1. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Geothermal Energy: Unlimited, Environmental-Friendly, and Economic

2. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Everywhere in the earth‘s crust, the temperature increases with depth. For example, in parts of Germany the temperature at 3 km depth is 120- 180  C.

3. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 A greenhouse A swimming pool An electric power station The heat stored in the rocks at depth can be used for direct heating, electricity production, or both. Source: Orkustofnun

4. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 The hot spring Strokkur in Iceland. For economic use of the heat stored in the rocks, detailed geological studies are absolutely necessary – even in areas such as Iceland where geothermal fields are common at the surface.

5. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Geothermal use of hot-dry rocks requires: a hole for injecting cold water; a stimulated, fractured reservoir; a hole for producing hot water; and a power plant.

6. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 To assess the geothermal potential of an area, we must make a) detailed geological and geophysical site studies, b) laboratory tests and studies, and c) numerical models. a b c

7. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 To minimise the risk and maximise the chance of success in geothermal projects, we begin by geological field studies. For example, we study extinct palaeogeothermal fields to understand current geothermal fields. Part of a palaeogeothermal field in sedimentary rocks in England.

8. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 To understand the permeability of a fractured geothermal reservoir, the host-rock fracture system must be known. Here is a part of a fracture system in the Bunter Sandstone, Göttingen.

9. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Much geothermal water is transported along faults, such as this one in the Muschelkalk in Göttingen.

10. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 The permeability of a geothermal reservoir in a fault zone depends partly on the fracture systems and properties of the fault zone, and partly on the local stress field. Field example from England.

11. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 An extinct geothermal system, consisting of mineral veins, in a fault zone in Iceland.

12. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Field studies must be complemented by laboratory studies of samples from the potential reservoir rocks to determine their properties. Strength tests Scanning-electron microscopy Texture analyses

13. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 HH P hh P HH hh Laboratory experiments on rock samples can be used to determine their permeabilities and how these relate to local stress fields and the fabric of the rock. Stress-dependent permeabilityFabric-dependent permeability

14. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Field and laboratory studies should be complemented by numerical models to forecast fracture propagation, interconnection, and fluid transport in the potential reservoir. These studies are also necessary for deciding on the type of stimulation needed to increase the reservoir permeability.

15. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Example of a simple numerical model showing the propagation, through many crustal layers, of a vertical fluid-driven fracture, that is, a hydrofracture.

16. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Numerical models on the stress concentration (left) and direction of hydrofracture propagation (right). The thin layers are soft, the thick layers are stiff. This difference in mechanical properties between the layers largely controls whether, and in which direction, a hydrofracture propagates. Tensile stress concentration Direction of hydrofracture propagation

17. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Based on the geological and geophysical investigations, a site is selected and the type of stimulation needed for the reservoir determined. The two basic stimulation methods are (a) hydraulic fracturing, and (b) massive hydraulic stimulation. Source: BGR

18. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Source: Smith & Shlyapobersky 2000 In hydraulic fracturing, a fluid under high pressure is injected into a certain layer – the reservoir. The fluid creates a fracture that increases the permeability of the reservoir.

19. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 In a massive hydraulic stimulation, natural fractures slip and open up, thereby generating a high-permeability reservoir between the injection and production drillholes. The fracture slip is monitored through numerous very small earthquakes (shown here by hypocentres). Source: Asanuma et al. 2002 (Tohoku University, Japan)

20. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Following the geological studies, the numerical modelling, and the stimulation experiments, tracers are used to test the permeability of the reservoir.

21. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Tracer tests also provide information on the contact area between the fractures and the surrounding rock in the resevoir, and thus how effectively heat is transported from the rock to the water. Tracer-Tests in Bad Urach

22. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Source: Geoforschungszentrum Potsdam Large parts of Germany (here indicated by light- green colour) offer suitable sites for man-made geothermal reservoirs. Some current geothermal drilling sites are indicated by large red dots. Groß Schönebeck Hamburg Hannover Thüringisches Becken Ober- rhein graben Dresden Leipzig Urach Soultz Stuttgart Frankfurt Köln Erding Straubing Bayerisches Molassebecken Norddeutsches Becken Neustadt-Glewe Berlin Genesys Horstberg Basel Speyer Offenbach Pullach Unterhaching Aachen

23. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Some 14% of the worldwide primary energy consumption is provided by renewable sources. It is predicted that non-renewable energy sources start to decline in the first half of this century. Source: Shell

24. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 For many decades now, there has been commercial production of electricity and direct use of geothermal energy at the scale of hundreds of mega-watts. More than 20 countries worldwide use geothermal steam to produce electricity. In several countries, 10-22% of the total electricity production is from geothermal sources. Photograph: a drillhole providing steam for the geothermal power plant at Nesjavellir, Iceland. Source: Fridleifsson 2002

25. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Conclusions  The experience from Iceland and other countries is of help when assessing the potential of geothermal-energy use in Germany.  Heat gradients in parts of Germany are similar to those in the older parts of Iceland, and thus quite high.  Trial-and-error methods in geothermal exploration are not used in Iceland and unlikely to be successful in Germany.  The main unknown scientific parameter for man-made reservoirs is the fracture-related permeability.  The permeability can be inferred from field data, natural analogies, laboratory and site tests, and numerical models.

26. GEOWISSENSCHAFTLICHES ZENTRUM GÖTTINGEN Hannover-Messe 2005 Geothermal power plants are environmental-friendly, and their surroundings can be used for various purposes. Example: the Blue Lagoon in Iceland.