Hydraulic fracturing of hard top coal and roof for controlling gas during the initial mining stages in longwall top coal caving: a case study

Due to the poor cavability of top coal, the connection between the high-level gas drainage lane and the gob is not sufficient during the initial mining stage in a extra-thick coal seam with hard roof and hard coal in longwall top coal caving. The gases gathering in the gob area will suddenly release and cause extrusion when the upper roof first collapses and falls, which will result in gas overrunning and affecting the safety production. Based on the propagation law of hydraulic fractures and the cause of gas overrunning, a new technology was put forward to solve the gas overrunning during the initial mining stage in longwall top coal caving operations. A method for designing the drilling hole parameters for hydraulic fracturing was formed. Hydraulic fracturing was carried out in the measure lane to weaken the top coal and roof and induce the mining pressure to break coal; through this the cavability of the top coal and roof could be improved. Simultaneously, the connection between the gob and the high-level gas drainage lane, and the permeability of the coal seam, could be improved; moreover, the gas drainage effect could be enhanced. The field test in the Tashan Coal Mine indicated that the high-level gas drainage lane was completely connected with the gob after hydraulic fracturing when the working face advanced 15 m, and the gas concentration at the upper corner of the working face was maintained at a level of 0.3% during the initial mining stage. Furthermore, the gas overrunning during the initial mining stages was avoided, and safety was guaranteed.