Nanoscale Distribution and Migration Dynamics of Oil–CO2–Water with Cosolvent in CO2 Soaking Alternating Gas Flooding

The application of the CO2–cosolvent soaking alternating gas (SAG) process in shale oil reservoirs is theoretically feasible. However, the migration characteristics of the oil–CO2–water system with a cosolvent under nanoconfinement remain poorly understood. To address this, we employed nonequilibrium molecular dynamics (NEMD) simulations to investigate the mobilization behavior of shale oil and the short-term CO2 retention in water-bearing nanopores, with particular attention to the coupling between water morphology and cosolvent-assisted SAG transport. Results indicate that structural differences in composite walls induce distinct electric fields, driving water to form either films or bridges. These water structures alter phase connectivity and fluid–wall contact, thereby introducing transport mechanisms that are absent from dry CO2–cosolvent SAG systems. The slip of oil molecules at the oil–water interface and the attenuation of strong pore-wall adsorption by the water film significantly promote the desorption and diffusion of oil. The stronger dipole interactions of ethyl acetate (EA) allow it to disrupt the water hydrogen-bond network and create penetration channels more effectively than dimethyl ether (DME), thereby promoting oil mobilization at high water content. Under the simulated conditions, DME performs better in low-water P–P–K nanopores, whereas EA generally shows greater oil mobilization in P–H–K nanopores and wetter P–P–K nanopores, suggesting that cosolvent effectiveness depends on water content and pore type. Although water reduces the overall amount of CO2 retained within nanopores, its sealing effect may contribute to the short-term retention of adsorbed and dissolved CO2. Formation water salinity further modifies oil recovery and short-term CO2 retention by regulating residual water distribution, ion hydration, and fluid–wall interactions, indicating that water chemistry is an important factor in evaluating CO2–cosolvent SAG performance. These findings provide molecular-level insights into the coupled mechanisms governing shale oil mobilization and short-term CO2 retention in water-bearing nanopores.