Page 183 - Geothermal Energy Systems Exploration, Development, and Utilization

3.9 Case Study Groß Sch¨ onebeck Well 159
• trouble shooting by geomechanical analysis (Moeck, Backers, and Schandelmeier,
2007; Backers, Stephansson, and Moeck, 2008);
• stress anisotropies/abnormal stresses in deep evaporites;
• temperature and/or temperature gradient lower than expected;
• low effective porosity/low matrix or fracture permeability in target formation, no
natural productivity.

3.8.5
Geotectonical Risks

Geotectonical risks can arise in critically stressed regions like, for example, the
Alpine foreland and the Oberrheingraben in Central Europe. Critically stressed
regions are featured by natural seismicity. In particular, in these regions fault and
fracture zones of certain orientation within the current stress field accommodate
high shear stresses. If the shear stress exceeds the rock’s frictional resistance, rock
failure occurs by slip along the fault plane generating an earthquake or earthquake
swarms. Drilling operations can be affected by earthquakes if the well is in the direct
vicinity of an active fault or can even induce seismicity by increasing formation
pressures. Possibly, fault activity can damage the well or the casing and results
in the worst case to loss of the well. Especially hydraulic stimulations as part of
work over operation in a well can induce seismicity. A detailed stress modeling
of known or suspected faults and a slip tendency analysis for initial and changed
fluid pressures contribute to the understanding of changing stresses and potential
seismicity of man-made geothermal reservoirs (Moeck, Schandelmeier, and Holl,
2009).

3.9
Case Study Groß Sch¨ onebeck Well

A production well (GrSk 4/05) for completing the in situ geothermal laboratory at
Groß Sch¨ onebeck has been drilled and stimulated 2006/2007.
The pay zones consisting of Rotliegend sandstones (about 80 m, average perme-
ability 35 mD) and underlaying volcanics reach 150 C at a depth of 4200 m. The
◦
investigations performed since 2000 in a reopened abandoned gas exploration well
(GrSk 3/90) showed the feasibility of an EGS basing on the flow in the pore space
between parallel situated hydrofrac planes (Huenges and Moeck, 2007).
After the stress field could be identified via the assessment of breakouts (Moeck
and Backers, 2006; Moeck, Schandelmeier, and Holl, 2009) and the direction of frac
ruptures in the first well (injection) the well path of the second well (production)
was designed to allow for the planned frac treatments to be performed properly
(Zimmermann et al., 2007) (Figure 3.25).
The planned borehole design consisted of 26 in. surface casing, 18 5/8 in. anchor
casing, a combined 16 in. × 13 3/8 in. production casing to the top of Zechstein,
9 5/8 in. liner to cover the 1500-m-thick rock salt layers, 7 in. liner for selective