-dimensional network of tubes (pore throats) connected to each other by pore bodies. The pore bodies are spheres and pore throats are cylinders. Numerical experiments are performed wherein typical experimental procedures for both static and dynamic measurements of capillary pressure-saturation curves are simulated. The value of the damping coefficient is determined for one realisation of our network model. Then, the effect of different averaging domains on capillary pressuresaturation curves is investigated.

113. Kieft, T. L., S. M. McCuddy, T. C. Onstott, M. Davidson, Li-Hung Lin, B. Mislowack, L. Pratt, E. Boice, B. S. Lollar, J. Lippmann-Pipke, S. M. Pfiffner, and T. J. Phelps, et al., 2005: Geochemically Generated, Energy-Rich Substrates and Indigenous Microorganisms in Deep, Ancient Groundwater. Geomicrobiology Journal, 22(6), doi:10.1080/01490450500184876 325-355
     [ Abstract ]

     Recent studies have shown that the biosphere extends to depths that exceed 3 km, raising questions regarding the age of the microbes in these deep ecosystems and their sources of energy for metabolism. Abiogenic energy sources that are derived from in situ, purely geochemical sources and thus independent from photosynthesis have been suggested.We sampled saline fracture water emanating from a 3.1-km deep borehole in a Au mine in the Witwatersrand Basin of South Africa and characterized the chemical constituents (including stable isotopes), groundwater age, and indigenous microorganisms. Salinity data and ratios of dissolved noble gases indicate that extremely ancient (2.0 Ga) saline fracture water has mixed with meteoric water to yield an average subsurface residence time of 20–160 Ma, the oldest age of any waters collected to date in the Witwatersrand Basin. H₂ isotope data suggest the water originated from a depth of 4 to 5 km. Sulfur isotope fractionation indicates biological sulfate reduction. Calculations of free energies and steady state energy fluxes based on water chemistry data also support sulfate reduction as the dominant terminal electron accepting process. Lipid and flow cytometry data indicate a sparse microbial community (103 cells ml⁻¹), despite the presence of relatively high concentrations of energy-rich compounds (H₂, CH₄, CO, ethane, propane, butane, and acetate). The H₂ can be explained by radiolysis of water. Stable isotopic signatures of the CH₄ and short chain hydrocarbons indicate abiogenic synthesis. The persistence of energy-rich compounds suggests that other factors are limiting to microbial metabolism and growth, e.g., availability of an inorganic nutrient, such as Fe or phosphate.

114. Nordbotten, Jan M., and I. Aavatsmark, 2005: Monotonicity Conditions for Control Volume Methods on Uniform Parallelogram Grids in Homogeneous Media. Computational Geosciences, 9(1), doi:10.1007/s10596-005-5665-2 61-72
     [ Abstract ]

     Control volume methods are frequently used in porous media flow. This article gives an example on how one method, the Multipoint Flux Approximation method (MPFA), fails to satisfy the maximum principle for strong anisotropies or grid skewnesses, and develops conditions for when the monotonicity property holds for uniform parallelogram grids in homogeneous media. The conditions developed are applicable to any nine-point scheme in 2D or 27-point scheme in 3D, and is useful when the method produces a system matrix which is not an M-matrix.

115. Nordbotten, Jan M., Michael Celia, , and H. Dahle, 2005: Semi-Analytical Solution for CO₂ Leakage Through An Abandoned Well. Environmental Science and Technology, 39(2), doi:10.1021/es035338i 602-611
     [ Abstract ]

     Capture and subsequent injection of carbon dioxide into deep geological formations is being considered as a means to reduce anthropogenic emissions of CO₂ to the atmosphere. If such a strategy is to be successful, the injected CO₂ must remain within the injection formation for long periods of time, at least several hundred years. Because mature continental sedimentary basins have a century-long history of oil and gas exploration and production, they are characterized by large numbers of existing oil and gas wells. For example, more than 1 million such wells have been drilled in the state of Texas in the United States. These existing wells represent potential leakage pathways for injected CO₂. To analyze leakage potential, modeling tools are needed that predict leakage rates and patterns in systems with injection and potentially leaky wells. A new semianalytical solution framework allows simple and efficient prediction of leakage rates for the case of injection of supercritical CO₂ into a brine-saturated deep aquifer. The solution predicts the extent of the injected CO₂ plume, provides leakage rates through an abandoned well located at an arbitrary distance from the injection well, and estimates the CO₂ plume extent in the overlying aquifer into which the fluid leaks. Comparison to results from a numerical multiphase flow simulator show excellent agreement. Example calculations show the importance of outer boundary conditions, the influence of both density and viscosity contrasts in the resulting solutions, and the potential importance of local upconing around the leaky well. While several important limiting assumptions are required, the new semianalytical solution provides a simple and efficient procedure for estimation of CO₂ leakage for problems involving one injection well, one leaky well, and multiple aquifers separated by impermeable aquitards.

116. Nordbotten, Jan M., Michael Celia, and , 2005: Injection and Storage of CO₂ in Deep Saline Aquifers: Analytical Solution for CO₂ Plume Evolution during Injection. Transport in Porous Media, 58(3), doi:10.1007/s11242-004-0670-9 339-360
     [ Abstract ]

     Injection of fluids into deep saline aquifers is practiced in several industrial activities, and is being considered as part of a possible mitigation strategy to reduce anthropogenic emissions of carbon dioxide into the atmosphere. Injection of CO₂ into deep saline aquifers involves CO₂ as a supercritical fluid that is less dense and less viscous than the resident formation water. These fluid properties lead to gravity override and possible viscous fingering. With relatively mild assumptions regarding fluid properties and displacement patterns, an analytical solution may be derived to describe the space–time evolution of the CO₂ plume. The solution uses arguments of energy minimization, and reduces to a simple radial form of the Buckley–Leverett solution for conditions of viscous domination. In order to test the applicability of the analytical solution to the CO₂ injection problem, we consider a wide range of subsurface conditions, characteristic of sedimentary basins around the world, that are expected to apply to possible CO₂ injection scenarios. For comparison, we run numerical simulations with an industry standard simulator, and show that the new analytical solution matches a full numerical solution for the entire range of CO₂ injection scenarios considered. The analytical solution provides a tool to estimate practical quantities associated with CO₂ injection, including maximum spatial extent of a plume and the shape of the overriding less-dense CO₂ front.

117. Nordbotten, Jan M., and G. T. Eigestad, 2005: Discretization on quadrilateral grids with improved monotonicity properties. Journal of Computational Physics, 203(2), doi:10.1016/j.jcp.2004.10.002 744-760
     [ Abstract ]

     Multi-point flux approximation (MPFA) discretization methods have been applied in the oil industry since the mid 1990s. The discretizations are based on a control volume formulation and the finite difference structure makes general skew grids and unstructured grids feasible in a fully implicit formulation. MPFA methods are therefore suitable for flow problems in realistic reservoirs. Monotonicity issues are known to arise for high aspect ratios combined with skewness of computational grids for MPFA methods. In this paper, we improve the MPFA discretization techniques for general quadrilateral grids, such that the above difficulties are handled to give a more robust discretization of the governing equations for fluid flow in porous media. Comparisons to the MPFA O-method are made, and the suggested discretization is shown to be an improvement in regards to monotonicity. For smooth solutions, the method performs equally well as the O-method when the convergence is examined.

118. Piri, M., Jean Hervé Prévost, and R. Fuller, May 2005: Carbon Dioxide Sequestration in Saline Aquifers: Evaporation, Precipitation and Compressibility Effects. Proceedings of the 4th Annual Conference on Carbon Capture and Sequestration, Alexandria, VA, http://www.princeton.edu/~cmi/research/Storage/Papers/CO2SequestrationSalineAquifers.pdf,
     [ Abstract ]

     A compositional reservoir simulator is developed that is capable of modeling multicomponent, isothermal multiphase flow of compressible fuids through deformable porous media. The simulator is used to study the detailed physics associated with the transport of dfferent components in a three-phase system encountered during Carbon Dioxide (CO₂) sequestration in deep saline aquifers. The model benefits from a robust three-phase (brine - solid salt - supercritical CO₂) flash module developed by Fuller et al. [1] to partition materials into different phases that are considered to be in thermodynamic equilibrium. As an example we model the radial injection of supercritical CO₂ in a deep saline aquifer taking into account dispersion and compressibility of °uids. The injected CO₂ resides in a dense liquid-like supercritical phase and also dissolves into the brine. The results illustrate the evolution of one- , two- and three-phase regions in the system. We encounter a dried region close to the injection well where salt precipitation into a solid phase is a direct consequence of complete evaporation of the residual liquid left behind the displacing front. Effects of evaporation on the amount of dissolved CO₂, salting out efect, is studied. We demonstrate that proper treatment of evaporation and precipitation provides solutions that deviate in the near term from scaling with the conventional similarity variable (R²/t) for radial flow. However, scaling does occur after all the transients have died out. Porosity change due to salt precipitation and matrix compressibility which in turn alters absolute permeability are also taken into account. Effects of fluid and matrix compressibilities and brine salinity on amount of CO₂ dissolved in the liquid phase are also analyzed. We also present results of two-dimensional simulations in which effects of hysteresis on trapping of CO₂ have been studied.

