Surface-downhole electrical resistivity tomography applied to monitoring of CO2 storage at Ketzin, Germany.

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. [ABSTRACT FROM AUTHOR]