Effect of temperature on the thermal conductivity of a granite with high heat production from Central Portugal

Abstract

We present new measurements on the thermal properties of granitic rocks collected from a nearly 1000-m deep well in Central Portugal, using a transient plane source technique with a hot disk sensor. At room-temperature, the granite has, on average, a thermal conductivity of 3.1 W m−1 K−1, a thermal diffusivity of 1.6 × 10−6 m2 s−1, a heat capacity of 1.9 × 106 J m−3 K−1 and a specific heat of 720.7 J kg−1 K−1. A nearly uniform distribution of thermal conductivity values was obtained across the well. We found that as temperature increases from 20 to 120 °C, the thermal conductivity decreases from 9 to 16%. Linear, exponential, power and logarithmic functions were used to fit the dataset and their validity studied by error analyses. This made it possible to obtain empirical relationships to extrapolate thermal conductivity from room temperature to higher temperatures. The application of these relationships to our dataset leads to a decrease in thermal conductivity of 17 to 19% within the temperature range of 20 to 200 °C. These temperature corrections for thermal conductivity will be used in future studies to calculate geotherms and to assess the deep geothermal potential in central Portugal.