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

Supercritical CO2 (scCO(2)), 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 scCO(2) displacement process. Herein, nonequilibrium molecular dynamics (NEMD) simulations were performed to study adsorption and translocation of scCO(2)/dodecane in shale inorganic nanopores at different scCO(2) injection rates. The injected scCO(2) preferentially adsorbs in proximity of the surface and form layering structures due to hydrogen bonds interactions between CO2 and -OH groups. A part of scCO(2) molecules in the adsorption layer retain the mobility, due to the cooperation of slippage, Knudsen diffusion, and imbibition of scCO(2). The adsorbed dodecane are separated partly from the surface by scCO(2), as a result the competitive adsorption between scCO(2) and dodecane, and thus enhancing the mobility of oil and improving oil production. In the scCO(2) 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 scCO(2) 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.