Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 459

462
− Geo-mechanical impacts. Around the injection well a cold water area spreads
out within the aquifer as injection progresses. This could lead to contractions of
the layer within the deeper underground, which theoretically may reduce the
layer thickness and cause the earth's surface to sink. However, simulations
have shown that if such lowering occurs, it only has a slight impact and only
occurs over a very long period of time. According to these investigations such
contractions amount to approximately 1 to 3 mm/100 m aquifer thickness.
Compared to the lowering which is common for deep mining of hard coal and
ore or for mineral oil or natural gas exploitation, or that occurs with regard to
the settlement of building grounds, the effects observed during the geothermal
heat production are almost negligible. Also geo-mechanical consequences dur-
ing the post-operation period are not expected /10-5/. Impacts on the earth's
surface, leading, for instance, to a deterioration of the building infrastructure
are thus very unlikely.
All in all, the environmental effects occurring during normal operation of hydro-
thermal heating plants are thus either clearly lower or comparable to those of fos-
sil fuel-fired plants (i.e. influence on the landscape, covered surface).
Malfunction. In case of malfunctions hot geothermal fluid may penetrate to the
earth surface. Due to the high salinity that is observed in some reservoirs the flora
and fauna may be deteriorated if geothermal fluid is discharged into the surface
water. However, by appropriate planning and monitoring (such as leakage moni-
toring systems, pressure balance, slop systems, etc.) the described risks are con-
siderably reduced. Theoretically, further environmental impacts may occur if, due
to the high mineral content of geothermal fluid, harmful chemicals are applied to
eliminate precipitations in, and obstructions of, pipelines. However, since the
chemicals are subsequently transferred back into the underground together with
the cooled geothermal fluid, the corresponding environmental impacts are limited
according to current knowledge. Furthermore, harmful substances may be re-
leased to the environment due to fires at electrical components (such as cables).
However, these fires are not specific for geothermal plants and can be avoided by
observing the relevant fire protection guidelines.
End of operation. To avoid detrimental effects on the environment with regard to
the end of operation, appropriate well sealing is of major importance. The bore-
hole seal must exclude any penetration of harmful substances from the surface
into the hole as well as any hydraulic short-circuit of different layers. The disposal
of the applied plant components does not raise extraordinary environmental prob-
lems, as it is comparable to the disposal of conventional machine parts having a
relatively slight impact due to extensive legal guidelines.