Simulation Study on the Effect of Coal Seam Hydraulic Fracturing to Increase Permeability

Abstract Hydraulic fracturing is mainly used to improve coal seam permeability, extract coal seam gas and prevent coal-gas outburst. However, in the aspect of anti-reflection effect, it is judged by the method of field inspection, and there is no good prediction method before the engineering application. In this work, PFC2D software is used to simulate the process of coal seam hydraulic fracturing, and the results are introduced into the mathematical coupling model of the interaction between coal seam deformation and gas flow for analysis. The results show that the particles are moving towards the direction of the minimum principal stress, the moving speed of the particles at the crack tip increases, and the crack will finally expand towards the direction of the maximum principal stress. The gas pressure decline rate and permeability increase rate of the fractured model are both higher than that of the unfractured model. With the increase of extraction time, the decrease rate of gas pressure and the increase rate of permeability both decrease rapidly and gradually approach 0. The longer the hydraulic fracturing time is, the more complex the fracture network is, and the faster the gas pressure drops. However, the difference between the horizontal and vertical gas pressure drops will become larger and larger, and the impact of fracturing on gas drainage effect will gradually become smaller. With the increase of fracturing time, the difference of permeability increase at different monitoring points becomes larger and larger, but with the increase of gas drainage time, the difference will gradually decrease. The higher the initial void pressure is, the faster the gas pressure drops and the greater the permeability increase. However, the influence of the initial void pressure on the permeability increase will gradually become smaller. The research results could provide certain guidance for the prediction of anti-reflection effect of hydraulic fracturing in underground coal mines.