Surface-downhole electrical resistivity tomography (SD-ERT) surveys were repeatedly carried out to image CO2 injected at the pilot storage Ketzin, Germany. The experimental setup combines surface with downhole measurements by using a permanent electrode array that has been deployed in three wells. Two baseline experiments were performed during the site startup and three repeat experiments were performed during the first year of CO2 injection. By the time of the third repeat, approximately 13,500 tons of CO2 had been injected into the reservoir sandstones at about 650 m depth. Field data and inverted resistivity models showed a resistivity increase over time at the CO2 injector. The lateral extent of the related resistivity signature indicated a preferential CO2 migration toward the northwest. Using an experimental resistivity-saturation relationship, we mapped CO2 saturations by means of the resistivity index method. For the latest repeat, CO2 saturations show values of up to 70% near the injection well, which matches well with CO2 saturations determined from pulsed neutron-gamma logging. The presence of environmental noise, reservoir heterogeneities, and irregularities in the well completions are the main sources of uncertainty for the interpretations. The degradation of the permanently installed downhole components is monitored by means of frequently performed resistance checks. In consistency with the SD-ERT data, these resistance checks indicate a long-term resistivity increase near the CO2 injector. In conclusion, the investigations demonstrate the capability of surface-downhole electrical resistivity tomography to image geologically stored CO2 at the Ketzin site.

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