Stepwise Diffusion Characteristics in Coal Mass under In Situ High-Stress Conditions

Abstract

The study of methane diffusion within coal is a critical theoretical foundation for coalbed methane extraction technologies. Currently, the development of coalbed methane resources is significantly constrained by the high-stress and low-permeability characteristics of deep coal seams. This necessitates an urgent focus on investigating the diffusion behavior of methane within the deep coal seams. This study investigates the stepwise diffusion characteristics of methane in coal under high-stress conditions. A series of experiments were carried out using a specially designed experimental apparatus aimed at this purpose to investigate the behavior of methane diffusion and to determine the dominant diffusion patterns in high-stress conditions. A diffusion model was constructed to calculate the diffusion coefficients of coal under high-stress conditions, which were then employed to perform numerical simulations of methane extraction based on the identified stepwise diffusion patterns. This analysis enabled the exploration of more economical and efficient methods of methane extraction. The results indicate that the adoption of a multistep diffusion pathway can effectively enhance methane desorption. When the diffusion pathways are uniform, the amount of desorption in the higher-level diffusion stage surpasses that in the lower-level diffusion stage. Additionally, the methane diffusion coefficient increases with a greater diffusion pressure gradient; however, simply increasing the diffusion pressure gradient does not necessarily lead to a significant increase in the desorption. Based on these findings, an intelligent extraction method was proposed, which enhances methane extraction efficiency while remaining economically viable and effective. The outcomes of this research provide theoretical support for understanding the mechanisms underlying methane extraction in high-stress coal seams.