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During test operation of a geological latent heat storage system as a potential option in the context of heat supply for heating and cooling demand, part of a shallow Quaternary glacial aquifer was frozen at the TestUM test site. In order to evaluate the current thermal state in the subsurface, the dimension of the frozen volume has to be known. As the target is too deep for high-resolution imaging from the surface, the use of borehole ground-penetrating radar (GPR) is being investigated. For imaging and monitoring of a vertical freeze–thaw boundary, crosshole zero-offset and reflection borehole GPR measurements are applied. The freezing can be imaged in the zero-offset profiles (ZOPs), but the determination of ice body size is ambiguous because of the lack of velocity information in the frozen sediment. Reflection borehole GPR measurements are able to accurately image the position of the freezing boundary through repeated measurements of ±0.1 m, relying on the velocity information from ZOPs. We have found that the complementary use of ZOPs and reflection measurements provides a fast and simple method to image freezing in geological latent heat storage systems. The presence of superimposed reflections from other observation wells and the low signal-to-noise ratio are problematic. The use in multiple observation wells allows an estimation of ice body size. A velocity model derived from multiple ZOPs enabled us to extrapolate geological information from direct-push-based logging and sediment cores to a refined subsurface model. [ABSTRACT FROM AUTHOR]

During test operation of a geological latent heat storage system as a potential option in the context of heat supply for heating and cooling demands a part of a shallow quaternary glacial aquifer at the "TestUM" test site is frozen. To evaluate the current thermal state in the subsurface the dimension of the frozen volume has to be known. With the target being too deep for high resolution imaging from the surface, the use of borehole Ground-Penetrating-Radar (GPR) is assessed. For imaging and monitoring of a vertical freeze-thaw boundary, crosshole zero-offset and reflection measurements are applied. The freezing can be imaged in ZOP, but determination of ice body size is ambiguous, because of lacking velocity information in the frozen sediment. Reflection measurements are able to image the position of the freezing boundary with an accuracy determined through repeated measurements of ±0.1 m , relying on the velocity information from ZOP. We found, that the complementary use of ZOP and reflection measurements make for a fast and simple method, to image freezing in geological latent heat storage systems. Problematic is the presence of superimposed reflections from other observation wells and low signal-to-noise ratio. The use in multiple observation wells allows for an estimation of ice body size. A velocity model derived from zero-offset profiles (ZOP) enabled to extrapolate geological information from direct-push based logging and sediment cores to a 3D-subsurface model. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: CO2 leakage from subsurface storage sites is one of the main concerns connected with the CCS technology. As CO2 leakages into near surface formations appear to be very unlikely within pilot CCS projects, the aim of this work is to emulate a leakage by injecting CO2 into a near surface aquifer. The two main questions pursued by the injection test are (1) to investigate the impact of CO2 on the hydrogeochemistry of the groundwater as a base for groundwater risk assessment and (2) to develop and apply monitoring methods and monitoring concepts for detecting CO2 leakages in shallow aquifers. The presented injection test is planned within the second half of 2010, as a joint project of the University of Kiel (Germany), the Helmholtz-Centre for Environmental Research (Leipzig, Germany) and the Engineering Company GICON (Dresden, Germany). The test site has been investigated in detail using geophysical methods as well as direct-push soundings, groundwater well installation and soil and groundwater analyses. The present paper presents briefly the geological and hydrogeological conditions at the test site as well as the planned injection test design and monitoring concept. [Copyright &y& Elsevier]