Microscopic Mechanisms and Applications of CO₂+Associated Gas with Tight and Shale Oils

Tight/shale oil is an important strategic resource for sustaining long-term stable oil and gas production in China and safeguarding national energy security. However, its development commonly faces low oil recovery and difficulty in mobilizing remaining oil, highlighting the urgent need for economical and efficient enhanced oil recovery (EOR) technologies. To address this issue, molecular dynamics simulations were employed to systematically investigate the miscibility behavior and transport characteristics of three binary gas mixtures, namely CO₂-CH₄, CO₂-C₂H₆, and CO₂-C₃H₈, with tight/shale oil in both bulk systems and quartz nanopores. The governing mechanisms by which gas-mixture type, gas composition, and oil composition regulate miscibility and displacement efficiency were clarified. The results show that the dissolution capacity and molecular diffusivity of gas mixtures govern their miscibility with crude oil and displacement efficiency, respectively. The miscibility capability follows the order of CO₂-C₃H₈>CO₂-C₂H₆>CO₂-CH₄, whereas the displacement efficiency shows the opposite trend. Injection of CO₂-C₂H₆ and CO₂-C₃H₈ mixtures provides the dual advantages of improving oil displacement efficiency and expanding the mobilized volume. Specifically, CO₂-C₂H₆ is more suitable for lighter oils, whereas CO₂-C₃H₈ is preferred for heavier oils, and both can significantly improve reservoir development performance. Based on these findings, the injection-gas composition can be dynamically adjusted in field development according to the EOR benefits and environmental benefits of CO₂ injection, thereby synergistically enhancing miscibility and displacement efficiency and improving development performance. This study helps improve resource utilization efficiency and extend the productive life of oilfields, providing a scientific basis for enhancing the overall benefits of the full CCUS chain.