Evolutionary pattern of pressure-relief gas transportation-storage area in fully mechanized mining face and its engineering practice in Huangling minefield

The evolution of overburden fracture structure induced by coal seam mining has a greater impact on the transportation and storage of pressure-relief gas in the mining zone. To determine the dynamic evolution law of pressure-relief gas transportation-storage area in the fracture network of the overlying rock strata on the condition of high-intensity mining in Huangling minefield, the change of supporting pressure, the rock strata movement, and the overlying rock fracture distribution are clarified through the physical similarity simulation experiment using the No. 213 working face of the No. 2 Coal Mine of Huangling Minefield as a basic prototype. Based on the fracture morphology of the pressure-relief gas transportation-storage area, the identification method of the pressure-relief gas transportation channel and storage area was clarified, and the evolution law of the pressure-relief gas transportation and storage area was obtained. The selection process of unloaded gas extraction technology based on the location of the pressure-relief gas transportation-storage area and the intensity of gas outflow is proposed. And the layers of conventional high-level boreholes and directional long-drill holes are optimized. The results show that the vertical stress distribution and the displacement distribution of the rock layer are symmetrical trapezoidal after overburden collapsed. The pressure-relief gas transportation-storage area can be divided into 4 zones which are the pressure relief gas storage advantage zone, transportation advantage zone, micro-seepage zone and transportation-storage advantage zone. Based on the historical gas outflow data of the adjacent workface and the gas content of the test workface, the pressure-relief gas extraction mode of directional long boreholes supplemented by high-level boreholes was selected, and the parameters of the borehole arrangement were optimized. The volume fraction of gas in the extraction boreholes can be maintained above 30% for a long period of time during mining. The volume fraction of gas in the working face, the return airway and the upper corner was controlled at 0.18%−0.46%, 0.2%−0.46% and 0.25%−0.84% respectively, which were far below the maximum value of the relevant regulations. The results of engineering practice show that the design of extraction boreholes based on the distribution characteristics of the pressure-relief gas transportation-storage area can effectively improve the efficiency of unloaded gas extraction and ensure the efficient production of the working face.