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4.10 Case Studies 213
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In a subsequent flow-back test, 250 m of water was produced within a time of
−1
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5 hours and a mean flow rate of 50 m h . This indicates in comparison with
tests after the sandstone fracturing treatment, a significant increase of productivity
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(Zimmermann et al., 2005). PI is above 4 m (h MPa) −1 during the whole test. This
is an indication that the massive water injection produced additional fractures, so
that the experiment was rated successful and represented roughly another doubling
of the PI. However, borehole breakouts occurred resulting in an obstruction just
at the upper part of the tested section at about 3900 m depth. Therefore, further
technical borehole operations were necessary. In October 2003, the obstruction in
the well was removed and the well was deepened to 4309 m and an additional liner
from 3850 m down to the final depth was installed. Prior to the liner installation,
an extensive logging program was performed in order to get information about the
geological structure and the lithology of the borehole section of interest. The liner
was installed with perforated tubes in the lower part beneath 4135 m installation
depth (diameter of holes 15 mm; 93 holes per meter circumferential) to ensure the
hydraulic contact to the formation. In the stabilized well, the massive water frac
experiment was continued in fall of 2003.
The injection treatment was continued with a pressure step-rate test to obtain the
fracture opening pressure (Huenges et al., 2006). Thereafter, a massive stimulation
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test of 108–144 m h −1 over several days and one short peak of up to 288 m h −1 for
approximately 2 minutes were performed (Figure 4.5). The total injection volume
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accumulated to 7291 m . Initially, it was intended to perform the last high rate flow
injection over 8 hours, but due to a cable break during the first few minutes the
test had to be abandoned. The pressure step-rate test indicates multiple fracture
opening events. Fracture closure pressure was determined by pressure decline
analyses during shut-in at 6.4 MPa above formation pressure.
The stimulation treatments were accompanied by a passive microseismic mon-
itoring in the five wells (50 m deep) in the vicinity of the drill site. Unfortunately,
micro-seismicity could not be observed during the various stimulations due to
attenuation between surface and reservoir in the Zechstein salt (Weber et al., 2005).

4.10.1.5 Hydraulic Fracturing Treatments in GrSk4/05
Three stimulation treatments were performed in the completed well GtGrSk4/05 in
August 2007 (Figure 4.12) starting from the bottom of the well within the volcanic
section. The whole well is cased and cemented with the exception of the last 40 m
(perforated liner and open hole section).
A first leakoff test in the volcanics was applied to obtain the fracture gradient.
Then a massive waterfrac treatment in the volcanics followed. After isolation
of this part of the well with a bridge plug, the lower sandstone horizon was
perforated and tested (injection test). The particular interval was selected due
to the interpretation of borehole measurements carried out before completion.
Subsequently, the gel-proppant stimulation followed. After isolating this interval
as well, the upper sandstone layer was perforated and stimulated with another
gel-proppant treatment. Finally, the bridge plugs were removed and a production
test was carried out to test all stimulated intervals.