The current phase of the European Hot Dry Rock
Project at Soultz-sous-ForÍts requires the drilling of
two additional deep wells to 5000 m depth into the
crystalline basement, to form a module consisting of
a central injector and two producers. The first well
GPK-2 was drilled to 5000 m in 1999 and
stimulated in 2000. The well GPK-3 (the injector)
was drilled in 2002 and targeted using microseismic
and other data. The bottom hole temperature was
200.6 ?C and separation between the two wells at the
bottom is around 600 m. GPK3 was then stimulated
to enhance the permeability between the wells. A
number of stimulation techniques were tried
including ìfocusedî stimulation, a novel method of
injecting simultaneously in two wells. Microseismic
monitoring, flow logging and other diagnostic
methods were used during these injections.
The ìsparseî microseismic network at the Soultz
site consists of a number of seismic sensors
deployed in wells between 1500 m and 3600 m deep
with bottom hole temperatures of 130-160 ?C.
A 48 channel, 22 bit data digitizing unit was used
for data acquisition in conjunction with proprietary
software to carry out automatic timing and location
in real time. This gave a real time decision-making
possibility and control of the reservoir. This was the
first time that such an interactive method had been
carried out at this site.
Around 90 000 micro-earthquakes were triggered
during these injections and about 9 000 events were
automatically timed and located in real time. These
stimulations created a total reservoir volume in
excess of 3 km 3 . This is the largest stimulated
volume in the development of HDR technology to
date.
The data suggest that ìfocusedî stimulation may
have a significant advantage over a single well
stimulation technique and may be a way forward for
efficient stimulation of larger separations between
It is recognized that the reservoir creation process
generates microseismic events but generation of
bigger events (30 events approaching 2ML & one
up to 2.9ML during this campaign) may retard the
acceptance of this technology in an urban
environment. This needs further studies to
understand the processes and find a procedure to
reduce the incidence of larger events.

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