Fracture initiation pressure and failure modes of tree-type hydraulic fracturing in gas-bearing coal seams

Tree-type hydraulic fracturing is a new technology that can uniformly increase the permeability of coal seams. The existing initiation pressure prediction models do not fully consider the characteristics of coal seams and are not suitable for tree-type hydraulic fracturing. In this paper, an initiation pressure prediction model considering coal seam joints and gas pressure is established. In this model, the fracture failure modes of coal seam fracturing are calculated separately to accurately predict the failure mode and initiation pressure. Statistics and reasons are obtained for the fracture initiation position and the failure mode of tree-type hydraulic fracturing under different conditions. The fracture failure modes of a gas-bearing coal seam are mainly tensile failure and coal matrix failure, and tensile failure along bedding planes is the most common. The effects of 7 factors, including coal seam geological parameters, fracturing hole arrangement parameters and tree-type branch borehole arrangement parameters, on the fracture initiation pressure of tree-type hydraulic fracturing are analyzed. Among them, the angle between the tree-type branch borehole and the maximum principal ground stress direction is the only major influential factor that can be artificially controlled. The fracture initiation pressure is lowest when the tree-type branch borehole lies along the direction of the minimum principal ground stress. The results of a field experiment show that the theoretical initiation pressure and the actual initiation pressure are within 6.3%. In addition, compared with traditional fracturing technology, tree-type hydraulic fracturing technology can reduce the initiation pressure. The proposed model can provide a reference for field applications of tree-type hydraulic fracturing technology.