Time-Dependent Model and the Evolution Mechanism of Dynamic Diffusion Coefficient for Gas Desorption in Gas-Containing Coal

Coalbed methane drainage has important significance for providing clean energy and reducing the risk of coal and gas outburst. Coalbed methane mainly exists in the adsorbed state in coal seam and diffuses from the pore network to the drainage pipelines. The diffusion coefficient is of strategic importance for the accurate prediction of the coalbed methane drainage process, while the currently reported dynamic diffusion coefficient models were found to lack systematic theoretical proof. Therefore, this study focuses on the dynamic diffusion coefficient model, which comprehensively adopts theoretical analysis, numerical calculation, and experimental verification. First, an evolution mechanism was proposed according to the fractal theory, the surface physical chemistry theory, and the diffusion theory in porous media. Then, a time-dependent model of dynamic diffusion coefficient was deduced based on the evolution mechanism. The numerical computation and experimental verification were then carried out to validate the established model. Results showed that the diffusion coefficient of gas desorption in gas-containing coal exhibited dynamic characteristics. The diffusion coefficient was negatively correlated with pore fractal dimension and gas desorption effect but positively correlated with coal matrix adsorption capacity. The pore structure plays a leading role in the dynamic characteristic of diffusion coefficient, followed by the adsorption capacity of the coal matrix, and the gas desorption effect was the weakest. The calculated results according to the proposed time-dependent model agreed well with the experimental data, with correlation coefficients above 96.0%. This research will provide a theory foundation for the in-depth understanding of the gas diffusion mechanism in coal.