Evaluation and development of options for remediation of CO2 leakage into groundwater aquifers from geologic carbon storage

There are many good reasons to have confidence in the long term security of carbon dioxide (CO2) storage in properly selected and operated projects. However, the possibility remains that CO2 leaks out of the formation, for example, up an abandoned well, into an overlying groundwater aquifer. In the event of leakage, a prudent operator should be prepared to quickly implement a remedial action plan. To date, remediation plans have focused on methods to plug leaking wells. In some cases, it may also be necessary to contain or remove the CO2 that leaked into the aquifer. Due to the importance of protecting drinking water resources and meeting permitting requirements, this study analyzes multiple remediation options for a range of hypothetical leakage scenarios. Three specific objectives for remediation are considered here, namely, reducing the amount of mobile CO2, in the aquifer, reducing the total quantity of CO2 in the aquifer, and reducing the aqueous phase concentration of CO2. First, we evaluate the processes and parameters controlling the size and shape of the plume of CO2 leaking into a groundwater aquifer using the multiphase flow simulator TOUGH2. Next, a systematic set of simulations are used to identify the multiphase flow processes and phase behaviors controlling extraction of CO2, such as buoyancy induced flow, capillary trapping, CO2 dissolution and exsolution. We then compare the effectiveness of various remediation options, including: (1) vertical and horizontal extraction wells to remove the CO2 in both gas and aqueous phase, (2) injecting water to dissolve the gaseous CO2 and increase capillary trapping, and (3) a combination of injection and extraction with multiple wells. Based on this study, multiple conclusions can be drawn about the effectiveness of various remediation scenarios. First, if it is necessary, it is possible to effectively remediate CO2 that has leaked into a groundwater aquifer, either by immobilizing and/or extracting it. Second, for small plumes of CO2 where a gravity tongue has not formed, a single vertical well located in the middle of the plume can be used to remove all of the CO2 over a several year period. In the case where a large gravity tongue has formed (a thin and large plume trapped under a confining layer), horizontal wells will be more effective at removing the plume, although this could take a decade or more. In fact, in these cases, injecting water to quickly immobilize and dissolve the plume can provide more effective containment in the short term. But, for larger plumes, the most effective remediation scenarios include a combination of sequential and/or simultaneous injection and extraction from multiple wells. In this case, even a large plume of CO2 can be contained and remediated effectively, with the time frame depending on the number and location of wells and flow rates. (C) 2011 Elsevier Ltd. All rights reserved.