Abstract: To address the challenge of low gas drainage efficiency via in-seam boreholes in deep,soft,and low-permeability coal seams,and to reveal the complex gas migration behavior,this study establishes a fluid-solid coupling numerical model based on the dual-porosity / dual-permeability medium assumption. This model comprehensively accounts for the Klinkenberg effect, coal matrix shrinkage,and a time-dependent dynamic diffusion coefficient,providing a more accurate description of the entire process of gas desorption,diffusion,and seepage within the coal mass. Three-dimensional numerical simulations were conducted using COMSOL Multiphysics,and the model was validated against field-measured data from the Wanfu Coal Mine. The results indicate that during the drainage process,the gas pressure field in the coal seam exhibits an elliptical distribution centered on the borehole and expands outward with time, with the gas pressure near the borehole decreasing significantly after 200 days of drainage. Dominated by the coal matrix shrinkage induced by the reduction in gas pressure, the fracture permeability at observation points A₁-A₆ increased by an average of 21. 7% over the drainage period,eventually stabilizing at about 1. 2 times their initial values. The borehole gas drainage rate decayed rapidly from an initial peak of 27. 77 m³ / d,with the decline trend subsequently slowing, and ultimately stabilizing at 13. 99 m³ / d. The simulated decay curve of gas drainage rate is highly consistent with the field-measured values (12. 49 m³ / d-14. 99 m³ / d),with an average relative error of only 4. 1%. This study confirms that the established model can accurately describe gas migration behavior in deep coal seams and elucidate its migration patterns,thereby providing a reliable theoretical basis for the design and performance prediction of gas drainage engineering.