High‐Saturation Gas Hydrate Reservoirs—A Pore Scale Investigation of Their Formation From Free Gas and Dissociation in Sediments

The enormous volume of gas stored in methane hydrate reservoirs has attracted worldwide interest. Hydrate reservoirs with hydrate saturations over 60% are considered viable targets for economic production. Although such high‐saturation hydrate reservoirs have been observed in nature, researchers have had difficulty in understanding how they may form and settling on best strategies for their production. Here we examined how high‐saturation methane hydrate deposits form from free gas in nature by synthesizing high‐saturation hydrate specimens in the laboratory by emulating natural conditions and monitoring hydrate formation using a high‐resolution micro‐computed tomography (micro‐CT) scanner. The results confirmed that free methane injection into water‐saturated sediments results in high‐saturation hydrate specimens. In addition, we explored gas production from high‐saturation specimens through a series of dissociation experiments. CT scan results show that water is expelled first during depressurization, while produced gas tends to remain trapped in the specimen. The volume of gas produced by thermal stimulation alone could be smaller than that of gas hydrate in deep reservoirs, which demands depressurization to expand the volume of the produced gas to extract gas from the reservoir. High‐saturation reservoirs are prone to flow path clogging during depressurization within hydrate stability zone, which can be mitigated by hydrate dissociation via thermal stimulation or further depressurization crossing the hydrate stability boundary.