Research on the anisotropic fracture behavior and the corresponding fracture surface roughness of shale

Understanding the anisotropic fracture mechanism of shale is of significance to assess the development of a stimulated reservoir volume. In this study, three-point bending tests were carried out using notched deep beam specimens of shale with 7 different bedding dip angles to investigate the anisotropic fracture behavior. The anisotropic characteristics of the fracture toughness, energy release rate, inelastic zone and crack propagation path were comprehensively analyzed and highlighted. The results showed that the fracture toughness decreases with increasing bedding dip angle. When the bedding dip angle is 0 degrees and 90 degrees, the fracture toughness reaches the maximum and minimum values, respectively, with a ratio of 1.60. The variation tendency of the energy release rate with the bedding inclination angle is consistent with that of the fracture toughness, but the ratio of the maximum value to the minimum value of the energy release rate is about 3.24. The size of the inelastic zone first decreases and then increases at different bedding inclination angles from 0 degrees to 90 degrees and reaches the maximum size of 1.51 mm when the bedding plane inclination angle is 45 degrees. The crack propagates along the bedding plane rather than straight along the initial crack direction when the bedding dip angle is higher than 45 degrees. As the bedding plane dip angle increases from 0 degrees to 45 degrees, the crack surface roughness of the tested shale first increases, then decreases and finally reaches the maximum. The discussion indicated that the presence of the Mode II fracture has a considerable influence on the crack surface roughness and crack propagation path, although it has little effect on the failure load. Compared with the maximum energy release rate (MERR) criterion, the maximum tangential stress (MTS) criterion seems more accurate in predicting the anisotropic Mode I fracture toughness. The applicability of these two classical crack initiation criteria of fracture mechanics has been analyzed based on our experimental results, which demonstrates the validity of the MTS criterion to assess the anisotropic fracture toughness of shales.

Automated summary

Studying the anisotropic fracture behavior of shale is crucial for evaluating the development of a stimulated reservoir volume. The fracture toughness decreases with increasing bedding dip angle, reaching maximum and minimum values at 0 degrees and 90 degrees, respectively, with a ratio of 1.60. The crack propagates along the bedding plane when the dip angle is higher than 45 degrees.