A novel method of orienting hydraulic fractures in coal mines and its mechanism of intensified conduction

Abstract

With 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.