Microscopic Mechanism for the Displacement of Shale Oil in a Nanochannel by CO₂ at Different Pressures

Aiming at the problems of unclear utilization degree of shale oil in different occurrence states. In this paper, the CO₂ displacement process of shale oil under different pressure conditions is simulated by the molecular dynamics method, the microscopic mechanism of CO₂ displacement of shale oil is clarified, and the influence of pressure on the utilization degree of different occurrence states of shale oil is revealed. The results show that CO₂ cannot form an effective flow channel under low pressure (1 MPa), and most of the oil and water remain in the channel. The water molecules form a water bridge at the outlet of the pore that CO₂ cannot break through under medium pressure conditions (5 and 10 MPa). CO₂ forms a stable flow channel under high pressure (15 MPa). With the increase of displacement pressure, the kinetic energy of oil and gas molecules in the channel increases. But limited by the system space, the diffusion capacity of shale oil system, the degree of oil and gas miscibility, and the competitive adsorption effect all show a trend of increasing first, then stabilizing, and rapidly increasing at last. The final recoveries of shale oil are 33.24%, 75.82%, 64.69%, and 83.03% under different displacement pressure conditions. Among them, the degree of oil and gas miscibility and shale oil utilization in the adsorbed layer is higher than that in the free layer under 1 MPa condition. And the shale oil utilization degree in the free layer is higher than that in the adsorbed layer under the other pressure conditions. This study can provide a theoretical basis for efficient development of shale oil.