Dynamic mode III fracture behaviors in shale: Unraveling the controlling role of bedding structures through experimental characterization and theoretical insights

Understanding the dynamic cracking mechanism of shale poses fundamental in the hydraulic fracturing engineering and controllable shock wave fracturing technique for shale oil and gas exploitation. Despite its importance, existing studies mainly focus on mode I and mode II fracture, experiments on the dynamic mode III fracture behaviors of shale remains scarce. In this study, using the novel axially notched flattened Brazilian disc, we systematically characterized the quasi-static and dynamic mode III fracture behaviors of Lushan shale and compared the rate-dependent anisotropic mode I, mode II and mode III fracture toughness. Experimental results show that the K_IIId values are slightly smaller than K_IId values, while significant larger than K_Id values of shale. The dynamic fracture toughness that penetrating the shale bedding plane is always higher than that along the shale bedding plane. Furthermore, we quantitatively analyzed the rate-dependent spatio-temporal mode III fracture morphology characteristics of shale by fractal dimension and joint roughness coefficient (JRC). The results indicate that the Arrester orientation samples exhibit rougher fracture surface and higher loading rate can reduce the roughness. Interestingly, two types of unconventional mode III fracture behaviors that violate the classical mode III fracture patterns of isotropic solids are firstly observed in shale: the ubiquitous mode III crack front segmentation phenomenon vanishes in the Short-transverse orientation and the famous facet coarsening phenomenon is suppressed in the Arrester orientation. Finally, these unconventional mode III fracture behaviors of shale are theoretically predicted and interpreted using an anisotropic strength criterion. The predicted facet tilting angle agrees well with the experimental results. Our findings highlight that the mode III fracture behaviors of shale are controlled by the bedding structure and loading rate. These results may hold significant implication for guiding the dynamic fracturing of shale reservoir using the controllable shock wave fracturing technique.