Guarded hot plate, optical scanning, transient divided bar: comparison of steady-state and transient methods to assess rock thermal conductivity.

• Main Authors: Miranda, Mafalda Alexandra, Chicco, Jessica, Giordano, Nicolo, Mandrone, Giuseppe, Raymond, Jasmin
• Format: Other/Unknown Material
• Language: English
• Subjects: géothermie; Bergen; Kuujjuaq; Norway; Peltier
• Online Access: https://espace.inrs.ca/id/eprint/11340/; https://espace.inrs.ca/id/eprint/11340/1/C3654.pdf

Thermal conductivity of rocks is a key parameter to model and design both deep and shallow geothermal systems relying on heat transfer simulations. However, in most cases, these models are based on literature data or laboratory measurements with high or unknown uncertainty. Three different laboratory techniques were compared in this work, trying to better understand analysis discrepancy related to the guarded hot plate, the optical scanning and the transient divided bar methods. The first method allows to assess thermal conductivity in steady-state when temperature equilibrium is reached in a small core sample placed between two parallel thermoelectric Peltier elements. The optical scanning technology adopts a moving infrared heat source and temperature sensors to scan diamond cut rock surfaces and thermal conductivity is measured in transient conditions at room temperature. The transient divided bar is a recent modification of the conventional steady-state apparatus and consists of two copper blocks of known conductivity, between which the specimen is interposed. By cooling the lower block with a thermostatic bath, the conductivity is derived from the rate at which the heat leaves the upper block. Rock specimens from two sites in Kuujjuaq (Québec) and Bergen (Norway) were collected to characterize the underground and to evaluate the efficiency of both deep and shallow geothermal systems. The Kuujjuaq samples belong to the Southeastern Churchill Province (1.8 Ga) and the Bergen ones to the Minor Bergen Arc (0.45 Ga). First results show the variability among the three devices ranging from 1 to 15%, with 7% average. The most representative value can be picked depending on the quality of the specimen and knowing advantages and limitations of each method.