The global expansion of subsurface CO₂ and hydrogen storage, alongside geothermal energy development, offers promising pathways for gigaton-scale CO₂ abatement. However, fluid injections and associated thermal effects can significantly alter reservoir stress states, risking fault reactivation and compromising caprock integrity. Direct stress measurements in the subsurface remain technically challenging, particularly beyond the near-wellbore zone. This study investigates how stress-induced changes in ultrasonic P- and S-wave velocities and amplitudes can serve as early indicators of irreversible rock deformation. Using triaxial cyclic and failure experiments on core samples from offshore Netherlands (depths: 3.1–4.2 km; porosity: 8–23 %), we demonstrate that wave velocities and amplitudes increase with axial loading in the elastic regime but decline progressively following crack initiation—well before mechanical failure. This trend reversal provides a reliable sonic precursor to failure. We propose a field-applicable traffic-light monitoring framework using sonic parameters to infer stress changes during injection operations. The observed inverse relationships between porosity and both mechanical strength and sonic velocity, along with the porosity-dependent velocity enhancement under confinement, present a novel opportunity to develop constitutive geomechanical models directly from reservoir sonic logs. This work advances non-invasive stress monitoring approaches and provides engineering geologists with robust tools to improve safety and predictability in subsurface energy storage projects. Moreover, such techniques can also be translated to integrity monitoring for underground mines and engineered structures. ... Read more