Petrophysics

Thermal conductivity

Thermal conductivity is a measure on how well heat is conducted in materials. Knowledge about thermal conductivity of the bedrock is important to achieve optimal design of ground-source heat plants, and for the estimation of heat flow in deep boreholes. An apparatus, mainly based on Middleton (1993), for measuring thermal diffusivity of rock samples has been developed at NGU. The thermal conductivity of a sample (k, [W/m K]) is estimated as a product of the measured thermal diffusivity (a [m2/s]), density (p [kg/m3]) and specific heat capacity (Cp, [Ws/kg K]).

k = Cp a p

A constant heat flow (radiation) is induced at the top of the rock sample by placing the heat source approximately 10 mm above the top surface of the sample. The temperature at the bottom of the sample is logged, and the thermal diffusivity is estimated from the linear part of the resulting temperature-time plot.
Today the equipment can handle four samples at the time, and each measurement takes approximately 200 seconds. Common diameter and height of the samples are 35 and 20 millimetres, respectively, but different diameters and shapes can be handled. Pyroceram, an accepted EU reference material, is used as reference material at NGU.

Reference:
Middleton, M.F. (1993): A transient method of measuring the thermal properties of rocks. Geophysics, Vol. 58, No. 3, p. 357-365.