Gas flow patterns in goaf and optimization of spatial parameters for directional borehole in deep thick coal seam mining

To address the complex gas flow and significant gas disaster threats in the goaf of deep thick coal seam mining, this study investigates the gas flow patterns and optimizes the spatial parameters for roof directional borehole in the goaf. By combining particle flow code and COMSOL numerical simulations, the study analyzes the stress-permeability evolution in the goaf and the gas migration patterns. The results reveal a strong correlation between the permeability of the goaf and the stress distribution, showing distinct zonal characteristics. The permeability decreases slowly in the coal wall-supported separation zone, drops rapidly in the transition zone, and stabilizes in the compaction zone. Without gas extraction, the gas concentration in the goaf can exceed 40% at 100 m from the working face and surpass 70% in the deepest parts. Additionally, gas accumulation at the upper corner can exceed 0.6%. The optimal spatial parameters for roof directional boreholes were determined as a 25 m from the return airway, a vertical height of 25–35 m, a horizontal spacing of 6 m, and an extraction pressure of 20 kPa. Field implementation confirmed that these parameters significantly improve gas extraction effect and reduce gas concentrations in the upper corner of the return airway, providing a reliable reference for gas control in similar mining conditions.