Monitoring CO2 migration in a shallow sand aquifer using 3D crosshole electrical resistivity tomography

Abstract Three-dimensional (3D) crosshole electrical resistivity tomography (ERT) was used to monitor a pilot CO 2 injection experiment at Vrogum, western Denmark. The purpose was to evaluate the effectiveness of the ERT method for detection of small electrical conductivity (EC) changes during the first 2 days of CO 2 injection in a shallow siliciclastic aquifer and to study the early-time behavior of a controlled small gaseous CO 2 release. 45 kg of CO 2 was injected over a 50-h period at 9.85 m depth. ERT data were collected using horizontal bipole-bipole (HBB) and vertical bipole-bipole (VBB) arrays. The combined HBB and VBB data sets were inverted using a difference inversion algorithm for cancellation of coherent noises and enhanced resolution of small changes. ERT detected the small bulk EC changes ( 2 and resistive gaseous CO 2 . The primary factors that control the migration of a CO 2 plume consist of buoyancy of gaseous CO 2 , local heterogeneity, groundwater flow and external pressure exerted by the injector. The CO 2 plume at the Vrogum site migrated mostly upward due to buoyancy and it also skewed toward northeastern region by overcoming local groundwater flow. The conductive eastern part is more porous and becomes the preferential pathway for the CO 2 plume, which was trapped within the slightly more porous glacial sand layer between 5 m and 10 m depths. The gaseous and dissolved CO 2 plumes are collocated and grow in tandem for the first 24 h and their opposite effects resulted in a small bulk EC increase. After raising the injection rate from 10 g/min to 20 g/min at the 24-h mark, the CO 2 plume grew quickly. The bulk EC changes from ERT agreed partially with water sample EC and GPR data. The apparent disagreement between high CO 2 gas saturation and prevailing positive bulk EC changes may be caused by limited and variable ERT resolution, low ERT sensitivity to resistive anomalies and uncalibrated CO 2 gas saturation. ERT data show a broader CO 2 plume while water sample EC had higher fine-scale variability. Our ERT electrode configuration can be optimized for more efficient data acquisition and better spatial resolution.

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