Three-Dimensional Numerical Simulation of Interaction Between Hydraulic Fractures and Natural Fractures in Deep Coal Reservoir

A large number of natural fractures develop in deep coal and rock reservoirs, resulting in a very complex fracture morphology after the interaction between hydraulic fractures (HF) and natural fractures (NFs). The HF morphology determines the effect of reservoir reconstruction and productivity, and the traditional two-dimensional fracture model cannot accurately describe the interaction characteristics of the two. In this paper, a three-dimensional HF propagation model containing NFs is established using the discrete lattice method, and the influence of natural fracture approximation Angle, horizontal stress difference, displacement, and multistage fracture types on HF propagation morphology is analyzed. The results show that: (1) The interaction between HF and NFs is more common in that HF pass through NFs and HF open NFs. (2) The size and approximation Angle of natural cracks mainly affect the difficulty of hydraulic cracks crossing natural cracks. When the size and approximation Angle of natural cracks are smaller, the difficulty of natural cracks crossing natural cracks is lower, and the fracture expansion pattern is more uniform. As the size and approximation Angle of natural cracks increase, the difficulty of natural cracks crossing natural cracks increases. HF tend to expand along the side that deviates from NFs. (3) The increase of injection displacement can significantly increase the reconstruction area of fractures, and to a certain extent increase the uniform expansion degree of fractures and the probability of crossing NFs; (4) Compared with single-stage fractures, the interaction behavior between HF and secondary and tertiary fractures is more complex. Due to the influence of stress shadows, HF have the characteristics of non-uniform propagation of fracture steering. The research results provide theoretical support for the fracture propagation law of deep coal rock reservoir and promote the efficient development of deep coal bed methane resources.