HEAT FLOW DENSITY DETERMINATIONS IN HUNGARY USING WELL LOGS

Abstract

The thermal conductivity of rocks can be deduced from available data of exploration wells such as core samples, cuttings, lithological descriptions and geophysical well logs. As the thermal conductivity of clastic sediments is lower than the conductivity of the crystalline basement, the sediments have a significant influence on the temperature distribution and heat flow density. We present a methodology for determining the thermal conductivity of clastic sediments using geophysical well logs and thermal conductivity data measured in laboratory. Our results are based on the data of 6 exploration wells. Several well log combi-nations and thermal conductivity measurements from 70 core samples were used to work out the method. The lithological composition consisting of shale, sand and water was identified and the volumetric fractions of these components were derived from wireline logging data such as natural gamma ray, resistivity, bulk density and neutron porosity logs. The lithological composition was determined with Bayesian inversion applying the weighted least squares method. The effective thermal conductivity was computed by applying an appropriate mixing law using the thermal conductivity values of the lithological components. The thermal conductivities derived from well logs were tested and new matrix thermal conductivity values were calculated using archive thermal conductivity measurements of core samples. The harmonic mean model proved to be the best mixing law, resulting in the best fit to laboratory measurements. The reliability of our model was also tested with the help of temperature measurements carried out in a well. Heat flow density determinations were carried out using the Bullard-plot technique with thermal conductivities calculated by our new method. In case of 6 wells, heat flow densities calculated by the new method are in the range of heat flow density values previously conducted but with substantially lower uncertainties.