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4.10 Case Studies 219
Table 4.6 Overview upon the massive hydraulic fracturing operations performed in Soultz.

Date Well Depth-TVD Volume Maximum rate Comment
−1
3
(m) (m ) (l s )
July 1991 GPK1 1 966−2 000 1 250/2 400 7/15 2 Fracs in a
Packer isolated
zone
September 1993 GPK1 2 850−3 400 25 300 36 Open hole frac
October 1993 GPK1 2 850−3 590 19 300 50 Open hole frac
June 1995/September GPK2 3 211−3 876 28 000/27 000 50/78 Open hole fracs
1996
June 2000 GPK2 4 403−5 026 23 400 50 Open hole frac
May 2003 GPK3 4 488−5 021 34 000 50 Open hole frac
September GPK4 4 479−4 972 9 300/12 300 30/45 Open hole fracs
2004/February 2005

3
Only frac operations are listed with an injected water volume of more than 1000 m .Maximum rate
means the maximum injection rate which was maintained for a period of 6 hours at least.

A characteristic feature of the Soultz wells are their long open hole sections. The
lower 400–600 m of the wells are uncased. In principle, the advantage of a long
open hole section is the access to several permeable structures.
The aim of the project is the development of a subsurface heat exchanger,
in a hydrothermally altered and fractured granite by means of hydraulic and
chemical stimulation, to be used for commercial geothermal electricity production.
A hydraulic link between the wells over a distance of several hundred meters has
to be created, to allow water circulation between the deep wells.
In low permeable crystalline rock, massive frac operations are the only means of
forming this link, to create a connection between the wells and a natural fracture
system, and to reactivate and expand a hydrothermally altered and fractured zone,
which acts as geothermal reservoir at Soultz.
Investigations at Soultz and other sites have shown that shearing is the dominant
fracturing process induced by massive water injections (Cuenot et al., 2006; Pine
and Batchelor, 1984). The dislocation of the fracture surfaces leads to a self-propped
fracture – the mechanism of which is assumed to be responsible for permeability
enhancement due to waterfracs. 1)
In the early project, some waterfracs were performed in packer isolated intervals
of the open hole section of GPK1 (Table 4.6). Since 1991, successful fracs were
conducted over the complete open hole sections.
Waterfrac operations in Soultz often commenced with the injection of brine
−1
(density up to 1.2 kg l ) to initiate the stimulation as deep as possible in the
borehole. Subsequently, large volumes of fresh water were injected at a constant

1) The terms waterfrac, frac and stimulation are
used synonymously.