Abstract
Subsurface fluid injections cause rapid thermal and stress changes near wells, potentially compromising reservoir integrity. Effective monitoring and mitigation strategies are essential to ensure safe operations. This study investigates highly porous North Sea sandstones under low confining pressures (1 and 3 MPa), replicating near-wellbore stress conditions for a saline aquifer during CO 2 injection. Using triaxial deformation experiments, we analyze the evolution of ultrasonic velocities, amplitudes, and acoustic emission (AE) signals to track deformation processes. Our results demonstrate that combined ultrasonic and AE monitoring effectively captures and forecasts the transition from elastic to inelastic deformation. An empirical relationship is proposed demonstrating a proof-of-concept correlation between microseismic event counts and ultrasonic velocity during irreversible deformation, which, upon calibration can be used for site-specific stress state monitoring. Higher confinement leads to increased AE activity due to intensified grain crushing and interparticle locking. At constant confinement, more porous samples also show greater AE activity, indicating enhanced damage accumulation. Spectral analysis reveals increased attenuation at higher confining pressures and a shift toward lower frequencies near failure, suggesting dominant shear deformation. Notably, deformation patterns vary with confinement: low confinement induces frictional slip and distinct failure planes, while high confinement promotes distributed damage and porosity reduction through grain crushing. This results in a non-linear failure envelope shaped by the dominant deformation mechanisms. These findings enhance our understanding of pore fluid-induced changes in reservoir rocks and offer new strategies for monitoring CO 2 storage and other subsurface injection operations, especially in heterogeneous, porous formations under varying stress regimes.