Experimental investigation on the dynamics of water vapor desorption–migration–adsorption in shale saturated with methane of various pressures

Water and methane coexist in shale reservoirs. Water desorption–adsorption dynamics at methane pressure is highly important for energy extraction and environmental protection. Water vapor desorption–adsorption experiments in shales saturated with CH₄ of different pressures were performed. A dynamic adsorption model for describing gas adsorption was developed. A method for calculating the effective water vapor permeability based on desorption–adsorption data was proposed. The experimental and computational results suggest that the water vapor permeability during desorption was greater than that during adsorption, and both decreased over time at different rates. Based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, interfacial forces were used to describe the interaction between water vapor and shale. The interfacial force first decreased and then increased with increasing water film thickness, which was great for pores with small sizes. The interfacial force magnitude characterized the adsorption strength and affected the desorption and adsorption rates. The early, middle, and late adsorption stages were controlled by gas flow, gas flow/interfacial force, and interfacial force, respectively. The influence of interfacial forces on the decrease in desorption rate with time was more pronounced than that of adsorption. The interfacial force effect on desorption increased, and that on adsorption decreased with increasing methane pressure. The water vapor permeability at desorption–adsorption equilibrium increased with increasing methane pressure.