Mechanism of desorption hysteresis in coalbed methane: Insights from microscopic pore properties and adsorption theory

Abstract

Desorption hysteresis, a phenomenon frequently observed in coal–methane desorption experiments, holds significant implications for coalbed methane extraction due to its influence on the volume of methane involved in migration. Yet, the precise mechanism behind desorption hysteresis remains incompletely understood. This study endeavors to elucidate the underlying mechanism of desorption hysteresis by considering pore characteristics and applying adsorption theory. The pore morphology was assessed using the physisorption method, and ad/desorption isotherms were measured using high-pressure volumetric methods, with the desorption hysteresis index (DHI) utilized to quantify the degree of hysteresis. The results pointed to the pivotal role of micropores (<1.5 nm) in the occurrence of desorption hysteresis. Enhanced specific surface area and pore volume, in conjunction with a decrease in the fractal dimension Ds of micropores, were found to correspond to higher DHI values. Conversely, an increase in meso/macropores (>1.5 nm) corresponded to reduced DHI. The Frenkel–Halsey–Hill fractal dimension showed no substantial correlation with DHI. In light of these findings, it is concluded that micropores, acting as gas entrapment spaces, primarily govern desorption hysteresis, while meso/macropores serve as migration pathways with lesser influence. Micropores exhibit robust adsorption capacity, leading methane molecules to preferentially occupy these spaces. However, during desorption, insufficient potential energy impedes the release of methane molecules from the coal surface, causing desorption hysteresis. These insights offer a fresh perspective on understanding the mechanism of desorption hysteresis in coal, which may prove valuable in optimizing methane extraction.