The measurement of thermal diffusivity of rock cores

Abstract Knowledge of the thermal diffusivity of a rock body is required for the prediction of its response to a transient heat source. Thermal diffusivity of a rock can be obtained indirectly from measurement of its conductivity, density and specific heat; indirectly by modelling, using assumed values of those parameters for each of its mineral constituents; indirectly by measurement of one or two parameters and assuming values for the remainder; or by direct measurement. The relatively unknown factor in the second two methods is specific heat, which cannot be assumed to be constant, particularly in minerals that are members of a range of chemical mixtures. Specific heat measured for a rock is generally higher than that modelled from the individual specific heats of the mineral components of the rock. Hence, direct measurement is preferred. It is desirable to measure conductivity and diffusivity of the same sample. A method has been developed that allows diffusivity to be measured of rock disks that have been cut to meet the requirements of the divided bar apparatus for conductivity determinations. The technique is a modification of Angstrom's method in that a thin sample disk is attached to a long matching rod of similar material, and diffusivity is obtained by measuring the amplitude decrement and phase lag of a sinusoidal temperature wave that travels through the assembly. A correction is made for the effect of thermal mismatch between disk and rod. Tests using standard materials suggest that diffusivity can be measured with an accuracy of ± 5%, and a repeatability of 3%.