Quantitative temperature monitoring of a heat tracing experiment using cross-borehole ERT

Abstract

The growing demand for renewable energy leads to an increase in the development of 1 geothermal energy projects and heat has become a common tracer in hydrology and 2 hydrogeology. Designing geothermal systems requires a multidisciplinary approach including 3 geological and hydrogeological aspects. In this context, electrical resistivity tomography 4 (ERT) can bring relevant, qualitative and quantitative information on the temperature 5 distribution in operating shallow geothermal systems or during heat tracing experiments. We 6 followed a heat tracing experiment in an alluvial aquifer using cross-borehole time-lapse 7 ERT. Heated water was injected in a well while water of the aquifer was extracted at another 8well. An ERT section was set up across the main flow direction. The results of ERT were 9 transformed into temperature using calibrated petrophysical relationships. These ERT-derived 10 temperatures were then compared to direct temperature measurements in control piezometers 11 collected with distributed temperature sensing (DTS) and groundwater temperature loggers.

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

“…The models were inverted with the same error level to the same inversion error. Data correspond to the heat tracing experiments performed by Hermans et al [55] in an alluvial aquifer with cross-borehole ERT. Hot water was injected in a well located 8 m upgradient on the whole thickness of the aquifer.…”

“…Dots show the position of borehole electrodes. Average groundwater temperature is around 13 °C (modified after [55]). …”

“…The tomography of Figure 2 shows a background snapshot of the resistivity distribution at a site in an alluvial plain [55]. The changes in resistivity reflect changes in the lithology.…”

“…These effects are linked to the modification of viscosity with temperature which modifies the mobility of charge carriers. By extension, the temperature changes observed on operating GWHP/ATES systems [4] are typically in the range of temperature changes that could be detected by ERT (~2 °C and more, see [55]). …”

“…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. Hermans et al [55] used a simple physical interpretation to assess the error level. They expected only decreases in resistivity in their time-lapse sections.…”