Pore-Scale Visualization of CH4 Gas Hydrate Dissociation under Permafrost Conditions

In this study, we investigate the effectiveness of combined pressure depletion and thermal stimulation to produce CH4 gas from CH4 gas hydrates at permafrost conditions. CH4 gas hydrate phase transitions were visualized using a high-pressure, water-wet, silicon-wafer micromodel with a pore network of actual sandstone rock. A set of eight experiments was performed in which CH4 gas hydrates were formed at a constant pressure between 60 and 85 bar and constant temperature between 0 and 4 degrees C. CH4 gas hydrates were then dissociated at a constant system temperature between -3 and -2 degrees C by pressure depletion to study the effect of hydrate and fluid saturation on the hydrate dissociation rate, self-preservation, and risk of ice formation. The hydrate dissociation rate and associated fluid flow were heavily affected by the hydrate saturation and initial hydrate distribution in the pore space. Specifically, the rate of CH4 gas production was low at T < 0 degrees C due to rapid formation of ice and secondary hydrate from the unfrozen liquid water that was liberated from the initial hydrate dissociation. The liberated CH4 gas was therefore immobilized and trapped and could not be produced without thermal stimulation. Thermal stimulation effectively melted the metastable hydrate and surrounding ice cover and thereby enhanced the CH4 gas production. Visual observation showed that self-preserved hydrates in the metastable state dissociated before ice at T < 0 degrees C, providing experimental evidence of recently discovered CH4 leaking from gas hydrates in permafrost-affected sediments. The reported results are important in order to understand and predict the influence of global warming on the dissociation of permafrost-affected gas hydrates.

Automated summary

The study investigates the effectiveness of combined pressure depletion and thermal stimulation to produce CH4 gas from CH4 gas hydrates under permafrost conditions. It was found that the rate of CH4 gas production is affected by hydrate saturation and initial distribution, and that thermal stimulation can enhance gas production by melting metastable hydrate and surrounding ice cover. These results provide important insights into understanding and predicting the impact of global warming on the dissociation of permafrost-affected gas hydrates.