Application of a Vertical Electrical Resistivity Array (VERA) for monitoring CO2 migration at the Ketzin site: First performance evaluation

Abstract At the CO2 storage site Ketzin, near Berlin (Germany), an integrated monitoring concept has been applied for characterization of the reservoir state before and during CO2 injection. Electrical Resistivity Tomography (ERT) is an essential part of this multi-method concept in order to image the spatial extent of the CO2 plume. ERT is shown to be sensitive to saturation changes caused by propagation of supercritical CO2 within a brine-filled reservoir. A permanent downhole electrode installation is placed behind casing in the three Ketzin wells (one for CO2 injection and two for observation). The so-called Vertical Electrical Resistivity Array (VERA) is deployed as crosshole configuration at a depth ranging from 590 to 735 meters, penetrating the Stuttgart formation, which hosts the target storage reservoir. For more than two years after finishing drilling and installation work the electrode array has been operating at a temperature of about 35 °C, an average reservoir pressure of about 75 bar, and a formation fluid pH of 5.5. These conditions have an effect on the life time of the components of the electrical monitoring system. Therefore, the performance of the VERA system has to be evaluated thoroughly during its long-term application. From the first results of the ongoing project we can conclude that crosshole ERT is indeed valuable as part of the CO2 monitoring program. Further use of the time-lapse resistivity data is in progress, e.g. support of CO2 distribution mapping in conjunction with results of seismic measurements. In the frame of an integrated monitoring concept, crosshole ERT can support the regulator’s work concerning safe reservoir operation and efficient risk management.