Joint interpretation of geoelectrical and soil-gas measurements for monitoring CO2 releases at a natural analogue

The development and validation of hierarchic monitoring concepts is essential for detecting and assessing possible leakages from storage formations, especially for carbon capture and storage (CCS) applications. Joint interpretation of various techniques (such as carbon dioxide (CO2) concentration and flux measurements, self-potential (SP) and geoelectrical surveys) showed that the combination of geophysical methods with soil-gas analysis for mesoscale monitoring of the shallow subsurface above geologic CO2 storages can be a valuable tool for mapping and monitoring potential CO2 spread in the subsurface. Three measurement campaigns were undertaken - May 2011, July 2011 and April 2012 - at an analogue site in the Cheb Basin, Czech Republic, with the aim of studying CO2 leakages and their temporal and spatial behaviour. Results of geoelectrical investigations give an insight into the structural features of the subsurface. CO2 discharge into the atmosphere is mostly impeded by shallow, clay-rich, partly water-saturated zones, which can be seen in the electrical resistivity tomography (ERT) results. Several transport processes can be identified based on SP measurements. The SP results highlight the complex behaviour of temporal variations for the flow patterns. In particular, coupled migration of gas and water plays an important influencing role in this process. Site-specific, near surface geological features and meteorological conditions seem to exert great influence on the degassing pattern and measured CO2 values. Therefore, soil-gas measurements represent a snapshot which illustrates both a distinct typical pattern of the soil-gas distribution in the near subsurface and certain differences caused by soil and meteorological conditions. Observed CO2 soil-gas anomalies and modelled results suggest that the occurrence of gas discharge is much more localized around restricted areas, often controlled by local permeability contrasts. Hence, our results show that a proposed monitoring concept should integrate SP, time-lapse ERT, meteorological parameters and soil-gas measurements to provide a comprehensive insight into the subsurface structures and processes.