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