In the past, the analysis of the surface heat flow and the measured temperatures in the subsurface has revealed clear geothermal anomalies in the Rhine graben of Rhineland-Palatine (e.g. Hurter and Schellschmidt, 2003). Partly, these geothermal anomalies have now been selected for geothermal energy production. They are in various stages of project development from initial exploration to failed, “dry” boreholes.
The aim of the new geothermal resource atlas of Rhineland-Palatine is to characterise the geothermal resources in terms of their potential evaluation and quantification for geothermal energy production. The large-scale evaluation is related a similar approach conducted for the Swiss geothermal atlas (Signorelli and Kohl, 2006). Similar findings are expected to the region of Rhineland-Palatine. The analysis is based on an approach integrating geological, thermal, hydrogeologic and surface utilization data. The geological model represents the basis of the interpretation. Temperature is evaluated by elaborating a 3D numerical thermal calibration model using the finite element program FRACTure (Kohl and Hopkirk, 1995). It represents the result of a fit from borehole temperature data accounting for effects caused by large-scale geological, topographical and partly even hydrogeological structures. The final results are reported for the known aquifer types (Muschelkalk, Buntsandstein, Crystalline basement). It accounts for the local hydrothermal situation. The findings are displayed in terms of geothermal productivity and energy.
The currently investigated area (2'000 km2) is located in the southern Pfalz and for practical reasons subdivided in three smaller areas of 25x28 km.
Topography is obtained from Shuttle Radar Topography Mission (SRTM, US Geological Survey). Geological models have been calculated using a potential field approach (Lajaunie et al., 1997), which incorporates geological layers as equipotential surfaces and the geological dip as gradient of the geological potential. In addition to own field observation, the following data were used to calculate the 3D geological model of the Landau area: geological maps (e.g. Griessemer, 1987, Steingötter, 2005), borehole information (Doebl, 1970; Doebl and Bader, 1971) and interpreted seismic sections (Doebl and Teichmueller, 1979). The geological model of the Landau area considers 11 different geological units and the major 28 faults (including approximately 80 relative age relation).
Temperature, heat and hydraulic conductivity data
The topographic and geological 3D model is used as an input model for a calibrated 3D numerical model of the temperature field in the investigated area. For calibration of the temperature model, temperature data in a 250x250x250 m grid covering a depth of 250-2500 m provided by the GGA Institute are used. The calibration of the temperature field aims (1) to identify thermal signatures (= potential aquifers) and (2) to extrapolate the temperature field to greater depth (where no measured data are available) based on a physical model.
Heat conductivity data are taken from the European geothermal project at Soultz and literature. Although the uncertainties in heat conductivity have a direct influence on the heat flux distribution they are less relevant to the calibrated temperature field.
Measured hydraulic transmissivity data are generally rare but further investigation is under way. In this regional resource analysis, we review the available data from literature and assume measured data to be representative for the complete geological unit. It should be noted, that for detailed local exploration this assumption is not appropriate.
Calculation of the temperature field
The calculation of the temperature field is carried out using Finite Element models derived from the geological models by triangulation of the surface and tethraedrisation of the geological units. The boundary conditions of the model are an altitude dependent ground surface temperature and a basal heat flux that is adapted in the calibration model. The geological units are characterised by different petrophysical parameters such as heat conductivity and heat production etc. The calculation of the temperature field is based on a diffusive heat transport. Heat transport by advection is considered only if evident from the measured temperature data.
Determination of the geothermal potential
The thermal productivity pth of the subsurface is coupled with the type of utilisation. For a doublette system Gringarten (1978) introduced an analytical solution:
pth = (rhocp)f x Q x (Tprod – Tinjec) [W]
where (rhocp)f is the specific heat capacity of the fluid [J m-3 K-1], Q is the production rate [m3 s-1] and Tprod – Tinjec is the temperature difference between production and injection [°C]. The production rate includes the geometry of the borehole and the geological unit, and the hydraulic transmissivity of the geological unit. It is assumed that the hydraulic transmissivity can be enhanced through stimulation. Factor of improvement of '10' for crystalline and '2.5' for sediments are taken that reflect recent achievements. The presented scheme accounts for the difference between total energy ("heat in place") and producible energy. By integrating the geothermal productivity with operation time the producible potential is calculated. Generally, recovery factors between 1% and 4% are obtained for the selected well-known aquifers.
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Darstellung auf Grundlage von geowissenschaftlichen Daten des Landesamtes für Geologie und Bergbau Rheinland-Pfalz, Kontrollnummer ..9/2006.., Bayrische GK 25 Manuskripte, Nr. 6713, 6714, 6813, 6834, 6913, 6914.
Landsat Mission, NASA Applied Sciences Directorate, https://zulu.ssc.nasa.gov/mrsid/mrsid.pl
This study has been financed by the Ministerium für Umwelt, Forsten und Verbraucherschutz in Rheinland-Pfalz. We would like to thank T. Charissé and F. Wellmann for the geologic modelling. R. Schulz and R. Schellschmidt (GGA Institute, Hannover) kindly provided the temperature data.