Abstract: To investigate the dynamic propagation process of the fracturing crack across the coal-rock interface, similar materials were used to prepare coal-rock combined specimens. Three-point bending tests and true triaxial hydraulic fracturing tests were carried out. By the digital speckle technology and the acoustic emission (AE) technology, the dynamic propagation characteristics of the fracturing crack were captured. The fracture pattern and its influencing factors were analyzed. The results show that in the three-point bending test, the crack can penetrate into the coal seam directly from the roof without changing direction at the interface. The peak stress required for the specimen fracturing is reduced while increasing the prefabricated crack length. In the true triaxial hydraulic fracturing test, due to the strong plasticity of the coal seam, the crack height and length in the roof are both larger than those in the coal seam, and the proportion of acoustic emission events in the roof is also higher than that in the coal seam. When the crack propagates across layers, increasing the distance between the horizontal well and the top surface of the coal seam will lead to the extension of the crack propagation time. Increasing the injection rate of the fracturing fluid can increase the penetration depth of the crack into the coal seam, but it is easy to cause the crack height to be out of control and the reduction of crack length. The fracturing method with variable injection rates was proposed. In the initial stage, the fracturing fluid injection with a large rate promotes the crack propagation across layers, and then the injection rate is reduced to promote the lateral propagation of the crack in the roof and coal seam. There is a competitive propagation phenomenon among cracks when multiple cracks are initiated synchronously, and part of the cracks can not propagate across layers. The research results can provide support for mastering the propagation characteristics of the crack across the coal-rock interface and optimizing the hydraulic fracturing parameters.

Key words: coal-rock interface, fracturing across layers, dynamic propagation, digital speckle, acoustic emission