Saltwater Monitoring Using Long-Electrode ERT

Saltwater rise is a possible result of CO2 injection into deep saline aquifers that could threaten aquifer quality. Hence, long-term monitoring of the groundwater system is a required task during and after the injection phase. However, point measurements in boreholes cannot describe large systems with sufficient precision. Therefore cost-efficient monitoring methods are needed, like surface geophysical techniques. Electrical Resistivity Tomography (ERT) is well-suited for saltwater problems and can be deployed using steel-cased boreholes as electrodes, so-called long electrode (LE) ERT. In the project Saltwater Monitoring using Long Electrode Geoelectrics (SaMoLEG) a research institute group focussing on numerical modelling cooperates with a private borehole logging company that has long been active in the region of interest, eastern Brandenburg, where CO2 injection was planned. A numerical framework for simulation and data analysis is developed and verified using a controlled laboratory experiment. Two differently scaled field sites in the Federal State of Brandenburg, Germany, where natural saltwater rise occurs were repeatedly investigated by LE-ERT. The medium-scale site was permanently wired and allows cost-efficient monitoring. The inversion results agree well with geology and measured in-situ fluid conductivities. The large-scale site, at the area of an existing water works, proves the applicability of the developed method in the catchment scale and gives valuable insight into deeper salinisation. The results show that the LE-ERT method is a suitable and cost-efficient method for saltwater rise and groundwater quality monitoring in the frame of geological CO2 storage and is ready for application at real storage sites.