The aim of the HDR project Soultz is geothermal power production based on an
artificially created heat exchanger at 5000 m depth. 
During the years 1999-2004 three wells were drilled down to 5000 m, one injection
(GPK3) and two production (GPK2 and GPK4) wells. After drilling each of the well was
stimulated individually by massive water injection. Water volumes up to 30000 m3 were
injected with a flow rate of 30 - 50 l/s. 
A downhole seismic network has been operated to monitor the spatial development of
the stimulated or artificially created fractures. The spatial orientation and the
spatial density of the seismic events vary significantly for the different
stimulation operations performed in Soultz. 
For GPK2 the seismic events are clearly aligned along a strike direction of NW-SE
(140°-150°) and the spatial density is very high. In contrast, for GPK3 almost no
preferential orientation could be observed. For GPK4 a high spatial density of the
seismic events was found again but the strike direction shows a tendency towards N-S. 
The high density of seismic events during stimulation of GPK2 and GPK4 indicate
effective stimulation operations here whereas for GPK3 a large portion of the
injected volume seems to be lost by diffusion into natural fractures without
significant stimulation. 
Hydraulic observations i.e. the characteristics of the stimulation pressure curves
support these statements. 
After the stimulation operations injection tests with significant lower rates were
performed to determine the injectivity (productivity) of the wells. For all three
wells a good agreement was obtained between the injectivity during stimulation and
after stimulation. Obviously, the stimulated flow pathes retain their hydraulic
conductivity completely after stimulation. It is very important to notice that this
correlation between injectivity during stimulation and after stimulation allows the
prediction of the productivity enhancement due to the massive water injections.

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