Permeability Analysis of Hydrate-Bearing Sediments during the Hydrate Formation Process

Permeability is a critical factor for controlling water and gas flow and influencing hydrate growth and distribution in hydrate-bearing sediments (HBS). In this study, we used in situ microfocus X-ray computed tomography to obtain three-dimensional information on microstructures of xenon hydrates in sedimentary matrices (Fujian sand) under excess-gas conditions. It was found that the hydrates in the specimen develop cementing and grain-coating pore habits, and the predominant hydrate pore habit is grain coating. We combined a pore network model simulation to analyze the effect of hydrate formation on the permeability in HBS. The results indicate that the water relative permeability decreases during the hydrate formation process. Gas relative permeability decreases at high water saturation (>0.20). The reason is that hydrate formation causes a decline in the pore space size and connectivity, and then, the gas and water flow are inhibited. Moreover, hydrate formation in HBS has an impact on its permeability anisotropy. Effective permeability decreases in x, y, and z directions with hydrate formation. The predominant hydrate pore habit in the pore space has a small influence on the evolution of the preferential flow direction of the water and gas. Predicting the dynamic permeability of the hydrate formation process would improve pore-scale water and gas flow analysis in HBS.