Mechanism of Hydraulic Fracture Propagation in Deep Coal-Rock Gas Reservoirs: Insights from Large-Scale True Triaxial Experiments

The mechanisms of hydraulic fracture propagation and key factors controlling high productivity in deep coal-rock gas wells remain unclear, limiting the optimization of fracturing technology and efficient resource development. To address the limitations of conventional laboratory experiments—such as undersized specimens (< 0.3 m3) and unrealistic injection rates (< 0.01 m3/min)—this study employs a novel large-scale (2.0 m × 2.0 m × 1.0 m, 4.0 m3) true triaxial hydraulic fracturing platform to simulate in situ stress conditions and field-scale operational parameters for deep coal-rock gas. Comparative experiments between coal-rock and sandstone were conducted, with fracture dynamics analyzed through integrated microseismic and stress-strain monitoring, tracer tracking, 3D reconstruction, and specimen dissection. Key findings include: (1) Coal exhibits 50% lower breakdown pressure and forms 2× more complex fracture networks than sandstone, dominated by shear failure and natural fracture activation. (2) Proppant distribution in coal is highly heterogeneous, with > 70% concentrated near the wellbore, limiting secondary fracture conductivity. This study pioneers field-scale experimental validation of deep coal-rock gas fracture network formation mechanisms, providing a theoretical foundation for fracturing technology and supporting efficient exploitation of deep coal-rock gas.