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Abstract:
To limit global warming to 2 °C above pre-industrial levels, our society
is confronted with the urgent need to make the transition to a
globally sustainable energy system (1). Geothermal energy is available
regardless of season or time and, unlike many other renewable
energies, is therefore suitable for base-load sytems. Geothermal energy
is regarded as renewable as heat flows back into the reservoir
due to temperature conditions and transport processes. It uses the
energy source from the earth’s interior, which is inexhaustible by
human standards. Geothermal energy can play an important role in
the decarbonization of the energy system in Germany.
In Central Europe, the greatest geothermal potential lies in the
crystalline basement with important hotspots in areas under tectonic
tension. These include the Upper Rhine Graben as a rift zone
with hydrothermal fluid flows and exceptional temperature anomalies in the deep underground (2). The technology “Enhanced Geothermal
Systems” (EGS) was developed to exploit the geothermal
potential in the crystalline (3). EGS use the deep fractured subsoil
as a natural heat exchanger. With at least two boreholes, a thermal
water cycle is created that brings geothermal energy to the surface
and makes it usable (4). However, since relatively high flow rates
(> 10 l/s) are required for economic operation, the natural permeability
of the rock in the crystalline – in contrast to hydrothermal
systems – must be increased by hydraulic or chemical stimulation
measures (reservoir engineering) to increase the flow rates.
A major challenge for EGS is to control and minimize the induced
seismicity generated in this process, both in the reservoir
engineering and operation phase and with a view to increasing
public acceptance. A profound understanding of the multi-physical
processes in the reservoir, such as the complex interactions of
the fluid with the reservoir at high flow rates, is indispensable for
this. New scientifically based strategies and technologies are urgently
needed to exploit the geothermal potential economically
and at the same time in an environmentally compatible way.
