Permeability Enhancement and Gas Drainage Effect in Deep High Gassy Coal Seams via Long-Distance Pressure Relief Mining: A Case Study

Coal 3 in group A is employed as a protective layer to release long-distance coal 4 in group B in Paner colliery (approximately 80 m vertical interval) as the mining depth extends downward, which is the first engineering test in the Huainan coal mining area. To evaluate the validity of the scheme, the permeability distribution, and evolution law, gas pressure distribution characteristics, swelling deformation, pressure relief range, and gas drainage volume of the protected coal seam are analyzed using a FLAC3D numerical simulation and field measurements. Therefore, different stress-permeability models are adopted for caved, fractured, and continuous deformation zones, and a double-yield model is applied in the goaf based on compaction theory to improve the accuracy of the numerical simulation. The results indicate that the extraction of coal 3 has a positive effect on permeability enhancement and pressure relief gas drainage. However, the dip angle of coal measurements causes asymmetric strata movement, which leads to the pressure relief and permeability enhancement area shifting to the downhill side, where the permeability enhancement effect of the downhill side is better than that of the uphill side. The permeability enhancement zone is an inverted trapezoid, but the effective pressure relief range is a positive trapezoid. The permeability of the protected coal seam in the pressure relief zone is significantly higher than that in the compressive failure zone. The permeability in the pressure relief zone will decrease again due to the recompaction of the coal seam with an advancement of the longwall face. Thus, pressure relief gas drainage is suggested during long-distance protective coal seam mining to eliminate gas hazards.