Temperature profiles in geothermal systems

L. Guillou-Frottier, BRGM, France

Engine Launching Conference, February 14th, 2006

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Temperature profiles in geothermal systems

> Examples

> Heat conduction processes

> Heat convection processes

> Conclusion

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Examples of temperature profiles

Several distinct processes can create shallow thermal anomalies...

After Muraoka et al., 2000

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At least 4 possible causes

- high heat producing granite

- insulating cover

- magma reservoir

- hot fluids at shallow depths

In these cases, the shallow thermal anomalyis not related to shallow convective processes.

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> Role of an insulating (sedimentary) coverCase of Soultz-sous-Forêts (France)

Pech.

Dol.

K + Mush

Mush+Bund.

Bund.

Granite

z

0

380

800

1000

1200

1400

Thermalconductivity

1.3

1.4

2.1

2.5

1.9

2.5

}

T° profile in the upper part can beexplained by thermal conductivity contrasts Schellschmidt & Clauser, 1996

Schellschmidt et al., 1997

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> Role of a high heat production granite

Grecksch et al., 2003

SOULTZ

High heat production can also accountfor one part of the excess temperature

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> Hypothesis of a cooling reservoirCase of Bouillante (Guadeloupe, French West Indies)

Negative T° gradient !BO-4(1996)

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> Hypothesis of a hot upwelling in a porous media

T(z) = f(u, k, ρ, Cp, z)

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Temperature profiles in a convective system

Laboratory experiments by Guillou & Jaupart, 1995

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Measurements of temperature profiles (lab. experiments)

Profiles at different locations(above and near one upwelling)

3 zones can be defined :

(1) the purely conductive zone = thermal boundary layer :

high temperature gradient

(2) zone of a decreasing T° gradient

(3) zone of a small / null / negativeT° gradient

note the existence of negative T° gradients...

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Local temperature profiles in geothermal systems

Same observations as in convective systems :

- 3 zones- local differences are constant over a large thickness

Schellschmidt & Clauser, 1996

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Up- and down-wellings can be inferred from local temperature profiles...

...however, drawing of isotherms is not sufficientto delineate the reservoir geometry...

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ReservoirReservoir geometrygeometry andand physicalphysical conditions conditions couldcould thusthus bebe betterbetter constrainedconstrained

by by carefulcareful analyses analyses ofof temperaturetemperature profiles.profiles.

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Conclusions

> Simple 1D conductive models show that partsparts ofvertical temperature profiles can be explained

> Curved temperature profiles at shallow depths mayresult from combination of several processes

> Scaling laws of thermal convection applied to temperature profiles may help in thecharacterization of geothermal systems