Experimental Investigation on the Slip Nucleation of Laminated Shale During Fracture Propagation

Slip usually occurs in the laminated structure of shale formations during hydraulic fracturing. In this paper, we used laminated shale to conduct macro and micro three-point bending experiments, as well as the visual hydraulic fracturing experiment to study the slip nucleation during fracture propagation, where the digital image correlation method (DIC) was used to monitor the interlaminar deformation. Based on the linear slip weakening model, a method to identify the slip nucleation zone (SNZ) using DIC was proposed, which could visually detect the formation of SNZ. In laminated shales from millimeter scale to decimeter scale, two asymmetric slip nucleation zones along the interface were observed on both sides of the opening-mode fracture tip before cohesionless slipping. The dominant path of slip could be predicted from the developed size of the conjugated SNZs. The linear slip weakening model considering the tensile fracture tip opening displacements ( w ) was discussed, according to which the SNZ length and interlaminar fracture energy would increase with w during quasi-static nucleation. After the cohesionless slipping, the residual slip zone was defined, which should be a part of the hydraulic fracture systems but was ignored in previous research. The findings of this study could help to understand the generation mechanism of slips in shale reservoirs during hydraulic fracturing.