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| Titel |
Microseismicity Observed at a Non-Pressure-Stimulated Geothermal Plant |
| VerfasserIn |
T. Megies, J. Wassermann |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069382
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| Zusammenfassung |
| The North Alpine Foreland Basin in south-eastern Germany provides remarkably favorable
conditions for the exploitation of geothermal energy resources. Scarce background seismicity
leads to low seismic hazard and the existence of a natural aquifer obviates the need for
high-pressure hydraulic stimulation. This hydrothermal usage was previously assumed to be
unproblematic with regard to induced seismicity and most of the currently operating
hydrothermal geothermal plants supply thermal baths or district heating with relatively low
flow rates and temperature drops. However, in February of 2008 two regionally recorded,
shallow magnitude Ml > 2 earthquakes occurred at a geothermal power plant which is
located in the municipality of Unterhaching south of Munich. One of the main differences of
this specific plant is their combined heat and electric power production which is accompanied
with much higher flow rates and thus larger volumes of circulated water. These events
showed that induced seismicity can not be ruled out even in this fortunate setting and
emphasized the need for a detailed analysis of the case, especially considering
that in 2012/13 a series of larger plants for power generation are about to go into
production. We present results from two years of data acquired with a local five
station seismic network. Overall, more than 100 events with magnitudes mostly
below 1 could be detected with a magnitude of completeness of around 0 and the
largest observed magnitude at 2.1. Absolute locations are calculated in a 3D velocity
model constructed from a high-quality 3D seismic survey and a simple two-layer
vp-vs model. As a result, the epicenters cluster tightly within 500 m around the
open-hole part of the injection well. The hypocentral depths are computed to be
1500 m below the well bottom but are less well constrained due to uncertainties in
the shear wave velocity model and the spatial distribution of the network. Several
indications point towards a necessary modification of the vp-vs model resulting in
shallower hypocentral depth estimates. Relative locations computed applying a nested
’Master-Event’ scheme show an alignment of earthquake locations along a strike that is in
good agreement with estimates of focal mechanisms and the main fault system
penetrated by the well. Preliminary results from the analysis of correlations with
operational data reveal a possible connection with sudden changes in the injection
parameters. The analysis furthermore suggests that in this case pore pressure changes
might be subordinate to thermal stresses. One aspect that is suspected to also have a
major influence on the observed seismicity is the favorable orientation of the pierced
fault system in the present day stress field. The presented results show that even
non-pressure stimulated geothermal plants operated hydrothermally at low injection
pressures and with very good hydraulic parameters are able to generate induced
seismicity under certain still to be investigated conditions and have led to a rethinking of
the demand for seismic monitoring at upcoming geothermal facilities in this area. |
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