Analysis of the stress–permeability coupling property in water jet slotting coal and its impact on methane drainage

The study analyzed the evolution equation of the effective stress–permeability coefficient and the mechanism of pressure relief and permeability enhancement by water jet slotting. A FLAC3D seepage module has been adopted for the computational analysis of the evolution of the coal effective stress, permeability enhancement rate and the pore pressure induced by the slots. Field experiment examined that the effects of slotting permeability enhancement and the dynamic features of methane drainage from boreholes. It was found by simulation that the effective stress in slotted coal was significantly reduced and the reduced area coverage was four to seven times larger than that in a conventional borehole. The pore pressure varied logarithmically with time during methane drainage. It was found in the field experiment that the methane drainage capacity was six times higher than a conventional borehole, and the significant region for methane drainage lay within 5.0 m, the transition region for methane drainage lay between 5.0 and 7.0 m, and the marginal region for methane drainage was above 7.0 m. The results of this research have provided guidance on the theories and parameters of applying water jet slotting technology to methane drainage from underground coal mines.