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4.10 Case Studies 217
2) The volcanic rocks should be stimulated by a waterfrac treatment in conjunction
with low proppant concentrations to support the sustainability of the produced
fractures. At the end of the treatment one can complete the same with a short
gel-proppant treatment to support the access to the fracture in the near wellbore,
where the highest pressure changes are expected during production. The design
of the treatment should be laid out in a way that the fracture propagates upwards
as well to connect the sandstone layers above the volcanic rocks and offers
the possibility to drain the high permeable sandstones from below and build
up a hydraulic connection to the other well. This can be achieved by a special
adjustment of the flow rate during the fracture treatment. The recommendation
is to carry out cyclic changes of flow rate with high and low stages to control
the fracture propagation in horizontal as well as vertical direction.
3) In the Rotliegend sandstone sections gel-proppant treatments should be per-
formed to give access to the high permeable layers. These treatments should
comprise high proppant concentration to obtain a multilayer proppant pack
and hence a high fracture conductivity.
4) To accomplish several treatments independently from each other, it is recom-
mended to drill a deviated well with a deviation in the reservoir rock. In this
case, no hydraulic interferences must be suspected.
5) Deviationinthe reservoir rockshouldbeinthe directionofminimum
horizontal stress as far as hydrothermal systems are concerned. This assumes
the existence of a high permeable layer between both wells to establish a
hydraulic connection. The distance between both wells should be far enough
to avoid a temperature short cut over the live cycle of the geothermal power
production, which is in the time frame of tens of years.

4.10.2
Soultz

4.10.2.1 Hydraulic Stimulation
The test site of the EGS project Soultz is located in France on the western edge
of the Rhine Graben, some 50 km north of Strasbourg near the German border
(Figure 4.15). This area is characterized by a thin continental crust resulting in a
high geothermal heat flow. The granitic basement at Soultz-sous-Forˆ ets lies beneath
approximately 1400 m of sedimentary rock (Hoijkaas, Genter, and Dezayes, 2006).
Soultz is located in the heart of an abandoned oil production area which was
intensively exploited at the beginning of the last century. Important reasons for the
selection of Soultz as location for a HDR project were
◦
• high geothermal gradient (about 110 C at 1000 m depth);
• good geological data basis from oil exploration.
The geothermal research program started at Soultz-sous-Forˆ ets in 1987 as a
joint German and French project. In three main consecutive phases, subsurface
reservoirs in granite were developed and tested (Hettkamp et al., 2004). First, a
reservoir at about 2 km depth was created and later at about 3 km. During the years