Research progress of natural gas hydrate exploitation with CO2 replacement: A review

Natural gas hydrate is a kind of new and clean energy source which has attracted much attention for safe and efficient energy recovery from hydrate reservoirs. Injecting CO2 into the gas hydrate deposits can not only realize CH4 production, but also seal up greenhouse gas CO2 and reduce the risk of geological hazards. This paper presents a comprehensive overview on the progress of CH4 recovery from gas hydrate by CO2 exchange. Two replacement mechanisms are analyzed and summarized according to the characteristics evolution of hydrate cage structure during the replacement process. Various key factors affecting CO2 replacement efficiency are illustrated, including the injected CO2 phase state, the thermodynamic replacement conditions, the type of auxiliary gases (N-2, H-2, and air), the hydrate structure, and the hydrate saturation. The replacement effect under the influence of multiple factors is also discussed. Meanwhile, the structural stability of the sedimentary layer is reviewed. It indicates that the strength of the CO2 hydrate layer is greater than that of CH4 hydrate, and the strength of the reservoir can be repaired after replacement. In addition, based on the current conclusions obtained, potential limitations and future research prospects of the replacement technology are discussed. This review summarizes the progress in CO2-CH4 replacement in recent years and provides valuable guidance for commercial hydrate exploitation and industrial applications of CO2 capture and storage in the future.