Geoelectrical Remote System for Monitoring Shallow Subsurface CO2 Migration

Abstract

The implementation of the first CO2 MMV field lab in Brazil, located in Florianópolis, Santa Catarina state, offered an excellent opportunity for running controlled release experiments in a real open air environment. After three CO2-controlled release experiments run from 2011 up to 2015, a new site with a more complex geology was selected to implement new experiments. The substrate of the PUCRS campus located in Viamão, state of Rio Grande do Sul is predominantly sandy clay. The purpose of this work is to present the results obtained by the geoelectrical remote time lapse, using 3D electrical imaging technique to monitor CO2 migration in both saturated and unsaturated clay-rich sediments in Viamão site. The CO2-controlled release occurred in 2016, covering a subsurface area of approximately 2.925 m2. The CO2 was continuously injected through an injection well, at 3 m deep, in a period of 31 consecutive days (24 hours/day), at a rate varying from 5 to 20 kg / day, totalizing 346 kg of injected CO2. While the CO2 was injected, 3D electrical images using dipole-dipole array were acquired in a daily base, totalizing 46 consecutive days. 3D (tridimensional) and 4D (time-lapsed) electrical imaging produced images reaching 17 m below the surface. Remote monitoring was used for the continuous characterization of the soil/sediment geoelectric responses, significantly increasing the accuracy of the geoelectric responses, such as changes in the injection rate. Comparison of post-injection electrical imaging results with pre-injection images shows changes in resistivity values consistent with released CO2 migration pathways. A pronounced increase in resistivity (up to ~ 1,900 ohm.m) with respect to the pre-injection values, was detected at shallow depths (~ 0.50 m) southeast of the injection well. Background values of 75 ohm.m have changed to 2,000 ohm.m, right after injection. On the same day of the resistivity increase CO2 flux measured using accumulation chambers also increased, reaching values 20 times greater than those observed during baseline measurements (7 mmol/m2/s). The increased CO2 concentration in the atmosphere compared to background-measured concentrations using carbon caps, also coincided with the results of the subsurface resistivity survey. Geoelectrical remote monitoring has also shown significant changes in resistivity values occurring in different portions of the area, probably related to the heterogeneous nature of the site lithology.