Numerical Study on the Field-Scale Criterion of Hydraulic Fracture Crossing the Interface Between Roof and Broken Low-Permeability Coal

In areas of broken soft and low-permeability coal, indirect hydraulic fracturing (HF) coal is a technology with great potential to increase coal bed methane (CBM) production. The key to success of the technology is that HFs can cross the coal–rock interface from the roof and propagate into the coal. To this end, this article focuses on the roots that hinder the HFs propagating enough in the coal, that is, the significant inelastic deformation and abundant discontinuities of the coal. Based on CT experiments and nonlinear mechanics-seepage theory, a 3D discontinuities network model and plastic-nonlinear fracture-seepage coupled constitutive equations were established, respectively. On this basis, 115 numerical calculation models were established to study the field-scale criterion of HFs crossing coal–rock interface under the influence of three factors, including the distance between horizontal well and interface (D_op), interface friction coefficient (f_c,i) and in-situ stress difference (Δσ). The results show that the critical f_c,i is positively correlated with D_op and negatively correlated with Δσ. Once the D_op exceeds 2 m, more than 80% of the hydraulic energy will be consumed in the nonlinear mechanical behavior and mixed fracture mode of coal, which will lead to the difficulty of HFs propagation. In-situ experiments show that, by reducing D_op, optimizing the landing of the horizontal well (i.e. increasing Δσ and f_c,i), and maintaining high hydraulic energy (or injection rate), the daily CBM production by indirect fracturing technology is 10 times higher than that without parameter optimization, exceeding 3000m³/d. The "field-scale criterion" obtained in this article will provide theoretical support for increasing the success rate of indirect fracturing coal technology in areas of broken low-permeability coal.