Christoph Clauser

The thermal regime of the crystalline continental crust: Implications from the KTB

An extensive geothermal research program within the German Continental Deep Drilling Program (KTB) covered an almost complete spectrum of experimental and theoretical aspects. The main results and conclusions are as follows: (1) Equilibrium temperature is 118.6°C at 4000 m in the KTB pilot borehole (KTB-VB) and will be around 260°C at 9100 m in the KTB main borehole (KTB-HB). Time required for thermal equilibration of the KTB-HB to within 1% of the initial perturbation will be about 13–16 years. (2) The failure to predict temperature correctly for the KTB was mainly due to an unaccounted perturbation by a transient ground surface temperature history. (3) Pleistocene surface temperature variations affect the present-day crustal temperature between 1.3 to 2.7 K up to a depth of 4000 m. Accordingly, present-day heat flow density is systematically too low down to approximately 1500 m. Model simulations indicate that groundwater flow does not eliminate paleoclimatic signals, even though it may translate them to a depth incompatible with both their diffusive age and their amplitude. These results emphasize the importance of an adequate consideration of paleoclimatic effects for the interpretation of thermal data. (4) Lateral heat transport is significant when steep inclination and folding of the rock formations coincide with contrasts in thermal conductivity. This is indicated by typical variations in the vertical components of temperature gradient and heat flow density, such as in the KTB-HB. In contrast, thermally relevant advection of heat is confined to the top 500–1000 m. In the vincinity of the KTB, this is about twice the maximum difference in elevation. (5) In the deeper crust, free convection systems require permeabilities greater than 10−17 m2 for large rock volumes, but simple numerical models indicate that the associated temperature regimes are imcompatible with KTB borehole data. (6) Heat production rate shows no systematic variation with depth and is related to lithology at the KTB as in other deep boreholes in crystalline rock. Numerical models, using heat production rate derived from seismic velocities, yield temperatures compatible with KTB borehole data.

Thermal conductivity from core and well log data

Abstract The relationships between thermal conductivity and other petrophysical properties have been analysed for a borehole drilled in a Tertiary Flysch sequence. We establish equations that permit us to predict rock thermal conductivity from logging data. A regression analysis of thermal conductivity, bulk density, and sonic velocity yields thermal conductivity with an average accuracy of better than 0.2 W ( m K ) - 1 . As a second step, logging data are used to compute a lithological depth profile, which in turn is used to calculate a thermal conductivity profile. From a comparison of the conductivity–depth profile and the laboratory data, it can be concluded that thermal conductivity can be computed with an accuracy of less than 0.3 W ( m K ) - 1 from conventional wireline data. The comparison of two different models shows that this approach can be practical even if old and incomplete logging data are used. The results can be used to infer thermal conductivity for boreholes without appropriate core data that are drilled in a similar geological setting.

Numerical simulation of flow and heat transfer in fractured crystalline rocks: Application to the Hot Dry Rock site in Rosemanowes (U.K.)

Abstract This study examines heat transfer during forced water circulation through fractured crystalline rock using a fully 3-D finite-element model. We propose an alternative to strongly simplified single or multiple parallel fracture models or porous media equivalents on the one hand, and to structurally complex stochastic fracture network models on the other hand. The applicability of this “deterministic fracture network approach” is demonstrated in an analysis of the 900-day circulation test for the Hot Dry Rock (HDR) site at Rosemanowes (U.K.). The model design with respect to structure, hydraulic and thermal behavior is strictly conditioned by measured data such as fracture network geometry, hydraulic and thermal boundary and initial conditions, hydraulic reservoir impedance, and thermal drawdown. Another novel feature of this model is that flow and heat transport in the fractured medium are simulated in a truly 3-D system of fully coupled discrete fractures and porous matrix. While an optimum model fit is not the prime target of this study, this approach permits one to make realistic long-term predictions of the thermal performance of HDR systems.

