Effect of natural filling fracture on the cracking process of shale Brazilian disc containing a central straight notched flaw

Abstract

Abstract The interaction between hydraulic fractures and natural fractures is a key factor in the forming of complex networks during shale fracturing. In spite of abundant natural filling fractures having been found in practical shale layers, previous studies have either ignored the effects of natural fractures or highly simplified them. In order to investigate how the natural filling fracture effect on crack propagation, in this paper, the central straight notched Brazilian discs (CSNBD) of shale under different loading angles were used to study the crack initiation and propagation via digital image correlation (DIC) method. The testing results reveal that the natural filling fracture leads to larger loading displacement, more active acoustic emission signals and lower strength, which indicates that the infillings may contain numerous micro-cracks, therefore, making the filling fracture a weak structure; The crack propagating path was unaffected when the filling natural fracture is parallel to loading line, while the natural filling fractures still on the risk of cracking due to the incompatible deformation between the natural infillings and the host rock; Besides, the initial cracking angle of the crack tip near the natural filling fracture increases while it decreases for the crack tip away from the natural filling fracture when the loading angle is 30°, 60° and 90°, nevertheless, for these discs without an infilling fracture, the two wing cracks are nearly symmetrical and initial cracking angles are almost equal. This research manifests that, on the laboratory scale, the failure modes and the cracking patterns of shale become more complicated when natural filling fractures exist. Therefore the influences of natural filling fractures could be even more significant when it is referred to the practical scale of on-site hydraulic fracturing, the related topics should be further studied in detail.