Acoustic and exact elastic impedance variations during CO₂ injection at the CaMI.FRS

Yichuan Wang, Donald C. Lawton

For seismic monitoring injected CO₂ during geologic CO₂ storage, it is useful to measure time-lapse (TL) variations of seismic impedance. Acoustic impedance (AI) and elastic impedance (EI) give direct connections to the mechanical and fluid-related properties of the CO₂ storage complex. However, evaluation of their subtle TL variations is complicated by the scaling and low-frequency uncertainties, and the various EI definitions involve different approximations and do not represent an elastic property of the medium. To solve these issues, we perform accurate waveform calibration for TL seismic data, and apply a robust impedance-inversion approach based on calibration of seismic records by using the well-log data. We also use an exact expression of EI in a matrix form, which truly represents the intrinsic physical property and accurately describes P- and S-wave propagations at arbitrary incidence angles. The above approaches are applied to TL DAS VSP data from the Field Research Station CO₂ injection project in southern Alberta, Canada. High-quality baseline, monitor and TL impedance-difference images are obtained. TL impedance variations are observed within the CO₂ injection zone, which are interpreted as being related to the CO₂ injection. AI and EI measured from this approach can be advantageous tools for monitoring the distribution and migration of CO₂ plumes within the CO₂ storage complex.