CO 2 migration in saline aquifers. Part 1. Capillary trapping under slope and groundwater flow

Injection of carbon dioxide (CO 2 ) into geological formations is widely regarded as a promising tool for reducing global atmospheric CO 2 emissions. To evaluate injection scenarios, estimate reservoir capacity and assess leakage risks, an accurate understanding of the subsurface spreading and migration of the plume of mobile CO 2 is essential. Here, we present a complete solution to a theoretical model for the subsurface migration of a plume of CO 2 due to natural groundwater flow and aquifer slope, and subject to residual trapping. The results show that the interplay of these effects leads to non-trivial behaviour in terms of trapping efficiency. The analytical nature of the solution offers insight into the physics of CO 2 migration, and allows for rapid, basin-specific capacity estimation. We use the solution to explore the parameter space via the storage efficiency, a macroscopic measure of plume migration. In a future study, we shall incorporate CO 2 dissolution into the migration model and study the importance of dissolution relative to capillary trapping and the impact of dissolution on the storage efficiency.