Characterizing temporal behavior of a thermal tracer in porous media by time-lapse electrical resistivity measurements

Abstract

Thermal tracer has been widely used as a substitute for salt tracers when investigating the subsurface, since NaCl may cause environmental pollution and cannot be used in aquifers already contaminated with salts. Traditional methods for monitoring thermal conditions, like distributed temperature sensing systems, have their strengths in high accuracy; however, such methods are limited by providing discontinuous information about the subsurface with a limited number of boreholes. Recently, electrical resistivity tomography (ERT) has shown great potential in monitoring temperature change and heat transport in the thermal affected zone. Applications of time-lapse ERT for monitoring thermal tracer transport have been undertaken in the field, but laboratory-scale experiments to validate the field observations are still lacking. In this study, a relationship between temperature and resistivity was derived in a preliminary experiment. The experiment quantitatively assessed the capability of electrical resistivity measurements for characterizing the temporal behavior of the thermal tracer in an experimental column, relying on the derived relationship. The results show that resistivity and the reciprocal of Kelvin temperature have a good quasi-exponential relationship. Time-lapse electrical resistivity measurements are able to capture the temperature change in the column during the continuous injection of hot water. There is a small deviation between the resistivity-derived temperature and the temperature obtained by a temperature sensor, suggesting absolute deviation <4.94 °C and relative deviation <9.69%. Moreover, the results indicate that the resistivity-derived temperature and actual temperatures have better fit in the cases with higher-temperature water and smaller particle size.