Time-lapse attenuation-attribute variations during CO₂ injection using DAS VSP data from 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 attenuation. Seismic attenuation directly connects to different petrophysical parameters of the CO₂ storage complex. We have developed an approach to derive smooth time-variant spectra from seismic signal by using sparse strongest signal peaks, and then measure two different attributes characterizing the path effects of seismic attenuation from the smooth spectra. This approach is straightforward and does not require sophisticated algorithm or parameterization scheme. We apply this approach to TL DAS VSP data from the Field Research Station (FRS) CO₂ injection project in southern Alberta, Canada. High-quality stacked reflection records are obtained from baseline and monitor DAS VSP surveys at the FRS and TL attenuation-attribute differences are derived from these reflection records. TL variations of attenuation attributes are observed within the injection zone at the FRS, which are interpreted as being related to the injected CO₂. Although there is always a significant trade-off between the accuracy and temporal resolution of the measured attenuation parameters, reliable attenuation measurements around the injection zone are still achieved with in-use reflection signal of a sufficient length and bandwidth. Attenuation attributes measured from this approach can be an advantageous tool for monitoring the distribution and migration of CO₂ plumes, especially for TL monitoring with DAS data that is with high noise level or with apparent acquisition or noise difference between different vintages.