A field investigation of the deformation of protected coal and its application for CBM extraction in the Qinglong coalmine in China

Protective coal layer mining with pressure-relief gas extraction from the protected coal layer is an effective and economical way of controlling coal and gas outbursts in underground mines. According to a field test of the deformation of the protected coal layer in the Qinglong coal mine, during the mining of the protective coal layer, the floor of the protective coal layer can be divided into five areas in the horizontal direction: the original stress area, the stress concentration area, the stress relaxed area, the stress recovery area, and the recompaction area. From coal bed methane (CBM) extraction with pressure relief, the floor strata can also be divided into five areas: the original gas flow area, the gas flow reduction area, the initial pressure relief area with increasing permeability and flow, the full pressure relief area with high permeability and flow, and the pressure recovery area with decreasing permeability and flow. Mining of a protective coal layer will result in the breakage and movement of the overlying strata as well as floor heaves in the floor strata. In these unloaded coal strata, relieving the stress leads to the production of new cracks, which increases the permeability. The coal deformation and gas desorption are remarkable in the protected coal seam. The maximum dilation and dilation rate were 13 mm and 4.19‰, respectively. The gas flow and permeability increased significantly. The maximum natural emission flow and permeability reached 0.194 m³/min and 19.75 m2/MPa2.d respectively, There were 27- and 42-fold increases in permeability, which indicated that the pressure relief in this region was complete and that fractures in the coal body developed. This emission is conducive to CBM migration and extraction.