STUDY ON ADSORPTION-DESORPTION CHARACTERISTICS AND MECHANISM OF GASEOUS WATER IN SHALE

Abstract

Studying the behavior of gaseous water adsorption and desorption in shale has significant theoretical and practical importance for exploring the micro-scale distribution of water and gas in shale reservoirs and deepening our insight into the mechanisms behind shale gas accumulation. The samples in this paper are shale samples collected from the Ningtiaota shallow coal mine in Shenmu County, Yulin City, Shaanxi Province, China. Based on scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and dynamic vapor sorption (DVS), and gas adsorption experiments combined with gas adsorption models, the study investigates the adsorption characteristics of gas-phase water on shale. In under 20-40°C conditions, isotherms and kinetic curves of gaseous water adsorption-desorption were tested in shale samples with a particle size of 60-80 mesh. Furthermore, the hysteresis behavior of gaseous water adsorption-desorption in shale was analyzed. Three different isotherm models and four adsorption kinetic models were used to explain how gaseous water adsorbed in shale. The results indicate that the Dent and bi-Langmuir kinetic models were the most suitable models for isotherm adsorption and adsorption kinetics, respectively. This suggests that the adsorption of gaseous water in shale follows a physical adsorption process from monolayer adsorption to multilayer adsorption and then to capillary condensation. Furthermore, this process is a first-order, two-stage kinetic process controlled by internal pore diffusion. The uncompleted adsorption-desorption hysteresis behavior is chiefly attributed to the difficulty of clay-bound water and cation-bound water to release at the experimental temperature. In addition, this study analyzed thermodynamic parameters, including ΔG, ΔH, and ΔS, and found that the adsorption behavior of gaseous water in shale is a spontaneous, exothermic process with a decrease in entropy. The study's conclusions serve as a foundation for reference for realizing the distribution patterns of gas and water in the Ningtiaota shale, as well as the flow behavior of shale gas.