Developments in geothermal energy in Mexico—part seventeen. Thermal conductivity of drill cores from the Los Azufres geothermal field: Experimental results and accurate prediction method

Abstract

The thermal conductivity of rocks is a basic parameter needed for a number of underground heat transfer studies. However, data are not readily available and its determination is not straightforward. Available predictive correlations have been developed for sedimentary rocks, while estimative methods are based on chemical or mineral analyses. It is thus convinient to have a method to predict or to estimate the thermal conductivity of igneous rocks from other known, easy to measure rock properties. As a contribution to the solution of this problem, a correlation for predicting the thermal conductivity of igneous rocks has been developed. Data were generated as part of a characterization project of drill cores from the Los Azufres geothermal field, which included physical, thermal and mechanical properties. For the correlation thermal conductivity, bulk density and total porosity data were determined on sixteen cores. Thermal conductivity varied between 1.05 and 2.34 W m −1 K −1, while density varied between 2050 and 2740 kg m −3 and porosity between 1.9 and 24.7%. The correlation consists of forming the logarithm of the product between conductivity and density and running a polynomial regression with porosity as the independent variable. First and second degree polynomials were tested, with the latter result being more satisfactory. The correlation obtained is simple mathematically and easy to use. For porosities between 0% and 25% the model predicts thermal conductivity within ±22%, with a residual mean square deviation of 0.00462. This is generally satisfactory for almost any engineering purpose. A further advantage of this correlation is that density and porosity may also be determined from drill cuttings, which are more readily available than cores.