Temperature profiles in geothermal systems L. Guillou-Frottier, BRGM, France

1. Temperature profiles in geothermal systemsL. Guillou-Frottier, BRGM, France Engine Launching Conference, February 14th, 2006

2. Temperature profiles in geothermal systems • Examples • Heat conduction processes • Heat convection processes • Conclusion

3. Examples of temperature profiles After Muraoka et al., 2000 Several distinct processes can create shallow thermal anomalies...

4. high heat producing granite • insulating cover • magma reservoir • hot fluids at shallow depths At least 4 possible causes In these cases, the shallow thermal anomaly is not related to shallow convective processes.

5. Role of an insulating (sedimentary) cover Case of Soultz-sous-Forêts (France) Thermal conductivity z 0 380 800 1000 1200 1400 } Pech. 1.3 1.4 2.1 2.5 1.9 2.5 Dol. K + Mush Mush+Bund. Bund. Granite Schellschmidt et al., 1997 T° profile in the upper part can be explained by thermal conductivity contrasts Schellschmidt & Clauser, 1996

6. Role of a high heat production granite SOULTZ Grecksch et al., 2003 High heat production can also account for one part of the excess temperature

7. Hypothesis of a cooling reservoir Case of Bouillante (Guadeloupe, French West Indies) Negative T° gradient ! BO-4 (1996)

8. Hypothesis of a hot upwelling in a porous media T(z) = f(u, k, r, Cp, z)

9. Temperature profiles in a convective system Labioratory experiments by Guillou & Jaupart, 1995

10. Measurements of temperature profiles (lab. experiments) 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... Profiles at different locations (above and near one upwelling)

11. Local temperature profiles in geothermal systems Schellschmidt & Clauser, 1996 Same observations as in convective systems : - 3 zones - local differences are constant over a large thickness

12. Up- and down-wellings can be inferred from local temperature profiles... ...however, drawing of isotherms is not sufficient to delineate the reservoir geometry...

13. Reservoir geometry and physical conditions could thus be better constrained by careful analyses of temperature profiles.

14. Conclusions • Simple 1D conductive models show that parts 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