Residual trapping, solubility trapping and capillary pinning complement each other to limit CO2 migration in deep saline aquifers

We derive a theoretical model for the post-injection migration of a CO2 gravity current in a confined, sloping aquifer under the influence of residual trapping, solubility trapping, and capillary pinning. The resulting model consists of two coupled partial differential equations that describe the local thickness of the buoyant CO2 current and the thickness of the mound of brine saturated with dissolved CO2 as a function of time. We apply this model to a representative geological formation and provide estimates of the lifetime of the buoyant CO2 current and its maximum migration distance. Our analysis shows that residual trapping, solubility trapping, and capillary pinning complement each other in limiting the ultimate migration distance of CO2 gravity currents. The relative contribution of residual trapping, solubility trapping, and capillary pinning varies as a function of the injection volume. Our model can be used as a screening tool to evaluate the potential of deep saline aquifers for large-scale CO2 sequestration.