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The Ketzin pilot site for geological storage of CO2 located about 25 km west of Berlin was the first European pilot site for onshore storage of CO2 in saline aquifers. A total amount of 67 kilotons of CO2 was injected without any safety issues between June 2008 and August 2013 when the injection ceased and the site entered its post-closure phase. Within this phase a multidisciplinary monitoring program continues and the five research wells are stepwise abandoned in order to address and finally close the entire life cycle of the storage site. In October 2014, a two-week field experiment was carried out with the aim to back-produce parts of the injected CO2 and to study the reservoir and wellbore behaviour. In this contribution, we present a comprehensive overview on the status of the Ketzin site in its second year of the post-closure phase and the main results of the field experiment on CO2 back-production.

At the Ketzin pilot site (Germany), a multi-disciplinary monitoring concept was established in order to image CO2 injected into a saline aquifer. Within this surveillance program a behind-casing borehole electrode array was deployed as permanent reservoir monitoring (PRM) tool. During the period from 2008 until 2014, weekly geo-electrical cross-hole data were recorded. This comprehensive archive of time-lapse resistivity data forms a respectable base for CO2 saturation estimation. Moreover, the long-term evaluation of the borehole electrode array provided the evidence for its endurance under subsurface conditions, reliability and usefulness for integrative geophysical data analysis.

Electrical resistivity tomography (ERT) has received consideration as a tool for permanent monitoring of saline storage reservoirs due to its high sensitivity to compositional pore fluid changes. The information content offered by geoelectrical data is ultimately limited by the electrode arrangement, and consequently, its full exploitation requires a well-conceived experimental design. We present a methodology to estimate an optimum number of electrodes as well as their specific locations along the borehole trajectories based on a maximization of the respective model resolution. Using a synthetic example analogous to the Ketzin site, Germany, we demonstrate that relatively sparse optimized setups with a refinement of the electrode spacings in the target horizon can offer comparable tomographic imaging capabilities with regard to rather dense arrays. The approach presented can assist practitioners with the design of high-resolution and cost-effective down-hole installations at future CO2 storage sites.

The first European onshore CO2 storage site has been set up at Ketzin (Germany). It is operated as a small scale pilot site, with a special focus on the validation of various geophysical, geochemical and microbial monitoring technologies. The site has been developed within the EU funded project CO2SINK, coordinated by the GFZ German Research Centre for Geosciences and supported by numerous national and EU initiatives. Injection started in June 2008. Since then, between 1.000 and 2.000 tons of CO2 per month have been injected into sandstone layers of the Triassic Stuttgart Formation at a depth of 620 m to 650 m. The total amount of injected CO2 was 45.000 tons in February 2011. Injection is planned to continue until up to 100.000 tons will have been injected into the Stuttgart Formation. Currently, food grade CO2 is being injected at Ketzin, however, the technical and contractual preparations are underway to inject a limited amount of CO2 from a sequestration pilot facility.