Evolution of hydrate habit and formation properties evolution during hydrate phase transition in fractured-porous medium

• Hydrate synthesis and decomposition occur simultaneously throughout phase transition process. • Hydrate growth habit is not monotonous change in fractured-porous medium sandstone at different stages of phase transition. • Hydrate occurences in fractured-porous medium differs from that in porous matrix system. • The preferential occurence of hydrates in fracture leads to significantly permeability changes. Natural gas hydrate, as an efficient and clean energy resource, are naturally distributed in porous and fractured-porous medium. With the most recent development of advanced micro-scale imaging techniques, hydrate habits evolution, hydrate occurrences, and pore structure evolution as well as seepage characteristics during hydrate phase transition in porous hydrate-bearing sediments have been studied extensively at pore scale. However, there are few studies on gas hydrates in fractured-porous sediment. In this work, xenon hydrate phase transition experiment by excess-gas method is carried out in a fractured sandstone core with in-situ micro computed tomography (micro-CT) scanning to explore the evolution of hydrate habits and physical parameters of the host sediment. The results indicate that hydrate-bearing sediment is a dynamic equilibrium system as hydrate synthesis and decomposition occur simultaneously at each moment of hydrate phase transition induced by pressure change. The hydrate occurrences in fractured hydrate reservoirs include contiguous-sheet, clustered and isolated, which are slightly different from that of porous hydrate formation; and the contiguous-sheet hydrate is the occurrence that dominantly determines the seepage characteristics of fractured hydrate-bearing sediments. In addition, the logic diagram for hydrate growth paths in fractured-porous medium is presented for the first time. These findings are significant for detailed understanding of pore-scale hydrate distribution throughout phase transition process and provide theoretical basis for precise modeling of permeability in host sediments.