119. Prévost, Jean H., R. Fuller, A. Altevogt, R. Bruant, and George Scherer, 2005: Numerical Modeling of Carbon Dioxide Injection and Transport in Deep Saline Aquifers. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, (GHGT-7), http://www.princeton.edu/~cmi/research/Storage/Papers/NumericalModelingCarbonDioxide.pdf, 2189-2193
     [ Abstract ]

     We describe the development of a compositional resevoir simulator capable of modeling multiphase transport of CO₂ in a brine aquifer, and results are provided which illustrate the evolution of a 2-phase fluid system in which the injected CO₂ resides in a dense supercritical phase and also dissolves into the liquid phase. Salt precipitation into a solid phase is found to occur close to the injection well where the gas phase dominates.

120. Puma, M. J., Michael Celia, I. Rodriguez-Iturbe, and A. J. Guswa, 2005: Functional Relationship to Describe Temporal Statistics of Soil Moisture averaged over Different Depths. Advances in Water Resources, 28, doi:10.1016/j.advwatres.2004.08.015 553-566
     [ Abstract ]

     Detailed simulation studies, highly resolved in space and time, show that a physical relationship exists among instantaneous soilmoisture values integrated over different soil depths. This dynamic relationship evolves in time as a function of the hydrologic inputs and soil and vegetation characteristics. When depth-averaged soil moisture is sampled at a low temporal frequency, the structure of the relationship breaks down and becomes undetectable. Statistical measures can overcome the limitation of sampling frequency, and predictions of mean and variance for soil moisture can be defined over any soil averaging depth d. For a water-limited ecosystem, a detailed simulation model is used to compute the mean and variance of soil moisture for different averaging depths over a number of growing seasons. We present a framework that predicts the mean of soil moisture as a function of averaging depth given soil moisture over a shallow d and the average daily rainfall reaching the soil.

121. Scherer, George, Michael Celia, Jean Hervé Prévost, , R. Bruant, A. Duguid, R. Fuller, S. Gasda, M. Radonjic, and W. Vichit-Vadakan, 2005: Leakage of CO₂ through Abandoned Wells: Role of Corrosion of Cement. The CO₂ Capture and Storage Project (CCP), Volume II, Chapter 10, 823-844
     [ Abstract ]

     The potential leakage of CO₂ from a geological storage site through existing wells represents a major concern. An analysis of well distribution in the Viking Formation in the Alberta basin, a mature sedimentary basin representative of North American basins, shows that a CO₂ plume and/or acidified brine may encounter up to several hundred wells. A review of the literature indicates that cement is not resistant to attack by acid, but little work has been reported for temperatures and pressures comparable to storage conditions. Therefore, an experimental program has been undertaken to determine the rate of corrosion and the changes in properties of oil well cements exposed to carbonated brine. Preliminary results indicate a very high rate of attack, so it is essential to have accurate models of the composition and pH of the brine, and the time that it will remain in contact with cement in abandoned wells. A model has been developed that incorporates a flash calculation of the phase distribution, along with analysis of the fluxes and pressure of the liquid, solid and vapor phases. A sample calculation indicates that wells surrounding the injection site may be in contact with the acidified brine for years.

122. Scherer, George, and I. Jimenez Gonzalez, 2005: Characterization of Swelling in Clay-Bearing Stone. Stone decay and conservation, 390, doi:10.1130/0-8137-2390-6.51 51-61
     [ Abstract ]

     Many sedimentary rocks contain clays that cause differential swelling upon exposure to moisture, and the resulting internal stresses are blamed for the deterioration of buildings and monuments. To predict the likelihood of damage from this mechanism, it is necessary to characterize the magnitude of the swelling and the mechanical properties of the stone. Stones that swell also exhibit viscoelastic behavior, probably owing to sliding of the clay layers. In this paper we discuss the characterization of the relaxation and swelling behavior and the estimation of stresses resulting from swelling. A new method for measuring swelling is introduced, in which warping of a plate of stone is produced by wetting one side. This method is faster than the traditional direct measurement of swelling pressure, and it also yields information about the permeability and the influence of wetting on the elastic modulus. Sample results are presented for Portland Brownstone.

123. Scherer, George, and J. J. Valenza II, 2005: Mechanisms of Frost Damage. Materials Science of Concrete, American Ceramics Society, VII(ISBN:978-1-57498-210), 209-246
     [ Abstract ]

     The mechanisms responsible for damage from internal freezing and salt scaling are reviewed. The primary cause of internal damage is crystallization pressure, and the role of the air voids is to provide sites for nucleation of macroscopic ice. The thermodynamics of the stress development are reviewed, and the predicted pressures are shown to be in quantitative accord with measured contraction of frozen bodies (including porous glass and cement paste). The origin of salt scaling damage is less clear. We examine two mechanisms that seem to account for most of the experimental observations: the bimaterial effect (i.e., thermal expansion mismatch between ice and cement paste) and salt-induced swelling. A sensitive experiment, in which a layer of water is frozen on top of a thin plate of cement paste and the deflection of the plate is measured, reveals the large stresses produced by these mechanisms. Cracking of the ice layer is promoted by brine pockets, and this may account for the pessimum concentration for scaling damage. Salt-induced swelling, which seems to result from a combination of crystallization pressure and ion exchange, also contributes to the superficial stresses and may exacerbate scaling.

124. Valenza II, J. J., and George Scherer, 2005: Evidence of anomalous thermal expansion of water in cement paste. Cement and Concrete Research, 35(1), doi:10.1016/j.cemconres.2004.08.022 57-66
     [ Abstract ]

     A comparative study of permeability measurement by thermopermeametry (TPA) and beam bending was performed on cement paste. To bring the two measurements into agreement, it is necessary to recognize that the pore solution has a thermal expansion coefficient about one and a half times that of bulk liquid and to account for viscoelastic stress relaxation during TPA experiments. The anomalous thermal expansion is not accounted for by the presence of ions in the cement paste pore solution.

125. Altevogt, A., and Michael Celia, 2004: Numerical Modeling of Carbon Dioxide in Unsaturated Soils Due to Deep Subsurface Leakage In , 40(W03509), doi:10.1029/2003WR002848
     [ Abstract ]

     A two-dimensional numerical model was utilized to explore the flux mechanisms governing CO₂ transport in the vadose zone. The simulations were set up to approximately correspond to a site of natural CO₂ leakage at Mammoth Mountain, California. The mass fraction gradient driving force, responsible for diffusive and slip fluxes, was determined to lead to less plume spreading than advection alone. Density-driven flow of CO₂ led to significantly greater spreading of the plume and greater storage of CO₂ within the vadose zone than if density contrasts were not accounted for. Exposure assessment simulations indicate that for the conditions of interest there may be no physically realistic domain that would lead to CO₂ levels below the criteria for human health impacts (sub 10%) in surface soils for the leakage rate present at Mammoth Mountain.

126. Baidya Roy, S., Stephen W. Pacala, and R. L. Walko, 2004: Can large wind farms affect local meteorology? Journal of Geophysical Research, 109(D19101), doi:10.1029/2004JD004763
     [ Abstract ]

     The RAMS model was used to explore the possible impacts of a large wind farm in the Great Plains region on the local meteorology over synoptic timescales under typical summertime conditions. A wind turbine was approximated as a sink of energy and source of turbulence. The wind farm was created by assuming an array of such turbines. Results show that the wind farm significantly slows down the wind at the turbine hub-height level. Additionally, turbulence generated by rotors create eddies that can enhance vertical mixing of momentum, heat, and scalars, usually leading to a warming and drying of the surface air and reduced surface sensible heat flux. This effect is most intense in the early morning hours when the boundary layer is stably stratified and the hub-height level wind speed is the strongest due to the nocturnal low-level jet. The impact on evapotranspiration is small.

127. Celia, Michael, , Jan M. Nordbotten, S. Gasda, and H. Dahle, September 2004: Quantitative Estimation of CO₂ Leakage from Geological Storage: Analytical Models, Numerical Models, and Data Needs. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, (GHGT-7), http://uregina.ca/ghgt7/PDF/papers/peer/228.pdf,
     [ Abstract ]

     Geological storage of CO₂ in mature sedimentary basins of North America requires special consideration of the large number of existing wells. Those wells represent potential leakage pathways for the stored CO₂, and must be analyzed in the context of an overall environmental risk assessment. Analysis of well patterns in the Alberta basin, Canada, indicates that injected CO₂ plumes are expected to contact from several tens to several hundreds of existing wells, depending on the local density of wells in the vicinity of the injection. Quantitative analysis of the impact of these wells requires an extensive data collection effort, analysis of materials used in well construction and abandonment, and different levels of computational modeling to ascertain the risk associated with these wells. New analytical solutions provide a promising avenue for leakage analysis at the large scale. Results from these models show accuracy comparable to more complex numerical simulators at a small fraction of the computational time. This allows many simulations to be run so that different parameters values can be explored. These large-scale models need to be coupled to smaller-scale detailed models of material behavior within the leaky well to provide a complete analysis of the problem.

128. Celia, Michael, H. Dahle, and S. M. Hassanizadeh, June 2004: Dynamic Effects in Capillary Pressure Relationships for Two-phase Flow in Porous Media: Insights from Bundle-of-Tubes Models and their Implications. Proceedings of the Computational Methods in Water Resources 2004 Conference, 1, 127-138
     [ Abstract ]

     Traditional multi-phase flow models use an algebraic relationship between capillary pressure and saturation. This relationship is based on measurements made under static conditions. However, this static relationship is then used to model dynamic conditions and evidence suggest that the assumption of equilibrium between capillary pressure and saturation may not be justified. Extended capillary pressure-saturation relationships have been proposed that include an additional term accounting for dynamic effects. In the present work, we study the underlying pore-scale physical mechanisms that give rise to this so-called dynamic effect. The study is carried out with the aid of a simple bundle-of-tubes model wherein the pore space of a porous medium is represented by a set of parallel tubes. We perform virtual two-phase flow experiments in which a wetting fluid is displaced by a non-wetting fluid. The dynamics of fluid-fluid interfaces are taken into account, and we consider systems in which viscosity differences influence the displacement process. From these experiments, we extract information about overall system dynamics, determine large-scale effects that are associated with viscosity differences between the two fluids. Based on these results, we then speculate about possible scale effects and the significance of the dynamic term.

