Methane hydrate exists abundantly under the seafloor of Japan's exclusive economic zone and has attracted attention as a domestic natural gas resource. Depressurisation is one of the methods for producing methane gas from methane hydrate under the seafloor, in which methane hydrate is dissociated into water by pumping up water and reducing the surrounding pressure. However, the production rate decreases after a certain period of time because the stratum is cooled by the endothermic dissociation of methane hydrate. To overcome this situation, an enhanced recovery method has been proposed: namely, CO2 is injected into the stratum to generate CO2 hydrate, and the surrounding area is heated by the exothermic formation of CO2 hydrate to promote the dissociation of methane hydrate. This combination of CO2 hydrate storage and enhanced recovery of methane hydrate is important because it can achieve both efficient methane gas production and the reduction of greenhouse gas emissions.

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