Effects of roughness on density-driven convection of dissolved CO2 in discrete fracture-matrix system

Dissolution trapping is a permanent and safe mechanism for geological carbon sequestration. An increase in density caused by the dissolution of CO2 into brine can trigger a density-driven convection process. In this study, the effect of fracture roughness on the density-driven convection process in heterogeneous saline aquifers is numerically analyzed by using a three-dimensional rough fracture-matrix model. The results show that the variation of the fracture aperture, a microscale heterogeneity phenomenon, can result in the change of the macroscopic flow characteristics. The rough fracture surface intensifies the convection inside the fracture and makes the migration path of the solute more tortuous. The spatial variability of the aperture generates the merging of convective fingers and suppresses the development of weak convective fingers, thus further enhancing the longitudinal extent of convective mixing. It is indicated that neglecting roughness during the flux growth stage can lead to an underestimation of dissolved fluxes by about 20%. The consideration of fracture roughness variation can contribute to the better understanding and prediction of the dynamics of dissolved CO2 in real reservoirs.