Molecular dynamics simulation on the displacement behaviour of crude oil by CO₂/CH₄ mixtures on a silica surface

Produced gas re-injection is an effective and eco-friendly approach for enhancing oil recovery from shale oil reservoirs. However, the interactions between different gas phase components, and the oil phase and rocks are still unclear during the re-injection process. This study aims to investigate the potential of produced gas re-injection, particularly focusing on the effects of methane (CH₄) content in the produced gas on shale oil displacement. Molecular dynamics simulations were employed to analyze the interactions between gas, oil, and matrix phases with different CH₄ proportions (0%, 25%, 50%, and 100%), alkanes and under various burial depth. Results show that a 25% CH₄ content in the produced gas achieves almost the same displacement effect as pure carbon dioxide (CO₂) injection. However, when the CH₄ content increases to 50% and 100%, the interaction between gas and quartz becomes insufficient to effectively isolate oil from quartz, causing only expansion and slight dispersion. Interestingly, the presence of CH₄ has a synergistic effect on CO₂, facilitating the diffusion of CO₂ into the oil film. During the gas stripping process, CO₂ is the main factor separating oil from quartz, while CH₄ mainly contributes to oil expansion. In addition, for crude oil containing a large amount of light alkanes, extracting light components through mixed gas may be more effective than pure CO₂. This study offers valuable insights for applications of produced gas re-injection to promote shale oil recovery.