Mechanisms for crack extension through the layer in transboundary hydraulic fracturing of coal seam roof

The extension mechanisms of penetrating fractures along the coal–rock interface during cross-layer fracturing in the roof of fragmented, soft, and low-permeability coal seams remain unclear. In this article, combined with the globally embedded cohesive elements technology, the cohesive zone model (CZM) is employed to investigate the primary controlling factors influencing the cross-material propagation of fractures in coal seam roofs. The results show that the vertical stress differential coefficient k controls the extension and steering of the crack, k ≤ 0.5, the cracks will extend along both sides of the interface or cease to extend when they reach the coal–rock interface, k > 0.5, and the crack can cross the interface and enter the coal seam. When the value of k is specific, the greater the strength of the coal rock interface, the longer the distance that cracks extend in the coal seam. Inject rate from 6 to 14 m3/min, the total length of the hydraulic crack increases by 3.05 m, an increase in about 57%. When the interface spacing between the horizontal well spacing and the coal–rock interface was increased from 0.5 to 3 m, the fracture length and width showed a tendency of increasing and then decreasing. This study proposes a globally embedded cohesive element method, and reveals the mechanism for complex cross-layer propagation behavior of hydraulic fractures within coal–rock composite structures. This obtained results display the scientific significance of the optimization of the cross-boundary penetrating fracturing in the coal seam roof.