With the increasing demand for the development of unconventional oil and gas resources, hydraulic fracturing has become a key technology for enhancing reservoir permeability. However, achieving controlled propagation of fracture networks remains a significant challenge under complex geological conditions. This study integrates theoretical analysis and finite-discrete element method (FDEM) simulations to investigate mixed-mode mechanisms, plastic zone evolution at fracture tips, and anisotropic mechanical responses of shale. Modified fracture criteria-including a T-stress-integrated Mohr-Coulomb criterion and maximum circumferential tensile stress criterion are derived and validated through uniaxial compression and Brazilian splitting tests on Longmaxi Formation shale. Results demonstrate that the modified Mohr-Coulomb criterion effectively predicts anisotropic fracture propagation by characterizing tensile-compressive strength differences, while the plastic zone evolution under maximum circumferential tensile stress is significantly influenced by T-stress: positive T-stress (45°–90°) expands the plastic zone, whereas negative T-stress (0°–45°) contracts it. Lower tensile-to-compressive strength ratios lead to larger plastic zones. An FDEM-based horizontal well fracturing model reveals vertical fracture propagation dominated by bedding plane and interbed fracture extension, forming complex networks, while horizontal fractures initially grow independently before deflecting and interconnecting under maximum principal stress. Sensitivity analysis of perforation spacing identifies 62.5 mm (16 holes/m) as the optimal configuration, achieving Mode Ⅱ-dominated fracture networks with superior connectivity and stimulation efficiency. Larger spacings (71.4–83.3 mm) result in reduced efficiency or isolated fractures. By coupling stress interference and fluid pressure field dynamics, this study establishes a methodology to balance fracture network complexity and reservoir stimulation efficacy. The findings provide theoretical insights and engineering guidelines for optimizing hydraulic fracturing designs in anisotropic shale gas reservoirs through advanced fracture criteria and FDEM-based multi-physics simulations.

Although significant progress has been made in the development of shallow natural gas, the exploitation of deep shale gas continues to face numerous challenges. Therefore, conducting research on deep shale gas extraction is crucial. The efficient exploitation is contingent upon a comprehensive understanding of the mechanical properties, fracturing behaviors, and transformation processes of deep reservoir formations. This paper initially delineates the geo-mechanical characteristics and key development challenges associated with deep shale gas reservoirs. It subsequently reviews recent advancements in laboratory experiments, numerical simulations, and field technologies. Finally, suggestions and strategies are proposed to enhance the efficiency of deep shale gas development. The perspectives offered in this paper aim to provide new insights into optimizing exploration and production in deep and complex geological environments.

In the last years, shale gas has gradually substituted oil and coal as the main sources of energy in the world. Compared with shallow shale gas reservoirs, deep shale is characterized by low permeability, low porosity, strong heterogeneity, and strong anisotropy. In the process of multi-cluster fracturing of horizontal wells, the whole deformation process and destruction modes are significantly influenced by loading rates. In this investigation, the servo press was used to carry out semi-circular bend (SCB) mixed-mode fracture experiments in deep shales (130, 160, 190 ℃) with prefabricated fractures under different loading rates (0.02, 0.05, 0.1, 0.2 mm/min). The fracture propagation process was monitored using acoustic emission. The deformation characteristics, displacement–load curve, and acoustic emission parameters of shale under different loading rates were studied during the mixed-mode fracture propagation. Our results showed that during the deformation and fracture of the specimen, the acoustic emission energy and charge significantly increased near the stress peak, showing at this point the most intense acoustic emission activity. With the increase in loading rate, the fracture peak load of the deep shale specimen also increased. However, the maximum displacement decreased to different extents. With the increase in temperature, the effective fracture toughness of the deep shale gradually decreased. Also, the maximum displacement decreased. Under different loading rates, the deformation of the prefabricated cracks showed a nonlinear slow growth–linear growth trend. The slope of the linear growth stage increased with the increase in loading rate. In addition, as the loading rate increased, an increase in tension failure and a decrease in shear failure were observed. Moreover, the control chart showing the relationship between tension and the shear failure under different temperatures and loading rates was determined.