Ultrasonic response of a brine-saturated reservoir rock during coupled stress and fluid perturbation during liquid-CO₂ injection

Abstract

CO₂ injection into porous sandstone reservoirs offers a promising pathway to curb anthropogenic carbon emissions, but poses risks of leakage and induced seismicity from stress perturbations and fault reactivation without meticulous monitoring. Here, we present a time-lapse monitoring approach based on laboratory measurements of ultrasonic Vp, Vs and corresponding peak amplitudes in critically stressed, partially saturated North Sea sandstones (porosity 9–23%). Our experiments show that Vp and Vs exhibit higher sensitivity (4–15%) to stress changes compared to fluid saturation changes (0.8–1%), whereas amplitudes are more responsive (30–500%) to saturation, showing staggered change when brine is displaced by CO₂. Under pure stress perturbation, amplitude variations are smaller (10–50%). During elastic deformation, the Vp/Vs ratio decreases while the ratio of their corresponding amplitudes increases, underscoring the need for both P- and S-wave measurements. Velocity and amplitude changes are more pronounced in high-porosity rocks. In a critically stressed state (beyond yield/before failure), the rise in pore fluid density from CO₂ injection boosts shear wave amplitudes, offsetting attenuation from inelastic deformation. Knowing the pre-injection stress state enables these velocity and amplitude trends to serve as robust indicators of reservoir conditions during and after CO₂ injection. This cost-effective approach can be adapted to reservoir-scale monitoring and extends beyond CCS, supporting enhanced detection of stress and fluid-induced changes in subsurface formations.