Processes in the vicinity of an injection well of a geothermal facility in the Malm Aquifer (Bavarian Molasse Basin)

Mark Lafogler1, Frank Wenderoth2, Jörn Bartels2, Gabriella Somogyi1, Thomas Hinkofer1, Katarina Hess1, Thomas Baumann1
1 Institut für Wasserchemie, TU München
2 Aquasoil GmbH

O 12.1 in Thermische Grundwassernutzung

14.04.2016, 10:15-10:30, Audimax A, Geb. 30.95

With high temperatures, high transmissivities and low salinities the
Malm Aquifer in the Bavarian Molasse Basin offers ideal conditions for
the exploration of geothermal energy. In 2011 the Pullach geothermal
facility was extended with a third geothermal well to account for the
increasing heat demand. In the course of this extension an injection
well was converted to a production well. Hence, for the first time in
the history of geothermal exploration of the Malm Aquifer, data became
accessible from the surrounding of an injection well which has been in
operation for more than 5 years. This data allows unique access to the
processes at the injection well and sets the baseline for an
assessment of the long term behavior of geothermal heat and power
plants in the Molasse Basin.

Starting 9 months before the scheduled conversion we injected 5 pulses
of fluorescent tracers in a push-pull-setting and monitored the
temperatures and hydrochemical composition of injected and produced
reservoir water.

The development of the production temperatures went faster than
expected, after 4 years of production the initial temperatures have
almost been reached. This can only be explained with a vertically
heterogeneous distribution of the transmissivity. In this setting, the
cold water forms a thin disc which extends much further from the
injection well. Thus, the effective area of the heat exchange with the
matrix of the aquifer is larger than in a homogeneous setting.

The breakthrough of the tracers was affected by an unexpected delay of
the start of the production. The regional flow led to a shift of the
injected tracer pulses with the innermost tracer pulse being entirely
transposed downstream of the injection well. The recovery rates mirror
the sorption coefficients of the individual tracers as determined in
batch tests and column tests. It became apparent, that the stagnation
phase led to a bias towards sorption with slow kinetics and
diffusion-limited matrix interactions. The distribution of an oil
phase along the main flow pathways is the most likely reason for a
complete retention of Eosine which has the highest Kow value of all
injected tracers.

The hydrochemical data showed a significant increase of the
concentrations of calcium, magnesium, and bicarbonate indicating a
dissolution of dolomite. The dissolution overcompensates the effects
of the increases viscosity of the injected cold water. Modeling
results indicate that lower temperatures and different
lithostratigraphy are contributing to the dissolution. These processes
would also occur if the water would be produced from a dolomite and
injected into a limestone, which explains why most facilities in the
Molasse Basin have recorded decreasing injection pressures.

With the results from the Pullach field lab, a number of hypotheses
have been experimentally proven, thus reducing the ambiguity of the
prediction of the long-term development and providing a positive
perspective for future exploration activities.

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