Pulse hydraulic fracturing is a new technology for coalbed methane exploitation. It can form a complicated fracture network in coal seam, and it is different from conventional hydraulic fracturing. During the fracturing process, periodic pulsating pressure is needed to produce stress wave in coal rock, and the coal rock can be destroyed under the action of pulsating stress and finally form a fracture network. However, the theoretical study on the fracture opening and propagation under the condition of orthotropic mechanical properties of coal rock has not been reported. Based on the elastic wave equation propagation theory and the superposition principle of the elastic theory, a numerical model for simulating the maximum tensile stress caused by pulsation stress in orthotropic coal rock is established. Then the effect of the coal rock orthotropic characteristics, amplitude, frequency, in-situ stress and fracture propagation direction on the maximum tensile stress at the tip of the fracture were studied, as well as the difficulty of opening and propagation of the fracture. The research results show that the maximum tensile stress increases with the increase of amplitude of pulsating load, and the decrease of in-situ stress difference hinders the propagation of the fracture; the effects of frequency and elastic modulus on the maximum tensile stress are similar, and both have an optimal value; the smaller the angle between the fracture and the horizontal maximum principal stress direction, the greater the maximum tensile stress. This work fundamentally reveals the mechanical mechanism of fracture opening and propagation of pulse hydraulic fracturing in coal rock, which can provide reference and basis for the parameters optimization design of coal rock pulse hydraulic fracturing.

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