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 (N-2) 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 N-2. This is mainly because N-2 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 N-2 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/N-2 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 N-2. The penetration depth of replacement is sensitive to the ratio of N-2 to CO2. The knowledge obtained in this study will be helpful for the effective utilization of CO2/N-2 mixtures to maximize the recovery percentage of CH4 from hydrate.