Application of Seismic Complex Decomposition on Thin Layer Detection of the CO₂ Plume at Ketzin, Germany

Summary

The seismic complex decomposition technique is a spectral decomposition method using inversion strategies to decompose a seismic trace into its constituent frequencies and corresponding complex coefficients. This method has high time-frequency resolution and it is not necessary to select a signal window in comparison to conventional spectral decomposition methods. The thickness of the reservoir at the Ketzin pilot site is relatively thin, making it difficult to resolve seismically due to the band-limited seismic spectrum. This study presents an application of seismic complex decomposition to the time-lapse 3D seismic datasets at the Ketzin pilot site for estimating the temporal thickness of the injected CO₂ within the thin reservoir via frequency tuning. Quantitative analysis for CO₂ thickness and mass is investigated. Comparison between the real recorded data and the estimates shows that our results are reliable in assessing the amount of the CO₂ in the plume at the Ketzin pilot site.

Introduction

The Ketzin pilot site is located west of Berlin, Germany, as an in situ laboratory for monitoring the storage of carbon dioxide (CO₂) in a saline aquifer. The project was initiated in 2004 with the aim to verify effective monitoring methods for mapping the injected CO₂ plume and to provide operational field experience of CO₂ geological storage (Martens et al., 2013; Martens et al., 2014). One injection/observation well (Ktzi 201) and two observation wells (Ktzi 200 and Ktzi 202) were drilled in 2007 prior to CO₂ injection. Over a 5-year period, up to 67 kt of CO₂ were injected into the target reservoir, the fluviatile and heterogeneous Upper Stuttgart Formation. It is characterized by alternating siltstones and mudstones with poor reservoir properties and sandstone channels with good reservoir properties. The main-reservoir sandstone unit is 9- 20 m thick in the three wells (Norden et al., 2010).

The time-lapse 3D seismic method has proven to be a successful technique to monitor the growth of the CO₂ plume at the Ketzin site. A 3D baseline seismic survey was acquired in autumn 2005 prior to CO₂ injection (Juhlin et al., 2007). Two 3D repeat seismic surveys were acquired in autumn 2009 and autumn 2012, after about 22 kt and 61 kt of CO₂ had been injected, respectively. Results from the time-lapse analysis (Figure 1) show conspicuous amplitude anomalies due to changes in the reservoir properties after CO₂ injection and a preferred westward trend of CO₂ migration, reflecting the internal heterogeneity of the reservoir.