Critical Contribution of Water in Hybrid CO₂/Water Enhanced Shale Oil Recovery: Insights from Molecular Dynamics

The development of unconventional reservoirs through gas injection has gained significant attention in recent years. CO2-enhanced recovery of shale oil is acknowledged to have great prospects but still faces challenges, such as unsatisfactory recovery efficiency. In this study, molecular dynamics (MD) simulations were performed to investigate the displacement behavior of shale oil using a hybrid CO2/water flooding system in calcite nanoslits under reservoir conditions. The simulation results demonstrate that pure CO2 is effective at displacing octane molecules but not efficient enough for displacing asphaltene molecules within the calcite nanoslits, which actually causes the essential obstacle for the effective enhanced oil recovery. Introducing a suitable amount of water into the CO2 phase significantly enhances the recovery of shale oil, including asphaltene components. It was found that water molecules have high adsorption capacity on the calcite surface, thereby playing a critical role in displacing asphaltenes, while the water phase has difficulties in permeating through the alkane oil layer, and pure water flooding could not achieve a good effect. By interacting with CO2 molecules, water molecules can easily permeate into the oil phase, and the potential of mean forces (PMFs) analyzation indicated that the desorption of asphaltene molecules from the calcite surface with water existing requires less energy with CO2-water coexisting than in a pure CO environment. The hybrid CO2/water flooding system could efficiently improve the recovery of asphaltene molecules and get high shale oil recovery. The effect of water contents and injection rate on flooding efficiency was also studied. This work illustrates the microscopic mechanism of hybrid CO2/water fluids for replacing shale oil in calcite nanoslits; the results offer new perspectives for optimizing current shale oil extraction techniques and might be helpful in finding more efficient ways to significantly improve the recovery of shale oil.