Study on the influence of stress difference coefficient on hydraulic fractures in deep shale reservoir and the formation method of complex fracture network

With the gradual shift of unconventional energy exploration to deep shale reservoirs, the difficulty in forming complex fracture networks and limited reservoir reconstruction scope under high in-situ stress have become key bottlenecks restricting efficient development. This study aimed to clarify the influence of horizontal stress difference coefficients on hydraulic fracture propagation in deep shale and explore a technical approach to construct complex fracture networks while controlling fracture direction. A multi-method integrated research framework was established: (1) True triaxial hydraulic fracturing experiments were conducted on deep shale outcrop samples under different horizontal stress difference coefficients to simulate high-stress geological conditions; (2) A rock brittleness evaluation method based on energy evolution (integrating pre-peak elastic energy ratio and post-peak energy release rate) was used to quantify the brittleness degradation law of deep shale under high confining pressure; (3) Fractal theory and energy release rate principle were combined to characterize hydraulic fracture morphology (fractal dimension) and analyze the interaction mechanism between fractures and bedding planes; (4) Hydraulic fracturing experiments with varying fracturing fluid viscosities and injection rates were designed, and multiple linear regression was applied to establish the coupling relationship between fluid parameters and fracture fractal dimension. Novel findings were obtained as follows: (1) Under high confining pressure, the shale brittleness index decreased to 0.435 (a 34.3% reduction compared to 20 MPa), indicating that deep shale’s enhanced plasticity significantly hinders complex fracture formation; (2) At low horizontal stress difference coefficients (0.2–0.22), hydraulic fractures easily communicate with shale bedding planes despite high overall triaxial stress, increasing fracture network complexity, but high stress still limits fracture expansion range; (3) The multiple linear regression model revealed the coupling effect of fracturing fluid viscosity (V) and injection rate (Dis) on fractal dimension (D). An optimized parameter combination was proposed. This study innovatively integrates true triaxial experimental simulation, energy-based brittleness evaluation, fractal characterization, and multiple linear regression to quantify the stress-fluid-fracture interaction mechanism in deep shale. The proposed fracturing fluid parameter optimization method provides a direct technical reference for efficient hydraulic fracturing in deep shale reservoirs.