The Fracture Behavior in the Vicinity of Interface Based on the Digital Speckle Experiment

Abstract Rock stratification develops widely in unconventional reservoirs, and it increases the non-uniform of the reservoir on vertical. The interfacial slip deformation can alter the fracture shape when it encounters the weak stratification or the natural fractures, so the rock stratification play an important role in the fracture propagation. In this paper, the crack extension experiment about the interface is established through the three-point bending. The digital speckle measurement technology was used in order to observe the change of the strain field, near the interface layer, in the process of cracks through the interface. At the same time, based on the maximum circumferential stress criterion, an interfacial slip model has been developed, and the extended finite element method which can reveal the distribution of stress and strain on the interface has been used in the numerical realization of the model. The numerical results of the strain field agree well with those of experiments. The experiment and numerical simulation results show that when a crack propagates the interface, the crack will break through the interface, or crack kink in the interface, or crack extend along the interface. The reason of the crack deflection, when a crack goes through the interface, is that the crack tip occurs blunt, and the crack length does not increase while the width increases. The tip blunt causes the interfacial shear strain which can turn the direction of the maximum circumferential stress. The crack deflection angle is affected extremely by the interface strength. The weaker the interface is, the greater of crack tip blunt is and the bigger the shear strain of the interface is, which cause the crack deflection angle increase. Based on the above analysis, it indicates that the blunt of crack tip in the interface is the mechanical cause of the crack to change direction extension. The weak bedding is the important factor causing the complexity of unconventional reservoir fracture morphology. This can be used as one of the standards to evaluate the fracturing effect of unconventional reservoir.