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

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.

KEYWORDS:

• Diffusion coefficient
• gas diffusion
• gas drainage
• Methane
• stress

Acknowledgements

The authors are grateful for the support from State Key Laboratory of Coal Mine Safety Technology and Henan Polytechnic University.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work is supported by the financial support from the Liaoning Provincial Natural Science Foundation research [2021-KF-23-09], Natural Science Foundation of Henan Province [232300420076], National Natural Science Foundation of China [52174172, 52074105, 52174175 and 52274192], Natural Science Foundation of Jiangsu Province [No. BK20190931], the Safety Discipline “double first-class” Creation Project of Henan Polytechnic University [AQ20230705] and China Postdoctoral Science Foundation [No. 2019M652023].

Notes on contributors

Weiwei Su

Weiwei Su is a researcher at China Coal Research Institute, Beijing, China. His research focuses on coal mine safety.

Fenghua An

Fenghua An is an associate professor at China University of Mining and Technology, Xuzhou, China. His research focuses on coal mine safety management.

Yanning Ding

Yanning Ding is a graduate student at College of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, China. Her research focuses on multi-field coupled gas seepage in deep coal mines.

Yujin Qin

Yujin Qin is a researcher at China Coal Research Institute, Beijing, China. His research focuses on coal mine safety.

Erlei Su

Erlei Su is an associate professor at Henan Polytechnic University, Jiaozuo, China. His research focuses on multi-field coupled gas seepage in deep coal mines.

Haidong Chen

Haidong Chen is an associate professor at Henan Polytechnic University, Jiaozuo, China. His research focuses on multi-field coupled gas seepage in deep coal mines.

Xiaolei Zhang

Xiaolei Zhang is an associate professor at Changzhou University, Changzhou, China. His research focuses on coal mine safety.