Current situation and development trend of pressure-relief gas extraction in the protective layer mining in coal mines in China

Deep coal mining in China is seriously restricted by factors including high stress, high gas pressure, and low coal seam permeability. Moreover, with the gradual deterioration of the mining conditions of deep coal seams, it is increasingly difficult to control the gas in these coal seams. Fortunately, the pressure-relief mining of protective layers serves as an efficient way to control gas and eliminate coal and gas outbursts. To meet the demand of practical projects, this paper reviews the studies on the mining technology for protective layers, the law of change in the stress of protected layers, the evolutionary law of the fracture system, the evolutionary law of permeability, and pressure-relief gas extraction technology of protected layers conducted in China in recent years. After summarizing and exploring the shortcomings in studies on the pressure-relief mining of protective layers, it is discovered out that small attention is paid to the pressure-relief effect of coals in the upper protected layers at an ultra-long distance within a bending subsidence zone, failing to meet the demand of practical projects. Based on the deformation characteristics of coals in the upper protected layers at an ultra-long distance during the pressure-relief mining of protective layers, as well as the opening and closing of transverse fractures in the coals, this study proposes the concept "time window" for pressure-relief mining of protective layers. Meanwhile, it points out that the important directions for the control of pressure-relief gas in the protected layers at an ultralong distance in the future include establishing the spatial-temporal relationships between the mining of protective layers and the evolution of the gas flow in protected layers, constructing the identification model for the "time window " of gas extraction, and building a spatial-temporal coordination pattern for the pressure-relief mining of protective layers and the control of gas in the upper protected layers at and ultralong distance. Finally, this study develops a method of pressure relief gas extraction from the upper protected layers using directional long-distance drilling and verifies the performance of the method in the Pingdingshan mining area, Henan Province.