Quantitative evaluation of thin-layer thickness and CO2 mass utilizing seismic complex decomposition at the Ketzin CO2 storage site, Germany

Abstract

Determining thin layer thickness is very important for reservoir characterization and CO2 quantification. Given its high time-frequency resolution and robustness, the complex spectral decomposition method was applied on time-lapse 3-D seismic data from the Ketzin pilot site for CO2 storage to evaluate the frequency-dependent characteristics of thin layers at the injection level. Higher temporal resolution and more stratigraphic details are seen in the all-frequency and monochromatic reflectivity amplitude sections obtained by complex spectral decomposition compared to the stacked sections. The mapped geologic discontinuities within the reservoir are consistent with the preferred orientation of CO2 propagation. Tuning frequency mapping shows the thicknesses of the reservoir sandstone and gaseous CO2 is consistent with the measured thickness of the sandstone unit from well logging. An attempt to discriminate between pressure effects and CO2 saturation using the extracted tuning frequency indicates that CO2 saturation is the main contributor to the amplitude anomaly at the Ketzin site. On the basis of determined thickness of gaseous CO2 in the reservoir, quantitative analysis of the amount of CO2 was performed and shows a discrepancy between the injected and calculated CO2 mass. This may be explained by several uncertainties, like structural reservoir heterogeneity, a limited understanding of the complex subsurface conditions, error of determined tuning frequency, the presence of ambient noise and ongoing CO2 dissolution.