REMEDIATION OF POSSIBLE LEAKAGE FROM GEOLOGIC CO2 STORAGE RESERVOIRS INTO GROUNDWATER AQUIFERS

Maintaining the long term storage of CO2 is important for a large scale geologic CO2 storage project. The possibility remains that the CO2 will leak out of the formation into overlying groundwater aquifers. A site specific remediation plan is important during the site selection process and necessary before storage begins. Due to the importance of protecting drinking water sources, we determined the optimal remediation scenario for various leakage conditions. We used the multiphase flow simulator TOUGH2 to analyze the plume at various leakage rates. At the depth of most groundwater aquifers a significant portion of the CO2 is in gas phase. Due to the difference between the density of the brine and the CO2, we found that the leakage rate and the quantity of CO2 have an important impact on the leakage plume. We then determined the physical processes that control removal of the CO2 plume. Important processes include capillary trapping as a result of hysteresis in the relative permeability and capillary pressure curves, dissolution and buoyancy induced flow. We found that the ease of removal was correlated with the shape of the plume. We next examined what occurred when we injected water to dissolve all the gaseous CO2 and reduce the overall concentration. With water injection, the main controlling factors were the residual gas saturation and the distance from the injection well. Based on the initial simulations, the characteristics to optimize are the extraction well depth, the number of wells, and the placement of the wells. We used three criteria to determine the optimal remediation scenario: the half life of the CO2 plume, the time until 95% is removed or dissolved, and the cost of remediation. Determining the optimal remediation scheme provides a starting point for planning groundwater remediation scenarios for possible leakage events at geologic storage sites.