Pore-scale numerical study of CO₂-oil miscible displacement with immiscible water in porous media

For carbon sequestration in deep reservoirs, carbon dioxide is miscible with oil, and immiscible water also exists around, forming a typical miscible flow process in immiscible surrounding. To investigate this process, three-phase color gradient lattice Boltzmann model is coupled with explicit velocity-difference model, and the wettability scheme of rotating color gradient is incorporated, which is validated by the diffusion process of binary component droplet on curved surface. Using this model, the miscible displacement processes in T-shaped structure, porous media and fractured porous media are simulated, and further analyzed by calculating mixing intensity, gas saturation distribution, gas front position and oil saturation. For T-shaped structure, the effects of diffusion coefficient, viscosity, surface tension and wettability are studied, showing that large diffusion coefficient, low oil viscosity, large surface tension ratio and strong water wettability is beneficial for mixing and oil recovery. For porous media, Péclet number and capillary number are changed, and branched, clustered and conical displacement patterns are identified. Oil recovery is enhanced from immiscible to miscible states, with the increase of capillary number, and with the decrease of Péclet number. Finally, the miscible displacements are conducted in fractured porous media. With the increase of diffusion coefficient, gas channeling is converted to non-gas channeling, resulting an increase of oil recovery.