Dynamic Characterization of Pore Structures in Hydrate-Bearing Sediments During Hydrate Phase Transition

Abstract Natural gas hydrate widely distributed in marine sediments and permafrost has brought great attention due to its large reserves. Unlike conventional reservoirs, the effective pore structures vary from time and space due to hydrate dissociation and secondary formation in the development, which produces significant impacts on gas flow and production. Therefore, figuring out the evolution of dynamic pore structures is of great importance for the efficient development of hydrate deposits. In this work, excess-water hydrate formation method was combined with micro-computed tomography to study hydrate transition effects on the evolution of dynamic pore structures. Gas state equation and chemical reaction dynamics were combined for separating the representative 3D images at different stages of hydrate formation into four phases, which are respectively hydrate, water, gas and solid skeleton. Hydrate pore habit evolution, formation characteristics, spatial distribution heterogeneity and its effect on the effective porosity variation were studied in detail. Afterwards, a modified maximal ball method was employed to extract hydrate-bearing pore networks at different stages of hydrate phase transition. Hydrate phase transition effects on the effective pore and throat radii distributions, pore and throat cross-sections, throat lengths and distance among connected pore bodies, as well as pore topology were further investigated based on the extracted networks. Results show that hydrate pore habit varies in porous media during hydrate formation with the main pore habit of pore filling mode. Hydrate spatial distribution exhibits some heterogeneity, causing diverse hydrate saturation at different layers during hydrate phase transition. Hydrate disrupted pore integrity to some extent, resulting in more extracted pore bodies and throats with increased hydrate saturation. In addition, hydrate phase transition reduces pore-throat radii and distribution regularity to different degrees, and results in more irregular pore-throat morphology, decrease of throat length and distance among connected pore bodies as well as poorer connectivity at the same time. This study provides a novel insight in better understanding the evolution of dynamic pore structures and lays a good foundation for the effective development of natural gas hydrate deposits.