Pore-scale study of multicomponent multiphase heat and mass transfer mechanism during methane hydrate dissociation process

Abstract

The understanding of the multiple physicochemical and thermal processes in methane hydrate dissociation is fundamental to accurately predict the methane recovery with the hydrate depressurization-induced technique. Pore-scale investigation can enlighten the accurate REV (Representative Elementary Volume) scale simulation models to take heat transfer and mass transfer limitation into account for better recovery forecast. In this work, a pore-scale numerical model based on the lattice Boltzmann method was developed to couple the multicomponent multiphase flow, heat and mass transfer, heterogeneous reaction, and solid structure evolution during the dissociation of gas hydrates. Numerical simulation of the methane hydrate dissociation in an idealized square cavity and a realistic porous structure was studied to understand the influence of mass transport, water saturation, and heat transfer. This study distinguished the hydrate dissociation characteristics in the kinetics-limited and diffusion-limited regimes. The mass-transfer-limitation in the diffusion-limited regime indicated that the enriched methane on the hydrate-water interface decreased the hydrate dissociation rate with the increasing water saturation. The methane concentration gradient across the water layer varied with the water layer thickness and contributed to the decomposition nonuniformity. The decreasing temperature gradually reduced methane recovery efficiency and yield during the endothermic decomposition due to decreasing the equilibrium pressure and intrinsic kinetic rate. Some implications were addressed from the pore-scale understanding of the transfer and kinetics characteristics. The local thermal equilibrium at the solid–fluid interface implied that the one-temperature equation is sufficient to model the heat transfer at the REV scale. The present study proposed a corrected reaction model based on the equivalent water layer thickness and introduced the implementation procedure in the REV-solution to account for the mass-transfer-limitation on the hydrate dissociation rate.