Effects of hydrogeochemistry on the long-term hydraulic behaviour of geothermal wells in the Molasse Basin, Bavaria

Mark Lafogler1, Alexandros Savvatis2, Frank Wenderoth3, Alexander Seitz-Gutmann4, Reinhard Niessner1, Thomas Baumann1
1 Institute of Hydrochemistry, Technische Universität München
2 Erdwerk GmbH
3 Aquasoil GmbH
4 Innovative Energie für Pullach GmbH

P 8.10 in Geothermal energy und geocooling

Geothermal exploration of the Malm aquifer in Bavaria is highly
successful. Data about the long-term operation, however, is still
scarce, although detailed knowledge about the processes occurring in
the aquifer is a key requirement to run geothermal facilities
efficiently and economically.
While there usually is a constant flow of data from the production
well (temperatures, hydraulic data, hydrochemical conditions, gas
composition) not even the temperatures in the immediate surrounding of
the reinjection well are accessible or known.
In 2011 the geothermal facility in Pullach was extended with a third
geothermal well reaching into the Malm aquifer which is now used as a
reinjection well. The former reinjection well was converted to a
production well after 5 years of operation. This setting offers a
unique opportunity to study the processes in the vicinity of a
reinjection well and provides the data base to describe the hydraulic,
thermal and hydrochemical performance of the reservoir.
As the viscosity of the injected cold water is at least 60% higher
compared to the hot water in den reservoir, one would expect an
increase of the reinjection pressure as the cold water plume spreads
around the reinjection well. Measurements, however, show a significant
decrease of the reinjection pressure at many geothermal sites in the
area of Munich, suggesting processes in the aquifer which positively
change the hydraulic properties and overcompensate the viscosity

When the well Th2 was transformed into a production well in 2012,
access was provided to injected thermal water. Not surprisingly many
dissolved ions like Na, K, and Cl showed similar concentrations in the
produced water compared to the injected water. However we also observe
a significant increase of Ca, Mg, and the alkalinity as well as a
decrease for sulfate. While the former indicate a dissolution process
in the aquifer, the latter points to increased microbial activity.

The dissolution processes might explain the overcompensation of
viscosity effects by either increasing the width of the flow paths in
the matrix or reducing the skin effect, or both.

last modified 2013-10-30