Mechanism and experimental investigation of the formation of hydro-fracture system by fracturing through the interface of large-size coal-rock

Taking the efficient exploitation of coalbed methane resources as the object,the mechanism and experiment on the formation of hydro-fracture system were investigated by the analysis of energy release rate of the fracture tip,the numerical analysis of the fracture tip yield fields and the experiments of fracturing through the interface of large-size coal-rock combination specimens,according to the characteristics of high storage,low permeability and difficult to form fracture in the coal seam in China. The results indicate that the plastic yield area of the fracture tip of the coal is larger than that of sandy mudstone under the same stress condition. The released strain energy of the fracture tip per unit length in the sandy mudstone could make 10. 69 to 25. 53 times area or length of fracture in the coal mass under the ideal conditions. It is convenient for the fracture propagation and the formation of the fracture system. In the numerical analysis,the maximum concentrated stress (7. 05×105 ) of the crack tip of the sandy mudstone in the XX orientation is about 3. 56 times of that of the coal (1. 98 ×105 ) when the critical pressure is reached. In the process of hydraulic fracturing from the hard sandy mudstone into the coal mass,the complex fracture system was formed on the coal-rock combination. The penetration,diversion or offset occur at the interface between the coal and the rock under the influ- ence of the interfacial cohesive force. The diversion or offset leading to the fracture patterns complex,will improve the fracture system.