Multiphase behavior and fluid flow of oil–CO2–water in shale oil reservoirs: Implication for CO2-water-alternating-gas huff-n-puff

Based on the CO2-WAG (water-alternating-gas) flooding for conventional reservoirs, CO2-WAG huff-n-puff in shale reservoirs is proposed. To clarify the phase behavior and fluid flow of oil–CO2–water in the CO2-WAG huff-n-puff process, a series of experimental studies are conducted under different injection sequences of CO2 and water. The results show that the saturation pressure of the oil–CO2–water systems is lower than that of the oil–CO2 systems since a portion of the CO2 is dissolved in water. In addition, CO2 injection followed by water can significantly reduce the dissolution of CO2 in the water. CO2 and water preferentially flow into the macropores and bedding fractures of the oil-saturated cores at the injection stage, and the oil in mesopores and micropores reflows into macropores and bedding fractures at the soaking stage. At the depressurization production stage, the oil in mesopores and micropores is gradually extracted. Compared to water injection followed by CO2, injecting CO2 first can avoid the barrier effect caused by the subsequent injection of water and promotes oil flow into mesopores and micropores in the matrix. At the end of production, the oil recovered from water injection followed by CO2 mainly originates from macropores, while that from CO2 injection followed by water primarily comes from mesopores.