129. Duguid, A., M. Radonjic, R. Bruant, T. Mandecki, George Scherer, and Michael Celia, 2004: The Effect of CO₂ Sequestration on Oil Well Cements. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, (GHGT-7), http://www.princeton.edu/~cmi/research/Vancouver04/GHGT7Duguid.pdf,
     [ Abstract ]

     Experiments were conducted to examine the effects of CO₂ sequestration conditions on cements used to construct and abandon oil and gas wells. The results showed that significant damage, complete loss of the calcium hydroxide phase, can take place over a time span as short as seven days.

130. Gasda, S., , and Michael Celia, 2004: Spatial characterization of the location of potentially leaky wells penetrating a deep saline aquifer in a mature sedimentary basin. Environmental Geology, 46(6-7), doi:10.1007/s00254-004-1073-5 707-720
     [ Abstract ]

     This work was motivated by considerations of potential leakage pathways for CO₂ injected into deep geological formations for the purpose of carbon sequestration. Because existing wells represent a potentially important leakage pathway, a spatial analysis of wells that penetrate a deep aquifer in the Alberta Basin was performed and various statistical measures to quantify the spatial distribution of these wells were presented. The data indicate spatial clustering of wells, due to oil and gas production activities. The data also indicate that the number of wells that could be impacted by CO₂ injection, as defined by the spread of an injected CO₂ plume, varies from several hundred in high welldensity areas to about 20 in low-density areas. These results may be applied to other mature continental sedimentary basins in North America and elsewhere, where detailed information on well location and status may not be available.

131. Guswa, A. J., Michael Celia, and I. Rodriguez-Iturbe, 2004: Effect of Vertical Resolution on Predictions of Transpiration in Water-limited Eosystems. Advances in Water Resources, 27, doi:10.1016/j.advwatres.2004.03.001 467-480
     [ Abstract ]

     Water-limited ecosystems are characterized by precipitation with low annual totals and significant temporal variability, transpiration that is limited by soil-moisture availability, and infiltration events that may only partially rewet the vegetation root zone. Average transpiration in such environments is controlled by precipitation, and accurate predictions of vegetation health require adequate representation of temporal variation in the timing and intensity of plant uptake. Complexities introduced by variability in depth of infiltration, distribution of roots, and a plant’s ability to compensate for spatially heterogeneous soil moisture suggest a minimum vertical resolution required for satisfactory representation of plant behavior. To explore the effect of vertical resolution on predictions of transpiration, we conduct a series of numerical experiments, comparing the results from models of varying resolution for a range of plant and climate conditions. From temporal and spatial scales of the underlying processes and desired output, we develop dimensionless parameters that indicate the adequacy of a finite-resolution model with respect to reproducing characteristics of plant transpiration over multiple growing seasons. These parameters may be used to determine the spatial resolution required to predict vegetation health in water-limited ecosystems.

132. Jimenez Gonzalez, I., and George Scherer, 2004: Effect of swelling inhibitors on the swelling and stress relaxation of clay-bearing stones. Environmental Geology, 46(3-4), doi:10.1007/s00254-004-1038-8 364-377
     [ Abstract ]

     Clay-containing stones such as Portland Brownstone (USA), Villarlod Molasse (Switzerland) and Tarifa Sandstone (Spain), are expected to weather as a result of wetting and drying cycles. During drying events, contraction of the drying surface leads to stresses approaching the tensile strength of the stone. However, we have found that the magnitude of these stresses is reduced by the ability of the stone to undergo stress relaxation. In this paper we describe novel methods to determine the magnitude of the stresses and the rate at which they develop and relax. We also discuss the influence of swelling inhibitors on the magnitude of swelling and the rate of the stress relaxation of these stones. The implications of our findings for the understanding of damage due to swelling of clays are discussed.

133. Miliani, C., and M. L. Velo-Simpson, et al., 2004: Sol-Gel Properties and Consolidation Effectiveness of three Particle Modified Consolidants: TiO₂-PMC, AI₂O₃-PMC and SiO₂-PMC. Proceedings of the EUROMAT 2003, Symposium P2 Materials and Conservation of Cultural Heritage, Not In Circulation
134. Nordbotten, Jan M., Michael Celia, and , 2004: Analytical Solutions for Leakage Rates through Abandoned Wells. Water Resources Research, 40(W04204), doi:10.1029/2003WR002997
     [ Abstract ]

     Disposal of waste fluids via injection into deep saline aquifers is practiced in a variety of industries. Injection takes place in sedimentary basins that often have a history of oil and gas exploration and production, which means that wells other than those used for waste disposal may exist in the vicinity of the injection site. These existing wells provide possible pathways for leakage of waste fluids toward the shallow subsurface and the land surface. For single-phase flows of liquids with essentially constant properties, the equations governing the system are linear, and solutions may be written using the superposition principle. Because leakage through existing wells produces a time-varying flux rate, the solution of the governing equations involves convolution integrals. Previous solutions have addressed the problem of one injection well, one existing (passive) well, and a simple geometry of two aquifers separated by an aquitard by use of Laplace transforms. Even for this simple case, inversion of the transform is difficult. Solutions involving more than one passive well have not been developed. Nor have solutions been developed for more than two aquifers and one aquitard. Realistic injection cases often involve layered systems with multiple aquifers and aquitards, as well as multiple passive wells, sometimes numbering in the hundreds. Solutions for the general case of multiple aquifers and wells may be developed through introduction of approximations to the well function and appropriate simplification of the convolution integral. Such a solution is computationally simple. Comparison to solutions using the full (Laplace transform) solution indicates that the new solution procedure produces excellent results. Application of the new solution to a case of multiple passive wells shows that the cumulative leakage flux in the passive wells is not a simple sum of the single-well case, owing to leakage induced drawdown around the passive wells. In addition, application to the case of multiple aquifers and aquitards demonstrates the importance of leakage into intervening aquifers as a mechanism to mitigate leakage into shallow zones, a process referred to as the ‘‘elevator model.’’ The new analytical solution provides a tool to analyze practical injection problems and forms a foundation on which more complex solutions, such as those involving injection of a nonaqueous fluid into a deep brine formation, may be based.

135. Reina, R., A. C. Leri, and S.C.B. Myneni, 2004: Cl K-edge X-ray Spectroscopic Investigation of Enzymatic Formation of Organochlorines in Weathering Plant Material. Environmental Science and Technology, 38(3), doi:10.1021/es0347336 783-789
     [ Abstract ]

     The contribution of halocarbons from plant weathering to the total organohalogen budget of terrestrial systems is gaining recognition. To evaluate the formation of such halocarbons, speciation of chlorine in Sequoia sempervirens (redwood) needles was examined in the presence of an external chloroperoxidase (CPO) enzyme using Cl K-edge X-ray absorption spectroscopy. The Cl forms in fresh and naturally weathered needles and in model laboratory reactions were compared. To provide a straightforward analogue to the enzymatic chlorination in plants, chlorination reactions were conducted for phenol, a common moiety of plant macromolecules. Plant material chlorination was also examined in the presence of hypochlorite in an ancillary mechanistic investigation. The dominant form of Cl in fresh, unreacted plant material was found to be inorganic Cl-, which was partially converted to organochlorine in the presence of CPO. Chlorination is affected by the nature of reactant (CPO, H₂O₂) addition, reaction time, and temperature. The organochlorines produced in these laboratory investigations closely resemble those produced during the natural weathering of redwood needles. A striking consistency in chlorine speciation observed among the various sample types suggests that (i) CPO produced by terrestrial organisms could play a vital role in the generation of organochlorines associated with the degradation of plant material and (ii) initial targets of enzymatic chlorination might include lignin-like macromolecules rich in aromatic character and hydroxyl groups. These findings lend further credibility to a significant biogenic contribution to the global organohalogen burden by elucidating a probable route of enzymatic chlorination of natural organic matter in terrestrial systems.

136. Scherer, George, 2004: Factors affecting crystallization pressure. Internal Sulfate Attack and Delayed Ettringite Formation, Proc. Int. RILEM 186-ISA Workshop, Paris, RILEM Publications, doi:10.1617/2912143802.009 139-154
     [ Abstract ]

     Crystallization pressure arises when the growth of a crystal is frustrated by an obstacle, such as a pore wall. At equilibrium, the crystallization pressure is inversely related to the size of the pore in which it grows. However, the equilibrium is unstable with respect to growth in a larger pore: if there is a path connecting small and large pores, small crystals will dissolve and the solute will diffuse to a larger pore where the chemical potential of the crystal will be lower. If the supersaturation is high, there can be abundant growth of small crystals that produce a high transient stress, while diffusion is transferring solute to the largest accessible pores. In the case of DEF, it is argued that expansion results from growth of crystals in mesopores, and macroscopic crystals appear subsequently as the solute redistributes.

