A novel technology for enhancing coalbed methane extraction: Hydraulic cavitating assisted fracturing

Low extraction ratio of coalbed methane (CBM) can not only cause the waste of energy resource, inducing environmental pollution, but also make CBM be a potential safety risk for underground coal mining. Increasing the permeability of coal seam is an effective means to solve these problems in low permeable seams. Given this situation, this paper proposes a highly efficient, widely applicable permeability enhancement technology (hydraulic cavitating assisted fracturing (HCAF)). The feasibility of the proposed technology to improve permeability was demonstrated theoretically. Furthermore, the effects of this technology on coal damage, crack development and methane extraction enhancement were simulated with RFPA and COMSOL. The results show that in coal seams, the HCAF technology can reduce the fracture initiating pressure below 15 MPa and increase the effective radius from 8 m to 12 m. Besides, this technology has an impact on controlling the development directions of fracture. Finally, field tests show that the HCAF technology can increase the maximum gas extraction flow rate from 0.56 m(3)/min to 0.81 m(3)/min and the gas flow rate is always greater than that of traditional hydraulic fracturing during extraction period. The area of methane extraction by HCAF technology was checked after increasing the borehole spacing from 12 m to 24 m, which showed that the amount of drilling cuttings s and initial speed of gas emission q were lower than the threshold limit value. Those indicate that the HCAF technology is efficient and reliable to improve methane recovery ratio and reduce methane emissions.