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284 5 Geothermal Reservoir Simulation
Those effects of residual apertures after loading and unloading of rough fractures
was observed, for example, by Brown (1987). We repeated the simulations with
increased storativity coefficient in the shut-off period by factors of 2 and 3. A better
fit of the shut-off period required an adaptation of the storativity coefficient, that is,
hydromechanical coupling effects become important.
Results of recent studies can be found, for example, in Kohl and Megel (2007);
Megel et al. (2006). Bruel (2002) investigated the impact of induced thermal stresses
during circulation tests in the Soultz site at the regional scale. More recently
(Baujard and Bruel, 2006) studied the influence of fluid density differences on the
pressure distribution in the reservoir during stimulation and fluid circulation tests.

5.9
KTB (Germany)

5.9.1
Introduction

The KTB, located at Windischeschenbach, Germany (Figure 5.1), comprising a
pilot borehole down to 4000 m and a main borehole down to 9101 m in southeast
Germany continues to provide a unique opportunity for the identification of impor-
tant factors and processes in deep seated fluid and energy transfer directly relevant
to the exploitation of geothermal energy (http://www.geozentrum-ktb.de/). In situ
stress conditions significantly impact flow, transport, and exchange characteristics
of fracture networks, which dominate the permeability of crystalline reservoir
rocks. To model such systems several scales of information need to be combined
to present a fully three-dimensional model of the principle KTB fault zones, and
linked to a geomechanical model describing the alteration of the hydraulic param-
eters with stress changes caused by fluid extraction. The concept of geomechanical
facies was introduced to define and characterize architectural elements in the
subsurface system. The evaluation of a long-term pump test in the KTB pilot hole,
June 2002 to July 2003, coupled with a geomechanical model gave an insight into
some of the elastic and nonelastic processes controlling hydraulic transport in the
basement rocks.
The geometrical basis for a three-dimensional fracture network model was
provided by the interpretation of several sets of data (Table 5.3). This included
large-scale geophysical surveys (Harjes, 1997) indicating the presence of reflectors
considered to be pathways for geofluids, geological investigations, particularly,
structural and tectonic interpretations (Hirschmann, 1996; 1997), and the interpre-
tation of fluid bearing fracture zones (Lodemann, 1998). Geologically individual
fault planes are seldom encountered, and in reality one is dealing with a whole
swarm of similarly orientated fractures extending sometimes over considerable
distances, within which the movement responsible for the formation of the frac-
turesisaccommodated asisflow within the fractures, for example, (Hoehn et al.,
1998; Talwani et al., 1999). As such the concept of shear zones to represent the
major geological tectonic features observed in the pilot and main boreholes was