Improving the temperature predictions of subsurface thermal models by using high-quality input data. Part 1: Uncertainty analysis of the thermal-conductivity parameterization

Highlights 3D steady-state thermal FE model using Feflow  Well-log based parameterization approach  Spatial parameterization of rock thermal conductivity  New heat-flow data  North German Basin (Danish-German border area) ABSTRACT We present a 3D numerical crustal temperature model and analyse the present-day conductive thermal field of the Danish-German border region. The model region covers the northernmost part of the North German Basin, a transition zone between the deep-reaching sedimentary rocks of the Glückstadt Graben (including complex salt structures) and the shallow crystalline basement of the Ringkøbing-Fyn High. The modelling approach is novel as it implements for the first time a comprehensive analysis of well-log data on a regional modelling scale. Those logs were used both to derive the spatial distribution of rock thermal conductivity across the study area, and to calculate heat flow values. New values of terrestrial surface heat flow are reported for eight deep boreholes in the North German Basin ranging from 72 to 84 mW/m² with a mean of 80 ± 5 mW/m². The values are computed from continuous temperature logs, carefully corrected BHT values, drill-stem tests and well-log derived rock thermal properties (thermal conductivity, radiogenic heat production) and were included in the setup of the numerical lower boundary conditions. New surface heat flow is up to 20 mW/m² higher than low values reported in some previous studies for this region. Heat flow from the mantle is 33 to 40 mW/m². The model temperature predictions are validated against 59 temperature observations from 24 wells. The prediction uncertainties between observed and modelled temperatures at deep borehole sites are small (rms = 3.5 °C, ame = 2.1 °C). Pronounced lateral temperature variations are predicted and found to be caused mainly by complex geological structures, including a large amount of salt structures and marked lateral variations in the thickness of basin sediments. The associated variations in rock thermal conductivity generate significant variations in model heat flow and large variations in temperature gradients.With regard to the utilization of geothermal energy, the Rhaetian and the Middle Buntsandstein sandstone reservoirs are found with temperatures within the range of 40-80 °C, suitable for low enthalpy heating purposes, in most of the area and locally also with higher temperatures.Temperatures above 120 °C, of interest for the production of electricity, are observed only in the very southeastern part of the study area.

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“…Further details to the amount and quality of the used database are reported in part one of this study (Fuchs and Balling, 2016, this issue).…”

Section: Figurementioning

confidence: 99%

Improving the temperature predictions of subsurface thermal models by using high-quality input data. Part 2: A case study from the Danish-German border region

Fuchs

Balling

2016

Geothermics

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“…Unfortunately, the absence of a large database for thermal conductivities of sedimentary rocks from the region implies that these heat flow density estimates are based on a limited number of thermal conductivity values, often determined for general lithologies that may not be fully representative of the formations within the graben (Flores Marquez 1992). This lack of data for the upper part of the sedimentary cover can lead to unreliable estimations of the local heat flow density (Fuchs and Balling 2016). Moreover, the thermal conductivity values considered for these early heat flow density estimations are for rocks in the dry state at ambient temperature and the absence of URG-specific models makes them difficult to port to in situ conditions.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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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.

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“…Although new techniques such as the optical scanning method (Popov et al 1999) allow rapid measurements of λ b , the generation of statistically reliable datasets covering large stratigraphic sections is very time-consuming and might require multi-year studies (c.f., Clauser et al 2007Clauser et al , 2009Fuchs and Föster 2010;Bär 2012;Jorand et al 2015;Fuchs and Balling 2016). Especially in the case of sandstones, the alternative use of textbookderived mean values for thermal modeling is not sufficient for accurate temperature predictions.…”

Section: Introductionmentioning

confidence: 99%

Pore-fluid-dependent controls of matrix and bulk thermal conductivity of mineralogically heterogeneous sandstones

Kämmlein

Stollhofen

2019

Geotherm Energy

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For a variety of geothermal engineering applications, the only indirectly determinable matrix thermal conductivity (λ m) is frequently used to convert the measured bulk rock thermal conductivity (λ b) of air-saturated sandstones to water-saturated conditions. However, the necessary assumption that the absolute value of λ m remains constant irrespective of the pore fluid present turns out to be not valid in practice and the explicit control factors on λ m have not been demonstrated yet for different pore fluids. A pore fluid-controlled change in the λ m value also questions the transferability of empirical proxy models for the estimation of water-saturated λ b when they were calibrated on air-saturated samples. This study applies a multiple regression analysis to quantitative mineralogical composition data and porosity (Φ) to identify the controls of the λ m for different types of pore fluids (water-vs. air-saturation). In addition, the differences in the calculated λ m values resulting from different calculation methods or input data (theoretical geometric mean model or mineralogical composition data) are examined. We further test the suitability of different sandstone properties as potential proxies for the estimation of the λ b , with respect to the pore fluid type. Differences in the absolute value of λ m of sandstones from different measurement conditions (air-or water-saturated) are most probably related to the formation of authigenic kaolinite in the pore space, originating from the alteration of alkali feldspar. In addition, the thermal properties of the rock matrix are mainly controlled by the volume fractions of the high thermally conductive mineral fractions quartz and dolomite. Empirical models that have solely Φ or P-wave velocity (v p) as variables are not suitable for the prediction of water-saturated λ b of sandstones. Instead, quantitative mineralogical data of high thermally conductive mineral phases such as quartz and dolomite have to be included. The easily measureable v p is proposed as a promising proxy for both pore fluid types tested: combined with the total quartz volume/porosity ratio for air-saturated sandstones, and combined with the quartz plus dolomite volume fractions for water-saturated sandstones.

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Improving the temperature predictions of subsurface thermal models by using high-quality input data. Part 1: Uncertainty analysis of the thermal-conductivity parameterization

Abstract: Geothermics 46, pp 42-54 [Elsevier] "Improving the temperature predictions of subsurface thermal models by using high-quality input data. Part 1: Uncertainty analysis of the thermal-conductivity parameterization"

Cited by 40 publications

References 38 publications

Improving the temperature predictions of subsurface thermal models by using high-quality input data. Part 2: A case study from the Danish-German border region

Improving the temperature predictions of subsurface thermal models by using high-quality input data. Part 2: A case study from the Danish-German border region

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

Pore-fluid-dependent controls of matrix and bulk thermal conductivity of mineralogically heterogeneous sandstones

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