Adsorption kinetics of water vapor in shale and their dependence on in-situ conditions and the composition of reservoirs

Abstract

Adsorption kinetic experiments were conducted to investigate the dynamic diffusion and adsorption behaviors of water vapor in shale gas reservoirs. Experimental results were fitted by a total of seven kinetic models to determine the optimum adsorption kinetic theory of water vapor. Furthermore, the kinetic parameters of water vapor diffusion coefficient and adsorption rate were calculated correspondingly. Then, the controlling and mechanism of relative humidity (RH) and experimental temperature on the adsorption kinetic process were discussed. Besides, two experimental groups of total organic carbon content (TOC) variation and clay content variation shale samples were set to clarify the influence of the major adsorption carriers on the adsorption kinetic behavior. The results indicate that Weber and Morris model (WMM) is the best fitting model, with a goodness of fit (GOF) is much higher than that of the other six models. Overall, diffusion coefficients of water vapor in shale are in the order of magnitude of 10⁻⁴ to 10⁻³ s⁻¹, increase with higher RH (0–0.9), whereas drop sharply for RH > 0.9. The adsorption rate exhibits a tripartite characteristic of rise (RH: 0–0.6), drop (RH: 0.6–0.9), and rise (RH > 0.9) with higher RH. The influence of experimental temperature on diffusion coefficient is complex with a non-monotonic correlation, whereas adsorption rate increases with higher temperature. Additionally, a higher TOC or clay content are conducive to water diffusion and adsorption processes in shale, with clear positive correlations. These results are of significant reference for determining the optimum kinetic theory for water adsorption and diffusion in shale gas reservoirs, and exploring the constrains of reservoir composition and in-situ multiphysics on water accumulation mechanism.