Insight into sweeping and oil washing mechanisms in CO2 immiscible and miscible flooding: A study of computational fluid dynamics

CO2 flooding is widely recognized as an effective method for enhancing oil recovery in tight reservoirs, primarily influenced by sweep efficiency and oil washing capability. Previous studies have focused on oil-gas seepage behavior in simplified pore models, but the underlying seepage mechanisms of sweeping and oil washing under different miscibility conditions remain insufficiently understood. In this work, a three-dimensional pore model was constructed using the digital rock method, and sweeping and oil washing behaviors were simulated using computational fluid dynamics. In immiscible flooding, high oil-gas interfacial tension induces significant Jamin effects, causing preferential displacement through larger pores and resulting in low sweep efficiency. The weak extraction ability of the injected gas on the adsorbed oil on the rock surface further limits oil washing capacity, leading to a recovery rate of only 66.99 %. In contrast, in miscible flooding, mutual diffusion between CO2 and oil forms a broad miscible zone at the displacement front, leading to more uniform displacement and higher sweep efficiency. CO2's superior extraction capability on the wall further enhances oil washing capacity, resulting in a recovery rate of 88.67 %. Additionally, the impact of injection velocity on seepage behavior was examined. In immiscible flooding, increasing the injection velocity improved sweep efficiency by enhancing injection capacity in small pores but had minimal effect on oil washing capability. In miscible flooding, sweep efficiency showed slight improvement, while oil washing capability significantly enhanced, resulting in a 13.32 % increase in recovery rate. This study provides new insights into enhancing oil recovery in tight reservoirs.