Page 245 - Geothermal Energy Systems Exploration, Development, and Utilization

4.10 Case Studies 221
suggest tensile fracturing as the dominant fracturing process – see stimulation of
GPK4 (Figure 4.18).
Indications about the flow outlets in the wells were derived from flowlogs (Evans,
Genter, and Sausse, 2005a; Evans et al., 2005b). A general feature of all Soultz wells
is that the flow is controlled by a few outlets only. In GPK3 (5 km) one dominant
outlet, which corresponds to a hydrothermally altered and fractured zone, takes
about 70% of the total flow. The flowlogs further demonstrate that the fractures
which were stimulated most show also the best performance after the treatment.
The flow distribution in the wells is stable after the stimulation.
Injection tests were carried out before and after the waterfrac operations in order
to determine the initial productivity and the productivity after fracturing. In these
injection tests, the rate was much lower than during stimulation to avoid any further
fracturing. Usually, the productivity of the wells could significantly be enhanced. A
20-fold increase in productivity of the wells GPK2 and GPK4 (5 km) was achieved
−1
up to 0.35 l s −1 bar −1 (GPK2) and 0.20 l s bar −1 (GPK4). The productivity of the
well GPK3 could only slightly be improved. But here the well was very productive
already before fracturing.
In these injection tests at 5 km depths the pressure continuously increased
at a constant rate but with decreasing slope. Steady state conditions have never
been observed. To allow the comparison between different injection tests, the
productivity has to be evaluated at a similar time period. The productivities were
usually determined after two to three days of injection.
In Soultz, the empirical rule was established that the injection period during
fracturing should last several days. Only after such long extended stimulations a
sufficient and persisting productivity enhancement was observed.
An important relationship was found between the injection rate during stim-
ulation and the productivity of the well after stimulation (Jung, 1999; Jung and
Weidler, 2000). The productivity of the wells appears to increase with the injection
rate during stimulation. Results from all the three reservoirs developed in Soultz
confirm this observation. In the deep reservoir (5 km), it could be shown that
the productivity of the well after stimulation is essentially the same as during
stimulation (Tischner et al., 2007). There is obviously no closing or relaxation of
the fractures after the long extended waterfracs and the productivity of the well
during stimulation persists after stimulation. This observation may be explained
by a self-propping effect of fractures, failing in shear under the local stress
regime.
As a consequence, the productivity enhancement due to the waterfrac operation
becomes predictable, simply by adjusting the injection rate during stimulation.
The predictability of the productivity enhancement is a great advantage of those
operations and must be emphasized compared to other stimulation methods.
Microseismic monitoring has evolved to the key technique to map the reservoir
in HDR projects (Niitsuma, 2004). In Soultz, six wells in the depth range of
1500–3500 m have been used as seismic observation wells (Figure 4.19). The
recorded and localized seismic events during hydraulic stimulation allow tracing
the development of the reservoir and serve as indication for the hydraulic connection