A Review of the Recent Advances in CH 4 Recovery from CH 4 Hydrate in Porous Media by CO 2 Replacement

With increasing attention paid to the development of natural gas hydrates, various mining methods have been studied. CO 2 -CH 4 hydrate replacement has become one of the key research topics in the field of natural gas hydrate mining because it can overcome the disadvantage of traditional mining methods that easily lead to reservoir collapse and realize CO 2 sequestration while extracting CH 4 . However, complex heat and mass transfer, as well as fluid migration, are involved in CO 2 -CH 4 hydrate in situ replacement, and this method has the drawbacks of slower reaction rates and a lower replacement efficiency compared to traditional methods. Therefore, a substantial amount of experimental and simulation research is still needed to advance this method. This paper reviews the current research on CH 4 recovery from CH 4 hydrate by CO 2 replacement. The main CO 2 -CH 4 hydrate replacement mechanisms are summarized according to whether the hydrate cage structure is disrupted. Numerical simulation studies based on the above replacement mechanisms are introduced and compared in detail. The effects of various replacement methods, such as soaking replacement and dynamic replacement, as well as factors including the presence of initial water, reservoir permeability, temperature, and pressure on the replacement reaction, are summarized. Additionally, existing pore-scale replacement studies are reviewed, highlighting the necessity of pore-scale research on CO 2 -CH 4 hydrate replacement reactions, pointing out the shortcomings of current pore-scale studies, and proposing suggestions for future research directions. This work provides a reference for the development of the CO 2 -CH 4 hydrate replacement method and the realization of its industrial applications.