Experimental and numerical analysis of the effect of stress change on methane diffusion in coal matrix

Abstract

ABSTRACT As a clean energy source and disaster gas, coal gas migrates not only under in-situ conditions, but also under stress disturbance due to mining. Although diffusion/desorption is key to gas extraction effects and gas disaster prevention, but the influence study of stress change is insufficient. This study aims to investigate the mechanism of action of stress change to gas diffusion, and a gas transport model considering stress effects is developed. Firstly, the diffusivity of anthracite coal under different stress conditions was measured using an experimental platform of coal-rock diffusivity. When the loaded stress for coal sample reduces 95%, the diffusion coefficient enhances 34%. Subsequently, the determining mechanism and rule of the pore size for gas diffusion in coal matrix were researched theoretically. The results suggest that the relationship between the diffusion flux and the matrix porosity is a power function, with the power exponent between 1 and 1.5, much smaller than the fractures. Experimental and theoretical studies have shown that the methane diffusivity minishes approximately linearly with increased effective stress. Then a methane transport model that involves the impact of stress change is constructed to analyze the effect on gas extraction. Numerical simulations show that the stress relief of a large diameter borehole has a significant effect on the diffusion, generating a larger drop of gas pressure near the borehole compared to the normal borehole. In the presence of regional stress relief, the diffusivity increases due to stress unloading, resulting in larger pressure drop around the borehole and gas drainage amount. The above results provide an important theoretical foundation for calculating gas migration with disturbed stress.