Fracture Toughness and Brittleness of the Mancos Shale, Utah.

Fracture toughness is a measure of a material’s resistance to dynamic crack propagation. For linear elastic materials it is defined by the critical stress intensity factor, KIc, beyond which catastrophic crack growth occurs. For materials which deviate from linear elasticity, cyclic loading of the specimen can be used to calculate the brittleness corrected Fracture toughness, KcIc.Fracture Toughness is an important control in the hydraulic fracturing of Gas-Shales, which have become a topic of interest since the US Shale Gas “Revolution”. The mechanical properties of Shales remain poorly constrained, with a wide range of reported property values. There is an extreme paucity of published data on the fracture toughness of soft sediments such as shales.KcIc values and a variety of supporting measurements have been made for the Mancos Shale in the three principle Mode-I crack orientations using a modified Short-Rod sample geometry. A very substantial anisotropy is observed in the loading curves and KcIc values for the three crack orientations (Arrester, Divider and Short-Transverse). The measured brittleness correction factor for Mancos Shales are higher than for any other rocks we have found in the literature, implying that the material is extremely non-linear.