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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