A fracture mechanics study of an anisotropic shale

The use of hydraulic fracturing to recover shale-gas has drawn attention to the fundamental fracture properties of gas-bearing shales. Such shales are strongly anisotropic, but there is a paucity of available experimental data on their mechanical and physical properties. Nevertheless, fracture propagation trajectories in these materials depend on the interaction between the anisotropic mechanical properties of the shale and the anisotropic in-situ stress field in the shallow crust. The mode-I stress intensity factor, KI, quantifies the concentration of stress at a crack tip under tensile loading. The fracture toughness of a linear elastic material is then defined as the critical value of this stress intensity factor; KIc, beyond which rapid, catastrophic crack growth occurs. However, shales deviate significantly from linear elasticity, and exhibit marked hysteresis during cyclic loading. Beneficially, this hysteresis enables the calculation of a ductility coefficient, which can be used to determine a ductility corrected fracture toughness value, KcIc. In Mancos shale this ductility correction can be as large as 60%. Here we report KcIc determined using a modified Short-Rod methodology in the three principal fracture orientations; Arrester, Divider and Short-Transverse, on Mancos shale. Experimental results for a range of other rock materials are also reported for comparison purposes. Significant anisotropy is observed in KcIc measurements at ambient conditions, with KcIc in the Divider and Arrester orientations being around 3.4 times that in the Short-Transverse orientation, where the fracture plane and propagation direction both lie parallel to the bedding. Here, a bimodal distribution in KcIc is recorded. This has been interpreted as being due to a progressing crack becoming trapped in one of the two types of layers within the material. KcIc measurements were also conducted on heat-treated samples, at elevated temperatures up to 150oC. After heat-treatment, we cease to observe the higher of the two Short-Transverse oriented KcIc values. We also observe a small increase in KcIc in the Arrester orientation with increasing temperature, causing a slight increase in anisotropy by 150oC compared to ambient conditions. Effective KIc was then measured as a function of confining pressure, with KIc increasing approximately four-fold between ambient conditions and 30MPa. KIc anisotropy is observed to decrease substantially with increasing confining pressure over the same range, suggesting that the material behaves near-isotropically at depth. Substantial difficulties were encountered with Arrester orientation fractures deflecting into the weaker Short-Transverse orientation during experiments. To analyse these deflections, fracture propagation criteria were developed based on the maximum circumferential stress and maximum energy release rate criteria. These criteria were trialled for the prediction of crack trajectories using a Singular Integral Equation methodology, and suggest that material anisotropy is able to cause sharp fracture deflection under stress states where no deflection occurs in isotropic material.