Page 244 - Geothermal Energy Systems Exploration, Development, and Utilization

220 4 Enhancing Geothermal Reservoirs
200
Wellhead pressure (bar), rate (l/s) 160 Pressure
GPK1, 3000m, 1993

120

80

Rate
40
0
1-Sep 5-Sep 9-Sep 13-Sep 17-Sep 21-Sep
Date, 1993

Figure 4.17 Wellhead pressure and injection rate during
stimulation of GPK1 in 1993 at about 3 km.
or an increasing rate. Especially in the reservoir at about 3 km depth, stepwise
increasing rates were applied during hydraulic stimulation (Figure 4.17). An
increasing injection rate is assumed to prefer shearing, first on fractures that are
most critically stressed, followed by the activation of fractures less exposed to shear
stress. In the deep reservoir, the concept of stepwise hydraulic stimulation was
only partly pursued, mainly for technical and economical reasons.
Compared to other sites the fracturing pressure in Soultz is low. At about
2 km depth the overpressure during fracturing was approximately 60 bar and in
5 km depth an overpressure of 130–180 bar was observed. The latter numbers
lead to a fracture gradient of approximately 26–34 bar km −1 at 5 km depths.
In most cases, only a weak dependence of the pressure on flow rate and time
was obtained. A slightly increasing fracturing pressure versus time may be an
indication for shearing as observed during the last two days of stimulation of
GPK2 (Figure 4.18) for instance. A continuously decreasing pressure however, may

200
Wellhead pressure (bar); rate (l/s) 160 Rate
GPK2; 2000
Pressure
GPK4; 2004
120
80

40

0
0 48 96 144 192
Elapsed time (h)
Figure 4.18 Wellhead pressure and injection rate during the
stimulation of GPK2 and GPK4 in the deep reservoir (5 km).