Effects of Varying Oil/Water Saturations on Miscible and Immiscible CO<sub>2</sub> Flooding in Heterogeneous Porous Media: A Lattice Boltzmann Method Simulation Study
Yongqi Wang, Jiarui Fan, Chunlong Yang, Kun He, Dayong Wang
Abstract
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 CO 2 -enhanced oil recovery (CO 2 -EOR), such distribution inevitably influences spatiotemporal evolution of oil, CO 2, and water distribution in porous rocks after CO 2 injection. However, there is currently a lack of specific studies on CO 2 flooding behavior under different initial oil/water saturations, and thus the effect of varying initial oil/water saturations on microscopic processes and efficiency of CO 2 -EOR still remains poorly understood. Here, we developed a lattice Boltzmann model for CO 2 –oil–water three-phase flow and conducted a pore-scale analysis of the effects of varying initial oil/water saturations on CO 2 flooding. Simulation results unveil the following insights: (1) initially, CO 2 /water displacement dominates the CO 2 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 CO 2 swept efficiency and prolong the time of CO 2 breakthrough. (2) Once CO 2 breaks through, immiscible flooding reaches steady state soon, and thus the displacement process prior to CO 2 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, CO 2 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 CO 2 -EOR, measures might focus on improving pore structure quality to reduce the amount of residual oil and reasonably arranging the CO 2 injection cycle based on the revealed temporal evolution characteristics of oil recovery in this study.