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

Abstract

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.

Static and Dynamic Moduli of Malm Carbonate: A Poroelastic Correlation

“…The permeability of the Malm carbonate was calculated to be to be 0.69 ×10 −18 m 2 at an applied confining pressure of 25 MPa (an average effective pressure of 12.5 MPa). The measured permeability value is in agreement with those reported by Homuth et al (2015) for Malm carbonate at high effective pressures. At low effective pressures Homuth et al (2015) reported a total shift of about 2-3 order of magnitudes in the permeabilty of the Malm carbonate.…”

From oil field to geothermal reservoir: First assessment for geothermal utilization of two regionally extensive Devonian carbonate aquifers in Alberta, Canada

“…Majorowicz and Weides, 2014;Weides et al, 2013;Nieuwenhius et al, 2015;Ardakani and Schmitt, 2016). However, an extensive data base on 5 parameters such as porosity, permeability, thermal conductivity, and others, is necessary for a geothermal assessment, especially with complete parameter sets for every single sample (Clauser, 2006;Sass and Götz, 2012;Homuth et al, 2015). This paper reports the results of a pilot study that forms the first phase of a larger project entitled 'MalVonian' (semantically amalgamated from Malm and Devonian), a joint undertaking by researchers from the Technische Universität Darmstadt and the University of Alberta.…”

“…The Malm-Aquifer in Southern Germany has already been proven to be suitable as a geothermal reservoir (Birner et al, 2012;Homuth et al, 2015;Wolfgramm et al, 2017). However, the overall subsurface data base is relatively scarce because it is mainly based on drill cores from exploration wells, which of there are few in areas without hydrocarbons.…”

“…Therefore, in order to obtain a large enough data base for upscaling and correlation of geothermal reservoir characterization. Homuth et al (2015) used samples not only from the four wells within the reservoir, but also from 19 outcrops in the shallow parts of the homocline. This approach is known as the '(outcrop) analogue study concept' (Homuth et al, 2015).…”

“…Homuth et al (2015) used samples not only from the four wells within the reservoir, but also from 19 outcrops in the shallow parts of the homocline. This approach is known as the '(outcrop) analogue study concept' (Homuth et al, 2015). This concept entails that properties of rock units in the target subsurface reservoir are compared to stratigraphically equivalent rock units in outcrop, which are an analogue to the subsurface target.…”