Measurement of Geomechanical Properties: Comparison Between Scratch Test and Uniaxial Compression Test Methods

The Scratch Test is a quasi-non-destructive method made up of pushing a tool across the surface of a weaker rock at a given penetration depth. The Uniaxial Compression Test is a destructive method of measuring the maximum axial compressive stress a rock can withstand before failing. The unconfined compressive strength (UCS), and Poisson's ratio (ν) are some of the geomechanical properties that can be obtained using the scratch test and uniaxial compression test methods. Previous studies have shown the scratch test method to be reproducible, non-destructive, and faster in juxtaposition to the “conventional” uniaxial compression test method, in addition to the high degree of correlation between rock strength measurements using these 2 methods. However, there is a need for further investigation on heterogenous shale rock samples to validate the degree of correlation in geomechanical properties using these 2 methods. This study experimentally investigated geomechanical properties of heterogeneous shale rocks using the scratch test and the uniaxial compression test methods, and compared the degree of correlation between these two methods of geomechanical measurements. This study utilized rock samples from 3 different shale formations, and subjected the samples from each geologic formation to the scratch test and the uniaxial compression test, respectively. The results from this study showed that in shale rocks, there is a high degree of correlation between the geomechanical properties (±20% UCS; ±10% ν) obtained from the scratch test and the uniaxial compression test methods. Further, the results show that the scratch test method can provide localized and detailed mechanical measurements of points along the longitudinal axis of the core samples, whereas the uniaxial compression test can provide larger-scale and bulk measurements of the core samples.