Molecular dynamics simulation on the displacement behaviour of crude oil by CO/CH 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 (CH 4 ) 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 CH 4 proportions (0%, 25%, 50%, and 100%), alkanes and under various burial depth. Results show that a 25% CH 4 content in the produced gas achieves almost the same displacement effect as pure carbon dioxide (CO 2 ) injection. However, when the CH 4 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 CH 4 has a synergistic effect on CO 2 , facilitating the diffusion of CO 2 into the oil film. During the gas stripping process, CO 2 is the main factor separating oil from quartz, while CH 4 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 CO 2 . This study offers valuable insights for applications of produced gas re-injection to promote shale oil recovery. The presence of CH 4 has a synergistic effect on CO 2 , facilitating the diffusion of CO 2 into the oil film. During the gas stripping process, CO 2 is the main factor separating oil from quartz, while CH 4 mainly contributes to oil expansion.