Comparative analysis of the production trial and numerical simulations of gas production from multilayer hydrate deposits in the Qilian Mountain permafrost

The focus of this paper is to determine the hydrate saturation of the natural gas hydrate (NGH) deposits in the Muri Basin of Qilian Mountain permafrost on the Qinghai-Tibet Plateau and to analyse the performance of different production methods using TOUGH + HYDRATE (T + H) simulator for simulation of the NGH production process trial. Both depressurisation and thermal stimulation methods, using a high-pressure submersible pump to keep the water level in the borehole below the hydrate layer and injecting either hot air or steam, were applied in the hydrate production trial. The simulation of depressurisation-induced gas production under different initial hydrate saturations showed that the gas production was 104.8 m3 for 84 h when the saturation of the hydrate layer was 1.0% and the hydrate abnormal layer was 0.5%. The simulated production was close to the actual value of 102 m3. The result for the NGH deposits indicated a production of 12.8 m3 for the hot air injection-induced gas for 11 h, which is slightly higher than the actual production 10.1 m3. The hot steam injection produced 7.1 m3 of gas for 6 h versus the actual production 3.3 m3. The difference was associated with the simplification of the simulated pressure. The simulation also revealed that larger pressure differences caused the dissociation front of the lower hydrate formation to advance faster than that of the upper formation. Further results also showed that the two heating methods had weak impacts on hydrate dissociation in this production trail since they were used mainly for accelerating the gas migration around the wall of the well, resulting in a slight growth of gas production rate. Depressurisation-induced production was relatively more effective for low saturation hydrate deposits in the Qilian Mountain permafrost.