The Influence of Confining Stresses on Formation Kinetics of Methane Gas Hydrates
Unconventional natural gas extraction has undergone rapid progress in recent years as an emerging energy source, and the amount of methane gas stored in gas hydrates is estimated to be larger than that of all other natural gas resources combined. In the process of finding suitable hydrate reservoirs for possible exploitation and other applications, it is vital to understand the effect of reservoir conditions on the formation kinetics of methane gas hydrates. The aim of this study is to investigate hydrate formation characteristics in a porous medium under different confining conditions, focusing on the effect of confining pressure on hydrate nucleation, growth, and saturation. A series of experiments was performed to simulate in-situ hydrate reservoir conditions in a custom-designed reactor to synthesise methane hydrates in a porous quartz sand medium under different confining and formation pressures. Temporal changes in gas pressure and temperature were incorporated to quantify the conversion of methane and water into gas hydrates. The results substantiate the extremely random nature of hydrate formation. Formation behaviour in the intermediate stages was observed to be highly individualistic due to the pore-scale configuration of hydrates. The pressure decrease observed during hydrate formation has a direct relationship with the timely progression of hydrate saturation. Moreover, the resultant hydrate saturation at the completion of the formation increases with confining pressure, while the initial formation pressure of methane gas also has a positive correlation with final hydrate saturation. Higher effective stress conditions on the porous medium contribute to greater hydrate saturation within the pores. 40% and 70% increments of effective stress cause the final hydrate saturation to be raised by 20 % and 30 %, respectively, complementing field observations where higher hydrate saturations were detected at the lowermost layers of the hydrate stability zone.
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