Permanent crosshole electrical resistivity tomography (ERT) as an established method for the long-term CO2 monitoring at the Ketzin pilot site

At the Ketzin pilot site, a permanent downhole electrode array has been utilized as part of the multi-disciplinary monitoring concept during all phases of the CO2 storage project. After more than seven years under the present CO2/brine prevailing subsurface conditions, including the five years of regular CO2 injection, a first long-term evaluation of the general technical performance of the electrode array can be given. This article reports on the application of the crosshole electrical resistivity tomography (ERT) technique considering the site specific requirements and challenges. It describes the installation procedure of the electrode array, field data acquisition schemes and automated pre-processing routines applied to the continuously growing data archive. It has been found that the evaluation of contact resistance measurements provides useful information about the current condition of the downhole installation. The 3D time-lapse inversion of one of the major observation planes yields resistivity distributions from various operational stages during the injection and post-injection phases of the CO2 storage reservoir. A simple saturation approach converts averaged resistivity signatures from the target reservoir zone into CO2 saturation estimates, which are in good agreement with saturation results measured by borehole logging campaigns. The permanent ERT array has shown a promising lifespan under downhole conditions, and is able to provide complementary information in conjunction with other monitoring systems. For further practical applications, a more standardized workflow for data acquisition and processing might be beneficial.