Reliability of ERT-derived Temperature - Insights from Laboratory Measurements

We 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.