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a b s t r a c tWith increasing coal mining depth, coal-bed methane (CBM) extraction becomes increasingly difficult, especially for underground CBM extraction. This paper proposes a method for the efficient extraction of CBM in underground coal mines that orients a borehole around a hydraulic fracturing borehole, uses a high-pressure water jet to perforate parallel to the coal bed inclination in the boreholes, and finally performs hydraulic fracturing. Because of shear failure at the perforation tip, cracks initiate at the perforation tip during hydraulic fracturing. Moreover, the perforation causes the maximum principal stress at an angle approximately parallel to the perforation axial, which leads to the orderly propagation of the crack in the coal. The initiation pressure of hydraulic fracturing with and without perforation is compared. Under the same stress combination, the initiation pressure is lower for boreholes with perforation than for boreholes without perforation. The initiation pressure also depends on both the ratio between the maximum horizontal principal stress and the vertical stress and the ratio between the maximum and minimum horizontal principal stresses. Additionally, perforation can effectively stress the coal seam, which extends the crack closure time. Field experiment results show that after hydraulic fracturing with the novel method, the gas drainage volume increased by a factor of 11.26, and the drainage concentration increased by a factor of 2.12 compared with the ordinary gas drainage method.

Initial cracking pressure and locations are important parameters in conducting cross-measure hydraulic fracturing to enhance coal seam permeability in underground coalmines, which are significantly influenced by in-situ stress and occurrence of coal seam. In this study, stress state around cross-measure fracturing boreholes was analyzed using in-situ stress coordinate transformation, then a mathematical model was developed to evaluate initial cracking parameters of borehole assuming the maximum tensile stress criterion. Subsequently, the influences of in-situ stress and occurrence of coal seams on initial cracking pressure and locations in underground coalmines were analyzed using the proposed model. Finally, the proposed model was verified with field test data. The results suggest that the initial cracking pressure increases with the depth cover and coal seam dip angle. However, it decreases with the increase in azimuth of major principle stress. The results also indicate that the initial cracking locations concentrated in the second and fourth quadrant in polar coordinate, and shifted direction to the strike of coal seam as coal seam dip angle and azimuth of maximum principle stress increase. Field investigation revealed consistent rule with the developed model that the initial cracking pressure increases with the coal seam dip angle. Therefore, the proposed