Carbon and oxygen isotope indications for CO2 behaviour after injection: First results from the Ketzin site (Germany)

Stable carbon isotopes may serve as useful tracers to monitor the fate and migration of injected CO2 gas in the subsurface. This is true as long as the already present dissolved inorganic carbon (DIC) has a different isotopic composition (δ13CDIC) than the injected CO2 (δCCO213). After dissolution and turnover of the injected CO2 to DIC, mixture of both sources (δCCO213 and δ13CDIC) enables isotope and mass balance calculations to quantify the degree of dissolution of the injected CO2 (geochemical trapping). Furthermore, δ13CDIC may be used as a sensitive tool to determine its source, as well as to indicate the arrival of CO2 at observation wells. Isotope measurements from before and after injection of CO2 at the Ketzin site near Berlin are presented here. Before injection bacterial activity, especially in the injection well, was observed. After injection, most of the DIC present originates from injected CO2 and, generally, an inverse trend exists between the expected increase in DIC contents and its corresponding, decreasing isotope signal. The stable isotopic composition of the water (δOH2O18), however, remained relatively homogeneous throughout the monitoring period. In order to shift the δOH2O18 signal, large amounts of CO2 are expected to be required. In order to verify the results, and to see how the observed trends evolve, further monitoring, including more isotope measurements combined with further biogeochemical data, will be necessary.