Evaluating thermal response tests using parameter estimation for thermal conductivity and thermal capacity

Thermal response tests (TRT) record the temperature variation of closed-loop shallow borehole heat exchangers (BHE) due to fluid circulation. The average change of fluid temperature is directly related to the rock thermal conductivity λ around the well. If environmental and experimental conditions satisfy the usual experimental standards, TRT can predict effective ground thermal conductivity within an error of approximately ±10%. This accuracy is generally accepted as sufficient for an appropriate prediction of the geothermal heat yield. However, the line source approach (on which the analysis of the TRT experiment is based) does not allow us to derive thermal capacity independently from thermal conductivity, and the soil thermal capacity ρc is usually assumed constant here. We calculate the response temperature of a synthetic TRT experiment as a reference for a subsequent joint estimation of rock thermal conductivity and thermal capacity. Within a reasonable computing time, a comprehensive parameter estimation is impossible, if coupled fluid flow and heat transport in the BHE tubes are explicitly simulated. Therefore, we substitute the BHE tube by a constant heat source with diffusive heat transport only. Although this simplification limits the application of the method to synthetic TRT data, we perform a systematic study of the methods accuracy to analyse thermal capacity with respect to data noise and test duration. Finding the minimum misfit with respect to the reference experiment, we obtain both thermal conductivity and thermal capacity, i.e. more information on ground thermal properties than the line source theory can provide. The effect of the additional information on the ground thermal capacity is demonstrated by a numerical simulation of a real TRT, where the fluid flow and the heat transport within the BHE tube are explicitly simulated. Nevertheless, thermal capacity is generally variable within ±20% for the same rock type. In our analysis, this uncertainty results in a variation of ±2% of the outlet temperature.

The thermal regime of the Eastern Alps from inversion analyses along the TRANSALP profile

A combination of petrophysical measurements and inverse modeling is used to estimate the 2-D, steady-state conductive thermal regime in the crust and heat flow at the Moho along the N–S trending TRANSALP profile across the continental collision zone of the Eastern Alps. The uncertainty to which each parameter in the simulation is known beforehand is expressed in terms of its variance, entering a Bayesian parameter estimation scheme. This approach is particularly attractive for geological applications–– where often information is available, if only with large uncertainty––because it allows to introduce soft information into a quantitative approach. Our inversion studies show that while the large a priori standard deviation of particularly the heat production rate in the upper crust can be significantly reduced a posteriori, the variance of the middle crust heat production rate remains comparatively large. Using two extreme models with maximum and minimum heat production rates in the middle crust the range of Moho temperatures and heat flow can be estimated. Depending on different assumptions about the composition of the middle crust we obtain maximum temperatures of around 900 C � 30% in the lower most parts of the European crust. In the Alpine root and in the Southern Alps, maximum temperatures are 700–800 C � 10% and 600 C � 10%, respectively. Moho heat flow varies from 5–25 mW m � 2 and is

Hydrothermal transients in Variscan crust: paleo-temperature mapping and hydrothermal models

Abstract This study combines experimental work and numerical simulations to reconstruct the thermal history of the Frankenwald Transverse Zone, which was formed by a granitic intrusion into a fault zone. Illite crystallinity, vitrinite reflectance, and geobarometric investigations reveal a metamorphic and paleo-temperature anomaly associated with the granitic intrusion. Results of numerical simulations adequately explain paleo-temperatures in that area. In order to be able to obtain a quantitative comparison between numerical model results and paleo-temperature as observed in the field, we propose an empirical relationship between illite crystallinity and the maximum paleo-temperature based on literature data of illite crystallinity and a combination of other temperature-dependent parameters like vitrinite reflectance, phase petrology and smectite-to-illite transformation. Application of this strategy to the Frankenwald Transverse Zone yields the following results: (1) The paleo-temperature anomaly can be explained by the cooling of a number of plutons which intruded into the center of the zone. No additional heat sources are required to explain the observed anomaly. (2) The diapiric shape of these plutons could be confirmed because, in contrast, dike-shaped bodies would produce much smaller paleo-thermal anomalies. (3) The resolution of paleo-temperatures obtained from the illite crystallinity data is not good enough to discriminate precisely between advective and conductive modes of heat transfer. According to our preferred model, conductive heat transport is more likely than fluid-driven advective heat transport.

