Exploration and monitoring of CO2 storage sites using controlled-source electromagnetics: modelling studies

Electrical and electromagnetic geophysical methods are sensitive to the presence or absence pore fluids. Where CO2 substitutes a fraction of saline pore fluid within a storage aquifer, bulk electrical resistivity increases significantly. Imaging the migration of an electrically resistive plume is therefore a key geophysical technology to trace CO2 in the subsurface. For instance, borehole electrical resistivity tomography has been successfully applied to monitor the migration of CO2 between injection and observation wells within a ~600 m deep storage aquifer at the in situ laboratory at Ketzin, Germany. However, the resolution power of cross-hole tomography is weak in regions beyond the boreholes and techniques with a wider spatial footprint are required. Controlled source electromagnetic (CSEM) techniques are powerful in detecting thin resistive layers and may provide a tool to explore and monitor a wider region around the injection site. Here, we present synthetic 1D and 3D modelling studies of CSEM measurements, aiming at simulating the conditions at the Ketzin site. We consider various borehole-to-borehole, surface-to-borehole, borehole-to-surface and surface-to-surface source-receiver configurations for a range of frequencies to evaluate the potential of CSEM for tracing CO2. Each of these configurations is capable of resolving resistivity variations within and beyond the reservoir and within the cap rock at different spatial scales. Our results show that borehole transmitters in combination with surface receivers located at some distance from the injection well are particularly well suited to increase the spatial footprint beyond the boreholes. Surface-to-surface (receiver and transmitter located at the surface) configurations are challenging, because the observable effects of CO2-related resistivity changes at depth are considerably smaller when compared to a borehole transmitter. On the other hand, surface-to-surface techniques are more flexible, as they do not require expensive and potentially invasive observation wells.