A new technique for preventing and controlling coal and gas outburst hazard with pulse hydraulic fracturing: a case study in Yuwu coal mine, China

Coal and gas outburst is one of the main gas hazards in coal mines, and pulse hydraulic fracturing is a new technique for preventing and controlling gas hazards in coal mines and is presented based on conventional hydraulic fracturing. S2107 is a coal seam with high gas content and coal and gas outburst hazard but with low gas concentrations when extracted. To safely drive air tunnels into S2107, the pulse hydraulic fracturing technique is used on cross-measure boreholes placed from a high-level roadway to the driving roadway to improve the effects of gas drainage and ensure safety during production. The mechanism of coal fracturing via pulsed pressure is analyzed. A variable frequency method is used to strengthen the fatigue damage effect, and the initial pressure is estimated based on previous studies. The layouts of fracturing borehole and guide borehole are designed according to the layer relation, and a new hole sealing method is developed. The results indicate the pressure changes during pulse hydraulic fracturing reflect crack extension, and it is conducive to extending and connecting cracks under pulse pressures. The initial pressure during pulse hydraulic fracturing is negatively related to the water volume and fracturing time but cannot directly influence the fracturing radius. Moreover, this pressure is 30-43 % less than the calculated value. After this application, the gas desorption index, K (1), at the driving workface decreases to below the critical value. The gas drainage volume of fracturing boreholes and their guide boreholes increases relative to normal boreholes by 3.32-fold and 3.07-fold, respectively. The new technique is promising for preventing and controlling gas hazards in the future.