RESERVOIR SANDSTONE SAMPLES FROM THE KETZIN PILOT SITE DURING LONG-TERM CO2-EXPOSURE EXPERIMENTS - MINERALOGICAL CHANGES AND GEOCHEMICAL MODELLING

Rock core samples of the Upper Triassic Stuttgart Formation from the Ketzin pilot CO₂ storage site were exposed to pure CO₂ and synthetic reservoir brine at simulated reservoir P-T conditions of 5 MPa and 40 °C. Autoclaves were opened and samples were taken after 15, 21, 24 and 40 months, respectively. The samples were analysed mineralogically and geochemically and compared to baseline data of untreated samples. XRD analyses with Rietveld refinement show no significant mineralogical changes for the studied intervals. On freshly broken rock fragments of the CO₂-treated samples, corrosion textures were found on plagioclase, K-feldspar and anhydrite surfaces. BSE images of the respective twin samples show (intensified) alterations of feldspar minerals. EMPA data display a change in plagioclase composition from intermediate to almost pure albite endmember compositions after CO₂ exposure. Inorganic fluid data show, besides others, highly increased calcium, potassium and sulfate concentrations [1]. The experimental observations were reproduced using the reactive geochemical modeling code PHREEQC.

The mineralogical-chemical measurements imply preferred dissolution of calcium out of plagioclase next to dissolution of K-feldspar and anhydrite. Due to the heterogeneous character of the Stuttgart Formation, which formed in a fluvial environment [2], it is often difficult to distinguish between natural variability and CO₂-related changes. Additional data is needed to interconnect the indicated changes during the experiments and to better understand CO₂-brine-rock interaction occurring within the Ketzin reservoir.

[1] Wandrey, et al. (2011). Monitoring petrophysical, mineralogical, geochemical and microbiological effects of CO₂ exposure – Results of long-term experiments under in situ conditions. Energy Procedia 4, 3644-3650, doi:10.1016/j.egypro.2011.02.295.

[2] Förster et al. (2006). Baseline characterization of the CO₂SINK geological storage site at Ketzin, Germany. Environ Geoscience, 13, 3, 145-161.