Characterization of rock thermal conductivity by high-resolution optical scanning

We compared three laboratory methods for thermal conductivity measurements: divided-bar, line-source and optical scanning. These methods are widely us ed in geothermal and petrophysical studies, particularly as applied to rese arch on cores from deep scientific boreholes. The relatively new optical sc anning method has recently been perfected and applied to geophysical proble ms. A comparison among these methods for determining the thermal conductivi ty tensor for anisotropic rocks is based on a representative collection of 80 crystalline rock samples from the KTB continental deep borehole (Germany ). Despite substantial thermal inhomogeneity of rock thermal conductivity ( up to 40-50% variation) and high anisotropy (with ratios of principal value s attaining 2 and more), the results of measurements agree very well among the different methods. The discrepancy for measurements along the foliation is negligible (<1%). The component of thermal conductivity normal to the f oliation reveals somewhat larger differences (3-4%). Optical scanning allow ed us to characterize the thermal inhomogeneity of rocks and to identify a three-dimensional anisotropy in thermal conductivity of some gneiss samples . The merits of optical scanning include minor random errors (1.6%); the ab ility to record the variation of thermal conductivity along the sample, the ability to sample deeply using a slow scanning rate, freedom from constrai nts for sample size and shape, and quality of mechanical treatment of the s ample surface, a contactless mode of measurement, high speed of operation, and the ability to measure on a cylindrical sample surface. More traditiona l methods remain superior for characterizing bulk conductivity at elevated temperature. (C) 1999 CNR. Published by Elsevier Science Ltd. All rights re served.