Displacement Mechanism of Oil in Shale Inorganic Nanopores by Supercritical Carbon Dioxide from Molecular Dynamics Simulations

Supercritical CO2 (scCO2), as an effective displacing agent and clean fracturing fluid, exhibits a great potential in enhanced oil recovery (EOR) from unconventional reservoirs. However, the microscopic translocation behavior of oil in shale inorganic nanopores has not been well understood yet in the scCO2 displacement process. Herein, nonequilibrium molecular dynamics (NEMD) simulations were performed to study adsorption and translocation of scCO2/dodecane in shale inorganic nanopores at different scCO2 injection rates. The injected scCO2 preferentially adsorbs in proximity of the surface and form layering structures due to hydrogen bonds interactions between CO2 and −OH groups. A part of scCO2 molecules in the adsorption layer retain the mobility, due to the cooperation of slippage, Knudsen diffusion, and imbibition of scCO2. The adsorbed dodecane are separated partly from the surface by scCO2, as a result the competitive adsorption between scCO2 and dodecane, and thus enhancing the mobility of oil and improving oil production. In the scCO2 displacement front, interfacial tension (IFT) reduction and dodecane swelling enhance the mobilization of dodecane molecules, which plays the crucial role in the CO2 EOR process. The downstream dodecane, adjacent to the displacement front, is found to aggregate and pack tightly. The analysis of contact angle, meniscus, and interfacial width shows that the small scCO2 injection rate with a large injection volume is favorable for CO2 EOR. The morphology of meniscus changes in the order convex–concave–CO2 entrainment with the increase of the injection rate. The microscopic insight provided in this study is helpful to understand and effectively design CO2 exploitation of shale resources.