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

Both surfactant solution imbibition and CO 2 flooding are widely applied oil recovery techniques in shale oil reservoirs. However, the synergistic oil recovery effect of water‐based medium imbibition and CO 2 flooding for shale oil reservoirs is rarely explored, and the mechanisms of synergistic enhanced oil recovery by water‐based medium imbibition and CO 2 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 CO 2 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 CO 2 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 CO 2 flooding can improve the total oil recovery rate by about 10%. (c) During slick water imbibition combined with subsequent CO 2 flooding, slick water primarily mobilizes oil from clay interlayer pores ( r < 10 nm), whereas CO 2 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, CO 2 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.