Combined low-frequency and micro-CT approach to reveal two-phase fluid saturations

One of the critical challenges in Carbon Capture and Storage (CCS) is to ensure that the CO₂ is stored safely in the reservoir over a long time. The approach is to monitor the stability based on seismic surveys that measure the change in seismic wave propagation as the amount of stored CO₂ increases. This is a well-established tool to monitor high CO₂ concentrations. To also access the regime of low CO₂ concentrations, imaging of two-phase fluid saturations and understanding the effect on the mechanical behavior on the laboratory scale is the key to ensure precise CO₂ monitoring. Low-frequency measurements give direct access to elastic properties within the seismic frequency band, which can be directly compared to seismic surveys. Additional µCT imaging can validate the actual saturations and give insight into dispersion mechanisms on the pore scale. 

To perform laboratory measurements at seismic frequencies and simultaneous capture processes inside the rock, we built a CT transparent triaxial stress apparatus. This allows for the validation and calibration of rock physics models for partially saturated media utilized for geological sequestration. The sample is saturated with water, and confining and pore pressure are increased. Then, CO₂-saturated water is flown in the sample, which is stepwise depressurized so that part of the CO₂ changes from liquid to gaseous CO₂. Rock mechanical properties are determined at low frequencies (0.5 – 150 Hz) and strain amplitudes from 10^-7 – 10^-5, measuring Young's modulus, Poisson's ratio and attenuation at seismic frequencies and ultrasonic P- and S-wave velocities. In addition,  µCT-imaging of the pore structure and fluids is performed. The results show differences in elastic properties between a dry, partially and fully saturated sample and the effect of partial CO₂-gas saturation at seismic and ultrasonic frequencies. P-wave velocities are reduced when free CO₂ gas is in the system. The change is more significant at seismic frequencies, which leads to an increased dispersion.

The apparatus allows for 3D pore-scale imaging of CO₂ in a fluid and gaseous phase while measuring frequency-dependent attenuation. Identifying fluid-solid interactions and estimating saturation via µCT-imaging is particularly critical when only a small amount of gas is present. Therefore, our combined approach allows precise determination of elastic properties at seismic frequencies for different CO₂ saturations to establish a rock physics model calibrated to partial CO₂ saturations to ensure safe CCS injection and monitoring.