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Hydraulic fracturing (HF) technology can safely and efficiently increase the permeability of coal seam, which is conducive to CBM exploration and prevent coal and gas outburst. However, conventional HF fractures tend to expand in the direction of maximum principal stress, which may be inconsistent with the direction of fracturing required by the project. Therefore, the increase direction of coal seam permeability is different from that expected. To solve these problems, PFC2D software simulation is used to study directional hydraulic fracturing (DHF), that is, the combination of slotting and hydraulic fracturing. The effects of different slotting angles (θ), different horizontal stress difference coefficients (K) and different injection pressure on DHF fracture propagation are analyzed. The results show that the DHF method can overcome the dominant effect of initial in-situ stress on the propagation direction of hydraulic fractures and control the propagation of fractures along and perpendicular to the slotting direction when θ, K and liquid injection pressure are small. When the DHF fracture was connected with the manual slotting, the pressure will shake violently, and the fracturing curve presents a multi peak type. The increase and decrease of particle pressure around the fracturing hole reflect the process of pressure accumulation and fracture propagation at the fracture tip respectively. Compared with conventional HF, DHF can not only shorten the fracturing time, but also make the fracture network more complex, which is more conducive to gas flow. Under the action of in-situ stress, the stress between slots will increase to exceed the maximum horizontal principal stress. Moreover, with the change of fracturing time, the local stress of the model will also change. Hydraulic fractures are always expanding to the area with large local stress. The research results could provide certain help for DHF theoretical research and engineering application.
Wang et al. (Sun,) studied this question.