Fracture toughness in shales using nano-indentation

Abstract

Fracture toughness property is often ignored but has a significant impact on the hydraulic fracture design, i.e. length and height. It represents the resistance of the rock to hydraulic fracturing and is a direct indicator of rock fracability. It is difficult to measure fracture toughness in shales using current experimental procedures designed for conventional rocks largely due to mechanical and chemical instability in shales. This study measures fracture toughness in shales using nano-indentation. Nano-indentation can, in fact, be used to derive both tensile and shear dominated fracture toughness. The observations in the study show that while crack-length method derived fracture toughness is close to Mode I (tensile) fracture toughness, energy-method based fracture toughness is closer to Mode II (shear) fracture toughness.Fracture toughness was calculated for 67 samples in different shale formations namely Marcellus, Wolfcamp, Woodford and Eagle Ford. The results show that mineralogy had a strong control on fracture toughness. Fracture toughness decreases with increasing porosity, TOC (Total Organic Carbon) and clay content. The fracture toughness also shows strong anisotropy with fracture toughness measured parallel to the bedding planes being generally 33% higher than the corresponding values measured normal to the bedding planes. The fracture toughness shows a direct correlation with other mechanical properties like Young's modulus, hardness and brittleness. Typically, samples with fracture toughness values greater than 3 MPa m0.5 were found to be brittle based on Young's modulus and Poisson's ratio cross plots. Evaluation of fluid effects on fracture toughness shows that it decreases by 10–37% due to spontaneous imbibition with low salinity brines. This could help understand the impact of hydraulic fracturing fluid on fracture propagation and stimulated reservoir volume (SRV) creation.