Methane Hydrate in Marine Sands: Its Reservoir Properties, Gas Production Behaviors, and Enhanced Recovery Methods

Since the discovery of methane hydrate in marine sands in 1999, our understanding of methane-hydrate-bearing sediments has progressed rapidly. Comprehensive studies of methane hydrate deposits in the Nankai Trough have made a great contribution to this research. Reservoir evaluation combining geophysical logging with pressure core analysis has become the standard protocol for exploring methane-hydrate-bearing sands, revealing that the effective permeability of methane-hydrate-bearing sands lies in the range of 1-100 md, even if the hydrate saturation reaches high value of 50-80 % in pores. Numerical and experimental studies of gas production have demonstrated that fluid flow and heat transfer are key rate-determining factors for field-scale depressurization-induced gas production. In that sense, the high permeability of methane-hydrate-bearing sands indicates the applicability of depressurization as a primary gas production method; however, a lack of heat limits hydrate dissociation and thus the recovery factor cannot exceed approximately 40 %. Thus, enhanced recovery methods are necessary to increase viability of commercialization. Experimental and numerical studies have shown that hydraulic fracturing enhances production rate, and that cyclic depressurization assisted by geothermal and deep depressurization using the latent heat of ice formation are possible means by which the enhancement of the recovery factor can be achieved while maintaining energy efficient production.