Pore-Scale Investigation on Dynamic Permeability Characterization of Hydrate-Bearing Sediments

Abstract Natural gas hydrates widely distributed in marine sediments and permafrost have attracted global attention as great potential energy resources. As an important parameter that critically affects the efficiency of gas hydrate production, reported permeability values are observed to be widely scattered due to multiple factors such as hydrate saturation, hydrate pore habit and so on, bringing a great challenge for accurate prediction. In this study, an unstructured hydrate-bearing network model with anisotropy is firstly constructed. Afterwards, a pore-scale flow network model considering hydrate pore shrinking habits is developed. Dynamic permeability evolution is then investigated under different hydrate saturations, pore connectivity, and pore throat size distribution conditions. Results show that the existence of hydrate changes the structure of the hydrate-bearing pore network, which reduces the pore and throat radius distribution as well as the distribution law. Since hydrate preferentially nucleates in large elements of the pore network, the deviation degree of the pore body radius is more significant than that of the throat radius. Dynamic permeability evolution with increased hydrate saturation shows a nonlinear decline trend due to the nonlinearity between pore structures and hydrate saturation variations. Owing to the two competing factors of pore structure variation and coordination number effects on pore-scale flow, dimensionless dynamic permeability under different pore connectivity shows a minor discrepancy. Pore throat size distribution possesses a significant impact on dynamic permeability evolution. Under the premise of fixed size distribution, number, and connectivity of the pore bodies, the smaller the initial throat radius, the more influence hydrate formation and dissociation on the network structure variation, resulting in smaller dimensionless permeability.