Numerical Simulation Of Gas Production From Hydrate Deposits In Qilian Mountain Permafrost By Depressurization

Natural gas hydrate is a special form of the existence of natural gas, which is mainly distributed both in the deep marine sediments and in the terrestrial permafrost regions. The Qinghai-Tibet Plateau permafrost in China has a vast expanse with a total area up to150×104km2. In recent years, a series of geological, geophysical, and geochemical investigations and studies were conducted in the Qinghai-Tibet Plateau permafrost, resulting in the confirmation of the favorable occurrence conditions and prospecting potential for gas hydrates in this region. In2008and2009, the scientific drilling project of gas hydrate in the Qilian Mountain permafrost was implemented in China, with the discovery of gas hydrate samples successfully.Based on the output characteristics of gas hydrates at the DK-3drilling site, we take advantage of currently one of the world’s most advanced hydrate simulator TOUGH+HYDRATE to numerically simulate the development potential of gas hydrate deposits in the Qilian Mountain permafrost, Qinghai-Tibet Plateau using a single vertical well by depressurization. In order to evaluate the gas production potential of hydrates in the model built by the field parameters comprehensively and systematically, we employ the absolute index and the relative index respectively, and set three different cases of wellbore pressure (1MPa,1.5MPa and2.5MPa) during the dissociation process by depressurization. The simulation results indicate that when the wellbore pressure is set to1.5MPa, the CH4production rate of hydrate dissociation and the total CH4production rate in the reservoir both show moderately, and the cumulative CH4volume produced from the reservoir is relatively the largest, with the development potential better than the other two cases obviously. Then we numerically simulate the evolution of spatial distributions of temperature, hydrate saturation, gas saturation and water salinity in the reservoir for the wellbore pressure of1.5MPa, which indicates that ice obstacle does not appear and secondary hydrate is not observed, however, a large volume of free gas remains in the reservoir instead of being produced. During the maximum dissociation time, the effective radius of gas hydrate dissociation in the reservoir is less than20m, and the actual quality of gas hydrate dissociation only accounts for2.3%of the total quality of gas hydrates in the simulation system. Through the above analysis and discussions, we can conclude that the method of a single vertical well by depressurization for the development of gas hydrate deposits in the Qilian Mountain permafrost, Qinghai-Tibet Plateau is not economically feasible.