A shallow geothermal experiment in a sandy aquifer monitored using electric resistivity tomography

Abstract

Groundwater resources are increasingly used around the world for geothermal exploitation systems. To monitor such systems and to estimate their governing parameters, we rely mainly on borehole observations of the temperature field at a few locations. Bulk electric resistivity variations can bring important information on temperature changes in aquifers. We have used surface electric resistivity tomography to monitor spatially temperature variations in a sandy aquifer during a thermal injection test. Heated water (48°C) was injected for 70 hours at the rate of 87 l∕h in a 10.5°C aquifer. Temperature changes derived from time-lapse electric images were in agreement with laboratory water electric conductivity-temperature measurements. In parallel, a coupled hydrogeologic saturated flow and heat transport model was calibrated on geophysical data for the conceptual model, and on hydrogeologic and temperature data for the parameters. The resistivity images showed an upper flow of heated water along the well above the injection screens and led to a new conceptualization of the hydrogeologic source term. The comparison between the temperature models derived from resistivity images and from the simulations was satisfactory. Quantitatively, resistivity changes allowed estimating temperature changes within the aquifer, and qualitatively, the heated plume evolution was successfully monitored. This work demonstrates the ability of electric resistivity tomography to study heat and storage experiments in shallow aquifers. These results could potentially lead to a number of practical applications, such as the monitoring or the design of shallow geothermal systems.

Geophysical Methods for Monitoring Temperature Changes in Shallow Low Enthalpy Geothermal Systems

“…It is non-invasive since it only requires electrodes at the ground surface. However, surface measurements suffer from poor resolution at depth [45,113,114] even if Robert et al [51] successfully managed to follow a salt tracer in fractures at a depth of 20 to 30 m. Hermans et al [74] propose guidelines deduced from their study case to design surface arrays for monitoring studies. Using 62 electrodes with an electrode spacing a, they successfully imaged a heat plume 3.33a thick, 4a wide and at a depth of 4a.…”

“…At the opposite, Hermans et al [74] went back to the idea developed by Benderitter and Tabbagh [103] to monitor changes in temperature related to the injection of heated water in aquifers. They successfully monitored with a surface profile the 3 days of injection of heated water (48 °C) at a relatively low rate (87 L/h) in a homogeneous sandy aquifer (10.5 °C, 2.5 m thick).…”

“…As an example, Hermans et al [74] had to correct their ERT-derived temperatures because a difference in specific electrical conductivity existed between formation and injection waters. They calculated the correction term using a simulation of the injection process, injecting less conductive water into the aquifer.…”

“…The resulting changes in resistivity are small and the maximum change is considered as the limit to interpret resistivity change. Hermans et al [74] estimated the level of noise with reciprocal measurements and generated 100 new data background data sets. Then, they evaluated the mean and standard deviation of the 100 inverted electrical resistivity models and proposed a conservative cut-off (two times the maximum standard deviation) to interpret time-lapse images.…”

“…This behavior was subsequently investigated in the laboratory by Robert et al [112]. Sand and water samples were collected on the site investigated by Hermans et al [74] to reproduce the heat and storage experiment in a saturated soil column.…”