Propagation behavior of hydraulic fracture across the coal-rock interface under different interfacial friction coefficients and a new prediction model

Indirect fracturing from rocks to coal is a promising technology of coalbed methane exploitation in soft and low-permeability coal seams. Central to this technology is the prediction of the propagation path of the fluid-driven fractures at the coal-rock interfaces, which requires a deep understanding of the role of the friction properties of coal-rock interfaces. In this paper, we performed hydraulic fracturing experiments using coal-rock blocks with interfaces that were not lubricated, lubricated by oil grease or Vaseline. The results show that the fracture behaviors at different interfaces depend mostly on the vertical stress and the interfacial friction coefficients. Only when the vertical stress reaches a certain threshold, can the fractures directly penetrate the interfaces, where the threshold value increases gradually as the interfacial friction coefficient decreases. Moreover, the abrupt change in the interfacial friction causes hydraulic fractures to deflect at the interface. The injection pressure evolution curves are significantly different due to different fractures behaviors at the interfaces. When hydraulic fractures penetrate the interface, the injection pressure evolution curve shows a significant secondary rise. A distinct prediction model of hydraulic fractures across the coal-rock interface that considers the interfacial friction, the stress state and the intersection angle between the hydraulic fracture and the coal-rock interface was established. This model shows high accuracy in predicting the propagation behavior of hydraulic fracture at different interfaces in hydraulic fracturing.