Mineralogical and geochemical analysis of Fe-phases in drill-cores from the Triassic Stuttgart Formation at Ketzin CO₂ storage site before CO₂ arrival

Reactive iron (Fe) oxides and sheet silicate-bound Fe in reservoir rocks may affect the subsurface storage of CO2 through several processes by changing the capacity to buffer the acidification by CO2 and the permeability of the reservoir rock: (1) the reduction of three-valent Fe in anoxic environments can lead to an increase in pH, (2) under sulphidic conditions, Fe may drive sulphur cycling and lead to the formation of pyrite, and (3) the leaching of Fe from sheet silicates may affect silicate diagenesis. In order to evaluate the importance of Fe-reduction on the CO2 reservoir, we analysed the Fe geochemistry in drill-cores from the Triassic Stuttgart Formation (Schilfsandstein) recovered from the monitoring well at the CO2 test injection site near Ketzin, Germany. The reservoir rock is a porous, poorly to moderately cohesive fluvial sandstone containing up to 2–4 wt% reactive Fe. Based on a sequential extraction, most Fe falls into the dithionite-extractable Fe-fraction and Fe bound to sheet silicates, whereby some Fe in the dithionite-extractable Fe-fraction may have been leached from illite and smectite. Illite and smectite were detected in core samples by X-ray diffraction and confirmed as the main Fe-containing mineral phases by X-ray absorption spectroscopy. Chlorite is also present, but likely does not contribute much to the high amount of Fe in the silicate-bound fraction. The organic carbon content of the reservoir rock is extremely low (<0.3 wt%), thus likely limiting microbial Fe-reduction or sulphate reduction despite relatively high concentrations of reactive Fe-mineral phases in the reservoir rock and sulphate in the reservoir fluid. Both processes could, however, be fuelled by organic matter that is mobilized by the flow of supercritical CO2 or introduced with the drilling fluid. Over long time periods, a potential way of liberating additional reactive Fe could occur through weathering of silicates due to acidification by CO2

Effect of pre-treatment and modification conditions of natural zeolites on the decomposition and reduction of N2O

The decomposition of N2O and reduction of N2O with NH3 on natural and modified zeolites were studied by a steady state reaction. Two different natural zeolites having different phase compositions were modified by ion exchange (0.5 M NH4NO3) and acid leaching (1 M HCl) and treated under low temperature, high temperature and steam conditions. It was observed that the surface modification of natural zeolites depends strongly on their structure and composition. The modification of zeolites by ion exchange and acid leaching increases the decomposition activity of N2O because of the formation of isolated iron and Fe-O-Al species. However, these modifications insignificantly affect the reduction of N2O with NH3. High temperature and steam treatments lead to dealumination, loss of crystallinity, sintering of phases and the formation of amorphous material in zeolites, resulting in a significant decrease in the decomposition and reduction activity of N2O. The kinetic evaluation for N2O decomposition predicts that the rate-limiting step is the recombinative desorption of molecular oxygen.Scientific and Technological Research Council of Turkey (TUBITAK,) [107M435]This work was supported by The Scientific and Technological Research Council of Turkey (TUBITAK,) under the Project Number of 107M435