Page 235 - Geothermal Energy Systems Exploration, Development, and Utilization

4.10 Case Studies 211
4.10.1.2 Hydraulic Fracturing Treatments in GrSk3/90
A series of stimulation experiments were carried out at the geothermal research well
in Groß Sch¨ onebeck (EGrSk 3/90) located in the north-eastern part of Germany.
The aim was the development of concepts for the productivity enhancement
of geothermal wells in that region. In a first attempt, hydraulic gel-proppant
fracturing treatments were conducted in two sedimentary reservoir zones with
high permeability at about 4 km depth. These treatments were performed under
◦
challenging conditions in the open hole section at a temperature of about 150 C.
On the one hand, they proved to be technically demanding and on the other
less successful than expected due to a suboptimal design. Most likely, the small
injection volumes combined with a low proppant density did limit the success of
these operations. Nevertheless, the productivity of the well could be increased by a
factor of two. The characterization of the inflow zones after the proppant fracturing
treatments and derived values for the minimal horizontal stress led to a completely
different fracturing concept. Massive waterfrac treatments were now applied over
the entire open section of the well below 3874 m to the final depth at 4294 m. Again,
a significant increase of productivity was achieved, demonstrating that waterfracs
can be a successful and effective stimulation concept for this geological situation.
Evidence of the creation and properties of a very long vertical fracture were
retrieved from pressure response analyses demonstrating a bilinear flow regime.
The stimulation effect in terms of a productivity increase was determined for the
described concepts and improvements are derived for similar field experiments.

4.10.1.3 Hydraulic Fracturing in Sandstones (Gel-Proppant Stimulation)
The first stimulation experiments were of the conventional kind, that is, on the
basis of expertise of the hydrocarbon industry. Two experiments were performed in
January 2002 using proppant-gel-frac techniques in two intervals of the Rotliegend
sandstones (Zimmermann et al., 2003; Legarth, Huenges, and Zimmermann,
2005). Experimental design comprised the isolation of the bottom boundary of the
interval of interest by filling the bottom of the well with sand. The top of the interval
was sealed with a mechanical packer. High viscosity fluid (gel) with proppant was
employed for stimulation. The flowmeter-log indicated a significant increase of
inflow due to this frac operation. Visualization by borehole televiewer (BHTV) and
formation micro imager (FMI) confirmed the creation of the new open vertical
fracture in the stimulated intervals with a height of more than 100 m and in the
◦
direction of the maximum horizontal stress (S H = 18.5 ± 3.7 )(Holl et al., 2003,
2004). Nevertheless, the observed flow rates were not sufficient for economic power
3
production (Zimmermann et al., 2003), but the PI could be enhanced to 2.2 m (h
MPa) −1 due to the stimulation treatments.
3
The mean flow rate obtained during the casing lift test (CLT) was 22.4 m h −1
at a differential pressure of 10.5 MPa at the end of the test. A total volume of 307
3
m was produced at the duration of the test of about 14 hours, which is a similar
time of production as during the previous CLT. Hence this result can be compared
to the previous test indicating a doubling of the PI of the well. Legarth et al. (2003)
conclude that the limited achievement was strongly influenced by the proppant