After primary and secondary oil recovery, CO 2 -enhanced oil recovery (EOR) has become one of the most mentioned technologies in tertiary oil recovery. Since the oil is confined in an unconventional reservoir, the interfacial properties of CO 2 and oil are different from in conventional reservoirs, and play a key role in CO 2 EOR. In this study, molecular dynamics simulations are performed to investigate the interfacial properties, such as interfacial tension, minimum miscibility pressure (MMP), and CO 2 solubility. The vanishing interfacial tension method is used to get the MMP (~10.8 MPa at 343.15 K) which is in agreement with the reported experimental data, quantitatively. Meanwhile, the diffusion coefficients of CO 2 and n-octane under different pressures are calculated to show that the diffusion is mainly improved at the interface. Furthermore, the displacement efficiency and molecular orientation in α-quartz nanoslit under different CO 2 injection ratios have been evaluated. After CO 2 injection, the adsorbed n-octane molecules are found to be displaced from surface by the injected CO 2 and, then, the orientation of n-octane becomes more random, which indicates that and CO 2 can enhance the oil recovery and weaken the interaction between n-octane and α-quartz surface. The injection ratio of CO 2 to n-octane is around 3:1, which could achieve the optimal displacement efficiency.

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