Unraveling Damage Variables in Hot Dry Rocks: An Optical Investigation of Geothermal Rocks from UTAH FORGE

Abstract

Geothermal resources are worldwide renowned for their sustainable and clean attributes. The US Department of Energy (DOE) estimates that harnessing only 0.1% of Earth's geothermal energy could support mankind for two million years. Enhanced geothermal systems (EGS) are highly acclaimed for their ability to extract heat from hot, dry rock. However, issues remain in assuring safe drilling and proper fracturing owing to a lack of knowledge of damage factors and fracture sequences, both of which are crucial for regulating crack propagation. Conventional laboratory approaches often fail to capture the nuanced variation seen in HDR. To address this, the present work uses a three-dimensional optical approach called digital image correlation (3D-DIC) to investigate damage factors in HDR. HDR samples from the DOE UTAH FORGE project's Well 16B (78)-32 are subjected to uniaxial (D 1.5", L 2") and diametrical (D 1.5", L 1") compression using a 100kN precision electro-mechanical load Instron frame at a constant displacement rate of 0.05mm/min. During the uniaxial and diametrical compression studies, a Trilion 3D-DIC image capturing system was used to monitor the samples at a rate of 10 frames per second in a non-contact manner. To monitor deformation during loading, a black-in-white speckle pattern is placed on the specimen. The GOM 3D-DIC system is used to process images, visualize data, and analyze HDR damage variables under various load circumstances. The findings showed that DIC-generated quantitative full-field strain preliminary maps (tension, compression, and shear) include all sequences involved in the damage process as well as discrete strain localization zones (SLZ). Damage factors are quantified using DIC maps to assess sample damage; the tension-compression ratio ranges between 2% and 8%. The damage evolution process of HDR specimens is divided into four phases that are assessed using damage variables: initial damage stage, linear elastic, elastic-plastic, and plastic damage stage. The damage variable regulates the deterioration of the material's stiffness, resulting in a nonlinear relationship between stress and strain. The damage to the SLZ demonstrates that it is bigger than the entire. The damage was at 65% in the yield stage and 35% in the first two stages. The 3D-DIC findings showed that the sample failed when the overall damage variable reached 0.25-0.35, and the damage in the SLZ region reached 0.8-near unity. The damage variable in HDR, which indicates fracture progression, is a unique characteristic in geothermal systems. These results have a substantial impact on our capacity to forecast the damage process in EGS. DIC outperforms CT, SEM, and AE methods in test range, cost-effectiveness, accuracy, and full-field monitoring. It improves our knowledge of damage factors in anisotropic and heterogeneous HDR, increases fracturing efficiency, and improves heat extraction from EGS.