A measurement study was conducted for the effective drainage radius of borehole along coal seam #9 of the Kaiyuan Coal Mine using the gas pressure method and gas flow method. The measurement results show that the effective drainage radius of borehole along coal seam #9 was 0.75 m, 27 days after drainage, and 1.5 m, 92 days after drainage. Experimental schemes were designed for the entire evolution of the stress in the coal mass around the borehole, and an experimental study on methane seepage in the coal mass around borehole was conducted. Fitting functions for the permeability of the coal sample and its vertical stress were obtained by fitting the experimental data. Based on the vertical stress–permeability functional relationship of coal masses around borehole, a numerical calculation model for methane seepage from coal masses around borehole was established, and the influences of drainage time, initial gas pressure, borehole diameter, and drainage negative pressure on the effective drainage radius of borehole were investigated. The numerical simulation results show that with the increase in initial gas pressure and borehole diameter, the effective drainage radius of borehole increases continuously but its increase amplitude decreases constantly. With the increase in drainage negative pressure, the effective drainage radius of borehole increases linearly but the increase amplitude is relatively small. The layout parameters of borehole along coal seam #9 of the Kaiyuan Coal Mine were optimized based on the numerical calculation results, and the reasonable drainage time, reasonable borehole diameter, and reasonable drainage negative pressure are 180 days, 120 mm, and 15 kPa, respectively, for the borehole along coal seam #9.

The traditional method to design coal pillar for lateral roof roadway was established based on the mining‐induced strata movement contour which is considered as a straight line, while ignoring the variations of the internal strata deformation law as well as stress distribution characteristics. In order to make up for this deficiency, in this study, evolution of mining‐induced stress in the overlying strata was simulated using physical and numerical simulations, and a method to design coal pillar for lateral roof roadway based on mining‐induced stress was proposed. The results indicate that the stress of the overlying strata is redistributed during excavation, and the stress distribution can be divided into a stress‐relaxation area, a stress‐concentration area, and an in situ stress area. The contour line of 1.05 times the in situ stress is used to define the mining‐induced stress contour. Stress inside the contour is redistributed while outside the contour the overlying strata are still within the in situ stress area. Mining‐induced stress contour presents a concave‐upward type from coal seam to the overlying strata that cannot be merged into a straight one due to their different characteristics of movement and deformation. With this in mind, this study proposed a method to design the width of coal pillar for lateral roof roadway according to the mining‐induced stress contour. According to mining‐induced stress contour, the width of coal pillar for lateral roof roadway of longwall panel 31100 is 160 m, and the maximum deformation of the roadway is 270 mm. The new method can definitely meet engineering demands.

Gas flowing fracture zone (GFFZ) is an active zone in the gob where pressure‐relieved methane can move freely. However, there are very few research findings on the horizontal development characteristics and internal methane seepage properties of GFFZs. In this paper, based on the development height of a GFFZ above the gob, the GFFZ was horizontally divided into the following: a lateral fracture subzone, an O‐ring fracture subzone, and a compacted subzone. The identification criteria for all of these horizontal subzones were given by analyzing the influence of the stress in the coal rock mass on the development of mining‐induced fractures. A numerical simulation study was conducted to determine the influences of the mining height and mining depth on the morphological development of all horizontal subzones of the GFFZ, and the simulation results showed that the mining height was the main factor influencing the development characteristics of horizontal subzones of the GFFZ. Both the maximum distance of the lateral fracture subzone beyond the mining boundary and the width of the O‐ring fracture subzone increased with the increase of mining height. A physical simulation experiment was performed for extraction of gob methane through a surface borehole, and the experimental results showed that the gas flow state was laminar within the range of the lateral fracture subzone and the compacted subzone but that the gas flow state was turbulent, not following Darcy’s law, within the range of the O‐ring fracture subzone.

Under the influence of advanced abutment pressure, the tightness of a borehole is destroyed, so secondary borehole sealing is needed; however, the opportune time for a secondary borehole sealing is unclear. In addition, current borehole-sealing devices do not have a secondary borehole-sealing functionality. In this paper, an opportunity election model of secondary borehole sealing was established, and a dynamic secondary borehole-sealing device was developed. In working face 3908 of the Kaiyuan Coal Mine, an in situ dynamic secondary borehole-sealing test was conducted. The test results show that the tightness of the boreholes sealed with high-water-expansion slurry is obviously better than that of the boreholes sealed with cement mortar and polyurethane. When a borehole is destroyed by the peak of the advanced abutment pressure, the secondary borehole-sealing measures can significantly improve borehole suction pressure, gas concentration, and gas drainage amount in the advanced abutment pressure loading stage.