Journal cover Journal topic
Geothermal Energy Science An open-access journal
Journal topic
Volume 3, issue 1
Geoth. Energ. Sci., 3, 41-49, 2015
https://doi.org/10.5194/gtes-3-41-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Geothermal play types – categories and worldwide cases

Geoth. Energ. Sci., 3, 41-49, 2015
https://doi.org/10.5194/gtes-3-41-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

  22 Jun 2015

22 Jun 2015

Reservoir characterization of the Upper Jurassic geothermal target formations (Molasse Basin, Germany): role of thermofacies as exploration tool

S. Homuth1, A. E. Götz2, and I. Sass3 S. Homuth et al.
  • 1Züblin Spezialtiefbau GmbH, Ground Engineering, Europa Allee 50, 60327 Frankfurt a. M., Germany
  • 2University of Pretoria, Department of Geology, Private Bag X20, Hatfield, 0028 Pretoria, South Africa
  • 3Technische Universität Darmstadt, Geothermal Science and Technology, Schnittspahnstraße 9, 64287 Darmstadt, Germany

Abstract. The Upper Jurassic carbonates of the southern German Molasse Basin are the target of numerous geothermal combined heat and power production projects since the year 2000. A production-orientated reservoir characterization is therefore of high economic interest. Outcrop analogue studies enable reservoir property prediction by determination and correlation of lithofacies-related thermo- and petrophysical parameters. A thermofacies classification of the carbonate formations serves to identify heterogeneities and production zones. The hydraulic conductivity is mainly controlled by tectonic structures and karstification, whilst the type and grade of karstification is facies related. The rock permeability has only a minor effect on the reservoir's sustainability. Physical parameters determined on oven-dried samples have to be corrected, applying reservoir transfer models to water-saturated reservoir conditions. To validate these calculated parameters, a Thermo-Triaxial-Cell simulating the temperature and pressure conditions of the reservoir is used and calorimetric and thermal conductivity measurements under elevated temperature conditions are performed. Additionally, core and cutting material from a 1600 m deep research drilling and a 4850 m (total vertical depth, measured depth: 6020 m) deep well is used to validate the reservoir property predictions. Under reservoir conditions a decrease in permeability of 2–3 magnitudes is observed due to the thermal expansion of the rock matrix. For tight carbonates the matrix permeability is temperature-controlled; the thermophysical matrix parameters are density-controlled. Density increases typically with depth and especially with higher dolomite content. Therefore, thermal conductivity increases; however the dominant factor temperature also decreases the thermal conductivity. Specific heat capacity typically increases with increasing depth and temperature. The lithofacies-related characterization and prediction of reservoir properties based on outcrop and drilling data demonstrates that this approach is a powerful tool for exploration and operation of geothermal reservoirs.

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