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Equilibrium and Kinetics of Water Vapor Adsorption on Shale

Water vapor adsorption and desorption isotherms and kinetics studies on three Sichuan Basin shale samples were performed at 298 K by an accurate gravimetric method. The adsorption equilibrium data were fitted using both Dent model and Modified Dent model to estimate the adsorption characteristic of water on the primary and secondary sites. The primary site adsorption is restricted to a monolayer while the secondary site adsorption is associated with multilayer sorption. A positive correlation was found between clay mineral content and monolayer sorption content. The isosteric heats of sorption of water were determined from the equilibrium data and they decreased with the increase of adsorption amount. The adsorption/desorption hysteresis were studied with the pore structure. The kinetics of water vapor adsorption was studied with the unipore model and linear driving force mass transfer (LDF) model. The effective diffusivity and kinetic rate constant varied with the increase of relative pressure, which suggested diffusion of water vapor on shale corresponding to a combination of adsorption on primary sites, adsorption on secondary sites, formation of water clusters, and capillary condensation.