Monitoring Approaches for Detecting and Evaluating CO2 and Formation Water Leakages into Near-surface Aquifers

The ambition to apply carbon capture and storage (CCS) requires the provision of effective monitoring approaches that can be applied to detect and to characterize a potential migration or leakage of CO2 and saline formation water into geological compartments between the storage formation and the protected groundwater resource. The sensitivity of monitoring techniques to detect such leakages into near-surface groundwater is therefore discussed in this study. The most significant geochemical processes following a CO2 leakage are the lowering of the pH due to the formation of carbonic acid and a rising of the electric conductivity (EC) due to mineral (especially carbonate) dissolution in the groundwater. It is shown that the variation in the EC is in principle detectable by geoelectric measurements. The detectability is reduced in non-calcareous aquifers, because the variation in the EC as a consequence of carbonate dissolution is at a lower level. Since the carbonate contents in such aquifers are barely known, a regionalization of carbonate contents in North German aquifers was only possible based on groundwater analyses. Although the geoelectric measurements can be in principle capable of detecting the effects of a CO2 leakage, their results can only cover a comparatively small area. The area-wide survey method of airborne-electromagnetics was tested for a base- line monitoring and may be suitable to detect CO2 leakages, but evaluating the sensitivity of this method with respect to variations in the geological parameters and boundary conditions of the CO2 leakage needs to be part of future works.