Experimental Characterization of Hysteresis in Cyclic Methane Hydrate Formation and Dissociation

초록

This paper experimentally analyzes hysteresis of cyclic methane-hydrate formation and dissociation using X-ray computed tomography (CT) images in unconsolidated sediments, and compares depressurization-induced production performances in irreducible gas saturation, i.e., with the assumption of no free gas in hydrate-bearing pores, with those under irreducible water saturation. The hysteresis of gas hydrate formation after dissociation highlights the stratigraphic variability of hydrate accumulation and the uncertainty in pore-scale saturation distributions. When methane gas is supplied to water-saturated pores to form hydrate, repeated formation and dissociation lead to elevated hydrate saturation at specific locations, forming roughly spherical features, as seen in CT images. During depressurization-based dissociation, free gas could not be distinguished from hydrate gas under irreducible water conditions. However, under irreducible gas conditions, water is produced before the dissociated hydrate gas, and subsequently, a period arises in which only gas is produced after water production ceases. This study experimentally demonstrates that, when hydrate formation and dissociation cycles are repeated, hydrates can become concentrated in specific regions of the sediments. Once the influence of free gas is minimized, dissociated hydrate gas and formation water interact to affect production behavior.

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