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Abstract
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 determines the optimal remediation scenario for various leakage conditions. The two objectives for remediation considered here are removing any mobile CO2 and reducing the quantity of CO2 in the reservoir. The main 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 and 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 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 a significant portion of the CO2 is in gas phase. Due to the large difference between the density of the groundwater and the CO2, we found that the leakage rate and the quantity of CO2 have a very important impact on the resultant 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 and capillary pressure curves, dissolution, and buoyancy induced flow. We compared the effectiveness of using vertical and horizontal extraction wells to remove the CO2. We next examined the processes that occurred during the second 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 controlling factor on the dissolution of CO2 was the residual gas saturation and the injection well flow rate. Also, the distance of the gaseous CO2 from the injection well impacted the amount dissolved over time.

Based on the initial simulations, the characteristics to optimize are the extraction well depth for vertical or horizontal wells, the extraction well rate, and the injection well rate. We considered the optimal scenario based on the effectiveness of meeting the two objectives of removing mobile CO2 and reducing the quantity of CO2 in the reservoir. Determining the optimal remediation scheme provides a starting point for planning groundwater remediation scenarios for possible leakage events at geologic storage sites.

Introduction
Carbon capture and storage (CCS) is considered as a viable option for significantly reducing anthropogenic CO2 emissions to the atmosphere from large point sources such as coal-fired power plants. A possible leakage event degrades the benefit of reducing emitted greenhouse gases, could result in forfeiture of carbon credits under a carbon trading system and, if not controlled, could ultimately result in the closure of the carbon storage project. Also, if the CO2 leaks into a groundwater aquifer utilized for drinking water or agricultural purposes it may pose human health risks or damage crops.