On Leakage and Seepage from Geologic Carbon Sequestration Sites: Unsaturated Zone Attenuation

Geologic carbon sequestration is the direct injection of CO2 into deep geologic formations for permanent disposal. Although numerous trapping mechanisms exist in the subsurface, it is possible that CO2 will leak from the primary sequestration target and seep out of the ground. The unsaturated zone has the potential to attenuate leaking CO2 and decrease seepage and near-surface CO2 concentrations. Attenuation processes include permeability trapping, ponding as dense CO2 spreads out on the water table, solubility trapping by infiltrating or residual water, and dilution through mixing with ambient soil gas. Numerical simulations of CO2 flowing upward through a thick model unsaturated zone were performed to investigate the sensitivity of various unsaturated zone properties on CO2 seepage flux and near-surface CO2 gas concentrations. These two quantities are considered drivers for health and environmental risk due to exposure to CO2. For the conceptual model considered, seepage flux and near-surface CO2 gas concentrations are most strongly controlled by the leakage rate at the water table, followed by the source zone radius. Permeability and permeability anisotropy, as well as porosity and infiltration rate are also important, although to a lesser degree. Barometric pumping causes local maxima in seepage flux and near-surface CO2 concentrations, but has negligible effect in a time-averaged sense. When the leakage source is turned off, the CO2 plume attentuates through dissolution into infiltrating water. For the case of a constant leakage rate, the unsaturated zone can attenuate low leakage fluxes but should not be expected to attenuate large CO2 leakage fluxes.