Monitoring of the gas composition and stable carbon isotopes during side track drilling in Ktzi 203 at the Ketzin CO2 storage pilot site, Germany

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.