Applicability of sonic velocities as a monitoring tool for subsurface CO2 plume migration and associated stress change

Porous reservoir rocks like sandstones have gained utmost importance in the last decade as a potential sink for CO₂. Most of the targeted reservoirs are depleted oil and gas fields, which have caprocks to ensure the containment of the injected CO₂. Injecting CO₂ into porous reservoirs increases the pore pressure, reducing the effective horizontal and vertical stresses. Depending on the pre-injection stress condition and permeability of the reservoir, careful monitoring should be in place to define the upper limit of CO₂ injection pressure to prevent any permanent damage to the reservoir, which can lead to leakage or induced seismicity. Lab-scale experiments provide key insights into the deformation behaviour of reservoir rocks under different stress conditions, which can be upscaled to understand reservoir-scale processes. To simulate the stress perturbation caused by CO₂ injection operations, we have subjected porous reservoir rocks (core plugs) collected from different depths of offshore North Sea under realistic reservoir stress and saturation conditions, with liquid CO₂ flow-through leading to failure. The P and S wave velocities along the core plugs were recorded every 15 s to assess the change in wave properties during deformation, fluid displacement and pore pressure build-up. It was observed that during each loading cycle, wave velocities are highest at the elastic-plastic transition zone, which can be attributed to the compression of pores and closure of microcracks perpendicular to the loading direction. The wave velocities and amplitudes decrease sharply after the onset of plastic deformation, which can be attributed to the formation of microcracks in the coreplug due to increasing load. During displacement of brine with CO₂, velocities and amplitudes drop sharply. These indicators are used to develop a traffic light scenario for CCS operations to maintain safe stress conditions in the reservoir. The consistent correlation between the wave properties and mechanical response of the reservoir rocks reveals that constant monitoring of wave velocities during CO₂ injection can act as a cheaper and more efficient tool for monitoring stress state and plume movement in the reservoir, facilitating safer CO₂ storage operations.