137. Scherer, George, 2004: Characterization of Saturated Porous Bodies. Materials and Structures, 37(1), doi:10.1007/BF02481624 21-30
     [ Abstract ]

     Measurement of the response of a saturated body to mechanical and thermal strains can be used to determine the permeability and viscoelastic properties of the body. For example, bending a saturated beam creates a pressure gradient in the pores, and as the liquid flows to equilibrate the pressure, the force required to sustain a fixed deflection decreases. Analysis of the kinetics of force relaxation yields the permeability, in addition to the elastic modulus of the body; if viscoelastic relaxation of the solid phase occurs, it can also be measured. This method permits measurement of very low permeabilities in minutes or hours, but it is useful only for structurally homogeneous materials (such as cement paste) that can be formed into slender beams. For concrete, it is more practical to find the permeability by analysis of thermal expansion kinetics. When a saturated body is heated, the liquid expands more than the solid, and the expansion of the liquid stretches the solid network like a spring; consequently, the apparent thermal expansion coefficient is high. During an isothermal hold, the solid phase squeezes the liquid out of the pores and the body contracts. Analysis of the kinetics of thermal dilatation yields the permeability of the body. Recent experiments reveal an anomalously high thermal expansion coefficient for the water confined in the small pores of cement paste.

138. Scherer, George, 2004: Thermal expansion kinetics: Method to measure permeability of cementitious materials: III, Effect of viscoelasticity. Journal of the American Ceramic Society, 87(8), 1509-1516
     [ Abstract ]

     When a porous material that is saturated with liquid is heated, the liquid expands much more than the solid phase. If the permeability is low, then the liquid may not be able to escape as it expands, so it expands within the pores and causes dilatation of the body. In that case, by analyzing the kinetics of dilatation during a change in temperature, it is possible to extract the permeability. The behavior of an elastic porous solid was analyzed previously. In this paper, we consider the case of a viscoelastic material; for cementitious materials, the effect of relaxation is shown to be significant, but the elastic analysis will usually provide sufficient accuracy. If significant viscoelastic relaxation occurs, it is revealed by comparing the data from samples of different sizes.

139. Scherer, George, 2004: Measuring Permeability of Rigid Materials by a Beam-Bending Method: IV. Transversely Isotropic Plate. Journal of the American Ceramic Society, 87(8), 1517-1524
     [ Abstract ]

     A simple beam-bending measurement can be used to determine the permeability, as well as the modulus, of a saturated porous material. The procedure is to apply a constant deflection and measure the load decay as the pressure in the pores equilibrates; alternatively, an oscillatory deflection can be applied and the phase delay measured. The analysis of the experiment has been previously presented for beams with square or round cross sections. In this article, we consider the case of a saturated porous beam with a rectangular cross section and transverse isotropy; that is, the permeability and elastic properties are uniform in the x(1) - x(2) plane, but have different values in the x(3) direction. This is the case for a sedimentary stone or laminated composite, for example. The kinetics of hydrodynamic relaxation depend on the orientation of the planes with respect to the bending axis; consequently, three different solutions are presented that allow determination of the properties in etch direction. Exact solutions are obtained for elastic beams; in approximation for the case of a viscoelastic anisotropic beam also is presented. The relaxation kinetics of an isotropic rectangular plate are obtained as a special case; that geometry is convenient to use for isotropic materials, such as cement paste or mortar.

140. Scherer, George, 2004: Stress from crystallization of salt. Cement and Concrete Research, 34(9), doi:10.1016/j.cemconres.2003.12.034 1613-1624
     [ Abstract ]

     The thermodynamic and kinetic factors influencing crystallization pressure are reviewed for cases including capillary rise and evaporation, cyclic wetting and drying, and hydration of cement. Under equilibrium conditions, where the crystal is surrounded by a film of solution, high stresses are expected only in small pores, but when that film is discontinuous (as may occur during drying), high stresses can arise even in large pores. High crystallization pressure requires a substantial supersaturation of the pore liquid. In the case of sodium sulfate, supersaturation results from the difference in solubility between the anhydrate and decahydrate phases; for ettringite, supersaturation may develop following the cooling from elevated temperatures. During the hydration of Portland cement, crystallization pressure may result from the growth of ettringite and/or calcium hydroxide.

141. Valenza II, J. J., and George Scherer, 2004: Measuring Permeability of Rigid Materials by a Beam-Bending Method: V. Isotropic Rectangular Plates of Cement Paste. Journal of the American Ceramic Society, http://www3.interscience.wiley.com/cgi-bin/fulltext/118743577/PDFSTART, 87(10), 1927-1931
     [ Abstract ]

     Beam bending is an excellent method for measuring low permeabilities (<10^-18 m²) in homogeneous materials, because it is fast, requires no high pressure, and provides a concurrent measurement of the modulus of the material. The method was previously analyzed and substantiated for cylindrical or square beams. Recently, the analysis was extended to include isotropic and transversely isotropic rectangular beams. In this paper, the analysis is applied to measurements performed on cement paste, and it is shown that the solution for isotropic rectangular beams accounts for changes in the hydrodynamic behavior caused by changing the aspect ratio of the sample. The permeability and elastic modulus results are verified through comparison to previous measurements on cylindrical beams.

142. Wang, S., and P. R. Jaffé, 2004: Dissolution of Mineral Phase in Potable Aquifers due to CO₂ Releases from Deep Formations, effect of dissolution kinetics. Energy Conversion and Management, 45(18-19), doi:10.1016/j.enconman.2004.01.002 2838-2848
     [ Abstract ]

     Injection of supercritical CO₂ into deep saline aquifers is a technique for sequestration of large amounts of CO₂ that is currently being investigated as a means to ameliorate the release of greenhouse gases into the atmosphere. Because complete characterization of these geological formations is not possible, the likelihood that some fraction of the injected CO₂ will leak into overlying aquifers needs to be considered. If the leaking CO₂ were to reach shallow groundwater aquifers, it could lead to geochemical alterations with detrimental effects on the water quality of these potable aquifers. Identification and assessment of these effects is necessary to analyze risks associated with geologic sequestration of CO₂ adequately. In order to assess if there is a potential of solubilizing trace metals, metalloids and/or selected radionuclides by CO₂ releases from deep formations into potable aquifers, a series of simulations were conducted. Numerical simulations were conducted for a series of CO₂ release scenarios and different aquifer geochemical properties. The effect of CO₂ induced pH changes as well as trace metal solubilization was assessed using a geochemical transport model. Results show that elevated CO₂ levels in freshwater aquifers can enhance the dissolution of trace metals so that concentrations may reach undesirable levels at the local scale. Transport models demonstrate the importance of assessing the areal extent of this CO₂ release, as well as the need to gain thorough understanding of the key kinetic processes related to CO₂ solubilization and the dissolution of a trace metal containing mineral phase.

143. Xu, S., G. Simmons, and George Scherer, 2004: Thermal Expansion and Viscosity of Confined Liquids. Materials Research Society, http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2631&DID=116771&action=detail, 790, 85-92
     [ Abstract ]

     The thermal expansion and viscosity of water and salt solutions in porous silica glasses have been systematically investigated, and the effect of salts on the properties of water in confined geometry has been addressed. A dilatometric method has been devised and utilized to measure the thermal expansion of confined liquids. A beam-bending method that was developed to study the permeability of porous bodies has been used to measure the relative viscosity of salt solutions to water inside the silica pores. This work has demonstrated that water when confined in nanopores shows anomalous behavior and its thermal expansion is higher than bulk water. This work has also suggested that the presence of ions in water could enhance the anomaly of water in confined space and the extent of the ion effect is dependent on the ion charge.

144. Zaman, M.S.U., L. A. Ferrand, and Michael Celia, 2004: Type Curves and Effective Parameters for Unsaturated Flow Systems with Structured Heterogeneities. Advances in Water Resources, 27, doi:10.1016/j.advwatres.2004.02.005 399-410
     [ Abstract ]

     Unsaturated flow simulation requires identification of soil parameters at length scales that usually subsume smaller-scale heterogeneities. The standard two-parameter constitutive model performs well for predictions of moisture plume evolution in homogeneous soils but may not be as successful in capturing the flow and dispersion of soil water in heterogeneous domains. The first step in evaluating the limitations of this model is to develop a clear understanding of the effects of the constitutive parameters on moisture plume evolution. One approach is to define type curves derived from multiple homogeneous simulations. These type curves are based on bulk measures of system behavior, suitable for comparison to responses of heterogeneous systems. Simulation results for a range of heterogeneities defined on a specific test system indicate that some heterogeneity patterns allow definition of effective parameters for use in the two-parameter constitutive model, while others do not. For those heterogeneity patterns, the mathematical structure of the governing equations applied at the large scale must have a form that is different from the equation that underlies the type curves.

145. Baidya Roy, S., G. C. Hurtt, C. P. Weaver, and Stephen W. Pacala, 2003: Impact of historical land cover change on the July climate of the United States. Journal of Geophysical Research, 108(D24), doi:10.1029/2003JD003565
     [ Abstract ]

     We used the Regional Atmospheric Modeling System (RAMS) model to investigate the possible impact of land cover change on the July climate of the coterminous United States over the last 290 years. Vegetation data were estimated using the Ecosystem Demography model. The observed change in land cover leads to a weak warming along the Atlantic coast and a strong cooling of more than 1 K over the Midwest and the Great Plains region. The precipitation signal is weaker and shows some reduction in the Midwest because of changes in the patterns of large-scale moisture advection.

