Microsecond simulation study on the replacement of methane in methane hydrate by carbon dioxide, nitrogen, and carbon dioxide–nitrogen mixtures

Replacement of methane (CH₄) in CH₄ hydrate by carbon dioxide (CO₂) can enable recovery of CH₄, which is a potential future energy resource, while sequestering CO₂ to mitigate the effects of global warming. However, little work has been done to address the effects of impurities on CO₂ replacement, and the detailed mechanisms. Here, microsecond molecular dynamics simulations were performed to understand the influence of nitrogen (N₂) gas on the process of replacing CH₄ in CH₄ hydrate with CO₂ at 280 K and 6 MPa. The results show that CO₂ molecules can penetrate more deeply into CH₄ hydrate phase when it is mixed with N₂. This is mainly because N₂ can favor the decomposition of CH₄ hydrate and expand the replacement area of CH₄ by guest molecules. We confirm that the replacement of CH₄ by CO₂ and N₂ preferably occurs in large and small cages, respectively. In most cases, a mixture hydrate reforms at the outmost layer of the hydrate surface. The CO₂/N₂ mixture shows an overall higher replacement efficiency than pure CO₂ case. Our work demonstrates that CH₄ recovery by CO₂ injection in CH₄ hydrate can be facilitated by N₂. The penetration depth of replacement is sensitive to the ratio of N₂ to CO₂. The knowledge obtained in this study will be helpful for the effective utilization of CO₂/N₂ mixtures to maximize the recovery percentage of CH₄ from hydrate.