Permeability anisotropy of methane hydrate-bearing sands: Insights from CT scanning and pore network modelling

Gas hydrate formation in porous media is random and inhomogeneous, which causes the permeability to be anisotropic. The absolute and relative anisotropic permeabilities of hydrate-bearing sands at different hydrate saturations were investigated using a CT scanning technique combined with a pore network modelling method. The absolute permeability is directly calculated based on the extracted pore network. The equivalent pore network extracted by a maximum ball (MB) method is used to calculate the relative permeability. The anisotropic permeability coefficient beta(2) becomes increasingly smaller than beta(1) with increasing hydrate saturation, which indicates that the absolute permeability decrease in the y direction is more significant than that in the x direction. The capillary pressure increases with increasing hydrate saturation. The relative permeability in the y direction is highest at 14% hydrate saturation. However, it becomes lowest at 51% hydrate saturation. The anisotropy evolution of the relative permeability is consistent with the evolution of the absolute permeability. Image analysis shows that more hydrates accumulate and distribute in the plane normal to y direction, which will induce the pores to be more irregular and increase the pore anisotropy in this direction. The pore anisotropy will cause the permeability to be anisotropic in different directions.