Page 250 - Geothermal Energy Systems Exploration, Development, and Utilization

226 4 Enhancing Geothermal Reservoirs
For example, a prior cleaning operation with a caustic soda solution to remove
significant amounts of drilling grease and drilling fragments from GPK-3 (injection
well), and from neighboring fractures, could have improved the injectivity index of
this well and allowed a best efficiency of the OCA treatment. A low seismic activity
has been observed during the chemical stimulation.
It is obvious that further on-site and laboratory experiments of chemical stim-
ulation must be carried out in the future and probably combined with moderate
hydraulic stimulation operations in order to reduce the injection pressures and the
possible resulting microseismic events, and improve the stimulation effects. Such
combined operations were not carried out on at Soultz.

4.10.3
Horstberg

4.10.3.1 Introduction
In Germany, hydrothermal reservoirs in which geothermal energy can be extracted
without hydraulic stimulation occur only in a few regions. Low permeable rock
dominates in the deep underground and hence, the utilization of geothermal
resources can be considerably extended, if concepts are available for heat extraction
from relative tight formations.
The investigations in Horstberg address the geothermal utilization of low per-
meable sedimentary rocks in the North German Basin.
For this objective, the experiences made in the EGS project ‘‘Soultz’’ serve as an
important basis. The concept of massive waterfracs as applied in Soultz is the basic
stimulation concept for the investigations in Horstberg too.
In Horstberg, single well schemes are investigated, where the well is simulta-
neously used for production and re-injection, in difference to the common dublet
scheme. Via large artificially created fractures a hydraulic link should be formed
between two layers at some vertical distance. This hydraulic link allows then the
circulation of water within one well. A second method of heat energy extraction
from one well is a Huff-Puff-scheme: Cold water is injected in a large fracture and
extracted as hot water after some recovery time. However, the basis for both single
well concepts is the creation of large fractures in a low permeable formation by
applying the waterfrac concept.
A one-well-concept can be operated economically even for a relatively low power
output in the order of few MW th and, is suitable for providing heat to large
buildings, or districts.
The scientific investigations at the test site ‘‘Horstberg’’ are an integral part of the
GeneSys-project. The GeneSys project finally aims at the realization of a one well
concept in Hannover to geothermally heat the buildings of the GEOZENTRUM
based on the experiences of ‘‘Horstberg.’’
The well Horstberg Z1 is located some 80 km north-east of Hannover
(Figure 4.21). It has been drilled into an inversion structure, striking NEE-SWW,
that is bound by saltdomes on either sides. The encountered stratigraphy is typical
for the Northern German Basin (Figure 4.22).