Remediation of Possible Leakage from Geologic CO2 Storage Reservoirs into Groundwater Aquifers
Keywords:  Department of Energy Resources Engineering
School of Earth Energy &amp; Environmental Sciences
Thesis

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. There are many groundwater remediation technologies available that could be applied for remediating CO2 leaks. 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. One technique to remediate the leak is to extract the CO2 in both the gaseous and dissolved phase. Another technique analyzed is to inject water to dissolve the gaseous CO2 in the groundwater, reduce the overall aqueous concentration, and immobilize CO2 by capillary trapping. Water injection is similar to the impact of regional flow in the reservoir. The final technique analyzed is the combination of water injection followed by extraction. 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. At the depth of most groundwater aquifers the pressure is shallow enough that CO2 is either in gas phase or dissolved. Due to the large contrast between the density of the groundwater and CO2, we found that the leakage rate, the quantity of leaked CO2, and the amount of time that the leak continues have a very important effect on the size and shape of the leakage plume. The second step was to determine the physical processes that expedite or hinder removal of the CO2 plume. Important 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 another remediation technique where we inject water to dissolve the gaseous CO2 and reduce the overall concentration and increase capillary trapping. With an injection well, the main factor controlling the dissolution of CO2 was the residual gas saturation and the injection well flow rate. Also, the spatial extent of the gaseous CO2 impacted the amount dissolved over time. Finally, we determined that water injection makes later extraction difficult because it displaces the CO2 from the initial leakage point. We also conducted a sensitivity analysis to examine the impacts on the effectiveness of the remediation when there is a heterogeneous layered reservoir and when the leak is allowed to continue after the remediation begins. The low permeability layers reduce the extent of the gravity tongue and allow for more effective remediation for the larger leakage cases analyzed. Allowing the leak to continue does not have a significant impact on the remediation effectiveness if the extraction well is able to remove more CO2 than is leaking. 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. Water injection is very 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. Finally, formulating the scenario very quickly after leakage is stopped leads to the most cost effective scenario.