146. Celia, Michael, and , 2003: Geological Sequestration of CO₂: Is Leakage Unavoidable and Acceptable? Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies (GHGT-6), http://www.princeton.edu/~cmi/research/kyoto02/celia&bachu.kyoto%2002.pdf, 1, 477-482
     [ Abstract ]

     Geological sequestration of CO₂ requires careful risk analysis to avoid unintended consequences of the subsurface injection. One potentially serious problem associated with injection into mature sedimentary basins is the possible leakage of injected CO₂ through or along existing wells. Over long time scales, these wells may serve as short-circuit pathways for leakage, with possible contamination of shallow subsurface zones, and ultimate leakage back into the atmosphere. Transport models for leakage analysis and overall risk assessment must include proper representation of the effects of existing wells. A multi-scale framework offers a guide for inclusion of existing wells into practical simulators.

147. Jaffé, P. R., and S. Wang, October 2003: Potential Effect of CO₂ releases from deep reservoirs on the quality of freshwater aquifers. Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies (GHGT-6), 1657-1660
     [ Abstract ]

     Injection of supercritical CO₂ into deep saline aquifers is a promising technique for sequestration of large amounts of CO₂. If some fraction of the injected CO₂ were to leak and reach shallow groundwater aquifers, it would lead to geochemical alterations that could have detrimental effects on the water quality. A mathematical model was developed to simulate the change in solution pH and the enhanced dissolution of trace metals as carbon dioxide dissolves into the groundwater. The model takes into account the buffering capacity as different mineral phases dissolved into the aqueous phase and includes the dissolution of minerals with the concomitant increase in dissolved species in the aqueous phase. A series of simulations were conducted for various CO₂ release scenarios and different aquifer properties. Results show that CO₂ dissolution in poorly buffered aquifers can solubilize trace metals to levels that exceed drinking water standards. This approach allows for a reasonable assessment of the risks on the quality of freshwater aquifers due to the escape of CO₂ from deep geological formations.

148. Nordhaug, H., Michael Celia, and H. Dahle, 2003: A Pore Network Model for Calculation of Interfacial Velocities. Advances in Water Resources, 26(10), doi:10.1016/S0309-1708(03)00100-3 1061-1074
     [ Abstract ]

     Two-phase flow in porous media is characterized by fluid–fluid interfaces that separate fluid phases at the pore scale. These interfaces support pressure differences between phases, and their dynamics lead to changes in phase saturation within the porous medium. Dynamic pore-scale network models mathematically track the dynamic position of each fluid–fluid interface through a pore network, based on imposed boundary conditions, fluid and solid properties, and geometric characteristics of the network. Because these models produce a detailed description of both phase and interface dynamics, results from these models can be volume averaged to provide values for many upscaled variables. These include traditional variables such as saturation and macroscopic capillary pressure, as well as non-traditional variables such as amount of interfacial area in the averaging volume. With appropriate geometric definitions in the dynamic pore-scale model, a new algorithm may be included in the pore-scale network model to calculate a new variable: average interfacial velocity. This algorithm uses local information in any pore that contains a fluid–fluid interface to estimate the velocity of that interface over a time step. Summation over all interfaces in the network provides a measure of average velocity. Computations for dynamic drainage experiments indicate that this average interfacial velocity is well defined and exhibits distinct behavior for stable and unstable displacements. Comparison of calculated interfacial velocities to a theoretical conjecture on the functional dependence of this macroscopic variable demonstrates another important use of pore-scale model, namely testing of new theories involving non-traditional variables.

149. Tsui, N., R. J. Flatt, and George Scherer, 2003: Crystallization damage by sodium sulfate. Journal of Cultural Heritage, 4(2), doi:10.1016/S1296-2074(03)00022-0 109-115
     [ Abstract ]

     Experiments demonstrate that a stone containing thenardite suffers great damage when exposed to water below the temperature limit of mirabilite stability. This is due to a transition between thenardite and mirabilite, and not to thenardite reprecipitation. Damage occurs whether or not thenardite was produced previously by mirabilite decomposition. Together with recent results from the literature, these results indicate that damage occurs because thenardite dissolution can produce solutions highly supersaturated with respect to mirabilite, so that precipitation of this mineral can lead to large crystallization pressures. Finally, it appears that there is a salt content threshold beyond which damage increases substantially.

150. Vichit-Vadakan, W., and George Scherer, 2003: Measuring permeability and stress relaxation of young cement by beam-bending. Cement and Concrete Research, 33(12), doi:10.1016/S0008-8846(03)00168-6 1925-1932
     [ Abstract ]

     When a saturated rod of a porous material is deflected in three-point bending, two types of time-dependent relaxation processes occur simultaneously: hydrodynamic relaxation, caused by the flow of liquid in the porous body, and viscoelastic (VE) relaxation of the solid network. By measuring the decrease in the force required to sustain a constant deflection, it is possible to obtain the permeability from the hydrodynamic relaxation function, in addition to the VE stress relaxation function of the sample. We report the early-age evolution of permeability, elastic modulus, and stress relaxation function for Type III Portland cement paste with water–cement (w/c) ratios of 0.45, 0.50, and 0.55. The stress relaxation function is shown to preserve its shape during aging; that function is numerically transformed into the creep function.

151. Wang, S., P. R. Jaffé, G. Li, S. W. Wang, and H. A. Rabitz, 2003: Simulating Bioremediation of Uranium-Contaminated Aquifers; Uncertainty Assessments of Model Parameters. Journal of Contaminant Hydrology, 64(3-4), doi:10.1016/S0169-7722(02)00230-9 283-307
     [ Abstract ]

     Bioremediation of trace metals and radionuclides in groundwater may require the manipulation of redox conditions via the injection of a carbon source. For example, after nitrate has been reduced, soluble U(VI) can be reduced simultaneously with other electron acceptors such as Fe(III) or sulfate to U(IV), which may precipitate as a solid (uraninite). To simulate the numerous biogeochemical processes that will occur during the bioremediation of trace-metal-contaminated aquifers, a time-dependent one-dimensional reactive transport model has been developed. The model consists of a set of coupled mass balance equations, accounting for advection, hydrodynamic dispersion, and a kinetic formulation of the biological or chemical transformations affecting an organic substrate, electron acceptors, corresponding reduced species, and trace metal contaminants of interest, uranium in this study. This set of equations is solved numerically, using a finite difference approximation. The redox conditions of the domain are characterized by estimating the pE, based on the concentration of the dominant terminal electron acceptor and its corresponding reduced species. This pE and the concentrations of relevant species are then used by a modified version of MINTEQA2, which calculates the speciation/sorption and precipitation/ dissolution of the species of interest under equilibrium conditions. Kinetics of precipitation/ dissolution processes are described as being proportional to the difference between the actual and calculated equilibrium concentration. A global uncertainty assessment, determined by Random Sampling High Dimensional Model Representation (RS-HDMR), was performed to attain a phenomenological understanding of the origins of output variability and to suggest input parameter refinements as well as to provide guidance for field experiments to improve the quality of the model predictions. By decomposing the model output variance into its different input contributions, RSHDMR can identify the model inputs with the most influence on various model outputs, as well as their behavior pattern on the model output. Simulations are performed to illustrate the effect of biostimulation on the fate of uranium in a saturated aquifer, and to identify the key processes that need to be characterized with the highest accuracy prior to designing a uranium bioremediation scheme.

152. Ataie-Ashtiani, B., S. M. Hassanizadeh, and Michael Celia, 2002: Effects of Heterogeneities on Capillary Pressure - Saturation - Relative Permeability Relationships. Journal of Contaminant Hydrology, 56(3-4), doi:10.1016/S0169-7722(01)00208-X 175-192
     [ Abstract ]

     In theories of multiphase flow through porous media, capillary pressure–saturation and relative permeability–saturation curves are assumed to be intrinsic properties of the medium. Moreover, relative permeability is assumed to be a scalar property. However, numerous theoretical and experimental works have shown that these basic assumptions may not be valid. For example, relative permeability is known to be affected by the flow velocity (or pressure gradient) at which the measurements are carried out. In this article, it is suggested that the nonuniqueness of capillary pressure–relative permeability–saturation relationships is due to the presence of microheterogeneities within a laboratory sample. In order to investigate this hypothesis, a large number of ‘‘numerical experiments’’ are carried out. A numerical multiphase flow model is used to simulate the procedures that are commonly used in the laboratory for the measurement of capillary pressure and relative permeability curves. The dimensions of the simulation domain are similar to those of a typical laboratory sample (a few centimeters in each direction). Various combinations of boundary conditions and soil heterogeneity are simulated and average capillary pressure, saturation, and relative permeability for the ‘‘soil sample’’ are obtained. It is found that the irreducible water saturation is a function of the capillary number; the smaller the capillary number, the larger the irreducible water saturation. Both drainage and imbibition capillary pressure curves are found to be strongly affected by heterogeneities and boundary conditions. Relative permeability is also found to be affected by the boundary conditions; this is especially true about the nonaqueous phase permeability. Our results reveal that there is much need for laboratory experiments aimed at investigating the interplay of boundary conditions and microheterogeneities and their effect on capillary pressure and relative permeability.

