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 anomaly is 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 be explained 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 account for 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, r, Cp, z)

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Temperature profiles in a convective system

Labioratory 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 / negative T° 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 sufficient to delineate the reservoir geometry...

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Reservoir geometry and physical conditions could thus be better constrained Reservoir geometry and physical conditions could thus be better constrained

by careful analyses of temperature profiles.by careful analyses of temperature profiles.

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Conclusions

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

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

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