Numerical investigation of the anisotropy role on carbon dioxide dissolution enhancement in saline aquifers

Geological storage of CO2 in saline aquifers is a promising method to reduce atmospheric CO2 concentration. When CO2 is injected in aquifers, it migrates upward due to its buoyancy and spreads under the cap rock. Afterward, CO2 will diffuse slowly into underlying brine. CO2-saturated brine is denser than fresh brine. This density difference may result in convection phenomena which enhances the dissolution of CO2 in aquifers. Previous studies were often focused on CO2 dissolution in homogeneous and isotropic aquifers; however, most aquifers are anisotropic in permeability. Therefore, dissolution mechanisms will be different in these kinds of reservoirs. It is expected that anisotropy will have strong effects on growth of instabilities and onset time of convection. Hence, understanding role of anisotropy on CO2 dissolution is crucial for safe storage of CO2 in aquifers. For this aim, a 2-Dimensional model was developed to study convective mixing during storage of CO2 in anisotropic aquifers. Results showed that anisotropy strongly affects convection mechanism and leads to enhanced CO2 dissolution. Furthermore, an appropriate definition for Rayleigh number of anisotropic systems was presented and then scaling relationships of convection parameters were developed. These results enable us to asses and select suitable injection sites for CO2 sequestration.