Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 446

Corrosion prevention and suitable materials. The equipment used within geo-
thermal fluid circuits has to meet certain requirements in terms of operation
safety, environmental protection and cost-efficiency.
− Wall openings due to corrosion and the resulting environmental contamination
need to be diligently prevented.
− Corrosion products damage the reservoir as the geothermal fluid is re-injected
and thus increases the filtration requirements.
Besides the described exclusion of oxygen, careful material selection is therefore
of major importance.
Some geothermal fluids are poor in oxygen but contain high contents of ag-
gressive carbonic acid. Furthermore, corrosion is mainly controlled by chloride
ions. Due to these circumstances corrosion becomes uncontrollable once oxygen
has entered the system. Experiences with unalloyed or basically alloyed steels
have shown, for instance, that corrosion of 0.05 to 2.0 mm/a in relation to volume
flow occurred due to a very small oxygen entry. This material removal is not
evenly distributed over the whole surface. Little lamina are removed and leaving a
scarred surface.
However, there is a wide range of suitable materials to be used for geothermal
fluid circuit. The choice is mainly determined by the nature and temperature of the
geothermal fluid, its pressure capabilities and required material manufacturing.
For instance, plastics, composite materials (plastics/glass fibre), coated and rub-
berised metals as well as several combinations of high-alloy steels are suitable.
Besides the requirements of the geothermal fluid circuit the demands of individual
components such as sensors and their connections, fittings and sealing materials
need to be carefully taken into consideration.
Leakage monitoring. It is important to protect the system against leakages and to
contain geothermal fluid quickly, if leakage occurs. The following combination
has to be provided
− anti-corrosive casing material,
− safe casing technology (stable casing connections, double casing systems etc.)
and
− leakage monitoring device (they have to ensure permanent control and must
quickly detect even small leakage's precisely and reliably).
Besides the observation of the geothermal fluid system related to the heat ex-
changer, the monitoring of the most deeply buried production and injection pipes,
connecting the heating station with the production and injection well, requires
special attention. For this purpose, control systems originally developed for dis-
trict heating systems are applied, which, however, require double-casing installa-
tion to prevent moisture from entering the control section. Alternatively, wireless
monitoring processes could be used.