Enhanced gas drainage technology by staged fracturing in long bedding borehole in hard coal bed of underground coal mine and its application

In view of the problems of rapid attenuation, long period and low efficiency of gas drainage in hard coal bed, the enhanced gas drainage technology of increasing the permeability of coal bed by staged fracturing in long bedding borehole was put forward for the medium-to-hard coal bed. Herein, research was conducted on No.4 coal bed of Shaanxi Binchang Mining Area. Specifically, the change characteristics of micro-pore structure of coal body before and after hydraulic fracturing were comparatively analyzed by high-resolution electron microscope (SEM) in the laboratory. Meanwhile, the stress characteristics of packer and the stability of borehole were simulated with Abaqus software, and the industrial test of hydraulic fracturing was carried out in the No.4 coal bed of Dafosi Underground Coal Mine in Binchang Mining Area. The results show that the pores and fractures in the coal body are increased in number, enlarged in size and enhanced in connectivity under the conditions that 15 MPa pressure is applied to the coal bed and held for 48 h. In particular, the connectivity between fractures is significantly improved. During fracturing, the packer will expand and deform under the joint action of the internal and external pressures. Besides, the borehole structure of hard coal could be kept intact and the maximum sealing friction would be generated when the internal pressure is 15 MPa and the external pressure is 10 MPa. In the engineering test, 3 long directional bedding boreholes with a depth of 540‒568 m for each were fractured by stage. Specifically, each borehole was divided into 8 stages, with a liquid injection volume of 910‒1 154 m³ for each single borehole and a cumulative liquid injection volume of 3 011 m³. After fracturing, the fracturing influence radius was determined through the transient electromagnetic test in the borehole to be 34‒46 m. The net amount of gas drained from the fractured boreholes is 0.72‒1.73 m³/min, with a volume fraction of 42.60%‒67.48%. Compared with the conventional boreholes in the test site, the volume fraction of gas drained is increased by 1.29‒2.49 times, and the net amount of gas drained in every 100 m boreholes is increased by 3.93‒10.03 times, realizing the advanced permeability improvement and pre-drainage of gas in the coal bed area of the test site. Generally, this process technology provides a technical reference for the advanced treatment of gas in large areas with similar geological conditions.