GENERAL AND TEMPORAL EFFECTS OF FRACTURE ON DENSITY-DRIVEN CONVECTION OF CARBON DIOXIDE

Global warming facilitated by the increase in the concentration of atmospheric carbon dioxide (CO₂) is recognized as a serious problem. Utilizing geological reservoir as a CO₂ storage site can be effective to mitigate CO₂ emission. Among four CO₂ trapping mechanisms, dissolution trapping sequestrates a great amount of CO₂ safely for a long-term period. CO₂-dissolved brine increases its density up to 1% greater than the fresh brine. It occurs gravitational instability and results in density-driven convection (DDC) represented by dynamic convective fingers. Unlike molecular diffusion that transports CO₂ slowly, DDC accelerates the mixing of CO₂ and brine. Although fractures are observed in many natural reservoirs, the effects of fractures on DDC are rarely considered. This study is aimed to understand the dynamic behavior of DDC in fractured porous media and to assess the effects of fractures on CO₂ dissolution trapping quantitatively. Both 2D and 3D numerical simulations were conducted and revealed that the fractures play a time-varying role. Especially the 3D model showed the spatio-temporal evolution of fingers at the fracture surface. The competition between the enhancement of convective mixing and the inhibition of finger growth by upward fresh flow led to complex flow system. We find that heterogeneity increases fracture-matrix mass transfer, and fracture connectivity enhances CO₂ transport at field scale. Additionally, relatively low-density fingers are prone to change their flow paths or disappear due to the upward flow. High-density fingers, however, are less affected by the upward flow, and tend to maintain their flow paths.