Shallow subsurface thermal structure onshore Denmark: temperature, thermal conductivity and heat flow

Abstract

Available measured temperatures and thermal conductivities covering Danish onshore areas to a depth of about 300 m have been compiled and analysed. Temperature data from 236 borehole sites, including 56 boreholes with detailed temperature profiles, are applied together with thermal conductivities mea- sured on samples collected at 34 well-characterised outcrops and on core material from 20 boreholes. Significant thermal variations in the shallow subsurface are observed. At a depth of 50 m, a mean temperature of 8.9 ± 0.8°C is found, close to the mean annual surface temperature. Higher mean values of 9.7 ± 1.1°C found at 100 m and 11.6 ± 2.2°C at 200 m reflect a general increase of temperatures with depth. In contrast to the assumption commonly held, we observe significant lateral variations both lo- cally and regionally. At a depth of 100 m, temperatures vary between 7.3 and 13.0°C across Denmark, and at 250 m between 9.6 and 17.9°C. Mean values of the thermal conductivities lie within a range of 0.6–6 W/(m·K) measured water- saturated at laboratory conditions. The majority of values are within the interval of 1–3 W/(m·K) and show a strong correlation with lithology. The content of quartz and the rock porosity (the content of water) are found to be two main factors controlling the observed variations. Characteristic temperature gradients are in the range 1–4°C/100 m. Following Fourier’s law of heat conduction, a clear correlation is observed between temperature gradients and thermal conductivities of different lithologies. Intervals of quartz-rich sand deposits with high thermal conductivity show low temperature gradients, chalk and limestone intervals with intermediate conductivity display intermediate gradients, while sections with fine grained clay deposits of low thermal conductivity show high gradient values. A correlation analysis provides an estimate of regional shallow heat flow of 37 ± 5 mW/m2, consistent with local, classically determined heat-flow values from shallow borehole data. However, it is significantly below deep background heat flow, and this is believed to be caused by long-term paleoclimatic effects. The shallow subsurface thermal regime across the Danish area is largely controlled by thermal conduction. Only locally, and in rare cases, do we observe temperature perturbations due to ground- water migration. In addition to general geoscientific purposes, our results are important for several applications including exploitation of shallow geothermal energy and the use of the subsurface for heat storage and cooling purposes.

Hydrogeologic and Thermal Effects of Glaciations on the Intracontinental Basins in Central and Northern Europe

“…Thereby, the ongoing thermal relaxation of subsurface temperatures observed beneath areas which were previously covered by the ice sheet could be related to both processes. Observations of the glacial thermal imprint can be found in numerous regions in Northern Europe (Kukkonen et al, 1994;Kukkonen and Jõeleht, 2003), i.e., in Udryn (Szewczyk and Nawrocki, 2011), Hannover and Ketzin (Fuchs et al, 2015) and several locations in Denmark (Balling, 2013;Møller et al, 2019), all areas which show temperature dissipation in our models as well.…”

Deep basin temperature and heat-flow field in Denmark – New insights from borehole analysis and 3D geothermal modelling

“…A significant amount of borehole temperatures was measured in the study area over the past decades. For the present study, high-precision continuous temperature logs (in thermal equilibrium unaffected from drilling perturbations) from five deep wells (50 values sampled; Aarhus University in-house data files; data on request) and from 16 shallow wells (Møller et al, 2019) are used. In addition, numerically corrected bottom-hole temperature (BHT) values from 41 deep wells (87 values; data and correction procedure described in Poulsen et al, 2012;2013) were applied.…”

“…we applied TC values based on measurements from crystalline rocks from the island of Bornholm (Kristiansen et al, 1982;Møller et al, 2019), which is adjacent to our model area, as well as literature values (e.g. Kukkonen et al, 2011;Fuchs et al, 2018).…”

“…Therein, constant matrix conductivities were applied for various lithological units, generalised porosity-depth functions and a constant lower boundary heat flow (65 mW•m -2 at 5 km depth). The present work is based on a nation-wide detailed log-based and core-validated parameterization of the formation thermal properties and is constrained by data from additional boreholes, including shallow accurate borehole temperatures from Møller et al, (2019) as well as new heat-flow determinations for deep wells. This improved set of input data, including the spatially variable TC and heat flow, is reflected in the better fit between modelled and measured temperatures (rms of 1.2°C) compared to already good results from Poulsen et al (2017).…”

Full-Scale Demonstration of Combined Ground Source Heating and Sustainable Urban Drainage in Roadbeds

“…The temperature boundary condition at the bottom of the model is set equal to the undisturbed value of T u (x = 50 m,t) = 8.9 • C, as estimated by [31].…”