Energy recovery enhancement from gas hydrate based on the optimization of thermal stimulation modes and depressurization

Natural gas hydrate is a potential and clean energy with abundant reserves in marine and permafrost areas. Efficient and safe extraction of the methane gas from gas hydrates has aroused worldwide attentions. In this work, the hydrate dissociation and gas production performances have been investigated in a high pressure reactor using two vertical wells by depressurization and its combinations with different injection modes, including warm water injection, room-temperature water flooding, and electric heating. The external heat supply rate is set identical in the cases with thermal stimulation. Results show that direct electrical heating combined with depressurization can dramatically increase the deposit temperature and eliminate the heat loss in the pipelines during fluid transportation, but a limited heat transfer radius exists in the vicinity of the heated wellbore due to the low thermal conductivity of the porous media. For the case with warm water injection, thermal convection becomes the key factor governing the heat transfer process, while the heat loss is inevitable during the transportation of the injected water in the pipelines. However, the heat can be more efficiently transferred to the hydrate-undissociated region through the water movement by enforced thermal convection than the electric heating. Comparatively, the novel tripartite strategy of electrical heating, room-temperature water flooding and depressurization shows the advantages of simultaneously reducing heat loss and enhancing heat transfer during hydrate exploitation, which results in the best energy recovery efficiency in this study. It suggests a good commercial exploitation value and shows important practical significance for future field studies.