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

•The addition of N2 to CO2 is effective for recovery of CH4 in gas hydrate.•The penetration capability of the replacement phase is sensitive to the ratio of N2 to CO2.•The function of N2 is not only replacement but also decomposition of CH4 hydrate. 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.