Integration of triaxial ultrasonic velocity and deformation rate analysis for core-based estimation of stresses at the Utah FORGE geothermal site

Core-based methods for in-situ stress estimation were applied using samples from 4 intervals within the Utah FORGE 16A(78)-32 well. At three of these locations, Triaxial Ultrasonic Velocity (TUV) tests were performed, resulting in experimentally-determined relationships between wave velocities and stresses. Non-monotonic increases in the velocity-stress relationships provide evidence of stress history and are therefore used to estimate in-situ stress magnitudes. Additionally, Deformation Rate Analysis (DRA) tests were run on core plugs from various orientations at each of the sampling locations. Results indicate the minimum stress gradient to be ∼0.61–0.74 psi/ft (14–17 kPa/m) and consistent with uniform tectonic strain of 150 microstrain in the Granitoid and tectonic strain that is around 50 % higher in the Gneiss than in the Granitoid. The vertical stress gradient is ∼ 1.09–1.12 psi/ft (24.7–25.3 kPa/m) for the two zones in which it could be inferred from data. The maximum stress gradient ∼ 0.90–1.01 psi/ft (20–23 kPa/m) in the granitoid and 1.32 psi/ft (30 kPa/m) in the Gneiss and consistent with tectonic strain around 300 microstrain through the Granitoid and over 900 microstrain in the Gneiss. The stress regime thus appears to be between normal faulting and strike-slip faulting, potentially becoming more strike-slip in nature with depth and/or flipping back and forth between the two regimes in the region penetrated by the well due to variability of rock properties, structures such as faults, and/or thermal anomalies. These are broadly consistent with other recently published stress models, and therefore provide valuable evidence to the stress state at the Utah FORGE geothermal site and a promising step in the direction of using TUV and DRA together as core-based stress estimation methods.