Pore Structure and Seepage Characteristics of Hydrate-Bearing Sediments

Permeability is a crucial parameter for gas hydrate-bearing sediments and plays a significant role in the exploitation of natural gas hydrates in sediments. Therefore, studying the seepage characteristics of hydrate-bearing sediments is of great significance. In this study, xenon gas hydrates were synthesized in quartz sand, and X-ray computed tomography (CT) was used to investigate the effects of the pore structure during hydrate formation and dissociation on the effective permeability and the relative permeability. The results indicate that hydrate particles form and grow in large pores and throats first during the hydrate formation, which is the primary cause of heterogeneity in the initial stage of hydrate formation. With the continuous formation of hydrate, other pores are also filled with hydrate, reducing the hydrate heterogeneity. During the dissociation process of hydrate, the heterogeneity can be attributed to layer-by-layer dissociation of hydrate, which does not affect the power function relationship between the effective permeability of hydrate-bearing sediments and hydrate saturation. The pore network model (PNM) was used to calculate the effective permeability, fractal dimension, tortuosity, and other relevant parameters such as pore and throat features of hydrate-bearing sediments for understanding the effect of hydrates on the seepage behavior of hydrate-bearing sediments at the pore scale. The gas–water two-phase flow in the pores was simulated by PNM, and the relative permeability of hydrate-bearing sediments was investigated. The major factor that affects the relative permeability is the dimension of flow channels. It is found that a smaller pore-throat radius and higher capillary pressures lower the water-phase relative permeability but do not affect the gas-phase relative permeability. Additionally, heterogeneity has a significant effect on the relative permeability. These results are the foundation for the efficient and economical exploitation of marine gas hydrate.