Our focus is on storage of CO2 in deep aquifers. We have two broad themes:

  • Development of a framework for leakage estimation in mature sedimentary basins;
  • Identification and characterization of specific environmental risks associated with CO2 leakage.

In combination, results from these two efforts can be used to develop risk maps and measures of relative risk that should help to guide decisions on CO2 injection strategies.

 


A framework for leakage estimation

In combination, results from these two efforts can be used to develop risk maps and measures of relative risk that should help to guide decisions on CO2 injection strategies.

For our field studies, we are using the Alberta Basin as our test basin, because of the availability of information through the provincial authorities. Within that basin we have zeroed in on the Viking Formation, which appears to be a good candidate for CO2 storage. In collaboration with Stefan Bachu and his team at the Alberta Geological Survey, we have estimated the ultimate capacity of the Viking Formation to be approximately 200 Gt CO2. We have identified all existing wells that penetrate this formation (approximately 190,000 wells) and their current status (active versus abandoned, with appropriate sub-categories). We have initiated analysis of the spatial statistics of the well locations, using procedures like cluster analysis. We are also pursuing a computational analysis that combines spatial statistics of well locations with physical (hydraulic) characteristics of the wells to estimate leakage potential as a function of system parameters.

Well characterization requires estimation of bulk hydraulic properties associated with the borehole and its immediate surroundings, which in turn depends on largely unknown features like well cement integrity. Our interest is in cement properties over the long-term time horizon of CO2 sequestration. We have formulated a laboratory-based experimental research plan to measure strength, permeability, and pore structure modification, as a function of time, temperature, a nd pressure. We have also completed a literature survey on deterioration of cement in acidic environments.

To help determine the long-term fate of injected CO2, we have done laboratory experiments to determine geochemical reaction rates, under high pressures and temperatures, associated with CO2-induced dissolution of rock minerals. We have used an array of analytical techniques (X-ray diffraction, diffuse reflectance infrared spectroscopy, and scanning electron microscopy) to investigate formation of new carbonate minerals. Associated with this work is a computationally based effort to determine how best to obtain effective (“upscaled”) reaction rate coefficients from coefficients based on smaller-scale measurements.

We have introduced multi-phase, multi-component transport capabilities into the geomechanics code, DynaFlow.

 


Specific environmental risks of CO2 leakage

We are exploring risks arising from CO2 leakage into the shallow zone. We have completed an initial study of possible mineral dissolution, and associated mobilization of metals, driven by pH changes associated with CO2 leakage. Based on shallow subsurface data taken from the U.S. Geological Survey and other sources, we have concluded that arsenic is a particularly important ground-water contaminant. We have also continued our studies at Mammoth Mountain, California, where recent magmatic CO2 leaks have significantly affected the land-surface ecosystem, including areas of significant tree kills.

At Mammoth Mountain, we found that within the high soil CO2 areas, while elevated soil CO2 either killed or severely altered the growth of large pine trees, small saplings of less than about eight years of age appear to be healthy. However, the large dead pine trees, as well as saplings in areas unaffected by CO2, have a different root structure and architecture than the small saplings growing in elevated CO2 areas. The implication for further growth of the affected saplings remains to be determined. Also, accelerated weathering in CO2 affected areas appears to lead to enhanced metal uptake in plants growing in these areas.