True Triaxial Physical Simulation Experiment on the Fracture Propagation Law of Hydraulic Fracturing for Horizontal Wells in the Roof of Soft and Low-Permeability Coal Seams

The key to exploiting coalbed methane in soft and low-permeability coal seams by deploying horizontal wells in roof strata lies in whether hydraulic fractures can propagate into the coal seam after initiating from the roof. True triaxial physical simulation experiments based on 15 artificially cast fracturing specimens were conducted to explore how in situ stress regimes, coal–roof interface strength, and roof mechanical properties control cross-layer propagation of hydraulic fractures. Results show that the strike-slip fault stress regime yields the highest cross-layer propagation success rate and the lowest average initiation pressure, followed by the normal fault stress regime, while the reverse fault stress regime significantly restricts vertical propagation of fractures. High-strength coal–roof interfaces can promote vertical fracture propagation and lower the fracture initiation pressure. High-strength roofs facilitate stable vertical fracture extension, while low-strength roofs containing microdefects lead to increased initiation pressure. The three geological factors exert hierarchical coupled control over the fracture propagation: in situ stress determines fracture propagation direction, coal–roof interfaces control fracture branching at stratigraphic interfaces, and roof mechanical properties regulate the complexity of induced fracture networks. To achieve high-quality reservoir stimulation, target zones with strike-slip or normal fault stress regimes, high-strength coal–roof interfaces, and matched roof types should be prioritized.