Understanding the mechanical behavior and damage evolution of rock specimens is a cornerstone for safe and efficient practices in various geoscience and energy sectors. Traditional laboratory methodologies cannot detect fine-scale variability in Hot Dry Rock (HDR) of Enhanced Geothermal System (EGS) and sedimentary structures. Characterizing damage variables is crucial because of their inherent discontinuities and natural fractures. This study examines damage variables for sedimentary and geothermal rock specimens using three-dimensional digital image correlation (3D-DIC).
A set of sedimentary rock (shale, sandstone, and carbonate) and HDR samples are examined under uniaxial (D 1.5", L 2") and diametrical (D 1.5", L1") compression utilizing a precision 100 kN Instron electro-mechanical load frame with a constant displacement of 0.05 mm/min. HDR samples are collected from the Department of Environment's Utah FORGE project core library. The samples displayed distinct discontinuities and natural fractures. 3D-DIC system was set up to take pictures of the samples at a rate of 20 FPS while the uniaxial and diametrical compression tests were being done. To track deformation while loading, a black-in-white speckle pattern is applied to the specimen. The 3D-DIC system is utilized for image, visualization, and analysis of damage variables under varied load conditions.
DIC-made quantitative full-field strain preliminary maps (tension, compression, and shear) show all the steps in the damage process, along with clear strain localization zones. To evaluate the sample damage, the tension-compression ratio is obtained at 5% – 15%. All rock specimens go through four stages of damage evolution, in which damage variables are evaluated: initial, linear elastic, elastic-plastic, and plastic damage stages. The DIC results revealed that the sample failed when the overall damage variables reached 0.45, 0.37, 0.24, and 0.27 for sandstone, carbonate, shale, and HDR, respectively.
The study reveals that DIC, a non-contact technique, offers advantages over CT, SEM, and AE in terms of test range, cost, precision, and full-field monitoring. It is widely used in rock mechanics to measure damage in various rocks, including HDR and sedimentary rocks. DIC is superior for understanding fracturing anisotropic and heterogeneous rocks and predicting stimulated reservoir volume. The findings will improve HDR fracturing efficacy, heat recovery from EGS, and fracture optimization of sedimentary formations.