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Between 2004 and 2017 the storage of CO2 in a deep saline aquifer was investigated in detail at the Ketzin pilot site close to Berlin. The series of research projects, coordinated by the GFZ German Research Centre for Geosciences, also incorporated an interdisciplinary scientific monitoring program during all phases of the storage process which provided a reliable insight into the overall behaviour of CO2 during storage in the underground. Prior to abandonment and cementing of the Ktzi 203 observation well, two side tracks were cored from 643.1 to 662.6 m and from 624.5 to 654.5 m, respectively. The drilled sections included the transition zone from the cap rock (Weser Formation) to the reservoir in the Stuttgart Formation which had been exposed to the injected CO2 for more than nine years. During the drilling operation continuous mud gas logging was performed by applying a mud gas separator, a mass spectrometer and a CO2 isotope-analyser for continuous separation and analyses of the extracted gas. Drill cores were shrink-wrapped with plastic foil to collect discharging gases. CO2 concentrations in the cap rock were relatively low and their isotopic compositions were in accord with typical Upper Triassic formation fluids providing no evidence for a significant CO2 infiltration from the underlying reservoir into the cap rock. A clear increase in the CO2 concentration and a shift in δ13CCO2 between the cap rock and the reservoir indicated the presence of the injected CO2 in the sandstone formation. The comparison with samples collected five years earlier in 2012 showed similar concentrations and isotopic depth trends which indicate a rather stagnant CO2 plume at depth and the tightness and integrity of the cap rock and the borehole cementing.

The pilot study at the Ketzin site close to Berlin (Germany) aims at in-situ testing of geological storage of CO 2 in a saline aquifer. Following site characterization and the drilling of one injection well and two observation wells, the in-situ field laboratory has been fully in use since CO 2 injection started in June 2008. After two years of operation, about 36,000 tons of CO 2 have been injected. This paper presents the key results from the second year of injection and the interdisciplinary monitoring concept in the frame of the European project CO2SINK (CO 2 Storage by Injection into a Natural Saline Aquifer at Ketzin) and accompanying projects.

The injection of CO2 at the Ketzin pilot site in Germany ended after five years in August 2013. We present the key results from site operation and outline future activities within the post-closure phase. From June 2008 onwards, a total amount of 67 kt of CO2 was safely injected into a saline aquifer (Upper Triassic sandstone) at a depth of 630 m - 650 m. The CO2 used was mainly of food grade quality (purity > 99.9%). In addition, 1.5 kt of CO2 from the pilot capture facility “Schwarze Pumpe” (power plant CO2 with purity > 99.7%) was injected in 2011. During regular operation, the CO2 was pre-heated on-site to 45C before injection in order to avoid pressure build-up within the reservoir. During the final months of injection a “cold-injection” experiment with a stepwise decrease of the injection temperature down to 10C was conducted between March and July 2013. In summer 2013, the injection of a mixture of 95% CO2 and 5% N2 was also tested. After ceasing the injection in August the injection facility and pipeline were removed in December 2013. Geological storage of CO2 at the Ketzin pilot site has so far proceeded in a safe and reliable manner. As a result of one of the most comprehensive R&D programs worldwide, a combination of different geochemical and geophysical monitoring methods is able to detect even small quantities of CO2 and map their spatial extent. After the cessation of CO2 injection a series of activities and further investigations are involved in the post-closure phase. The aim is that Ketzin will for the first time ever close the complete life-time cycle of a CO2 storage site at pilot scale. The five wells (1 injection/observation well, 4 pure observation wells) will be successively abandoned within the next few years while monitoring is continuing. The partial plugging of one observation well in the reservoir section was already completed in fall 2013. The new four-years project COMPLETE (CO2 post-injection monitoring and post-closure phase at the Ketzin pilot site) started in January 2014. Activities within COMPLETE include R&D work on well integrity, post-closure monitoring as well as two field experiments. One is a back-production test of the CO2 aiming at information on the physicochemical properties of the back-produced CO2 as well as the pressure response of the reservoir. The other experiment will focus on brine injection into the CO2 storage reservoir in order to study e.g. the residual gas saturation. Public outreach has been a key element for the project from the very beginning and accompanies the research on CO2 storage at Ketzin since 2004. Thus dissemination (e.g. www.co2ketzin.de) and activities at the visitor centre at the pilot site will continue within COMPLETE and along the entire life cycle of the Ketzin project.

At Ketzin, located west of Berlin, the GFZ German Research Centre for Geosciences operates Europe's longest- running on-shore CO 2 storage site. The Ketzin pilot site has been developed since 2004 and comprises three wells to depths of 750 m to 800 m and one shallow observation well, an injection facility and permanently installed monitoring devices. Since June 2008, CO 2 is injected into 630 m to 650 m deep sandstone units (Upper Triassic Stuttgart Formation) in an anticlinal structure of the Northeast German Basin. Until mid of May 2012, about 61,400 t of CO 2 have been stored safely. One of the most comprehensive monitoring concepts worldwide is applied and capable of detecting the behaviour of the CO 2 in the subsurface. The Ketzin project demonstrates safe CO 2 storage in a saline aquifer on a research scale and effective monitoring. This paper summarizes the key results obtained after four years of CO 2 injection.