Quantifying the impact of effective stress on changes in elastic wave velocities due to CO₂ injection into a depleted carbonate reef

Abstract

The interpretation of time‐lapse seismic monitoring of CO2 sequestration has typically focused on mapping the saturation and pressure changes. However, elastic wave velocity and associated seismic attribute variation also depend on the poroelastic response to CO2 injection. In this study, we investigated the impact of effective stress changes on elastic wave velocity responses during CO2 storage within a carbonate reef of the Michigan basin. The geomechanical multiphase flow modeling combined with the experimental measurements were used to investigate the impact of effective stress on changes in velocities. We measured the ultrasonic compressional (Vp) and shear (Vs) wave velocities in cores acquired from the carbonate formations under different stress‐pressure scenarios. Experimental scenarios were applied by changing the effective stress and pore pressure conditions simulating injection into the depleted reservoir. Experimental results show that incorporating stress changes of injection avoids overestimation of changes in velocities due to considering only pore pressure effects. Coupled multiphase flow – geomechanical simulations were performed to assess stress and saturation change during CO2 injection. We discussed expected changes in elastic wave velocities using combined simulation results with experimental ones. The simulation results show that both effective stress and saturation changes contribute to changes in velocities in the main reservoir. Effective stress is the dominant parameter that contributes to changes in velocities in the upper formations. Results of this work show that quantifying the poroelastic effect of stress on elastic wave velocities changes can lead to a better interpretation of changes in seismic attributes during CO2 injection. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.