153. Binning, P. J., and Michael Celia, 2002: A Forward Particle Tracking Eulerian Lagrangian Localized Adjoint Method for Solution of the Contaminant Transport Equation in Three Dimensions. Advances in Water Resources, 25(2), doi:10.1016/S0309-1708(01)00051-3 147-157
     [ Abstract ]

     The contaminant transport equation is solved in three dimensions using the Eulerian–Lagrangian Localized Adjoint Method (ELLAM). Trilinear and finite volume test functions defined by the characteristics of the governing equation are employed and compared. Integrations are simplified by forward tracking of integration points along the characteristics. The resulting equations are solved using a preconditioned conjugate gradient method. The algorithm is coupled to a block-centered finite difference approximation of the groundwater flow equation similar to that used in the popular MODFLOW code. The ELLAM is tested by comparison with 1D and 3D analytic solutions. The method is then applied with random, spatially correlated hydraulic conductivities in a simulation of a tracer experiment performed on Cape Cod, Massachusetts. The linear test function ELLAM was found to perform better than the finite volume ELLAM. Both ELLAM formulations were found to be robust, computationally efficient and relatively straightforward to implement. When compared to traditional particle tracking and characteristics codes commonly used with MODFLOW, the ELLAM retains the computational advantages of traditional characteristic methods with the added advantage of good mass conservation.

154. Bruant, R., D. E. Giammar, S.C.B. Myneni, and Catherine A. Peters, October 2002: Effect of pressure, temperature, and aqueous carbon dioxide concentration on mineral weathering as applied to geologic storage of carbon dioxide. Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies (GHGT-6), http://www.princeton.edu/~cmi/research/kyoto02/bruant%20et%20al.kyoto%2002.pdf,
     [ Abstract ]

     CO₂ mediated dissolution of silicate minerals and subsequent precipitation of carbonates in deep saline aquifers may allow permanent trapping of carbon dioxide. However, the time-scales and extents of the reactions are poorly understood for CO₂ receptor formation conditions. To address these shortcomings, experiments were conducted to investigate the effects of pressure, temperature, and aqueous solution composition on rates and mechanisms of silicate mineral dissolution and carbonate precipitation. A high pressure/high temperature flow-through reactor system was used to derive steady-state dissolution rates of crushed forsteritic olivine. The system allowed continuous monitoring of temperature, pressure, and pH, and periodic sampling of effluent fluids for dissolved ion concentration analysis. Preliminary measurements of dissolution rates indicate good agreement with previously published measurements at ambient conditions. Increasing the pressure from 1 to 100 bar under constant CO₂ conditions increased the dissolution rate by ~80%. The same reactions were studied in batch systems using an array of analytical techniques to investigate dissolution mechanisms and secondary precipitate formation. The extent of olivine dissolution in the batch reactors increased with temperature, PCO₂, and surface area. Precipitation of magnesium-rich carbonates on reacted olivine was observed at initial magnesite saturation indices greater than 1.6.

155. Bruant, R., A. J. Guswa, Michael Celia, and Catherine A. Peters, 2002: Safe Storage of Carbon Dioxide in Deep Saline Aquifers. Environmental Science and Technology, 36(11), doi:10.1021/es0223325 240A-245A
     [ Abstract ]

     Over the past 420,000 years, global average atmospheric CO₂ concentrations have fluctuated narrowly between 180 and 280 parts per million by volume (ppmv), but since the Industrial Revolution, CO₂ concentrations have increased to ~370 ppmv. This increase is believed to be contributing to risingmean global temperatures (1, 2). Average annual global anthropogenic CO₂ emissions during the 1990s were ~27 GtCO₂/yr (1 GtCO₂ = 109 metric tons of CO₂ = 10¹² kg of CO₂ = 0.27 GtC). The Intergovernmental Panel on Climate Change estimates that under a “business-as- usual” energy scenario, global emissions will reach ~77 GtCO₂/yr by 2100, and the average atmospheric CO₂ concentration will reach ~750 ppmv (2). To stabilize atmospheric CO₂ concentrations at 550 ppmv, which is approximately twice preindustrial concentrations, global emissions must be continuously reduced so that by 2050, global emissions are 15 GtCO₂/yr less than the business-as-usual projection, and by 2100, emissions are 50 GtCO₂/yr less (2, 3).

156. Celia, Michael, 2002: How Hydrogeology Can Save the World (An Editorial). Ground Water, 40(2), doi:10.1111/j.1745-6584.2002.tb02495.x 113
     [ Abstract ]

     I have read with interest recent editorials and articles in Ground Water discussing the future of hydrogeological research. Is it the beginning of the end, or is it simply time for a shift in focus? What can we infer from citation trends of from a move toward more practical applications? These questions lead to interesting debates. Definitive resolution of these debates is unlikely. However, there does appear to be a sense in the community that new directions, with applications that address truly important problems, are needed to revitalize hydrogeological research.
     While ground water contamination was a highly visible and important environmental concern for much of the 1980s and early 1990s, this concern has faded considerably, and the current problems that dominate the broad environmental landscape are carbon dioxide, greenhouse gas emissions, and global warming. A recent editorial in Ground Water discussed how ground water resources, and the field of hydrogeology, might be impacted by global warming. While this type of passive role needs to be planned for, I submit that hyddrogeology may have a central, active role to play in solving the CO₂ problem, and that CO₂ sequestration represents a new and potentially important problem area for hydrogeologic research.

157. Celia, Michael, and A. J. Guswa, 2002: Hysteresis and Upscaling in Two-Phase Flow through Porous Media. Proceedings of the Joint Summer Research Conference on Fluid Flow & Transport in Porous Media, American Mathematical Society, 295, 93-104
     [ Abstract ]

     Modeling two-phase flow in porous media is difficult because of the strong nonlinearities in the governing equations and the presence of hysteresis in some of the constitutive relationships. Further difficulties are introduced by the need to upscale many of the parameters to achieve scale consistency in a model. Computational studies provide insights into the behavior of these systems under the process of upscaling. In this work, we focus on behavior of water flow in unsaturated soils, including the dominant effects of evaporation and transpiration. Numerical simulations are used to upscale constitutive relationships. We demonstrate how upscaling can lead to hysteresis in constitutive relationships for cases when no small-scale hysteresis is included. This shows how upscaling and hysteresis are linked for nonlinear systems such as two-phase flow in porous media.

158. Dahle, H., Michael Celia, S. M. Hassanizadeh, and K. H. Karlsen, 2002: A Total Pressure - Saturation Formulation of Two-Phase Flow incorporating Dynamic Effects in the Capillary Pressure - Saturation Relationship. Proceedings of the XIV Intl Conf on Computational Methods in Water Resources, Delft, 1067-1074
     [ Abstract ]

     New theories suggest hat the relationship between capillary pressure and saturation should be enhanced by a dynamic term that is proportional to the time rate of change of saturation. This so-called dynamic capillary pressure formulation is supported by laboratory experiments, and can be included in various forms of the governing equations for the two-phase flow in porous media. An extended model of two-phase flow in porous media may be developed based on fractional flow curves and a total pressure - saturation description that includes the dynamic capillary pressure terms. A dimensionless form of the resulting equation set provides an ideal tool to study the relative importance of the dynamic capillary pressure effect. This equation provides rich set of mathematical research questions, and numerical solutions to the equation provide insights into the behavior of two-phase immiscible flow. For typical two-phase flow systems, dynamic capillary pressure acts to retard infiltration fronts, with responses dependent on system parameters including boundary conditions.

159. Gielen, T., S. M. Hassanizadeh, Michael Celia, and H. Dahle, 2002: Study of Pc-Sw Relationship using a Dynamic Pore-Scale Network Model. Proceedings of the XIV Intl Conf on Computational Methods in Water Resources, Delft, 1099-1100
     [ Abstract ]

     Current theories of multiphase flow rely on capillary pressuer and saturation relationships that are commonly measured under static conditions. Recently, new multiphase flow theories have been proposed that include an extended capillary pressure-saturation relationship that is valid under dynamic conditions. In this relationship, the difference between the two fluid pressures is called dynamic capillary pressure, and is assumed to be a function of the saturation and its time rate of change.
     In this work, we test this relationship using a pore-scale network model. Our model consists of a three-dimensional network of tubes (pore throats) connected to each other by pore bodies. Both pore bodies and pore throats are assumed to have square cross sections. We perform numerical experiments wherein typical experimental procedures for both static and dynamic measurements of capillary pressure-saturation curves are simulated. We determine the value of the dynamic coefficient τ for our network model.

160. Guswa, A. J., Michael Celia, and I. Rodriguez-Iturbe, 2002: Models of Soil-Moisture Dynamics in Ecohydrology: A Comparative Study. Water Resources Research, 38(9), doi:10.1029/2001WR000826
     [ Abstract ]

     An accurate description of plant ecology requires an understanding of the interplay between precipitation, infiltration, and evapotranspiration. A simple model for soil moisture dynamics, which does not resolve spatial variations in saturation, facilitates analytical expressions of soil and plant behavior as functions of climate, soil, and vegetation characteristics. Proper application of such a model requires knowledge of the conditions under which the underlying simplifications are appropriate. To address this issue, we compare predictions of evapotranspiration and root zone saturation over a growing season from a simple bucket-filling model to those from a more complex, vertically resolved model. Dimensionless groups of key parameters measure the quality of the match between the models. For a climate, soil, and woody plant characteristic of an African savanna the predictions of the two models are quite similar if the plant can extract water from locally wet regions to make up for roots in dry portions of the soil column; if not, the match is poor.

