Distribution and reformation characteristics of gas hydrate during hydrate dissociation by thermal stimulation and depressurization methods

Pore-scale distribution and reformation characteristics of gas hydrate in porous sediments can provide invaluable information on macroscale production behaviors. In this work, the X-ray computed tomography (X-ray -CT) has been conducted to detect distribution characteristics of the hydrate-bearing sample during hydrate formation and dissociation. Experimental results indicate that, during hydrate formation, mass and heat transfer can lead to the transformation of grain-attaching (grain-cementing and grain-contacting) hydrate to pore-filling hydrate, as well as the heterogeneous distribution of gas hydrate in pores. During hydrate dissociation, whether the thermal stimulation stage or the depressurization stage, the hydrate decomposition initiates from the ablation of the hydrate-gas interface, and the grain-cementing hydrate remains intact until the hydrate cracks into particles and collapses. In addition, during the thermal stimulation stage, the migration of "memory water" under the equilibrium hydrate formation condition leads to the hydrate reformation, and the hydrate reformation promotes the homogeneous distribution of gas hydrate in pores. During the depressurization stage, gas hydrate is reformed below the hydrate "dissociation front" because of the endothermic process of hydrate dissociation and the pressure-driven fluid flow. The reformed grain-cementing hydrate provides fluid flow channels instead of plugging the pores and throats. However, the shut-in time after the depressurization-induced gas production should not be too long to prevent the pore plugging by the further growth of the reformed hydrate. Additionally, the depressurization process finally leads to the grain migration and may reduce the sediment strength under the loose grain filling condition.