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.