Numerical analysis of stress distribution due to upper protective layer mining and its impact on gas extraction

Pressure-relief gas extraction through the floor directional long boreholes is an advanced and effective gas control technology for the upper protective layer of contiguous coal seams. This study systematically analyzed the effects of mining activities on stress distribution in the underlying strata and developed an analytical equation to calculate the permeability distribution of the protected layer under mining-induced conditions. The results indicate that the effective extraction radius of directional long boreholes increased by 186% as the mining distance of the upper protective layer extended from 50 to 150 m. Furthermore, increasing the borehole diameter from 89 to 153 mm led to a 14.8% improvement in gas extraction efficiency, while raising the negative pressure from 15 to 35 kPa resulted in a 19.6% increase in the effective extraction radius. These findings provide valuable guidance for optimizing borehole design parameters and gas extraction efficiency, ensuring safer and more effective gas control in coal mining operations.