Miscibility Characteristics of CO₂-Oil in Tight Sandstone Reservoirs: Insights from Molecular Dynamics Simulations

Identifying the microscopic mechanism of CO₂-oil miscibility is significant for the CO₂ enhanced oil recovery (CO₂-EOR) in tight sandstone reservoirs. In this work, the effects of oil composition, formation pressure, temperature, and methane content on the characteristics of the CO₂-oil miscibility were systematically studied by molecular simulation methods. According to the change of oil-gas centroid displacement, the CO₂-oil miscibility behavior was divided into four stages: rapid diffusion, CO₂ dissolution and oil swelling, competitive adsorption and oil film detachment, and complete miscibility or dynamic equilibrium stability. The results showed that light or medium component oil is more easily miscible with CO₂ under reservoir conditions. The changes in temperature and pressure will greatly influence the oil-gas miscibility. Increasing the temperature is conducive to reducing the adsorption energy between oil and quartz, thus improving the miscibility of CO₂ and heavy component oil. However, the static swelling effects of CO₂ alone cannot effectively displace the heavy component oil on quartz. The CO₂ diffusion coefficient perpendicular to the quartz surface does not increase continuously with the temperature increase due to the adsorption of oil and quartz. There is a critical temperature range of 320-340 K, which makes the miscibility effects the best. A small amount of CH₄ can enhance the interaction energy between the two phases of oil and gas, thus promoting the miscibility of CO₂ and oil at the interface. However, it is not conducive to oil film detachment, with the CH₄ content increasing.