On the effect of CO₂ on seismic and ultrasonic properties: A novel shale experiment

Carbon capture and storage (CCS) by geological sequestration comprises a permeable formation (reservoir) for CO₂ storage topped by an impermeable formation (caprock). Time-lapse (4D) seismic is used to map CO₂ movement in the subsurface: CO₂ migration into the caprock might change its properties and thus impact its integrity. Simultaneous forced-oscillation and pulse-transmission measurements are combined to quantify Young’s modulus and Poisson’s ratio as well as P- and S-wave velocity changes in the absence and in the presence of CO₂ at constant seismic and ultrasonic frequencies. This combination is the laboratory proxy to 4D seismic because rock properties are monitored over time. It also improves the understanding of frequency-dependent (dispersive) properties needed for comparing in-situ and laboratory measurements. To verify our method, Draupne Shale is monitored during three consecutive fluid exposure phases. This shale appears to be resilient to CO₂ exposure as its integrity is neither compromised by notable Young’s modulus and Poisson’s ratio nor P- and S-wave velocity changes. No significant changes in Young’s modulus and Poisson’s ratio seismic dispersion are observed. This absence of notable changes in rock properties is attributed to Draupne being a calcite-poor shale resilient to acidic CO₂-bearing brine that may be a suitable candidate for CCS.