Remediation of possible leakage from geologic CO2 storage reservoirs into groundwater aquifers

Maintaining the long term storage of CO2 is an important requirement for a large scale geologic CO2 storage project. Nevertheless, the possibility remains that the CO2 will leak out of the formation into overlying groundwater aquifers. A site specific remediation plan is also important during the site selection process and necessary before storage begins. Due to the importance of protecting drinking water resources, this study analyzes the optimal remediation scenario for various leakage conditions. The three objectives for remediation considered here are removing any mobile CO2, reducing the quantity of CO2 in the reservoir, and reducing the aqueous phase concentration of CO2. The first part of our research was to determine the processes that control the size and shape of the leakage plume in the groundwater aquifer. We used the multiphase flow simulator TOUGH2 with CO2 leakage from a point source to analyze the plume at various leakage rates. We next determined that during remediation the important physical processes include capillary trapping as a result of hysteresis in the relative permeability curves, dissolution, and buoyancy induced flow. We compared the effectiveness of using vertical and horizontal extraction wells to remove the CO2. We next examined injecting water to dissolve the gaseous CO2 and reduce the overall concentration and increase capillary trapping. Finally, we analyzed the combination of water injection and extraction with multiple wells to determine the optimal spacing and flow rate. Based on the simulations analyzed for this study, multiple conclusions can be made on the effectiveness of various remediation scenarios. With one vertical extraction well the optimal scenario for the larger leakage cases is a multistep extraction process that removes mobile CO2 from the areas with high gas saturation first. A horizontal extraction well in the middle of the aquifer is much more efficient than vertical wells at removing CO2. Water injection is effective at quickly reducing the mobile phase CO2 with tradeoffs between injection rate and increases in pressure. The most effective scenario over a longer time period includes injection for a short time followed by extraction from four vertical wells. To reduce the CO2 most rapidly, four injector wells with high flow rates and one extraction well is the most effective for the large leakage cases. (C) 2011 Published by Elsevier Ltd.