ISRM Suggested Methods for Determining Thermal Properties of Rocks from Laboratory Tests at Atmospheric Pressure

Thermal properties—thermal conductivity, thermal diffusivity, specific heat, volumetric heat capacity, and thermal effusivity—are fundamental physical properties of rocks and rock-forming minerals. They have clear physical meanings, and two of them (thermal conductivity and volumetric heat capacity) are used in Fourier’s heat conduction equation for homogeneous solid: cq oT ot k o T ox2 þ o T oy2 þ o T oz2 1⁄4 F ð1Þ

Parameter identification and uncertainty analysis for heat transfer at the KTB drill site using a 2-D inverse method

Abstract The German continental deep-drilling program (KTB) provided a unique opportunity for studying heat transfer processes in deep continental crust. We used an inversion technique based on the Bayesian parameter estimation to constrain parameters involved in crustal heat transfer at the KTB site and their standard deviations. Measurements in the two deep KTB boreholes and on rock material recovered from these holes were used to compile a set of a priori information for a 2-D cross-section of 40 km × 30 km. Thermal and hydraulic parameters were determined in a steady-state inverse finite-element model for a simplified geological structure. The inversion algorithm yielded a posteriori information for thermal conductivity, permeability, and heat production rate as well as for temperature and hydraulic head. Because of the large temperature range (up to 800°C), we introduced the variation of thermal conductivity with temperature into the numerical algorithm. While thermal conductivity could be well resolved, the distribution of heat production rate was relatively poorly resolved. The uncertainties for the parameters varied depending on the number of free parameters. If heat production rate was constrained tightly, the resolution of the thermal conductivity was improved. A zone of high heat production rate between 3.8 km and 10 km combined with a reduced thermal conductivity above it provided the best fit to the measured temperatures in the borehole. The steady-state inversion yielded a better solution when paleoclimatic temperature perturbations, such as those caused by the last ice age, were removed from the temperature data.

Mobile NMR for porosity analysis of drill core sections

We apply a novel mobile nuclear magnetic resonance (NMR) scanning system, the NMR-MOUSE? (NMR-MOUSE (Nuclear Magnetic Resonance Mobile Universal Surface Explorer) is a registered trademark of RWTH Aachen University), for measuring porosity of geological drill core sections. The NMR-MOUSE? is used for transverse relaxation measurements on water-saturated core sections using a CPMG sequence with a short echo time. A regularized Laplace-transform analysis by the UPEN program yields the distribution of transverse relaxation times. The signal amplitudes and the distribution integrals correlate directly with the porosity of the cores, in spite of the influence of diffusion in the strong field gradient of the NMR-MOUSE?, which is discussed. The method is particularly attractive because it neither requires a volume calibration nor the samples to be machined to fit the coil, and because the device is mobile and particularly attractive for field use such as on logging platforms and research vessels.

Maximum potential for geothermal power in Germany based on engineered geothermal systems

We estimate the maximum geothermal potential in Germany available for exploitation by operated engineered geothermal systems (EGS). To this end, we assume that (a) capabilities for creating sufficient permeability in engineered deep heat exchange systems will become available in the future and (b) it will become possible to implement multiple wells in the reservoir for extending the rock volume accessible by water circulation for increasing the heat yield. While these assumptions may be challenged as far too optimistic, they allow for testing the potential of EGS, given the required properties, in countries lacking natural steam reservoirs. With this aim, we model numerically the thermal and electric energies which may be delivered by such systems by solving coupled partial differential equations governing fluid flow and heat transport in a porous medium. Thus, our model does not represent the engineered fractures in their proper physical dimension but rather distributes their flow volume in a small region of enhanced permeability around them. By varying parameters in the subsurface, such as flow rates and well separations, we analyze the long-term performance of this engineered reservoir. For estimating the maximum achievable potential for EGS in Germany, we assume the most optimistic conditions, realizing that these are unlikely to prevail. Considering the available crystalline landmass and accounting for the competing land uses, we evaluate the overall EGS potential and compare it with that of other renewables used in Germany. Under most optimistic assumptions, the land surface available for emplacing EGS would support a maximum of 13,450 EGS plants each comprising 18 wells and delivering an average electric power of 35.3 MW e. When operated at full capacity, these systems collectively may supply 4155 TWh of electric energy in 1 year which would be roughly seven times the electric energy produced in Germany in the year 2011. Thus, our study suggests that major scientific, engineering, and financial efforts are justified for developing the drilling and stimulation technologies required for creating the permeabilities required for successful EGS. Then, EGS will have great potential for contributing towards national power production in a future powered by sustainable, decentralized energy systems.