Visualization of CH4 Hydrate Dissociation Under Permafrost Temperature Conditions Using High-Pressure Micromodel

Methane (CH4) gas hydrate formation, dissociation, and stability in permafrost sediments are essential to model these systems concerning global warming and in schemes of CH4 recovery and/or carbon dioxide (CO2) storage. It is known
that CH4 hydrate is thermodynamically less stable than CO2 hydrate due to the lower activation energy of the decomposition. However, recent studies show that CH4 hydrate’s dissociation slows down in subzero temperature due to the self-preservation mechanism. Thus, a fundamental understanding of CH4 hydrate distribution, dissociation mechanism, and self-preservation in sediments at the pore-scale level, is essential to optimize the CH4 gas production method from permafrost-affected hydrate reservoirs. In this study, CH4 hydrate dissociation was visualized using a high-pressure, water-wet, silicon-wafer based micromodel with pore network of actual sandstone rock. A total of nine runs were  performed, and CH4 hydrate was formed between 60-85 bar, and between 273.15 K-275 K. CH4 hydrate was dissociated between 270-275K by pressure depletion to evaluate the effect of hydrate and fluid saturation on dissociation rate, self-preservation, and risk of hydrate reformation. Below 273.15K, the CH4 gas production was limited due to rapid formation of ice from liquid water liberated from initial hydrate dissociation. The liberated CH4 gas was immobilized and trapped by the formed ice. Consequently, we demonstrate the ineffectiveness of depressurizing CH4 hydrate without thermal stimulation. The results highlight the importance of initial hydrate/ice/gas saturations and free gas availability in characterizing hydrate dissociation patterns.