Modeling and analysis on thermal stimulation of gas transport considering hydrodynamics and geomechanics coupling in coal

Thermal stimulation can activate adsorbed gas, enhancing cross-scale gas transport (CSGT) in coal. Nevertheless, the thermal stimulation mechanism of CSGT remains unclear. In this study, He and CH4 desorption experiments under different temperatures were conducted, and propagation characteristics of longitudinal and transverse waves of gas-containing coal before and after thermal stimulation were tested. Based on the results, the intrinsic relationship between gas diffusion parameters and thermal stimulation temperatures was elucidated, and the dynamic mechanical responses of gas-containing coal induced by thermal stimulation were analyzed. It has been revealed that more gas of surface diffusion state transfers into the gas of bulk diffusion state as the thermal stimulation temperature rises, and the gas-containing coal occurs three phenomena, i.e., inward expansion-fracture compression, outward expansion-fracture enlargement, and outward expansion-coal damage. Additionally, a CSGT model was constructed based on the micro-scale fractal geometry of coal, and how changes in pore-fracture scale, matrix gas pressure, and adsorbed gas content influence the multi-modal gas diffusion conductivity under varying temperatures were analyzed. The results suggest that the gas emission rate is sequentially controlled by the free gas diffusion, the desorption of mono-layer adsorbed gas in diffusion pores, and the surface diffusion of adsorbed gas in filling pores. Ultimately, the mechanical response of gas-containing coal induced by thermal stimulation was analyzed considering hydrodynamics and geomechanics coupling, fining that the critical transformation and instability temperature have a significant influence on stimulation mechanisms. The research can provide a theoretical reference for thermal stimulation of gas drainage in deep coal seams.