Numerical investigation of convective mixing in impure CO2 geological storage into deep saline aquifers

Dissolution of CO2 into formation brine could lead to density-driven convective mixing which enhances the transfer rate of free-phase CO2 into saline aquifers and thus plays an important role in efficient and secure long-term geological storage. In practical projects, CO2 is always co-injected with multiple impurities to reduce capture costs or to dispose hazardous species like SO2 and H2S alongside. Numerical simulations in this study reveal that, compared with the pure CO2 case, only the co-injection of SO2 would strengthen the convective mixing process in the formation brine while all the other common impurities would result in delayed onset and weakened convection. Particularly, single impurity such as N-2 may represent all the air-derived impurities in predicting the evolution of convective mixing process. With the exception of SO2, the diffusivity of most co-injected impurities is not likely to have noticeable impact on the occurrence and development of the density-driven convection. However, the effective CO2 diffusivity should be measured or estimated when SO2 is coinjected. Several scaling relations characterizing the onset, strength, and decay of the convective mixing process in impure CO2 geological storage are also established, which would provide reference for screening potential reservoirs for impure CO2 geological storage.