To develop Enhanced Geothermal Systems (EGS) in unconventional areas, one of the main goals in MEET is the modelling of representative geothermal reservoirs in granitic as well as in folded and thrusted metasedimentary rocks of the so-called Variscan Orogen –an old Paleozoic mountain belt, which comprises the basement of large parts of Europe.
Since model quality directly depends on the quality of the input, in the first half of the project it was crucial to explore the geology, i.e. the kind of rocks, their distribution, the structures they form as well as to measure the petrophysical parameters of the rocks in the lab. At all our four demo sites with existing or envisaged geothermal plants, the targeted rock units are generally covered by younger sediments. Even though, we have information from wells at some sites, for a better understanding there is the need to explore the rock units in so-called analogue areas, i.e. where outcrops of the rocks at the surface allow direct access.
We just completed the evaluation of the suitability of our selected outcrop analogue sites. The sites with granitic rocks as target horizons, are the Carnmenellis granites and the Cornubian batholith in Cornwall for the United Downs Deep Geothermal Project site (UDDGP) in Southwest England and the Southern Death Valley Fault Zone in California/US for the Soultz-sous-Forêts site with an existing geothermal plant in the Alsace, Western France. Outcrop analogue areas for the 5,000 m deep Havelange well site in Belgium are situated in the Ardennes and for the University Göttingen campus site in Germany the Western Harz Mountains as well as the Eastern Rhenish Massif were selected. As different as these regions, the geotectonic settings and the technological readiness levels in regard of an existing or potential geothermal are, as different are the individual exploring approaches applying analogue studies. Figure 1 summarizes the lithological and structural elements of all sites we are quantitatively characterizing. A crucial discussion is always focusing on the data transferability from the analogue site to the reservoir to be explored as well as of the methodical restrictions. This discussion will be part of the public presentation of our analogue studies during the German Geothermal Congress 2020 (Der Geothermikongress – DGK) in November 2020 in Forum 4 “Deep Geothermal: Geoscientific Studies in Central Europe (in english)”.
To be able to proceed with the challenging and unconventional conceptual reservoir modelling, we also have the petrophysical database of all samples from all demo and/or analogue sites completed. All these experiments e.g. on the thermal and mechanical properties were carried out in the labs of our MEET partner Technical University of Darmstadt.
Figure 1: Conceptual geotectonic model representing the different lithological and structural settings and elements of the variscan basement explored within MEET project. (1) mineralisations in slates within fracture and cleavage planes, (2) representation of a fault zone within a late large scale transcurrent zone (3) close up on a damage zone along a brittle normal fault affecting granitic basement rocks.