geowissenschaftliches zentrum gÖttingen hannover-messe 2005 geothermal energy: unlimited,...

Hannover-Messe 2005

G GZ GEOWISSENSCHAFTLICHESGEOWISSENSCHAFTLICHESZENTRUM GÖTTINGENZENTRUM GÖTTINGEN

Geothermal Energy:

Unlimited, Environmental-Friendly, and Economic

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-

180C.

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A greenhouse A swimming pool

An electric power stationThe heat stored in the rocks at

depth can be used for direct

heating, electricity production,

or both.

Source: Orkustofnun

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The hot springStrokkur inIceland.

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.

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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.

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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

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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.

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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.

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Much geothermal water

is transported along

faults, such as this one

in the Muschelkalk in

Göttingen.

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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.

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An extinct geothermal system, consisting of mineral

veins, in a fault zone in Iceland.

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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

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H

P

h

P

Hh

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 permeability Fabric-dependent permeability

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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.

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Example of a simple

numerical model showing

the propagation, through

many crustal layers, of a

vertical fluid-driven fracture,

that is, a hydrofracture.

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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

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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

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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.

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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)

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Vin Vout

Injection phase

Tracer 1, Tracer 2

Production phase

Diffusion front(immobile water)

Pulse-input

Back diffusion

Following the geological studies, the numerical modelling, and the

stimulation experiments, tracers are used to test the permeability of the

reservoir.

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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

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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

ThüringischesBecken

Ober-rhein

graben

Dresden

Leipzig

Urach

SoultzStuttgart

Frankfurt

Köln

Erding

Straubing

BayerischesMolassebecken

NorddeutschesBecken

Neustadt-Glewe

BerlinGenesys Horstberg

Basel

Speyer

Offenbach

Pullach

Unterhaching

Aachen

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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

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• 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

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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.

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Geothermal power plants are environmental-friendly,

and their surroundings can be used for various purposes. Example: the Blue Lagoon in Iceland.

geowissenschaftliches zentrum gÖttingen hannover-messe 2005 geothermal energy: unlimited,...

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