Mechanisms of Synergistically Enhanced Oil Recovery by Water‐Based Medium Imbibition and CO2 Flooding in Shale Oil Reservoirs

Both surfactant solution imbibition and CO2 flooding are widely applied oil recovery techniques in shale oil reservoirs. However, the synergistic oil recovery effect of water‐based medium imbibition and CO2 flooding for shale oil reservoirs is rarely explored, and the mechanisms of synergistic enhanced oil recovery by water‐based medium imbibition and CO2 flooding are still not clear. In this paper, core flooding experiments combined with NMR tests are conducted to explore the synergy between slick water imbibition and CO2 flooding in enhancing shale oil recovery and the governing mechanisms. The results show that (a) the average oil recovery rate of slick water imbibition is 38.86%, and after subsequent CO2 flooding, the oil recovery rate is further increased by 21.93%. (b) Compared with slick water imbibition combined with subsequent slick water flooding, slick water imbibition combined with subsequent CO2 flooding can improve the total oil recovery rate by about 10%. (c) During slick water imbibition combined with subsequent CO2 flooding, slick water primarily mobilizes oil from clay interlayer pores (r < 10 nm), whereas CO2 preferentially displaces oil trapped in pores with radii >10 nm, and the large pore (r > 150 nm) oil recovery rate can reach almost 100% when the miscibility is obtained. (d) During subsequent flooding, compared with slick water, CO2 can improve oil recovery rates in pores with radii >10 nm by more than 10%. Additionally, it enhances imbibition recovery of slick water by improving the hydrophilicity of shale core samples.