Geoelectrical methods for monitoring geological CO sub(2) storage: First results from cross-hole and surface-downhole measurements from the CO sub(2)SINK test site at Ketzin (Germany)

The feasibility of monitoring CO sub(2) migration in a saline aquifer at a depth of about 650 m with cross-hole and surface-downhole electrical resistivity tomography (ERT) is investigated at the CO sub(2)SINK test site close to Ketzin (Germany). The permanent vertical electrical resistivity array (VERA) consists of 45 electrodes (15 in the injection well Ktzi201 and 15 in each of the two observation wells Ktzi200 and Ktzi202), successfully placed on the electrically insulated casings, in the depth range of about 590-740 m with a spacing of about 10 m. The three Ketzin wells are arranged as perpendicular triangle with distances of 50 and 100 m. First synthetic modelling studies indicate an increase of the electrical resistivity of about 200% caused by CO sub(2) injection, corresponding to a bulk CO sub(2) saturation of 50%, which is in good agreement with laboratory studies. Finite difference inversion of field data delivers three-dimensional resistivity distributions between the wells which are consistent with the reservoir modelling studies. To increase the limited observation area provided by the cross-hole measurements, additional surface-downhole measurements were deployed. A main CO sub(2) migration in SE-NW direction is deduced from surface to downhole resistivity experiments. The first cross-hole time-lapse results show that the resolution and the coverage of the electrode array in the Ketzin setting are sufficient to resolve the expected resistivity changes on the characteristic length scale of the electrode array. Significant resistivity changes could be measured, however, detailed information on the CO sub(2) plume could not be resolved yet by VERA under the existing geological circumstances.