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Scalings in the Thermal Water System of a Geothermal Power Plant in the Malm Aquifer

Moritz Herbrich1, Nadine Frank2, Christian Pletl2, Franz Barenth2, Reinhard Niessner1, Thomas Baumann1
1 Institut für Wasserchemie, Technische Universität München
2 SWM Services GmbH

P 8.9 in Geothermie und Geocooling

The Malm aquifer in the Molasse basin, Bavaria, Germany, is ideally
suited for the exploration of deep hydrogeothermal energy. Several
ambitious projects for combined heat and power production were
initiated over the last few years. Although the general conditions of
the Malm aquifer are very favorable for geothermal power production
with volume fluxes exceeding 100 L/s, temperatures above 140 °C, and
low salinity, the technical details for long-term operation are far
from trivial. Recent research projects have shown that higher
concentrations of methane and H2S can occur in the area around Munich
and that carbonate equilibria are likely to be disturbed during
production.

During the retrieval of the thermal water pump at the combined heat
and power station at Sauerlach, operated by the Stadtwerke München
GmbH, scalings were found on the production unit, inside the stand
pipes, and in the ground level thermal water system.

The thickness of the scalings was high immediately at the pump's
outlet, decreased to minimum thickness roughly 400 m above the
pump and increased again to the surface. The morphology
and mineralogical composition of the scalings analyzed by SEM/EDX and
XRD reveals mainly magnesium-calcite and some iron sulfides in the
lower part of the stand pipes. The size of individual crystals in the
scalings increases from the pump to the surface.

The results indicate a disturbance of the carbonate equilibria due to
degassing. Indeed, gas bubbles can be observed in the thermal water at
ground level inspection windows. These findings, however, are in
contrast to model calculations under static conditions, which predict
complete dissolution of all gases in the thermal water system under
the given pressure conditions. A likely explanation to this
discrepancy is a local pressure drop in the pump which leads to the
formation of gas bubbles which, once formed, dissolve only very slowly
in the thermal water flow due to little differences in the local
partial pressures.

Scalings inside the pipes of the ground level facility where also
found after filter systems and show little adhesion to the pipes due
to the presence of an oil film. Although easily removable during
maintenance cycles, these scales can also be mobilized by sudden
pressure changes when starting and stopping the pumps. They might then clog the heat exchangers and reduce their efficiency significantly.

This project is funded by the Federal Environment Ministry under the
project number 0325280B.

Letzte Änderung 31.10.2013