Effect of heterogeneous hydrate distribution on permeability: Pore filling hydrate presents high permeability

To explore natural gas hydrate in sediment, the permeability plays a vital role in estimating fluid flows and production potential. Although natural gas hydrate exhibits a heterogeneous distribution with different pore habits, the combined effects of heterogeneity and pore characteristics have not been studied well. In this study, a two-dimensional homogeneous porous medium was constructed to simulate single-phase fluid flow with different sizes and number of grain coating (GC) or pore filling (PF) hydrate particles. The results showed that under the same saturation of PF hydrate, heterogeneous hydrate distributions exhibited higher permeability than homogeneous distributions. This higher permeability can be attributed to the critical pore radius of the preferential flow paths. The heterogeneous distribution left some pores unoccupied, not reducing the critical pore radius. In contrast, for GC hydrate, heterogeneous hydrate distributions exhibited lower permeability than homogeneous hydrate distributions. Despite the presence of preferential flow paths unoccupied by gas hydrate, the continuous blockage induced by GC hydrate decreased the permeability. Furthermore, we analyzed the tortuosity and heterogeneity degree of the hydrate-bearing sediment. GC hydrate exhibited higher tortuosity under heterogeneous distributions. A modified model based on a classic theory showed good correlations between permeability and tortuosity. The heterogeneity degree exhibited a negative correlation with both hydrate saturation and tortuosity. This study shows the importance of preferential flow paths in the permeability, explaining the high permeability characteristics of natural hydrate-bearing sediment.