Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 447

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Slop system. The slop system collects geothermal fluid that is outside of the geo-
thermal fluid circuit. This system reprocesses these fluids and transfers them back
into the circuit. Slop waters result from
− rinsing out the production well and the aboveground system after first start-up
and prolonged standstills,
− filter replacements,
− repairs,
− evacuation of the casing system,
− at some sealing of e.g. pumps and filters, and
− leakages within the overall system.
The main slop container is located very close to the injection well. This container
is designed to be able to absorb the volume of geothermal fluid to be held back
necessarily to protect the reservoir rock within the injection well during the start-
up operation. It is furthermore equipped with a sedimentation tank for particulate
matter.
Additional slop containers are located at other operating locations of the overall
geothermal heating station. They are needed for leakages collected by floor in-
takes.
Re-injection of geothermal fluid. During the re-injection of the geothermal fluid
into the reservoir rocks, injection well pressure losses inside of the injection well
casing need to be overcome. This is also true for the required overpressure be-
tween the injection well and the uninfluenced reservoir. For a given configuration,
these two variables are dependent on the volume flow.
During standstill the geothermal fluid reaches a certain water level inside the
injection well. This level depends on the temperature of the geothermal fluid in-
side the well and thus on its density. During plant operation, however, the fluid
level inside the injection well rises according to the given pressure losses. Up to a
certain volume flow there is thus the risk of negative pressure at the wellhead,
which has to be avoided by appropriate measures (e.g. such as foot valves inside
the well).
If the volume flow to be re-injected continues to rise, the required wellhead
pressure can be built up to a certain level. This pressure level is pre-determined by
the pressure to be built up by the borehole motor pump installed within the pro-
duction well as well as the allowed pressure level of the overall aboveground sys-
tem (in most cases up to 5 bar). Only if a higher pressure is needed, injection
pumps are integrated into the geothermal fluid circuit close to the injection well.
Under these circumstances the piping downstream of the pumps has to be de-
signed for this pressure.
District heating systems. Except for the supply of industrial consumers, charac-
terised by a high low-temperature heat demand, household customers in conjunc-
tion with district heating systems can use geothermal heat for the provision of