Permeability characteristics of hydrate-bearing sediments during hydrate formation and depressurization decomposition processes

Abstract

The permeability of hydrate-bearing sediment (HBS) reservoirs governs fluid migration and directly influences the efficiency of gas hydrate extraction. In this study, HBS samples with varying hydrate saturations were prepared using two distinct methods: direct hydrate formation (Method A) and depressurization-induced decomposition (Method B). Permeability measurements were conducted under effective confining pressures ranging from 1.5 to 4.5 MPa. Hydrate growth modes during formation and decomposition were characterized using the Kozeny grain model (KGM), enabling mechanistic analysis of permeability evolution. Key findings reveals that the permeability decreases with increasing hydrate saturation and effective confining pressure. Under an effective stress of 4.5 MPa, permeability values decline to 3.76 mD (at S_h = 29.30 %), 9.44 mD (at S_h = 19.66 %), and 13.3 mD (at S_h = 11.27 %), respectively, closely matching field-scale observations. Notably, HBS samples subjected to hydrate formation and subsequent decomposition exhibit higher permeability than those without hydrate decomposition, highlighting the irreversible impact of hydrate dynamics on pore structure. Method B, simulating depressurization mining conditions, induces distinct hydrate growth modes, which critically alter permeability behavior. This method better replicates field-scale hydrate dissociation processes, demonstrating its superiority in predicting reservoir responses during gas extraction.