161. Hassanizadeh, S. M., Michael Celia, and H. Dahle, 2002: Dynamic Effects in the Capillary Pressure - Saturation Relationship and their Impacts on Unsaturated Flow. Vadose Zone Hydrology, 1, 38-57
     [ Abstract ]

     Capillary pressure plays a central role in the description of water flow in unsaturated soils. While capillarity is ubiquitous in unsaturated analyses, the theoretical basis and practical implications of capillarity in soils remain poorly understood. In most traditional treatments of capillary pressure, it is defined as the difference between pressures of phases, in this case air and water, and is assumed to be a function of saturation. Recent theories have indicated that capillary pressure should be given a more general thermodynamic definition, and its functional dependence should be generalized to include dynamic effects. Experimental evidence has slowly accumulated in the past decades to support a more general description of capillary pressure that includes dynamic effects. A review of these experiments shows that the coefficient arising in the theoretical analysis can be estimated from the reported data. The calculated values range from 104 to 107 kg (m s)-1. In addition, recently developed pore-scale models that simulate interface dynamics within a network of pores can also be used to estimate the appropriate dynamic coefficients. Analyses of experiments reported in the literature, and of simulations based on pore- scale models, indicate a range of dynamic coefficients that spans about three orders of magnitude. To examine whether these coefficients have any practical effects on larger scale problems, continuum-scale simulators may be constructed in which the dynamic effects are included. These simulators may then be run to determine the range of coefficients for which discernable effects occur. Results from such simulations indicate that measured values of dynamic coefficients are within one order of magnitude of those values that produce significant effects in field simulations. This indicates that dynamic effects may be important for some field situations, and numerical simulators for unsaturated flow should generally include the additional term(s) associated with dynamic capillary pressure.

162. Held, R. J., W. Kinzelbach, and Michael Celia, 2002: Characterization of Stable and Unstable Flow Regimes via Dynamic Pore-Scale Network Simulation. Proceedings of the XIV Intl Conf on Computational Methods in Water Resources, Delft, 1059-1066
     [ Abstract ]

     Dynamic network models for two-phase flow in porous media describe the physics of fluid motion and propagation of interfaces. The resolution is at the scale of single pores. A network structure representation in the model allows for simulation of macroscopic flow phenomena under variable influence of gravitational, viscous and capillary forces. Macroscopic averages such as specific interfacial areas and macroscopic interfacial velocities are investigated and effective interfacial tensions according to Chuoke et al. may be computed from the simulations. Quantification of such variables affords us the exploration of a system characteristic length scale for displacement stability. An example of viscous instability is presented.

163. Myneni, S.C.B., 2002: Formation of stable chlorinated hydrocarbons in weathering plant material. Science, 295, doi:10.1126/science.1067153 1039-1041
     [ Abstract ]

     Though several chlorinated organic compounds produced by humans are carcinogenic and toxic, some are also produced by the biotic and abiotic processes in the environment. In situ x-ray spectroscopy data indicate that natural organic matter in soils, sediments, and natural waters contain stable, less volatile organic compounds with chlorinated phenolic and aliphatic groups as the principal Cl forms. These compounds are formed at rapid rates from the transformation of inorganic Cl during humification of plant material and, thus, play a critical role in the cycling of Cl and of several major and trace elements in the environment and may infuence human health.

164. Myneni, S.C.B., 2002: Soft X-ray spectroscopy and spectromicroscopy studies of organic molecules in the environment. Rev. Mineral. Geochem. Appl of Synchrotron Radiation in Low-Temp Geochemistry and Env Science, 49, doi:10.2138/gsrmg.49.1.485 485-579
     [ Abstract ]

     Organic molecules are found everywhere and play an important role in almost all biogeochemical processes occurring on the surface of the Earth (Aiken et al. 1985; Thurman 1985; Schwartzenbach et al. 1993; Senesi and Miano 1994). They are found in soluble and insoluble phases, coatings on mineral and colloidal particles, and in gas phase molecules in soils, sediments and aquatic systems. The activities of macro- and micro- fauna and flora release organic molecules of various sizes and composition. Photochemical reactions in the atmosphere also add certain small chain molecules to the organic carbon content in the environment. Significant compositional variations occur in natural organic molecules, which include small chain carboxylic acids, alcohols and amino acids; and polymeric, polyfunctional and polydisperse macromolecules such as humic and fulvic acids. The behavior of small chain molecules and their influence on different geochemical reactions is well understood. However, understanding of the chemistry of biopolymers and their role in different biogeochemical processes in the environment is poor, which may be attributed to the unavailability of instrumentation to examine the chemistry of natural organic molecules in their pristine state.

165. Russell, T. F., and Michael Celia, 2002: An Overview of Research on Eulerian-Lagrangian Localized Adjoint Methods (ELLAM). Advances in Water Resources, 25(8-12), doi:10.1016/S0309-1708(02)00104-5 1215-1231
     [ Abstract ]

     For problems of convection–diffusion type,Eulerian–Lagrangian localized adjoint methods provide a methodology that maintains the accuracy and efficiency of Eulerian–Lagrangian methods,while also conserving mass and systematically treating any type of boundary condition. In groundwater hydrology,this framework is useful for solute transport,as well as vadose-zone transport,multiphase transport,and reactive flows. The formulation was originated around 1990 by the authors,Herrera and Ewing,in a paper that appeared in Advances in Water Resources [Adv. Water Resour. 13 (1990) 187]. This paper reviews the progress in the development,analysis,and application of these methods since 1990,and suggests topics for future work.

166. Vichit-Vadakan, W., and George Scherer, 2002: Measuring Permeability of Rigid Materials by a Beam-Bending Method: III. Cement Paste. Journal of the American Ceramic Society, 85(6), doi:10.1111/j.1151-2916.2002.tb00309.x 1537-1544
     [ Abstract ]

     The evolution of permeability and elastic modulus for Type III portland cement pastes with water/cement ratios varying from 0.4 to 0.6 were measured using a beam-bending method. Young’s modulus was independently verified by measuring the ultrasonic pulse velocity. The permeability ranged over 2 orders of magnitude, depending on the water/cement ratio and the age of the samples. The advantage of the beam-bending method is that the permeability results are obtained in a few minutes to a few hours, whereas conventional techniques take hours or days to measure permeability of this order of magnitude. More importantly, there is no need to maintain high pressure during the measurement period, so leaks are not a problem.

167. Ai, H., J. F. Young, and George Scherer, 2001: Thermal expansion kinetics: Method to measure permeability of cementitious materials: II, Application to hardened cement pastes. Journal of the American Ceramic Society, http://www3.interscience.wiley.com/journal/118965150/abstract, 84(2), 385-391
     [ Abstract ]

     A new approach to determine the permeability of cementitious materials is presented in this paper. The method involves the measurement of the thermal dilatation of thin slabs of saturated cement paste. On heating and subsequent isothermal holding, the sample first expands, then gradually contracts over time. The paste is treated as a porous solid with low permeability in which the initial expansion is partly due to the liquid expanding within the pores, putting both solid and liquid under stress. The time-dependent contraction is due to the liquid flowing out of the pores to restore the pressure to equilibrium. By analyzing the kinetics of the thermal expansion and contraction, the liquid permeability of the porous solid can be calculated. The theoretical analysis (presented in a previous paper) is applied to determine the water permeability coefficients of selected cement pastes modified with silica fume and made with different water/solid ratios.

168. Ataie-Ashtiani, B., S. M. Hassanizadeh, M. Oostrom, Michael Celia, and M. D. White, 2001: Effective Parameters for Two-Phase Flow in a Porous Medium with Periodic Heterogeneities. Journal of Contaminant Hydrology, 49(1-2), doi:10.1016/S0169-7722(00)00190-X 87-109
     [ Abstract ]

     Computational simulations of two-phase flow in porous media are used to investigate the feasibility of replacing a porous medium containing heterogeneities with an equivalent homogeneous medium. Simulations are performed for the case of infiltration of a dense nonaqueous phase liquid (DNAPL). in a water-saturated, heterogeneous porous medium. For two specific porous media, with periodic and rather simple heterogeneity patterns, the existence of a representative elementary volume (REV). is studied. Upscaled intrinsic permeabilities and upscaled nonlinear constitutive relationships for two-phase flow systems are numerically calculated and the effects of heterogeneities are evaluated. Upscaled capillary pressure–saturation curves for drainage are found to be distinctly different from the lower-scale curves for individual regions of heterogeneity. Irreducible water saturation for the homogenized medium is found to be much larger than the corresponding lower-scale values. Numerical simulations for both heterogeneous and homogeneous representations of the considered porous media are carried out. Although the homogenized model simulates the spreading behavior of DNAPL reasonably well, it still fails to match completely the results form the heterogeneous simulations. This seems to be due, in part, to the nonlinearities inherent to multiphase flow systems. Although we have focused on a periodic heterogeneous medium in this study, our methodology is applicable to other forms of heterogeneous media. In particular, the procedure for identification of a REV, and associated upscaled constitutive relations, can be used for randomly heterogeneous or layered media as well.

169. Brown, D. G., and P. R. Jaffé, 2001: Effects of Nonionic Surfactants on Bacterial Transport Through Porous Media. Environmental Science and Technology, 35(19), doi:10.1021/es010577w 3877-3883
     [ Abstract ]

     Nonionic surfactants of the form C_xE_y, where x is the number of carbons in the alkyl chain and y is the number of ethylene oxide units in the polyoxyethylene (POE) chain, were studied for their ability to alter the transport of Sphingomonas pacilimobilis through an aquifer sand. The surfactants C₁₂E₄ (Brij 30) and C₁₂E₂₃ (Brij 35) were the focus of this study. Through a systematic study, it was shown that these nonionic surfactants were able to enhance the transport of this bacterial culture through porous media. The magnitude of the enhancement increased with decreasing solution ionic strength and increasing POE chain length. The mechanism of this enhanced transport appears to be due to expansion of the electric double layer about the bacteria and aquifer sand through displacement of the counterions by the sorbed surfactant. This expanded electric double layer increases the electrostatic repulsion, with a resultant reduction in the collision efficiency and an increase in the Langmuirian blocking parameter. Application of the colloid filtration theory with the experimental parameters of this study shows that nonionic surfactants have the potential to significantly enhance the bacterial travel distance, especially for low ionic strength systems.

