Investigation on the influence of bedding planes on hydraulic fracture propagation in shale based on true triaxial experiments and numerical simulation

The propagation behavior of hydraulic fractures in laminated shale is governed by the interplay between bedding plane weakness and in-situ stress fields; however, the quantitative effects of operational parameters and geological factors on trans-bedding fracture capability remain not fully elucidated. Large-scale true triaxial hydraulic fracturing experiments were conducted on natural Longmaxi Formation shale specimens from the Changning area, southern Sichuan Basin, combined with three-dimensional discrete lattice numerical simulations, to systematically investigate the effects of injection rate, fluid viscosity, bedding mechanical properties, and tectonic stress state on fracture propagation behavior. Experimental results show that low injection rates (6 mL/min) produce single planar fractures confined to bedding interfaces, while higher rates (50 mL/min) significantly enhance trans-bedding capability and generate complex volumetric fracture networks. High-viscosity fluid (200 cp) sustains elevated net pressure promoting large-scale bedding-parallel propagation, whereas low-viscosity fluid yields more tortuous trans-bedding fracture paths. Numerical simulations further reveal that bedding planes reduce breakdown pressure by approximately 16% relative to homogeneous models, and a transitional cementation strength regime governs (approximately between 0.6 and 2.4 MPa) the transition from bedding-controlled to trans-bedding propagation modes. High-density bedding suppresses vertical propagation but activates multiple bedding interfaces, yielding comparable total stimulated area to low-density conditions. Fault type systematically modulates the competition between stress-field guidance and bedding weakness: strike-slip conditions produce the most complex fracture networks favorable for volumetric stimulation, while reverse fault conditions promote large-scale horizontal spreading but severely limit fracture height. These results establish that the relative magnitude of net pressure versus effective bedding tensile strength is the fundamental criterion controlling fracture propagation mode transition, providing a theoretical basis for differentiated fracturing parameter optimization in laminated shale reservoirs.