Monitoring of cap-rock integrity during CCS from field data at the Ketzin pilot site (Germany): Evidence from gas composition and stable carbon isotopes

Cap-rock integrity is a key issue in Carbon Capture and Storage (CCS) technologies. At the Ketzin pilot site, new drill cores of sealing- and reservoir formations were retrieved from a depth range of 620-701 m below ground level (b.g.l.). These strata have been exposed to injected CO2 under field conditions for more than four years. Here we present data on cap-rock and reservoir lithologies, CO2 concentrations and their stable carbon isotope ratios (δCCO213). CO2 concentrations within the mudstone-dominated cap-rock were <1% (i.e. 10 000 ppmV) with corresponding carbon isotope values between −18.6‰ and −29.4‰. Below the boundary between the cap-rock and the reservoir, CO2 concentrations of more than 90% with mean δCCO213 values around −36.5‰ were measured. Below this section, between 648 and 655 m b.g.l., CO2 concentrations decreased again in the clayey and silty lithology of the reservoir to less than 2%, but maintained a depleted δCCO213 value of around −34.2‰. At depths below 662 m b.g.l., the CO2 concentrations decreased to values of less than 10 000 ppmV and showed corresponding increases to δCCO213 values between −15.9‰ and −27.6‰. Both isotope and CO2 concentration confirm that no CO2 from the reservoir penetrated the cap-rock at the Ketzin pilot site after four years of injection.