EVALUATION OF THE SORPTIVE CO2 STORAGE POTENTIAL OF CLAY-RICH CAPROCK LITHOTYPES AND CLAY MINERALS

Long-term underground carbon dioxide storage is being evaluated internationally as an option to reduce CO₂ emissions that are considered to promote global warming. The long term sealing efficency and retention potential of caprocks are a key criterion for the selection of appropriate underground CO₂ storage sites. In the course of the CO2SEALS project, which is incorporated into the GEOTECHNOLOGIEN¹ R&D program, funded by the German Federal Ministry for Education and Research (BMBF) and the German Research Foundation (DFG), laboratory experiments have been performed to evaluate the sorptive CO₂ storage potential of different clay-rich caprock lithologies and “pure” clay minerals. High-pressure CO₂ sorption experiments were conducted on 17 caprock samples from five different locations. Experiments were carried out on dry samples at 45°C and pressures up to 25 MPa. A positive correlation was found between the CO₂ sorption capacity of shales and BET surface area as well as illite/smectite content. No statistically significant correlation was found between CO₂ sorption capacity and total organic carbon content (TOC). Sorption capacities of clay-rich shales ranged from 0.2 to 0.6 mmol/g. Quarz-rich sandstones and calcite rich marl had only minor CO₂ sorption capacities of 0.03 and 0.12 mmol respectively. The CO₂ excess sorption capacity of a dry sodium smectite (Na-WyS-2) was higher (0.4 mmol/g) than for a dry kaolinite (0.3 mmol/g). This is in accordance with the significantly higher BET surface area of smectite (32 m²/g) as compared to kaolinite (8 m²/g). Sorption measurements on smectite and kaolinite revealed significant differences in isotherm shapes, indicating substantially different sorption mechanisms for the two clays. Future experiments will address the influence of moisture content on CO₂ sorption for smectite by moisturizing the sample at different relative humidity.