Abstract: The interfacial properties of shale reservoirs significantly influence the three-dimensional propagation behavior of hydraulic fractures. This study prepares artificial shale specimens with varying interfacial strengths using an interlayer interval casting method and conducts true triaxial hydraulic fracturing tests. The center deviation index (CDI) and radial non-uniformity (RNU) are established to quantitatively characterize the three-dimensional geometric features of fractures. By analyzing injection pressure curves and acoustic emission parameters, this study investigates the influence of interfacial strength on hydraulic fracture propagation. The results demonstrate that interfaces induce deflection effects in hydraulic fractures, causing them to twist or propagate along these interfaces, resulting in asymmetric three-dimensional morphologies with noticeable shifts toward the interfaces. Under a normal stress ofσ_z=15 MPa, as the interfacial cohesion and the internal friction angle decrease, the calculated interfacial shear strength decreases from 19.9 MPa to 10.1 MPa. Consequently, the fracture morphology transitions sequentially, exhibiting characteristics of V-shaped penetration, Y-shaped branching, and ultimately T-shaped or H-shaped capture. The RNU value increases from 0.36 to 1.06, while the CDI value decreases from 0.42 to 0.29, indicating that lower interfacial strength restricts outward fracture propagation. Interfacial strength governs both the energy release modes and fluid migration pathways. Weaker interfaces reduce the initiation pressure and promote large-scale fluid loss along these interfaces, creating favorable conditions for complex fracture network formation. A reduction in interfacial shear strength significantly alters the fracture failure mode, where high-strength interface conditions predominantly produce tensile fractures, while low-strength interfaces induce a transition to a composite tensile-shear failure mechanism.

Key words: hydraulic fracturing, interfacial strength, fracture morphology, artificial shale specimen, failure mechanism