Pore-Scale Modeling of Miscible CO₂-SAG (Soaking-Alternating-Gas) Flooding in Heterogeneous Porous Media

CO2 miscible flooding is a very promising technology for further increasing oil production in water-flooding reservoirs. However, the low viscosity and density of injected CO2 can easily lead to gas channeling when the reservoir is strongly heterogeneous, resulting in a diminished capacity for CO2 to enhance oil recovery. In particular, the underlying mechanisms of oil displacement by CO2 flooding under various miscible conditions remain unclear, which severely limits the application of the CO2 miscible flooding to maximize crude oil recovery. By incorporation of an interfacial tension (IFT) term into the open-source twoLiquidMixingFOAM solver, a pore-scale mathematical model of CO2 flooding considering various miscible conditions is established in this paper. After validating the reliability of the proposed method, the effects of injection rate, IFT, diffusion coefficient, and injection pattern, including continuous CO2 injection and CO2- soaking-alternating-gas (SAG) injection, on the oil recovery performance of CO2 miscible flooding in a dual-permeability porous media are further investigated. The results show that, as the injection rate increases, the degree of miscibility gradually decreases, indicating that oil recovery is not invariably improved with the increase of injection rate. When the injection rate is relatively low, the decrease in IFT and the increase in the diffusion coefficient cannot effectively recover remaining oil in the low-permeability zone. Increasing the injection rate can gradually increase oil production, while the overall efficiency is still very low. In the miscible CO2-SAG process, some remaining oil occupied in the low-permeability zone can migrate into the high-permeability zone, where it mixes with CO2 and is ultimately recovered. Under identical conditions, the oil recovery rate achieved by miscible CO2-SAG injection is higher than that of the continuous CO2 injection by 16.5%. Thereafter, an innovative multisoaking miscible CO2-SAG flooding method is simulated. It is observed that after the first cycle of miscible CO2-SAG, more remaining oil in the low-permeability zone can diffuse into the high-permeability zone during the second soaking cycle. Compared to the first cycle of miscible CO2-SAG injection, the twice-soaking miscible CO2-SAG flooding method can achieve a higher oil recovery rate by 13.8%.