Experimental Study on the Impact of Multiple Sets of Natural Fractures on Hydraulic Fractures

Shale reservoirs commonly exhibit well-developed bedding planes and multiple sets of natural fractures, resulting in complex and variable hydraulic fracture propagation paths. Previous research unveiled the interactions when the hydraulic fracture encounters a single natural fracture, however, in cases of multiple sets of natural fractures the interactions remain unclear. This study investigates the fracture propagation behaviors and fracture mechanisms of hydraulic fractures in shale with multiple natural fractures under different treatment conditions, explores the influence of multiple natural fractures on hydraulic fractures, and provides theoretical guidance for field hydraulic fracturing. Using full-diameter shale cores with multiple natural fractures, triaxial experiments of varying bedding plane angles and hydraulic fracturing were conducted to analyze fracture propagation for diverse geological features and treatment parameters, clarify the impact of multiple natural fractures on hydraulic fractures, and identify fracturing mechanisms. Additionally, a 3D scanner was employed to extract fracture surface data, characterizing fracture morphology and surface roughness, and further analyzing post-fracturing fracture topography. Study indicates that the compressive strength of shale with multiple sets of natural fractures decreases and then increases with bedding plane angles, with three stages (0–30°, 30–60°, and 60–90°), and the strength varies significantly within the 0–30° and 60–90°, which is different from shales of a single natural fracture. At 0° bedding angle, hydraulic fractures penetrate bedding planes to form straight fractures. As the bedding angle increases, fractures propagate stepwise along bedding planes, leading to increased surface roughness and complexity. This behavior aligns with stress state variations near bedding interfaces during fracture propagation. High-viscosity fluids and injection rates elevate net pressure within fractures, altering local stress states and creating complex fracture networks with stepwise bedding-aligned propagation, accompanied by shifts in failure mechanisms. This study elucidates the mechanism behind strength variations in shale with multiple natural fractures as bedding angles change, revealing that localized stress state alteration near fractures is the primary driver of propagation path deviations. Variations in geological conditions and treatment parameters modify stress states around fractures, leading to transitions of failure mechanisms. A novel fracture mechanism is proposed, where hydraulic fractures evolve from pure tensile failure to tensile-shear mixed failure.