Numerical simulation of gas production from hydrate deposits using a single vertical well by depressurization in the Qilian Mountain permafrost, Qinghai-Tibet Plateau, China

In November 2008, gas hydrate samples were recovered during the scientific gas hydrate drilling project conducted in the Qilian Mountain permafrost located in the Qinghai-Tibet Plateau, China. This region is expected to become a strategic gas hydrate exploitation area in China. Based on the gas hydrate characteristics at the DK-3 drilling site located in this region, we used using Tough + Hydrate to numerically simulate the gas production potential of the gas hydrate deposits using a single vertical well by depressurization. The simulation results indicated that for a 1.5 MPa wellbore pressure, the average CH4 production rate of hydrate dissociation was approximately 188 ST m3/d, the reservoir average total CH4 production rate was approximately 539 ST m3/d, and the cumulative CH4 volume produced from the reservoir was approximately 35% and 39% larger than those for wellbore pressures of 1 MPa and 2.5 MPa. Moreover, we numerically simulated the spatial distribution evolution of temperature, hydrate saturation and gas saturation in the reservoir for a 1.5 MPa wellbore pressure; the simulation indicated that a large volume of free CH4 remained in the reservoir. During the dissociation time, the gas hydrate dissociation effective radius in the reservoir was less than 20 m, and the actual dissociated gas hydrates only accounted for 2.3% of the total gas hydrates in the simulated system. The results may suggest that the single vertical well by depressurization method is not optimal for the development of gas hydrate deposits in the Qilian Mountain permafrost. Other production strategies, such as a horizontal well design or the combination of depressurization and thermal stimulation, may be more economically feasible.