Study on the influence of well closure and production pressure during dual-gas co-production from hydrate-bearing sediment containing underlying gas

Hydrate reservoirs with underlying gas are promising for commercial exploitation because the dual-gas co-production from hydrate and underlying free-gas zones is expected to increase gas yields. In addition, the exploitation of natural gas hydrate deposits mostly suffers from unexpected well pore clogging due to ice generation, hydrate reformation, and sand production. A novel method for forming Class 1G deposits with enriched underlying free gas is proposed and the influence of well closure and production pressure during dual-gas co-production is investigated. Operational well closure enhances temperature recovery, induces sediment re-expansion, and inhibits gas production. In addition, with a delay in well closure, the above-mentioned influences weaken. The production pressure influences the internal temperature variation during gas production, and a higher production pressure weakens the aforementioned influence of well closure. A lower production pressure and later well closure operation simultaneously enhance gas production, inhibit temperature recovery, and bring a more pronounced sediment shrinkage, implying a competition between reservoir safety and economic benefits of hydrate exploitation. The obtained heat transfer, sediment deformation, and gas production relationships resulting from depressurization and well closure can be used as well-plugging indicators and as a reference for identifying optimum depressurization and well-closure strategies for hydrate reservoir exploitation.