4D Seismic Monitoring of CO₂ Storage During Injection and Post-closure at the Ketzin Pilot Site

At the Ketzin pilot site for geological CO₂ storage, about 67,000 tons of CO₂ were injected during the period June 2008 – August 2013. Since August 2013, the site is in its post-closure phase. Before and during the injection phase, a comprehensive monitoring programme was established. In the early post-injection phase, a majority of the monitoring activities have continued. The stepwise abandonment of the pilot site, which is planned to be accomplished in 2018, marks also the termination of most monitoring activities. Four 3D seismic surveys were acquired between 2005 and 2015 for characterizing the reservoir structure and its overburden and for monitoring the propagation of the injected CO₂ in the storage formation. The first and second repeat surveys revealed the lateral extension of the CO₂ plume after injecting 22 and 61 ktons, respectively. In autumn 2015, the third 3D repeat seismic survey, serving as the first post-injection survey, was acquired. The survey was acquired using the same acquisition geometry as for previous surveys, consisting of 33 templates with five receiver lines and twelve source profiles perpendicular to the receiver lines. Seismic processing of the recently acquired data has resulted in preliminary observations which can be summarized as follows: As in previous seismic repeat surveys, a clear CO₂ signature is observed at the top of the storage formation. No systematic amplitude changes are observed above the reservoir which might indicate leakage. Compared to the second repeat survey acquired in 2012, the lateral extent of the CO₂ plume seems to have been reduced, which may be an indication for ongoing (and relatively fast) dissolution of the CO₂ in the formation brine and diffusion into very thin layers indicating pressure release.