Effective CH4 production and novel CO2 storage through depressurization-assisted replacement in natural gas hydrate-bearing sediment

In this study, the guest exchange behaviors in the hydrate-bearing sediment of a one-dimensional reactor specially designed for depressurization-assisted replacement were experimentally investigated for CH4 produc-tion and CO2 storage. The longitudinal distributions of vapor compositions, replacement efficiency, and hydrate weight fractions, as well as the average efficiency for depressurization-assisted replacement, were examined using gas chromatography and powder X-ray diffraction (PXRD). The immediate re-formation of gas hydrates after CO2 injection implied a rapid recovery of the geo-mechanical strength of the sediment. The unique changes in the gas compositions in the pore space caused by the re-formation of gas hydrates resulted in a distinctive longitudinal distribution of the replacement efficiency in the hydrate-bearing sediment. PXRD analysis of the hydrate samples revealed that the gas hydrate saturation nearly recovered after depressurization-assisted replacement. Despite CH4 re-enclathration, the replacement efficiency was remarkably enhanced through depressurization-assisted replacement, and a larger amount of CO2 was stored in the hydrate-bearing sediment than the amount of CH4 produced. Furthermore, the production rate of CH4 through depressurization-assisted replacement was significantly higher than that through replacement only, and the guest exchange rate increased with an increase in the initial hydrate dissociation ratio. The experimental results demonstrated that depressurization-assisted replacement could solve the weakening of the geo-mechanical strength of the sediment for depressurization only and the slow production rate for replacement only; thus, it would be useful for low -carbon energy production from natural gas hydrate-bearing sediments.