Modelling of steady-state vs transient heat transfer in the COSC-1 drillhole, Central Sweden
Geophysics
Final Report Abstract
The drilling of the ~2.5 km COSC-1 well, which took place in the former Fröå mine near Åre in 2014, aims at deepening the understanding of the Caledonian Orogeny. The fully cored COSC-1 scientific well provides an excellent and unique opportunity to study rocks from the deeper section of the highly eroded orogen and to determine HF within the penetrated Seve Nappe situated in the Åre syncline. To do so, 104 core samples, selected in an approximately even distance and representing all major lithologies, were tested for their thermal properties, i.e. specific heat capacity, thermal diffusivity and thermal conductivity. Furthermore, hydraulic parameters were determined from selected cores indicating that heat transport in the Seve Nappe is mainly driven by conduction. Thermal properties were determined via two fundamentally different measurement approaches, a commercial optical scanner based on Popov et al. (1985) and a modified Ångström device developed at the Ruhr-Universität Bochum (RUB). Employing these two measurement devices allowed for detailed studies on directional and temperature dependencies of the thermal properties. Median λ values amount to 2.9 ±0.1 W m^-1 K^-1 and variations with temperature range from+16 % at 5 °C and -11 % at 55 °C. In addition to the experiments conducted at RUB an interlaboratory test was initiated, where nine samples were tested for the aforementioned thermal properties by five research institutes with six different testing devices. For fairly isotropic samples results were in good agreement, however, for anisotropic samples systematic deviations were identified. Temperature was logged many times in COSC-1, from these logs nearly undisturbed thermal gradients were calculated. Together with the determined thermal conductivity a detailed HF profile is calculated along the well. To further investigate the distribution of HF in the vicinity of COSC-1, 3D numerical geomodels were constructed based on seismic and potential field data interpreted by Hedin et al. (2013, 2016). Ground surface temperatures were varied according to a simplified temperature model and basal HF as well as the heat generation (HG) in the poorly constrained basement were varied systematically. Heat flow calculated along the COSC-1 well path was matched against the one determined in the well, resulting in a linear relation for possible combinations of basal HF and HG. Based on this relation, it is possible to estimate the HF from the basement into the Caledonian Nappes, i.e. 67 mW m^-2. Subsequently, the undisturbed HF was calculated for the vicinity of COSC-1 along with the undisturbed surface HF, which amount to 75 and 78 mW m-2, respectively. Both values are in excellent agreement with the HF determined in the lower section of COSC-1, i.e. 79 mW m^-2. The undisturbed HF values represent a significant extension to the knowledge and understanding of HF in west-central Sweden and contribute substantially to future research ranging from studies on the Caledonian orogeny to the reconstruction of the paleoclimate.
Publications
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2015. COSC-1 – drilling of a subduction-related allochthon in the Palaeozoic Caledonide orogen of Scandinavia, Scientific Drilling, 19, 1-11
Lorenz, H., Rosberg, J.-E., Juhlin, C., Bjelm, L., Almqvist, B.S.G., Berthet, T., Conze, R., Gee, D.G., Klonowska, I., Pascal, C., Pedersen, K., Roberts, N.W.W., and Tsang, C.-F.
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2016. Analysing the geothermal regime of the scientific COSC-1 well bore, west central Sweden. IODP/ICDP Kolloquium Heidelberg, 14. – 16.03.2016
Löwe, Richard; Pascal, Christophe; Renner, Jörg
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2016. Investigations on the geothermal state of the ICDP COSC-1 well bore. EGU General Assembly 2016, held 17-22 April, 2016 in Vienna Austria, id. EPSC2016-1176
Löwe, Richard; Pascal, Christophe; Renner, Jörg
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2017. Analysing the geothermal state of the ICDP COSC-1 well bore, Central Sweden. AGU Fall Meeting, New Orleans, 11-15 Dec. 2017
Löwe, Richard; Pascal, Christophe; Renner, Jörg
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2020. Steady-state vs. transient heat-transfer in the COSC-1 drill hole, Central Sweden. Thesis Ruhr University Bochum, 324 p.
Löwe, Richard H.
