Page 285 - Geothermal Energy Systems Exploration, Development, and Utilization

5.5 Groß Sch¨ onebeck 261
GrSk3/90 was tested to investigate scenarios of enhancing productivity of thermal
fluid recovery from the underground (Legarth et al., 2005; Reinicke et al., 2005;
Zimmermann et al., 2005). In order to complete the doublet system, a second well
Gt GrSk4/05 with a total depth of −4198 m had been drilled in 2006, followed
by three stimulation treatments to enhance productivity. In order to increase the
apparent thickness of the reservoir horizon, the new well is inclined in the reservoir
◦
section by 48 and was drilled in the direction of the minimum horizontal stress
◦
(S h = 288 azimuth) for optimum hydraulic fracture alignment in relation to the
stimulated preexisting well E GrSk3/90. Hence, the orientation of the fractures
◦
will be 18 azimuth in the direction of the maximum horizontal stress (Holl et al.,
2005). During the elapsed time of the geothermal project, it became obvious that
an appropriate numerical model becomes increasingly important for planning the
well path and fracture design, for interpretation of hydraulic tests and stimulations
as well as for prediction of reservoir behavior during the time of geothermal power
production.
To satisfy the requirements of the simulations, the model should implement
all the acquired knowledge of the reservoir. This includes the reservoir geology
and structure; the geometry of wells and fractures; the hydraulic, thermal, and
mechanical conditions of the reservoir; and generated fractures due to changes of
the reservoir conditions.

5.5.2
Model Description

5.5.2.1 Geology
The reservoir is located at a depth of −3850 to −4258 m within the Lower
Permian of the northeast German Basin. The reservoir rocks are classified into
two rock units from base to top: volcanic rocks (Lower Rotliegend of the Lower
Permian) and siliciclastics (Upper Rotliegend of the Lower Permian) ranging from
conglomerates to fine grained sand-, silt- and mudstones. These two main units can
be subclassified depending on their lithological properties, which are, in particular,
of importance for the hydraulic-thermal-mechanical (HTM) modeling:

• I: Hannover formation – silt- and mudstone
(−3850 to −3996 m true vertical depth subsea (TVDSS))
• IIA: Elbe alternating sequence: siltstone to fine grained sandstone
(−3996 to −4026 m TVDSS)
• IIB: Elbe base sandstone II: fine grained sandstone
(−4026 to −4086 m TVDSS)
• IIC: Elbe base sandstone I – fine to medium-grained sandstone
(−4086 to −4133 m TVDSS)
• III: Havel formation – conglomerates from fine sandstone to fine grained gravel
(−4133 to −4178 m TVDSS)
• IV: Volcanic rocks – ryolite and andesite
(−4178 to −4258 TVDSS)