Coal permeability-improving mechanism of multilevel slotting by water jet and application in coal mine gas extraction

Multilevel slotting by water jet is one of the effective methods used to improve the coal seam permeability. As the example of the 1603 working face of Guhanshan Coalmine, first, using acoustic emission technology, we analyzed the mechanical properties and crack propagation characteristics of the multilevel slotting coal under the condition of uniaxial loading. Then, using the particle flow software, PFC2D, we numerically simulated the fracture, stress, and porosity evolutions of the multilevel slotting coal, and analyzed the coal permeability-improving mechanism of the slot crack network. Meanwhile, the borehole gas extraction slotted by water jet underground was field tested. The study found that the coal's body strength could be weakened significantly when the number of slots was increased, and the multilevel distribution of the slots induced a great amount of crack growth and extension, which formed the slot crack network; reduced the stress concentration; and increased the coal porosity. Field tests demonstrated that the gas extraction effect underground was significantly improved, the ability of gas extraction of multilevel slotting borehole was enhanced by 5-10 times compared with the conventional borehole, and the coal permeability coefficient was increased by about six times.