Insights into CO2-caprock interaction through seismic and ultrasonic monitoring: An experimental study

The sealing capacity and structural integrity of caprocks determine the safe and long-term storage of carbon dioxide (CO₂) in a reservoir. To assess the risk of CO₂ leakage, it is important to evaluate changes in the physical properties of the caprock caused by CO₂-caprock interactions and monitor CO₂ plume migration. In this study, we exposed upper Miocene caprock mudstones of the Vagn Formation in the Danish Central Graben to CO₂-saturated brine under in-situ stress conditions to simulate CO₂ injection at a laboratory scale. The goal was to observe the evolution of elastic stiffness parameters, such as Young's modulus and Poisson's ratio, as well as acoustic velocities (P- and S-waves) during different stages of fluid exposure to identify changes, serving as a proxy for time-lapse seismic surveys. Dynamic Young's moduli and Poisson's ratios were measured using the low-frequency (1–143 Hz) forced-oscillation method, while acoustic velocities were estimated through high-frequency (250 kHz / 500 kHz) pulse transmission. A 3–6 % reduction in both elastic and acoustic properties was observed after CO₂ exposure. Mineralogy and porosity likely influenced how the rock interacted with carbonated brine. Notably, due to the limited impact of CO₂ on these mudstones, they have the potential to function as an effective seal for long-term CO₂ storage. These findings are valuable for reducing risks in prospective CO₂ storage sites and for developing improved seismic monitoring strategies for active operational sites.