Facilitating gas hydrate dissociation kinetics and gas migration in clay interlayer by surface cations shielding effects

Abstract

Knowledge of the kinetics of the gas hydrate dissociation in microporous sediments is a critical and fundamental issue for improving the gas recovery from fine-grained natural gas hydrate (NGH) reservoirs. Herein, molecular dynamics (MD) simulations were performed to investigate methane hydrate dissociation in clay sediments. Two initial configurations which comprised liquid bulk and clay mineral phases were adopted, and the later phase comprised two illite sheets and occupied by a hydrate phase with clay cations in and out of the interlayer. The results show that methane nanobubbles form at liquid water/hydrate interface for the cations in system, not like forming spherical cap-shaped nanobubbles on the mineral surface for the cations out system. This phenomenon can be attributed to the cations stably adsorbing on the clay surface, the cation film and ion hydration effect shielding the dissociated methane close to the solid surface. An increasing amount of released methane (approximately three times) diffuses from the pore phase to the bulk phase as driven by the Brownian motion in the cations in the system, as compared with the dissociation in cations out system and sandy pores; however, the diffusion of the released water is the opposite. Additionally, the injection speeds of heat flow against porosity were estimated to maintain a constant dissociation rate in the illite mineral hydrate reservoir. This study provides guidance for the scientific optimization of hydrate reservoirs, and for subsequent gas production from sediments with low permeabilities.