Characterizing mechanical anisotropy in rocks using instrumented scratch test

This study demonstrates a successful application of scratch test to characterizing geomechanical anisotropy in weakly to moderately anisotropic sedimentary rocks. Scratch tests were performed in three orthogonal directions on four sedimentary rock samples: one limestone and three shales, and the scratch-derived mechanical properties were compared to anisotropic parameters calculated from acoustic velocities. Results showed a similar pattern of anisotropy in the scratch-derived unconfined compressive strength (UCS) and fracture toughness (K_IC), and velocity-derived Thomsen’s parameters. The measured values also compared well to previous studies of anisotropy in shales. The UCS and K_IC for shale samples were correlated: shales with higher UCS were also characterized by higher K_IC, and vice versa. The lowest values for both were observed perpendicular to bedding planes. All parameters consistently indicated that three out of four samples were transversely isotropic, and one sample exhibited more orthotropic behavior. Overall, our results indicate that scratch tests provide a simple and reliable tool for screening and potentially quantifying mechanical properties of anisotropic rocks. Compared to the majority of conventional geomechanical methods, which are destructive and require multiple directional samples, scratch test provides an opportunity to characterize mechanical anisotropy with very limited rock material and/or on a single sample.