Optical characterization of full-field deformation and damage evolution in hot dry rock under mechanical loading

Geothermal energy continues to emerge as a reliable, low-carbon resource with strong potential to support long-term clean energy goals. However, the development of enhanced geothermal systems is often limited by incomplete understanding of fracture initiation, propagation, and associated damage evolution within hot dry rock. Accurately characterizing these processes is essential for predicting stimulation outcomes and ensuring reservoir stability. This study employs high-resolution three-dimensional digital image correlation (DIC) to quantify strain localization and damage progression in hot dry rock samples collected from the Department of Energy Utah FORGE project’s Well 16B (78)-32. Laboratory tests conducted under uniaxial compression and Brazilian tensile loading provide a controlled framework for observing mechanical responses under contrasting stress regimes. The DIC-based analysis identifies four distinct deformation stages: Initial closure, linear elastic response, elastic-plastic transition, and a final plastic phase associated with unstable fracture growth. Across both loading conditions, deformation is governed by tensile-dominated strain fields, with fractures consistently initiating and propagating along the specimen centerline. Damage variables derived from strain statistics range from approximately 0.25±10% in Brazilian tensile tests to 0.30±20% in uniaxial compression, with all samples falling within a broader interval of 0.25 to 0.40. These values reflect meaningful variations in internal degradation influenced by loading mode and specimen geometry. Overall, the findings advance the fundamental understanding of hot dry rock fracture mechanics and provide quantitative parameters that can enhance geomechanical modeling, improve stimulation design, and contribute to more effective and reliable geothermal reservoir development.