Page 252 - Geothermal Energy Systems Exploration, Development, and Utilization

228 4 Enhancing Geothermal Reservoirs
The well was originally intended for the production of gas but no economic
production was achieved. The well was abandoned as gas well and transferred to
Federal Institute for Geosciences and Natural Resources (BGR) as a geothermal
test well. Before takeover, the gas bearing formation ‘‘Rotliegend’’ was sealed by
plugging back the well from final depth of 4918−4120 m.
For geothermal investigations the Middle Bunter (3636–3926 m) was selected
because of its widespread occurrence in the Northern German Basin and because
of the high temperature at that depth level. The Middle Bunter is dominated by clay
and siltstone and only a few sandstone subformations (Volpriehausen-, Detfurth-,
and Solling) are embedded (see Figure 4.23). These sandstone layers are in the
focus of investigations. The porosities of these layers vary from 3 to 11%. Due to
the low porosities and permeability, an economic production rate over a relevant
time period cannot be maintained by simply producing these layers.
‘‘The well is equipped with a 7’’ casing that is cemented from final depth to
2035 m. These specifications allow injection operations at high pressures and
flow rates, and separate perforation of single horizons. Waterfracs were performed
in two formations, Volpriehausen at about 3920 m depth and Dettfurth at about
3790 m after individual perforation.
In 2004, a packer was installed at 3770 m depth in order to separate the two
subformations Solling and Detfurth as a precondition for circulating water between
both layers.
Information on the stress field was not available before fracturing. No indicators
for stress (borehole breakouts, drilling induced fractures) were detected during
drilling. Besides, it is known that on a regional scale the stress field is very
inhomogeneous because of the salt tectonics. Hence, the evaluation of stress
indicators from other wells around is likely not representative for the test site
Horstberg.

4.10.3.2 Fracturing Experiments
Waterfrac-operations were performed through the perforated intervals in the
Volpriehausen-sandstone and later, in the Detfurth-sandstone. Pure freshwater
without any additive was applied at these frac operations.

Volpriehausen The waterfrac tests in the Volpriehausen comprised several injec-
tions with increasing flow rate.
In a first injection, a breakdown pressure of about 430 bar was recorded at a flow
−1
rate of only 0.17 l s . These values indicate a low initial productivity and an almost
impermeable formation.
3
During the most extensive fracturing test the volume of 600 m was pumped at a
rate of 7 l s −1 and at a wellhead pressure of 460 bar. As the technical equipment was
not dimensioned for this high pressure a flow rate larger than 7 l s −1 could not be
injected. Therefore, the stimulation of the Volpriehausen ended ahead of schedule.
Production tests after fracturing revealed still a poor productivity of the Vol-
priehausen. A low rate of about 0.5 l s −1 led to a pressure drop of more than 300
bar after a few hours of production.