Hydraulic fracturing is a widely-used solution to improve continuity and connectivity across thin layers and to bypass near-wellbore damage. However, predicting and controlling the initiation and cessation of a hydraulic fracture remains a challenge due to compositional and poromechanical heterogeneity, which causes stress concentration, and due to inelasticity at the grain scale. The presence of organic matter also affects the rock-mechanical properties and directionality in fracture initiation and propagation processes. Overall, understanding the fracturing behavior of a rock at the microscale plays a critical role in predicting the performance of hydraulic fracturing during and after the “frac job”. In this paper, we propose a method to investigate the fracture initiation and propagation behaviors in tight sandstones and shales by estimating fracture toughness and directionality using micro-scale mechanical scratch tests. Scratch tests provide a means to account for grain-scale heterogeneity and inelasticity during measurement of fracture properties in different directions. Three uniformly-spaced, consecutive scratch tests are performed in a sample for a representative coverage of the sample surface in terms of the number of grains. The characterization of scratch track using Scanning Electron Microscopy provides a means of identifying the initiation and propagation of failures at critical points during the loading process. The fracture toughness and fracture directionality values of tight sandstone and shale samples are compared to understand the effect of lithology on fracture initiation and propagation processes, respectively. We discuss the influence of the packing density of matrix, porosity and pore size distribution on the fracture processes.
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