Evaluation of gas production from Qilian Mountain permafrost hydrate deposits in two-spot horizontal well system

In 2008–2009, gas hydrate deposits were confirmed to exist in the Qinghai–Tibet Plateau permafrost (QTPP) during the Scientific Drilling Project of Gas Hydrate in Qilian Mountain permafrost. Gas hydrate samples were successfully collected from three drilling sites: DK-1, DK-2, and DK-3. Based on the limited geological data from site measurements, gas was produced from the hydrate deposits of DK-2 zone in a two-spot well system through numerical simulation, and its commercial viability was evaluated. The two wells were placed on the same horizontal plane in the middle of the hydrate-bearing layer (HBL), and the depressurization and thermal stimulation methods were employed simultaneously in this system. Simulation results showed that desirable gas production performance could be obtained under suitable operation conditions when using this kind of well design. During the production process, large amount of free gas accumulated in the vicinity of the injection well until the flow channels between the two wells were gradually opened. It was found that the gas production performance was more favored with larger depressurization driving force, while the heat injection rate should be determined cautiously. Dependences of gas production performance on the various properties of hydrate deposits were also assessed.