Abstract
To stimulate the reservoir for a "hot dry rock" geothermal project
initiated by a private/public consortium in the city of Basel, Switzerland,
approximately 11,500 m3 of water were injected at high pressures
between 2 December and 8 December 2006 into a 5-km-deep well below
Kleinhüningen (Häring et al. 2008). A six-sensor borehole
array, installed by the operators of the project at depths between
317 and 2,740 meters around the well to monitor the induced seismicity
recorded more than 10,500 seismic events during the injection phase.
Hypocentral locations could be calculated for more than 3,000 of
these events. The gradual increase in flow rate and wellhead pressure
was accompanied by a steady increase in seismicity, both in terms
of event rates and magnitudes. In the early morning hours of 8 December,
after water had been injected at maximum rates in excess of 50 l/s
and at wellhead pressures of up to 29.6 MPa for about 16 hours (Häring
et al. 2008), a magnitude ML 2.6 event occurred within the reservoir.
This exceeded the safety threshold for continued stimulation, so
that injection was stopped prematurely. In the afternoon and evening
of the same day, two additional events of magnitude ML 2.7 and 3.4
occurred within the same source volume. As a consequence, the well
was opened and the water allowed to flow back. In the following days
about one third of the injected water volume flowed back out of the
well (Häring et al. 2008). Though the seismic activity declined
rapidly thereafter, even more than two years later sporadic microseismicity
was being detected in the stimulated rock volume by the downhole-instruments.
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