Migration characteristics and local capillary trapping mechanism after the CO2 leakage out of saline aquifers

CO2 may leak from the wellbore, cross-section of faults, fractures, and pore space due to geological tectonic activity or human-induced activity during the CO2 geological storage in deep saline aquifers. To evaluate the CO2 leakage risk and select safe saline aquifers, it is necessary and crucial to predict the CO2 migration characteristics and trapping mechanism after its leakage. Although some previous studies have shown that heterogeneous capillary pressure can change CO2 migration path, its impact on the migration and storage after CO2 leakage is still unclear. Therefore, a simulation model of CO2 leakage and migration in saline aquifers is established by adopting stochastic modeling in geology and two-phase seepage modeling in the fluid. Based on the model, the influence of capillary pressure heterogeneity and caprock factors (thickness and leakage area) on the leaked CO2 migration, distribution, and trapping is analyzed quantitatively. The research result shows that heterogeneous capillary pressure in the saline aquifers can effectively trap the leaked CO2 and enlarge the lateral swept volume of leaked CO2. With the increase of capillary pressure and its heterogeneity, the upward migration of CO2 slows down but the local capillary trapping increases. Both of these can weaken the risk of CO2 leakage to the surface. Under the local capillary trapping above and below the caprock, the increasing of caprock thickness and leakage area have little effect on CO2 leakage amount, but may increase the local capillary trapping and make the formation safer. Therefore, priority should be given to selecting saline aquifers with thicker caprock to reduce the potential risk of CO2 leakage. In order to ensure the safety of CO2 storage, a saline aquifer with strong capillary pressure heterogeneity and good trapping performance should be selected.