170. Brown, D. G., and P. R. Jaffé, 2001: Effects of Nonionic Surfactants on the UV/Visible Absorption of Bacterial Cells. Biotechnology and Bioengineering, 74(6), doi:10.1002/bit.1138 476-482
     [ Abstract ]

     Nonionic surfactants are used in a number of different microbiological applications, including solubilization of cell membranes, washing bacterial cultures prior to experimentation, and enhancing biodegradation of low-solubility compounds. An important consideration in these applications is the potential for the surfactant to alter the cell membrane. One potential means to monitor the impact of surfactants on the bacterial cell membrane is through monitoring the absorbance spectrum of the bacterial suspension. This is due to the colloidal nature of bacteria, where the absorbance of a bacterial suspension is related to the size and refractive index of the bacterial cells. Through a systematic study it was shown that there can be a significant change in the bacterial absorbance spectrum due to the presence of nonionic surfactants, with the effect a function of surfactant structure and concentration, solution ionic strength and cation valence. The effects were most pronounced with Na+ as the cation, with surfactants having midrange hydrophile-lipophile balance (HLB) values, and with surfactant concentrations above the CMC. The results indicate that measurement of the absorbance spectrum of bacterial cultures can provide a means to monitor the effects of nonionic surfactants on the bacterial cell membrane. In addition, depending on the specific application, appropriate selection of surfactant structure and media composition can be made to enhance or minimize the effects.

171. Bruant, R., R. J. Held, Catherine A. Peters, and Michael Celia, 2001: Pore Scale Network Simulation of Single and Multiple Component Non-Aqueous Phase Luquid (NAPL) Dissolution. American Geophysical Union,
     [ Abstract ]

     A computational three-dimensional pore-scale network model was used to quantify residual single- and multi-component non-aqueous phase liquid (NAPL) dissolution driven by aqueous-phase advection. The pore network was discretized into spherical pore bodies and biconical pore throats to represent the effective void space and void distribution of a fine-grained Ottawa sand. Fluid saturations, positions, and interfacial areas, in addition to aqueous-phase flow, were established by externally applied pressure gradients. Mass transfer from the NAPL to the aqueous phase was computed as a local flux across each interface using a stagnant boundary layer Fickian diffusion model. Subsequent mass transport in the aqueous phase was simulated by a volume-conserving characteristic method along streamlines. The model dynamically calculated interface retraction resulting from mass transfer between the non-aqueous and aqueous phases, and concurrently tracked physical changes in NAPL saturation, NAPL composition, and interfacial geometry. The model avoids scale inconsistencies, allowing pore-scale through continuum-scale description of NAPL dissolution. In this presentation, results from NAPL dissolution simulations will be compared (as a function of saturation and location) to laboratory experiments and implications for up-scaling mass transfer coefficients will be discussed. Dependence of multi-component NAPL composition on mass transfer phenomena and differences between single- and multi-component systems also will be highlighted.

172. Held, R. J., and Michael Celia, 2001: Pore-Scale Modeling Extension of Constitutive Relationships in the Range of Residual Saturations. Water Resources Research, http://www.agu.org/journals/wr/v037/i001/2000WR900234/2000WR900234.pdf, 37(1), 165-170
     [ Abstract ]

     A pore network model is used to describe constitutive relationships between saturation, capillary pressure, relative permeabilities, and interfacial areas over the full range of saturations. We focus on residual nonwetting-phase saturations, that is, the saturation range between main drainage and primary drainage. Interphase mass transfer and dynamic miscible transport are modeled to generate the range of saturations over which the extended constitutive relationships are calculated. To accommodate all saturations in a consistent way, we define capillary pressure as the areal average of local capillary pressures associated with each fluid-fluid interface. The extended constitutive relationships provide input for continuum-scale equations.

173. Held, R. J., and Michael Celia, 2001: Modeling Support of Functional Relationships between Capillary Pressure, Saturation, Interfacial Area, and Common Lines. Advances in Water Resources, 24(3-4), doi:10.1016/S0309-1708(00)00060-9 325-343
     [ Abstract ]

     Computational pore-scale network models describe two-phase porous media flow systems by resolving individual interfaces at the pore scale, and tracking these interfaces through the pore network. Coupled with volume averaging techniques, these models can reproduce relationships between measured variables like capillary pressure, saturation, and relative permeability. In addition, these models allow nontraditional porous media variables to be quantifed, such as interfacial areas and common line lengths. They also allow explorations of possible relationships between these variables, as well as testing of new theoretical conjectures. Herein we compute relationships between capillary pressure, saturation, interfacial areas, and common line lengths using a pore-scale network model. We then consider a conjecture that definition of an extended constitutive relationship between capillary pressure, saturation, and interfacial area eliminates hysteresis between drainage and imbibition; such hysteresis is commonly seen in the traditional relationship between capillary pressure and saturation. For the sample pore network under consideration, we find that hysteresis can essentially be eliminated using a specific choice of displacement rules; these rules are within the range of experimental observations for interface displacements and therefore are considered to be physically plausible. We find that macroscopic measures of common line lengths behave similarly to fluid-fluid interfacial areas, although the functional dependencies on capillary pressure and saturation differ to some extent.

174. Vichit-Vadakan, W., and George Scherer, 2001: Beambending method for permeability and creep characterization of cement paste and mortar. Creep, Shrinkage and Durability Mechanics of Concrete and Other Quasi-Brittle Materials, Amsterdam, Elsevier, 27-32
     [ Abstract ]

     When a saturated rod of Portland cement paste or mortar is deflected in 3-point bending, two types of time dependent relaxation processes occur simultaneously: hydrodynamic relaxation caused by the flow of liquid in the porous body and viscoelastic relaxation of the solid network. By measuring the decrease in the force required to sustain a constant deflection, it is possible to obtain the permeability and Young's modulus from the hydrodynamic relaxation function, in addition to the stress relaxation function of the sample. As shown in previous papers, the total relaxation can be very closely approximated as the product of the hydrodynamic and viscoelastic relaxation functions. We are reporting the evolution of permeability, elastic modulus, and stress relaxation function for Type III Portland cement pastes with water-cement ratio of 0.5 at early ages.

175. Rodriguez-Iturbe, I., 2000: Ecohydrology: A Hydrologic Perspective of Climate-Soil-Vegetation Dynamics. Water Resources Research, http://www.agu.org/journals/wr/v036/i001/1999WR900210/1999WR900210.pdf, 36(1), 3-9
     [ Abstract ]

     The hydrologic mechanisms underlying the climate-soil-vegetation dynamics and thus controlling the most basic ecologic patterns and processes are described as one very exciting research frontier for the years to come. In this personal opinion I have concentrated on those processes where soil moisture is the key link between climate fluctuations and vegetation dynamics in space and time. The soil moisture balance equation at a site is shown to be the keystone of numerous fundamental questions which may be instrumental in the quantitative linkage between hydrologic dynamics and ecological patterns and processes. Some of those questions are outlined here, and possible avenues of attack are suggested. The space-time links between climate, soil, and vegetation are also explored from the hydrologic perspective, and some exciting research perspectives are outlined.

176. Crandell, Lauren E., B. R. Ellis, and Catherine A. Peters, 0000: Dissolution Potential of SO₂ Co-Injected with CO₂ in Geologic Sequestration. Environmental Science and Technology, American Chemical Society, (44), doi:10.1021/es902612m 349-355
     [ Abstract ]

     Sulfur dioxide is a possible co-injectant with carbon dioxide in the context of geologic sequestration. Because of the potential of SO₂ to acidify formation brines, the extent of SO₂ dissolution from the CO₂ phase will determine the viability of co-injection. Pressure-, temperature-, and salinity-adjusted values of the SO₂ Henry’s Law constant and fugacity coefficient were determined. They are predicted to decrease with depth, such that the solubility of SO₂ is a factor of 0.04 smaller than would be predicted without these adjustments. To explore the potential effects of transport limitations, a nonsteady-state model of SO₂ diffusion through a stationary cone-shaped plume of supercritical CO₂ was developed. This model represents an end-member scenario of diffusion-controlled dissolution of SO₂, to contrast with models of complete phase equilibrium. Simulations for conditions corresponding to storage depths of 0.8—2.4 km revealed that after 1000 years, 65—75% of the SO₂ remains in the CO₂ phase. This slow release of SO₂ would largely mitigate its impact on brine pH. Furthermore, small amounts of SO₂ are predicted to have a negligible effect on the critical point of CO₂ but will increasephasedensity by asmuchas12% for mixtures containing 5% SO₂.

177. Kutchko, B., and N. Thaulow, et al., in press: Structure and composition of cement from a well at Teapot Dome. In , , . 0/00.
178. Michael, K., and , in press: AAPG Studies 59 – Carbon Dioxide Sequestration in Geological media – State of the Art. In , , . 0/00.
179. White, S. J. O., and M. Hay, et al., 0000: Influence of elevated soil-CO₂ on mineral weathering and plant growth: A limitation for geological sequestration of CO₂? In , Unpublished

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