Numerical Study on Seepage Characteristics of Hydrate-Bearing Sediments: A Pore-Scale Perspective

Abstract

Natural gas hydrate is one of the most promising energy resources. The permeability of hydrate-bearing sediments (HBS) is a critical parameter that affects gas production efficiency. In this study, by carrying out two-dimensional seepage models of HBSs with uniformly round particles, the effect of factors including pore habit, hydrate saturation, and particle size on the permeability of HBS is investigated. Two pore habits, seven hydrate saturations, and three matrix particle sizes were considered to complete the numerical simulation. The results show that the permeability of HBSs decreases with increasing hydrate saturation and decreasing matrix particle size because of the formation of smaller flow channels. The pore-filling HBSs present a considerable reduction of permeability at low hydrate saturation than that of grain-coating HBSs. This can be attributed to the fact that hydrates formed in the center of pores restrict fluid flow more than those on the surface of particles. As a result, the permeability assumes a zero value at hydrate saturation close to 68%, though it is supposedly only possible at 100%. This discrepancy may account for the variations of pore spaces between the two-dimension and three-dimension, the ignorance of heterogeneity in HBSs, and the shapes of hydrates and soils. For coarse-grained HBSs, models developed to account for features as pore geometry can capture the primary trend of HBSs permeability and reveal its seepage characteristics.