Experimental study on the dual-gas co-production from hydrate deposit and its underlying gas reservoir

Short gas production periods and high production costs are two main challenges for the commercial exploitation of natural gas hydrates (NGHs). The NGHs deposit with enriched underlying free gas has drawn more attention, because the dual-gas co-production from hydrate zone and underlying free gas zone are expected to increase gas yields. We designed a method to prepare this kind of hydrate deposits that a hydrate layer over a free gas layer, and performed a series of dual-gas co-exploitation experiments to investigate its gas production characteristics via depressurization method. Results demonstrate that the temperature of hydrate layer and free gas layer exhibit different variation tendency during gas production process and lower production pressure attributes to higher gas production rate and gas re-covery ratio. The wellbore set at underlying free gas layer is recommended because it can avoid lower initial local permeability around the well and prevent ice plug or hydrate reformation during gas pro-duction process. Meanwhile, it enhances the movement of hydrate decomposition front and further improves gas production efficiency. Compared with the NGHs reservoir with a single hydrate zone, the dual-gas co-production has higher gas production efficiency and lower temperature decrease to maintain the driving force of hydrate dissociation. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Short production periods and high costs are obstacles for commercial exploitation of natural gas hydrates (NGHs). This study explores a method to prepare NGHs deposits with an underlying free gas layer and a hydrate layer on top, and conducts experiments to investigate its gas production characteristics. The results show that the temperature of the hydrate layer and the free gas layer exhibit different variations during gas production, and lower production pressure leads to higher gas production rate and recovery ratio. Setting the wellbore at the underlying free gas layer enhances hydrate decomposition and improves gas production efficiency.