Coalbed gas production contributes to energy diversification and effectively mitigates the risk of mine gas outbursts. However, the complexity and nonlinear characteristics of multifield coupled gas migration in deep coal seams pose significant challenges that traditional prediction and control methods struggle to address. This paper explores the effects of coupled multi-physics fields on gas migration and reviews a numerical simulation method that integrates fractal theory with discrete fracture network modeling, aiming to overcome the limitations of conventional models in capturing the interactions among seepage, heat transfer, stress distribution, and gas adsorption/desorption. The study highlights the interactions between fractures and pores, as well as the coupling effects between fluids flow, heat transfer, and solid mechanics. It further presents a more accurate prediction method to enhance the simulation accuracy of gas migration in deep coal seams.

The structure of fractures and pores has a dominant impact on the heat transfer-seepage-deformation process of a coal seam. Previous models have primarily used the cubic permeability model to characterize coal seam permeability properties. In this study, we developed a new multi-field coupling model, which includes fracture and pore structure, coal seam temperature, effective stress and gas seepage. Two major extraction scenarios were simulated: the unconstrained plane strain state and the uniaxial plane strain state. In addition, two microstructural parameters were applied to characterize coal permeability: the maximum fracture length and the fractal dimension for the fracture. The results show that the fractal seepage model provides a more realistic and reliable characterization of resource migration and extraction processes in unconventional reservoirs than the cubic-law permeability model. Compared with the cubic-law permeability model, the permeability calculated by the model proposed in this paper changes about 17.09%-91.56%. Furthermore, coal seam permeability is proportional to the maximum fracture length and the fractal dimension for the fracture. The permeability changes about 17.09% and 17.18% with the different fractal dimension, and about 87.17% and 91.56% with the different maximum fracture length. However, the fractal dimension and coal seam permeability are inversely proportional to seam temperature.