An investigation on the permeability of hydrate-bearing sediments based on pore-scale CFD simulation

Abstract

The distribution pattern of CH4 hydrate in hydrate-bearing sediments (HBS) significantly impacts the permeability, which is one of the critical flow properties controlling the fluid production in the exploitation of natural gas hydrate (NGH) reservoirs. Thus, it warrants investigation on the key influencing factors, i.e., (a) hydrate saturation; (b) hydrate distribution patterns; (c) sediment particle size on the associated permeability of HBS especially at pore-scale. In this study, a series of two-dimensional geometric models were constructed that best represent the commonly-observed hydrate distribution patterns. The pore-scale flow behavior of the resulting models was simulated through computational fluid dynamics. The simulation results suggest that the reduction of permeability is dependent on the distribution pattern of hydrate. The permeability of grain-coating HBS decreases by a maximum of five orders of magnitude at SH = 0.3. Additionally, increasing SH results in new pore throats and induces clogging in fluid flow that controls permeability reduction. A strong log-log linear relationship was identified between sediment particle size and the simulated permeability. A modified empirical model for the estimation of HBS permeability was proposed considering both tortuosity and effective porosity. Moreover, a heterogeneous sediment particle distribution model was constructed for the first time that accurately described the particle size distribution of a hydrate-bearing core sample recovered from Japan Nankai Trough. Compared to the homogeneous model, a heterogeneous model results in a more substantial reduction of permeability with SH and better represents the permeability of the HBS core sample. The finding of this study can be significant in understanding the pore-scale flow behavior in HBS and are meaningful for the design of safe and effective production methods in the exploitation of NGH reservoirs.