Simulating Remediation of CO2 Leakage from Geological Storage Sites

One strategy to reduce net greenhouse gas emissions is to inject carbon dioxide (CO₂) deep into subsurface formations where presumably it would be stored indefinitely. Although geologic storage formations will be carefully selected, CO₂ injected into a target formation may unexpectedly migrate upwards and ultimately seep out at the ground surface, creating a potential hazard to human beings and ecosystems. In this case, CO₂ that has leaked from the geologic storage site is considered a contaminant, and remediation strategies such as passive venting and active pumping are needed. The purpose of this study is to investigate remediation strategies for CO₂ leakage from geologic storage sites. We use the integral finite-difference code TOUGH2 to simulate the remediation of CO₂ in subsurface systems. We consider the components of water, CO₂ and air, and model flow and transport in aqueous and gas phases subject to a variety of initial and boundary conditions including passive venting and active pumping. We have investigated the time it takes for a gas plume of CO₂ to be removed from the vadose zone both by natural attenuation processes and by active extraction wells. The time for removal is parameterized in terms of a CO₂ plume half-life, defined as the time required for one-half of the CO₂ mass to be removed. Initial simulations show that barometric pressure fluctuations enhance the removal of CO₂ from the vadose zone, but that CO₂ trapped near the water table is difficult to remove by either passive or active remediation approaches. This work was supported by a Cooperative Research and Development Agreement (CRADA) between BP Corporation North America, as part of the CO₂ Capture Project (CCP), and the U.S. Department of Energy (DOE) through the National Energy Technologies Laboratory (NETL), and by the U.S. Department of Energy under contract DE-AC03-76SF00098.