Experimental Assessment of Combined Contribution of Coal Mass Diffusion and Seepage to Methane Migration under In Situ Stress Conditions

Gas diffusion and seepage characteristics can reflect the transport capacity of coal mass pores and fracture channels and are also important parameters to determine the mechanism of methane migration. Currently, research on the contribution of diffusion and seepage to methane migration is carried out mainly by fixing one parameter and discussing the effect of the other. However, few studies are focused on the dynamic combined effect of diffusion and seepage on methane migration under in situ stress conditions, which is mainly attributed to difficulties in obtaining the parameters of diffusion characteristics of coal mass under stress. This paper presents a study on the dynamic combined effect of coal mass diffusion and seepage on methane migration under different in situ stress conditions. First, the physical structure model of coal mass under high and low stresses was constructed based on the variations of coal mass porosity, permeability, and computed tomography under different stresses. Meanwhile, based on the methane desorption and diffusion curves of coal mass under different stresses, the diffusion parameters of coal mass were obtained according to the corresponding diffusion model. Furthermore, the coupled multifield model was used to calculate the methane migration in coal mass during the process of borehole extraction with different stresses. The results suggest that the difficulty in methane extraction is directly proportional to the stress, but a huge discrepancy exists in methane migration under high and low stresses principally due to the change of dominant factors in methane migration of coal mass under different stresses. Additionally, given the essential factors controlling the overall methane migration of coal mass under high and low stresses, a joint technical method for promoting methane flow rate under multicoal seam conditions in the same mining area was proposed, i.e., hydraulic fracturing and sand injection (increasing permeability) in coal seams under low stress and simultaneous application of hydraulic fracturing and sand injection and multistage cavitation pressure relief (increasing diffusion) in coal seams under high stress. The research results obtained in this paper provide theoretical support for understanding the methane migration mechanism in coal seams under in situ stress and stimulated gas production.