Numerical Simulation of Cross-Layer Propagation Mechanisms for Hydraulic Fractures in Deep Coal-Rock Formations

Hydraulic fracturing serves as a critical technology for reservoir stimulation in deep coalbed methane (CBM) development, where the mechanical properties of gangue layers exert a significant control on fracture propagation behavior. To address the unclear mechanisms governing fracture penetration across coal-gangue interfaces, this study employs the Continuum-Discontinuum Element Method (CDEM) to simulate and analyze the vertical propagation of hydraulic fractures initiating within coal seams, based on geomechanical parameters derived from the deep Benxi Formation coal seams in the southeastern Ordos Basin. The investigation systematically examines the influence of geological and operational parameters on cross-interfacial fracture growth. Results demonstrate that vertical stress difference, elastic modulus contrast between coal and gangue layers, interfacial stress differential, and interfacial cohesion at coal-gangue interfaces are critical factors governing hydraulic fracture penetration through these interfaces. High vertical stress differences (>3 MPa) inhibit interfacial dilation, promoting predominant cross-layer fracture propagation. Reduced interfacial stress contrasts and enhanced interfacial cohesion facilitate fracture penetration across interfaces. Furthermore, smaller elastic modulus contrasts between coal and gangue correlate with increased interfacial aperture. Finally, lower injection rates effectively suppress vertical fracture propagation in deep coal reservoirs. This study elucidates the characteristics and mechanisms governing cross-layer fracture propagation in coal–rock composites with interbedded partings, and delineates the dynamic evolution laws and dominant controlling factors involved. The findings provide critical theoretical insights for the optimization of fracture design and the efficient development of deep coalbed methane reservoirs.