Pore-scale investigation of permeability evolution during hydrate formation using a pore network model based on X-ray CT

Permeability in hydrate-bearing sediment critically governs fluid flow and determines hydrate nucleation, growth, and distribution, making it important to characterize the evolution of permeability with respect to water and gas during hydrate formation. This study uses CT scanning of krypton hydrate formation in silica sand using the excess gas method, together with a pore network model, to investigate variations in hydrate morphology and associated permeation. The results show that during hydrate formation, the growth habit mainly varies from grain-coating to patchy with an increase in hydrate saturation; however, at relatively low saturation, hydrate preferentially grows as a grain-cementing habit. Theoretical models of a capillary tube and of a Kozeny grain both predict that permeability in grain-coating hydrates will be higher than in patchy hydrates. We thus recognise a correlation of permeability and hydrate saturation for multiple growth habits that is of interest for gas production from hydrate reservoirs. Under lower water saturation, there is a decrease in relative permeability to water but an increase in relative permeability to gas, due to the reduction in pore shape factors. Conversely, an upward shift in relative permeability to water and a downward shift in relative permeability to gas is found with hydrate formation under higher water saturation, owing to the Jamin effect. Steeper curves of relative permeability to gas and water with increasing hydrate saturation resulted in a reduction of the co-cementation zone for water and gas, even if or though no gas migrates. The results suggest that, due to the low relative permeability of gas, higher water saturation sediments result in an excess water yield accompanied by low gas production that is not desirable for natural gas hydrate production. Therefore, improving the permeability and weakening the Jamin effect are critical for gas production in marine hydrate reservoirs, especially in low permeability sediments.