Effects of Varying Oil/Water Saturations on Miscible and Immiscible CO₂ Flooding in Heterogeneous Porous Media: A Lattice Boltzmann Method Simulation Study

Oil/water saturations vary significantly among different oil reservoirs, which, in combination with pore structure, determines the oil and water distribution in pores. As an initial condition for CO2-enhanced oil recovery (CO2-EOR), such distribution inevitably influences spatiotemporal evolution of oil, CO2, and water distribution in porous rocks after CO2 injection. However, there is currently a lack of specific studies on CO2 flooding behavior under different initial oil/water saturations, and thus the effect of varying initial oil/water saturations on microscopic processes and efficiency of CO2-EOR still remains poorly understood. Here, we developed a lattice Boltzmann model for CO2–oil–water three-phase flow and conducted a pore-scale analysis of the effects of varying initial oil/water saturations on CO2 flooding. Simulation results unveil the following insights: (1) initially, CO2/water displacement dominates the CO2 flooding process under both miscible and immiscible conditions, forming a preferential migration path. Thus, water content influences displacement path and rate. High initial water saturation tends to increase CO2 swept efficiency and prolong the time of CO2 breakthrough. (2) Once CO2 breaks through, immiscible flooding reaches steady state soon, and thus the displacement process prior to CO2 breakthrough actually determines final oil recovery. Due to the small swept efficiency and high oil content, the saturation of final residual oil becomes higher at the lower initial water saturation. For miscible flooding, CO2 injection can still increase oil recovery in the short term. Subsequently, the flooding process approaches steady state. (3) Some certain pores have narrow entrances that either block the entry of external fluids or hinder the increase in local pressure gradients, leading to failure of effective displacement of internal water or oil under both miscible and immiscible conditions. In order to further optimize efficiency and cost of CO2-EOR, measures might focus on improving pore structure quality to reduce the amount of residual oil and reasonably arranging the CO2 injection cycle based on the revealed temporal evolution characteristics of oil recovery in this study.