M. Chandler

Fracture toughness anisotropy in shale

The use of hydraulic fracturing to recover shale-gas has focused attention on the fundamental fracture properties of gas-bearing shales, but there remains a paucity of available experimental data on their mechanical and physical properties. Such shales are strongly anisotropic, so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in-situ stress field in the shallow crust. Here we report fracture toughness measurements on Mancos shale determined in all three principal fracture orientations; Divider, Short-Transverse and Arrester, using a modified Short-Rod methodology. Experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes. Significant anisotropy is observed in shale fracture toughness measurements at ambient conditions, with values, as high as 0.72MPam1/2 where the crack plane is normal to the bedding, and values as low as 0.21MPam1/2 where the crack plane is parallel to the bedding. For cracks propagating non-parallel to bedding, we observe a tendency for deviation towards the bedding-parallel orientation. Applying a maximum energy release rate criterion, we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed-mode loading conditions. We find for Mancos shale that the fracture should deviate towards the plane with lowest toughness regardless of the loading conditions.

Experimental Determination of the Fracture Toughness and Ductility of the Mancos Shale, Utah

Hydraulic fracture treatment of Gas-Shales is increasingly being investigated across Europe. Understanding the propagation of hydraulic fractures under in-situ conditions is important for treatment design, both to maximise gas accessed, and to minimise the risk of fracture overextension. Fractures will always propagate along the path of least resistance, but the direction and extent of this path is a complex relationship between the in-situ stress-field, the anisotropic mechanical properties of the rock, and the pore and fracturing pressures. The anisotropic mechanical properties of gas-shales remain poorly constrained. In particular, there is an extreme paucity of published data on the Fracture Toughness of shales. Mode-I Fracture Toughness is a measure of a material’s resistance to dynamic tensile fracture propagation. Defects such as pre-existing microcracks and pores in a material can induce high local stress concentrations, causing fracture propagation under substantially lower stress than its bulk strength. We report anisotropic Fracture Toughness values made on the Mancos Shale in the three principle Mode-I crack orientations (Arrester, Divider and Short-Transverse) using a modified Short-Rod sample geometry. A substantial anisotropy is observed in the loading curves and Fracture Toughness values for the three crack orientations, and the material is seen to be extremely non-linear.

Effect of temperature on the fracture toughness of anisotropic shale, and other rocks

Abstract Fracture toughness was measured for a range of rock materials as a function of temperature between ambient temperature and 150°C. Measurements were made along all three principal crack orientations for the transversely isotropic Mancos shale and in single orientations for the more isotropic Darley Dale sandstone, Indiana limestone and Lanhelin granite. Fracture toughness was measured using a modified short-rod method with the sample and loading equipment enclosed within an elevated temperature chamber. A slight increase in KIc was observed in Lanhelin granite with increasing temperatures up to 54°C, before a steady decrease at higher temperatures. For the sandstone and limestone, little change was observed in KIc over the studied temperature range. In measurements on Mancos shale at elevated temperatures. Fracture toughness was seen to increase slightly with increasing temperature in the arrester orientation over this range, while remaining constant in the other two orientations. These observations can be explained in terms of the development of thermally induced microfractures parallel to the bedding planes in this material. A bimodal distribution of KIc values in the short-transverse orientation was not observed, as it has been for previously published measurements at ambient conditions.