Modeling and experimental study on methane diffusivity in coal mass under in-situ high stress conditions: A better understanding of gas extraction

Abstract

Gas desorption and diffusion in pore is not only an important behavior to control the law of gas migration, but also an important theoretical basis for the proposal and optimization of extraction technology. At present, researches on gas diffusion in coal were mainly conducted on granular coal under the stress-free condition. However, on-site gas desorption and diffusion in coal seams occur in coal mass under the in-situ stress condition. In particular, high stress exerts a notable effect on gas diffusion in the deep mining stage. Therefore, experimental study and theoretical model establishment concerning the diffusion characteristics of coal mass under high stress require further investigation. In this work, first, with respect to the desorption and diffusion characteristics of coal mass under high stress, desorption and diffusion experiments were performed on coal mass under different high stress conditions with the aid of a self-developed experimental device, and the corresponding gas desorption laws were obtained. In addition, the law of coal mass permeability evolution under different stresses was obtained through the transient measurement method. Moreover, a coal mass diffusion model which conformed to the characteristics of high stress was established based on reasonable assumptions, a new physical structure model of coal under high stress and the relationship between diffusion length and stress. Moreover, this model, according to the laws of gas desorption under different conditions, it is found that the diffusion coefficient increases with the rise of adsorption equilibrium pressure, and stress is significantly proportional to the diffusion coefficient. Besides, with reference to the relationship between diffusion coefficient and stress value, the fundamental parameter that affects the desorption and diffusion amount, i.e., effective diffusion coefficient, was analyzed. Based on the variations of the effective diffusion coefficient with stress, the mechanism of how the classical pressure relief mining technology to improve gas extraction efficiency was further elaborated. The research results provide a more comprehensive theoretical support for the application of the pressure relief mining technology in deep high-stress coal seams.