Model development

Over the next two years, Jean Prevost and postdoctoral researcher Bruno Huet will complete the implementation of a geochemical module in Dynaflow that includes the constituents of cement. Dr. Huet has developed an extensive database of chemical properties (solubility and stability) of the constituents of cement. The geochemistry module has been developed and successfully tested in 2006. The effort now is focused on improving the kinetic aspects by coupling of a transport module with the geochemistry module. In 2006, the 1st version of the reactive transport module has allowed qualitative simulations of Andrew Duguid’s experiments. The remaining challenges concerning the dynamic aspects are 1), the prediction of the change in diffusion rate as the porosity is changed by corrosion, and 2) the quantification of the advective flow driven by volume changes following chemical reactions.

The multiphase flash calculation will further be modified to allow for pressure and temperature changes as the plume rises toward the surface, including thermal effects. Dick Fuller, who was responsible for the development of this module, will be leaving the project by early February, 2007. We will hire a post-doctoral researcher to continue this aspect of the work.

The modular nature of the software is particular important. Dick Fuller has developed a module for non-reactive equilibration of the components (i.e., flash calculation) that includes all the CO2 -rich phases, and is supported by a separate module that calculates all the thermodynamic properties of the system. Bruno Huet has assembled the most extensive database in existence for reactions involving components of phases in cement, and can calculate the composition of the solution in equilibrium with any assemblage of cement phases between 0 and 100°C. All of these modules are currently integrated by Jean Prévost into Dynaflow, which has exceptional ability to couple geomechanics (including fracture) with geochemistry. However, all of these modules could be used with other systems, such as Eclipse.


Simulation of injection environments

Once the basic geochemistry is in place, Bruno Huet and Jean Prevost will simulate attack on cement by carbonated brine rising through an annular gap around the cement in a well. Multiple leakage scenarios will be examined, including the impacts of varying brine composition, initial gap width, and depth at which the leak originates. The simulations will also be extended to include corrosion of the steel casing in the well.

In summary, at the end of about 2 years from today, we will be able to put bounds on the risk of leakage under unfavorable conditions (brine in a sandstone formation encountering a preexisting annular gap in typical cement). Should we discover that the brine is quickly neutralized as it flows through the gap, so that the leak is not significantly expanded over the course of a century, then the behavior of the wells under more benign conditions is academic. On the other hand, if our simulations indicate that leaks worsen at a significant rate in sandstone formations, then we will need to see whether there is any such risk under more favorable circumstances (such as limestone formations). That information will emerge from the thesis work to begin this fall; the results will start flowing in about two years, and final results will be available by year 4 or 5 (that is, by the end of the project).

The results from the combined experimental and geochemical modeling should predict the most important variables affecting leakage rates within a well. This information will be combined with statistical information about well properties to provide input for the semi-analytical and coarse resolution models developed by Mike Celia and colleagues for risk assessment (see below).