Study on the Synergistic Enhancement Mechanisms of CO2/CH4 Single-Phase and Alternating Flooding Based on Molecular Dynamics

Abstract

To mitigate the greenhouse effect, promoting the clean utilization of fossil fuels and advancing carbon capture, utilization, and storage (CCUS) technologies have become critical pathways. Among these approaches, CO₂ and CH₄ flooding, as key enhanced oil recovery (EOR) techniques, exhibit dual benefits in both emission reduction and production enhancement. In this study, molecular dynamics simulations are employed to systematically investigate the oil displacement behaviors of CO₂ and CH₄ in porous media reservoirs. Simulations are conducted under two temperature conditions: 300 K (ambient temperature) and 383 K (elevated temperature). The heat and mass transfer characteristics, as well as multiphase interaction mechanisms, are comparatively analyzed under three injection scenarios: CO₂ single flooding, CH₄ single flooding, and alternating CO₂-CH₄ injection. The results indicate that CO₂ single flooding at 383 K achieves the highest molecular displacement fraction; however, it is accompanied by pronounced gas channeling. In contrast, under ambient temperature conditions (300 K), alternating injection exhibits significant synergistic effects. In particular, the alternating flooding strategy initiated with CO₂ injection enhances molecular displacement fractions while effectively suppressing gas channeling compared with single-gas flooding. At elevated temperature, although alternating injection mitigates the risk of gas channeling, the increased gas diffusion coefficient weakens intermolecular interactions, leading to a reduced displacement efficiency relative to single-phase flooding. By establishing a multidimensional analytical framework that integrates temperature distribution, molecular interactions, and reservoir adsorption characteristics, this study elucidates the intrinsic mechanisms governing gas migration under multi-physical field coupling. The findings provide molecular-scale theoretical insights for optimizing gas injection strategies in unconventional oil reservoirs.