Isostructural and cage-specific replacement occurring in sII hydrate with external CO2/N2 gas and its implications for natural gas production and CO2 storage

A replacement technique has been regarded as a promising strategy for both CH4 exploitation from gas hydrates and CO2 sequestration into deep-ocean reservoirs. Most research has been focused on replacement reactions that occur in sI hydrates due to their prevalence in natural gas hydrates. However, sII hydrates in nature have been also discovered in some regions, and the replacement mechanism in sII hydrates significantly differs from that in sI hydrates. In this study, we have intensively investigated the replacement reaction of sII (C3H8 + CH4) hydrate by externally injecting CO2/N2 (50:50) gas mixture with a primary focus on powder X-ray diffraction, Raman spectroscopy, NMR spectroscopy, and gas chromatography analyses. In particular, it was firstly confirmed that there was no structural transformation during the replacement of C3H8 + CH4 hydrate with CO2/N2 gas injection, indicating that sII hydrate decomposition followed by sI hydrate formation did not occur. Furthermore, the cage-specific replacement pattern of the C3H8 + CH4 hydrate revealed that CH4 replacement with N2 in the small cages of sII was more significant than C3H8 replacement with CO2 in the large cages of sII. The total extent of the replacement for the C3H8 + CH4 hydrate was cross-checked by NMR and GC analyses and found to be approximately 54%. Compared to the replacement for CH4 hydrate with CO2/N2 gas, the lower extent of the replacement for the C3H8 + CH4 hydrate with CO2/N2 gas was attributable to the persistent presence of C3H8 in the large cages and the lower content of N2 in the feed gas. The structural sustainability and cage-specific replacement observed in the C3H8 + CH4 hydrate with external CO2/N2 gas will have significant implications for suggesting target gas hydrate reservoirs and understanding the precise nature of guest exchange in gas hydrates for both safe natural gas production and long-term CO2 sequestration.