Reliability of ERT-derived Temperature - Insights from Laboratory Measurements

Abstract

SUMMARYWe performed laboratory measurements on fully saturated sand samples in the context of deriving reliable temperature from time-lapse electrical resistivity tomography (ERT). The experiment consisted in monitoring an increase of temperature in sand samples with electrical resistivity measurements. We neglected the effect of surface conductivity since experiments showed two orders of magnitude between surface and fluid conductivities. We show that using simple linear relationship between fluid electrical conductivity and temperature alone does not allow reliable temperature estimates. Indeed, chemical analyses highlight the importance of accounting chemical reactions occurring when temperature changes, including dissolution/precipitation processes. We performed two experiments based on typical in-situ conditions. We first simulated the injection of a less conductive tap water and second, the injection of heated formation water. In the second case, minerals solubility decreases and precipitation occurs, leading to an increase of bulk resistivity. This mechanism competes with dissolution of minerals when tap water is injected, since tap water is not in equilibrium with the medium. In any case, further research is needed to fully understand the mechanisms and to develop a fully integrated law to derive better temperature estimates.

Time-lapse electrical resistivity imaging of the thermally affected zone of a Borehole Thermal Energy Storage system near Torino (Northern Italy)

“…imaging of heated water injection (Benderitter and Tabbagh, 1982;Hermans et al, 2015), monitoring of gas-phase formation due to heat storage (Lüders et al, 2016), geothermal exploration (Bruno et al, 2000;Garg et al, 2007), monitoring the performance of low enthalpy ground source energy systems (Fragkogiannis et al, 2008;Firmbach et al, 2013) and localizing the burning front of underground coal fires (Revil et al, 2013) to name but a few. Nevertheless, more examples are particularly necessary in real scale applications, since calibrated, robust and reliable correlations between electric resistivity and temperature are not available yet (Robert et al, 2013). Indeed, several uncertainties stand at field scale where different factors do affect the underground resistivity distribution.…”

Heat as a Proxy to Image Dynamic Processes with 4D Electrical Resistivity Tomography

“…Their results show a very good agreement with direct temperature measurements in piezometers (maximum discrepancy of 20%, i.e., 1.5 • C). Robert et al [25] showed that for medium (T > 40 • C) and higher injection temperatures, ERT-derived temperature estimates do not follow a linear relationship anymore as shown by Hayley et al [18]. Indeed, at this temperature range, calcite, if close to saturation, starts to precipitate and the induced temperature change does not only increase the mobility of ions anymore but also modify the composition of dissolved minerals.…”

Geophysical monitoring for shallow geothermal applications – two Italian case histories

“…Some of these applications involved a quantitative evaluation of the thermal anomaly. Nevertheless, Robert et al (2013) under laboratory conditions highlighted problems of ERT-derived temperatures owing to temperature-related chemical reactions occurring within porous media, both on fluid and solid phases. They observed a divergence between the resistivity and temperature curves related to the increasing solubility of some minerals and the increasing fluid conductivity with increasing temperature.…”