Pore-scale flow simulation on the permeability in hydrate-bearing sediments

Abstract

Gas hydrate exploitation involves complex physical and chemical processes. Formation and dissociation of hydrate break the pressure-temperature phase equilibrium and cause change of pore-structure. The permeability of hydrate-bearing sediments plays a significant role for the fluid flow. The permeability change depends on the evolution of pore habit which is affected by hydrate phase behavior. Quantitative analyzing the relationship between pore structure and permeability still keeps a challenging work. To study the microscopic mechanism of hydrate effect on flow characteristic, the pore-scale simulation was carried out considering hydrate morphology and distribution. Five kinds of hydrate occurrence patterns including grain-coating, pore-filling, throat-filling, grain-cementing, and load-bearing in hydrate-bearing sediments are established. The permeability of hydrate-bearing sediments was obtained by solving the Navier-Stokes equation from the direct numerical simulation (DNS). The effect of factors related to the skeleton grain and hydrate distribution on permeability variation was analyzed. The results indicate that the permeability of hydrate-bearing porous media always changes with the variation of hydrate saturation. In most instance, the permeability reduction curve obeys exponential distribution. Permeability decreases as the hydrate saturation increases. The permeability variation is highly sensitive to the grain size, porosity, pore-throat size, hydrate occurrence pattern, and hydrate distribution morphology. From the perspective of the pore-scale, the formation and growth of hydrate cause the reconstruction and complication of pore structure in porous media. The permeability reduction caused by the hydrate formation is pronounced in the porous media with a larger initial pore-throat size. Considering different hydrate occurrence patterns and distribution morphology, the curve shape of the normalized permeability shows different trend. This new workflow has a potential application for analyzing the microscopic flow mechanism of hydrate-bearing sediments.