Regional shallow heat flow in Denmark from borehole temperatures and thermal conductivities of main lithologies

Abstract

Exploitation of shallow geothermal energy requires knowledge of the geothermal conditions like the temperature field, thermal conductivity of the earth materials and the heat flow. New and existing temperature data from 50 wells of 100-300 m depth, covering the Danish onshore areas have been analysed for temperature gradient variations and correlation with lithology and thermal conductivity. The temperature gradients were grouped according to lithology over which they were measured. Similarly, thermal conductivity data measured on outcrop and core material were grouped into main sediment classes. Following Fourier’s law of heat conduction (heat flow = thermal conductivity x temperature gradient) a regional estimate of characteristic shallow heat flow in Denmark was obtained by linear regression of mean temperature gradient and the inverse of mean thermal conductivity for the main sediment classes. An estimated heat flow value of 35 mW/m2 is in good agreement with local, classically heat-flow determination from shallow borehole data. Due to long-term palaeoclimatic effect, this value is significantly below deep background heat flow.