Hydraulic fracture propagation research in layered rocks based on 3D FEM modeling and laboratory experiments

The complex morphology of hydraulic fractures (HFs) has been reported in layered formations due to the frequent occurrence of bedding planes (BPs) and natural fractures. The problem of hydraulic fracturing is the unpredictable propagation path of HFs in the exploitation of layered formations. This study employs a fully coupled 3D finite element model to simulate HF propagation in layered rocks and its interaction with BP. To simulate natural discontinuities effectively, artificial BP was created by bonding river sand with epoxy resin in laminated blocks. Furthermore, artificial BP was created by epoxy resin bonding river sand in laminated blocks, effectively simulating natural discontinuities. The bonding strength of artificial BPs can be altered by varying the particle size of the river sand used. True triaxial hydraulic fracturing experiments were conducted on laminated specimens made of artificial BP materials bonded with different outcrops, which simulate layered formations. Experiments and numerical simulations were employed to investigate the influence of various factors, including in-situ stress, BP friction coefficient, BP conductivity, fracturing fluid viscosity, and injection rate. In general, When HFs encounter BPs in layered rocks, interface effects can impact HF morphology significantly, leading to HF patterns resembling a "+" "T" or "I" shape. The laboratory experimental results were compared with numerical simulations at the experimental scale, to validate the model reliability. Numerical predictions match well with true triaxial fracturing experiments. The influence patterns associated with the interaction between HFs and BPs are revealed. The ability of fractures to traverse BPs relies on the interplay of in-situ stress, fracturing fluid properties, and intrinsic BP characteristics. Factors such as increased vertical stress and BP friction coefficient favor fracture penetration. Rougher interfaces with greater strength facilitate fracture propagation by enabling fractures to overcome interlayer stress differences. In the Linxing gas field, where the BP friction coefficient is around 0.65, a Delta sigma h of 3 MPa can effectively prevent HFs from penetrating BPs. However, when HFs encounter BPs with permeability exceeding 50 md or when there is a larger initial opening at the millimeter scale, a substantial amount of fracturing fluid leaks into BPs. The excess leakage into BP restricts HF growth significantly. Increasing the pumping rate and/or fluid viscosity proves effective in reducing the impact of the BP and facilitating the growth of the HF, ultimately driving the HF to penetrate through the BPs. The analysis identifies varying pump rates and viscosity of the fracturing fluid as effective methods for enhancing the efficiency of hydraulic fracturing treatments.