Page 226 - Geothermal Energy Systems Exploration, Development, and Utilization

202 4 Enhancing Geothermal Reservoirs
4.8
Outcome

4.8.1
Hydraulic Stimulation

4.8.1.1 Hydraulic Stimulation – Soultz
The three 5000 m deep geothermal wells in the Soultz granite were stimulated
using the water frac technique where the 500 m long open hole sections were
treated on the whole. Each hydraulic stimulation started with the injection of up
3
to 800 m of heavy brine with a density of up to 1.2 g cm −3 in order to reactivate
natural fractures having low initial permeability, as deep as possible in the well.
The main stimulation consisted of a fresh water injection as summarized in
Table 4.2.
GPK2 and GPK4 were stimulated by a single-well-injection whereas in GPK3, the
concept of a dual injection was tested. In order to achieve a pressure concentration
and therefore fracture concentration between the two wells to be connected, the
fresh water was injected simultaneously into GPK2 and GPK3.
The pressure curves of the GPK2 and GPK4 (2004) stimulation indicate an
effective stimulation which could improve their productivities by a factor of
20 (Table 4.1). The wellhead pressure of GPK3 stimulation is more flow rate
dependent and indicates a less effective stimulation in which the productivity could
be improved by a factor of 1.5.
An important relationship was found between the injection rate during stim-
ulation and the productivity of the well after stimulation (Jung and Weidler,
2000). The productivity of the wells appears to increase linearly with the in-
jection rate during stimulation. Moreover, the series of stimulations at 5 km
depth has shown that the productivity as determined during the stimulation
persists completely after stimulation. Thus, a ‘‘perfect’’ self-propping was ob-
served.
In a deep geothermal system with low initial permeability, the reservoir will be
developed by stimulation. To control the reservoir development the operation of
a seismic network is essential. The located events serve to monitor the reservoir
Table 4.2 Summary of the four stimulation operation on the Soultz wells.

Well Duration Volume Flow rate Initial Improvement Located
3
–1
(days) (m ) (l s ) productivity factor microseismic
−1
(l s –1 bar ) events
GPK2 6 23 400 50 0.02 20 14 000
GPK3 11 34 000 50 0.2 1.5 21 600
GPK4 3.5 9 300 30 0.01 20 5 700
4 12 300 45 – – 3 000