Heat flow density estimates in the Upper Rhine Graben using laboratory measurements of thermal conductivity on sedimentary rocks

Abstract

The Upper Rhine Graben (URG) has been extensively studied for geothermal exploitation over the past decades. Yet, the thermal conductivity of the sedimentary cover is still poorly constrained, limiting our ability to provide robust heat flow density estimates. To improve our understanding of heat flow density in the URG, we present a new large thermal conductivity database for sedimentary rocks collected at outcrops in the area including measurements on (1) dry rocks at ambient temperature (dry); (2) dry rocks at high temperature (hot) and (3) water-saturated rocks at ambient temperature (wet). These measurements, covering the various lithologies composing the sedimentary sequence, are associated with equilibrium-temperature profiles measured in the Soultz-sous-Forêts wells and in the GRT-1 borehole (Rittershoffen) (all in France). Heat flow density values considering the various experimental thermal conductivity conditions were obtained for different depth intervals in the wells along with average values for the whole boreholes. The results agree with the previous heat flow density estimates based on dry rocks but more importantly highlight that accounting for the effect of temperature and water saturation of the formations is crucial to providing accurate heat flow density estimates in a sedimentary basin. For Soultz-sous-Forêts, we calculate average conductive heat flow density to be 127 mW/m 2 when considering hot rocks and 184 mW/m 2 for wet rocks. Heat flow density in the GRT-1 well is estimated at 109 and 164 mW/m 2 for hot and wet rocks, respectively. Results from the Rittershoffen well suggest that heat flow density is nearly constant with depth, contrary to the observations for the Soultz-sous-Forêts site. Our results show a positive heat flow density anomaly in the Jurassic formations, which could be explained by a combined effect of a higher radiogenic heat production in the Jurassic sediments and thermal disturbance caused by the presence of the major faults close to the Soultz-sous-Forêts geothermal site. Although additional data are required to improve these estimates and our understanding of the thermal processes, we consider the heat flow densities estimated herein as the most reliable currently available for the URG.

Petrophysical properties of the Muschelkalk from the Soultz-sous-Forêts geothermal site (France), an important lithostratigraphic unit for geothermal exploitation in the Upper Rhine Graben

“…11a). This range of thermal conductivity values are consistent with the large-scale thermo-hydro-mechanical modelling of the Soultz-sous-Forêts (Vallier et al 2019) and Rittershoffen (Vallier et al 2018) geothermal sites, with laboratory data for a wide range of sedimentary rocks from the Upper Rhine Graben (Fuchs and Förster 2010;Haffen et al 2013;Harlé et al 2019), and with thermal conductivity maps performed on an 18-cm-long piece of core from the Buntsandstein (from borehole EPS-1) (Haffen et al 2017). The very low porosity of the anhydrite samples and the high value of thermal conductivity of anhydrite (~ 5 W/mK; Clauser and Huenges 1995) have contributed to the high thermal conductivity of these samples.…”

“…However, we found that the permeability of all the prepared Muschelkalk samples was below the resolution of our permeameter (≪ 10 −18 m 2 ). Thermal diffusivity and thermal conductivity were measured (and specific heat capacity per unit volume calculated) for each depth interval using two 20-mm-diameter cylindrical samples either side of a Kapton sensor (3.189 mm in radius) and a Hot Disk TPS 500 Thermal Constants Analyser (more details are provided in Gustavsson et al 1994;Harlé et al 2019). A series of four consecutive measurements was performed on the different sample surfaces.…”

“…The thermal properties (thermal conductivity, thermal diffusivity, and specific heat capacity) for rocks within a geothermal reservoir can inform, for example, thermohydro-mechanical modelling designed to better understand large-scale fluid circulation, heat flow calculations, and temperature estimations (e.g., Fuchs and Förster 2010;Blöcher et al 2010;Rühaak et al 2015;Fuchs and Balling 2016;Vallier et al 2018Vallier et al , 2019Harlé et al 2019). Although our thermal properties ( Fig.…”

“…Indeed, more than fifteen geothermal wells have been drilled in the Upper Rhine Graben since the 1980s (Vidal and Genter 2018). The geology of the region consists of a fractured granitic basement (e.g., Ledésert et al 1993;Genter and Traineau 1996;Genter et al 1997;Hooijkaas et al 2006;Sausse et al 2006;Dezayes et al 2010;Genter et al 2010;Villeneuve et al 2018;Glaas et al 2018) overlain by a sequence of Permian and Triassic sedimentary rocks (Buntsandstein, Muschelkalk, and Keuper; e.g., Haffen et al 2013;Vidal et al 2015;Griffiths et al 2016;Aichholzer et al 2016Aichholzer et al , 2019Heap et al 2017;Kushnir et al 2018a, b;Heap et al 2018Heap et al , 2019Harlé et al 2019), Jurassic sedimentary rocks, and Tertiary to Quaternary graben fill (e.g., Berger et al 2005;Hinsken et al 2007Hinsken et al , 2011Aichholzer et al 2016; (Fig. 1c).…”

Novel Experimental Device to Monitor the Ground Thermal Exchange in a Borehole Heat Exchanger

“…Specifically, the effect of groundwater flow on the thermal performance is assessed in this experiment, finding that the EGRT deviates drastically to the laboratory outcomes under groundwater flow. Currently, studies are also focused on measurements of thermal conductivity on sedimentary rocks and thermally enhanced materials [21,22]. Still, the outcomes reveal that additional data is required to improve the reliability of the estimates and understanding of the thermal processes [22].…”