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.