Abstract
Gas hydrates are ice-like crystalline solids that form in marine sediments or permafrost layers. The inhomogeneous distribution of the hydrate will form different hydrate patterns such as grain-coating and pore-filling hydrate, which will further cause permeability anisotropy in different directions. In this study, X-ray computed tomography (CT) test and image analysis by AVIZO were used to investigate the permeability anisotropy of hydrate-bearing sands. The image analysis revealed the following results: (1) For both hydrate patterns, the hydrate formation induced the largest permeability decrease in the y direction and the smallest one in the z direction. (2) The degree of anisotropy of pores after the pore-filling hydrate formation is larger than that after the grain-coating hydrate formation. A decrease in the pore anisotropy ratio will induce a significant increase in permeability anisotropy. (3) Although the increase in the pore shape factor ratio after the pore-filling hydrate formation is smaller than that after the grain-coating hydrate formation, the permeability anisotropy increase is much larger. (4) The tortuosity increase is the largest in the y direction and the smallest in the 2 direction, which is consistent with the permeability variations. Pore-filling hydrate formation induces a larger variation in the tortuosity ratio, which causes a higher permeability anisotropy.
NSTL