Experimental investigation of thermodynamic and kinetic behaviors from water vapor adsorption on four typical clay minerals

Clay minerals within the shale reservoirs mainly exist in the water-bearing state in situ. The water adsorption behavior and behind mechanisms on the clay minerals are essential for the evaluation of shale gas reservoirs. In this work, combined experiments of water vapor adsorption (WVA) and nitrogen (N2) adsorption were conducted on four typical clay minerals, including montmorillonite (Mnt), illite (Ilt), chlorite (Chl), and kaolinite (Kln). Our results show that the various pore size distributions derived from the WVA and N2 adsorption data indicate that pores with a diameter smaller than 10 nm contribute 54%-82% to the total amount of water uptake. Fitting parameters obtained from five mathematical models, including the Guggenheim-Anderson-de Boer (GAB), Dent, Oswin, Freundlich, and Frenkel-Halsey-Hill (FHH), can illustrate physicochemical characteristics behind the WVA isotherm data. Compared to other models, the FHH model is more optimal to fit and predict the WVA isotherms on the four typical clay minerals. In addition, three kinetic models were utilized to evaluate the WVA kinetic behavior for the four typical clay minerals, including Pseudo-first-order (PFO), Pseudo-second-order (PSO), and Double-exponential (DE) model. Water adsorption process in the clay minerals is a two-stage pro-cess and is mainly controlled by the slow pore-diffusion process, as indicated by the best fitting results from DE model. Finally, the differences of the fitting parameters in the GAB model (the most versatile model on WVA experiments for shale rocks) and adsorption kinetics between clay minerals and shale in the WVA process were discussed, which can provide guidance for investigates of moisture distribution in the shale. Overall, the findings of this work are of great significance to determine the water distribution in the shale reservoirs and shale gas content evaluation in the actual moisture condition.