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.

In order to research on the issue of high‐pressure gas blasting fracture deeply, a high‐pressure gas blasting cracking experimental system was established; the effects of gas pressure and gas volume on the morphology of crack growth were studied; the p‐t curve of pressure in the blasting process with time was obtained; and the mechanism of high‐pressure gas blasting cracking was analyzed in this paper. The conclusion has shown that the stage of effect of high‐pressure gas blasting on the test block includes three stages: the gas jet impact stage, the crack initiation and development stage, and the perforation fracture stage. In the design of gas pressure of 5 MPa, the experimental block only produces one longitudinal main crack. As the gas pressure increases to 7.5 MPa and 10 MPa, besides forming one longitudinal main crack, a transverse main crack is formed with further expanded 4–6 secondary cracks. In the same design of gas pressure regardless of volume size, each pressure variation stage has the same length, and the experimental block of the cracking pressure is basically the same. With the higher design pressure, the cracking pressure and the fracture pressure are higher. In the same design of gas pressure, with the larger gas volume, the fracture pressure required for the experimental block is smaller.

A coal mine underground reservoir, composed of a large number of goaf groups, is effective in achieving the protective utilization of coal mine water. Mastering the hydraulic connection between the goaf groups and the law of seepage is the key to the design and safe operation of coal mine underground reservoirs. Coal/rock sample seepage tests were conducted in Wanli No. 1 Mine in the Shendong mining area according to the evolution law of the lateral supporting pressure of the coal pillars. From the tests, the function of vertical stress in relation to the permeability of coal/rock samples was obtained and the distribution law of permeability of porous media in the goaf was revealed. On this basis, the computational fluid dynamics (CFD) numerical calculation of seepage properties of the coal/rock pillars was conducted. The results show that with the decrease of water level in the goaf and the increase of coal pillar width, the flow range and flow velocity keep decreasing. At the water level of 18.65, 28.65, and 38.65 m, the critical coal pillar widths for the goaf water to penetrate into the adjacent goaf are 30, 50, and 70 m, respectively. When the coal pillar width is less than 10 m, the water can bypass the elastic core area at the bottom coal seam to the adjacent goaf area; when the width exceeds 20 m, the water cannot enter the adjacent goaf area through the bottom coal pillar. On the basis of CFD simulation study on seepage properties of coal/rock pillars between goafs, this paper conducts calculations for all the goafs in the 4-2 coal-panel block of Wanli No. 1 Mine and reveals the seepage path and seepage law in the underground reservoir of this mine.

Coal mining results in strata movement and surrounding rock failure. Eventually, manual mining space will be occupied by the destructed coal rock, making it difficult to conduct field tests of the coal seam to explore gas seepage and transport patterns. Therefore, computational fluid dynamics (CFD) numerical computation is an important tool for such studies. From the aspect of gas pre-drainage, for layer-through boreholes in the floor roadway of the 8,406 working face in Yangquan Mine 5 in China, reasonable layout parameters were obtained by CFD optimization. For effectively controlling the scope of boreholes along coal seam 9 in the Kaiyuan Mine, CFD computation was performed. The results revealed that the horizontal spacing between boreholes should be ≤2 m when a tri-quincuncial borehole layout is used. Optimization of the surface well position layout for the fault structure zone in the Xinjing Mine of the Yangquan mining area indicated that the horizontal distance between the surface well and the fault plane should be <150 m. From the aspect of gas drainage with mining-induced pressure relief, CFD computation was performed for pressure-relieved gas transport in the K8205 working face of Yangquan Mine 3. The results showed that forced roof caving should be used before the overhang length of hard roof reaches 25 m in the K8205 working face to avoid gas overrun. From the aspect of gas drainage from the abandoned gob, surface well control scopes at different surface well positions were computed, and an O-ring fissure zone is proposed as a reasonable scope for the surface well layout. CFD computation has been widely applied to coal and gas co-extraction in the Yangquan mining area and has played a significant role in guiding related gas drainage engineering practice.