The Rotliegend of the North German basin is the target reservoir of an
interdisciplinary investigation program to develop a technology for the generation of
geothermal electricity from low-enthalpy reservoirs. An in-situ downhole laboratory
was established in the 4,3km deep well Groß Schönebeck with the purpose of developing
appropriate stimulation methods to increase permeability of deep aquifers by
enhancing or creating secondary porosity and flow paths. The goal is to learn how to
enhance the inflow performance of a well from a variety of rock types in low
permeable geothermal reservoirs.
A series of stimulation experiments with different fracturing concepts were carried
out. In a first attempt open hole hydraulic proppant-gel fracturing treatments were
conducted in two pre-selected sedimentary reservoir zones in Rotliegend sandstones in
about 4km depth.
They proved to be on the one hand technically demanding and on the other hand less
successful than expected due to a suboptimal design.
Nevertheless, the main inflow zones could be clearly identified. In a second step the
concept of zonal selection and proppant application was abandoned and massive
waterfrac treatments were applied over the entire open hole interval of the well
below 3,874m to the final depth at 4,294m. Evidence of the creation and properties of
vertical fractures were retrieved from pressure response analyses and demonstrated a
bilinear flow regime in the reservoir. Therefore, the stimulation effect in terms of
a productivity increase can be determined for the described concepts and improvements
can be recommended for similar field experiments.

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