Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 458

which might be recognised by a minor earthquake. However, these effects have
only rarely occurred in the past. Also, cooling of the aquifer may lead to changes
of the reservoir chemistry with all resulting effects, such as precipitation of min-
erals. However, since the aquifer is located far below the surface and is generally
not connected to the biosphere, environmental effects on flora and fauna have not
been reported so far.
Furthermore, gases such as CH 4 , NH 3 , H 2 S, CO 2 , etc. may be emitted, and min-
erals may be released by subterranean fluid circulation. However, since geother-
mal fluid is transported within a closed circuit in the geothermal heating station,
no material releases are observed during normal operation. This could be different
when the geothermal fluid is used for balneology purposes. However, the de-
scribed effects can be and are avoided by appropriate technical measures. Solid
precipitations separated by filters integrated within the geothermal fluid circuit
must be disposed of according to the applicable regulations.
Within normal operation, environmental effects are thus mainly restricted to
thermal effects on the upper and lower layer of the exploited reservoir (i.e. the
rock bed above and underneath), the rocks surrounding the production and injec-
tion wells, and geo-mechanical impacts during long-term operation.
− Thermal impacts on the aquifer. By re-injection of the cooled geothermal fluid
into the aquifer during the operation of a hydrothermal plant, the initial aquifer
temperature is continuously reduced, which leads to a temporarily varying
temperature gradient between the storage and its surrounding layers /10-5/. The
result is a conductive heat flow from the overburden into the reservoir, partly
reheating the injected waters and simultaneously cooling the overburden. The
calculation of the temperature changes within the surrounding rocks has shown
that in a worst case scenario, after 30 years of operation only a depth of 160 m
has been thermally affected while temperature reductions exceeding 10 K have
only been observed in depths up to 70 m around the well /10-5/. Moreover, no
direct environmental impacts on the biosphere have been observed that are at-
tributable to a cool-down of the underground and there has not been reported
any organic life underneath the reservoir that could be affected.
− Thermal impact on the rocks surrounding the well. The well transmits heat or
cold to the environment and influences it thermally. An analysis of the well
surrounding of a typical well used at the geothermal heating station of Neus-
tadt-Glewe/Germany shows that the maximum heat transmission to the direct
environment amounts to 230 kW at the beginning of operation and to 180 kW
after 30 years of operation. After this time period and the assumption of an un-
interrupted heating the thermal impact, characterised by the ratio of the fluid
temperature to the undisturbed rock temperature, would reach a radius of 60 m
around the well. The thermal impact would be reduced to 56 % at a distance of
10 m and to only 34 % at a distance of 20 m. Hence, there is no far-reaching
thermal impact of the underground due to geothermal wells. Moreover, only
the circular area around the well could have a slight impact on flora and fauna.
However, up to now no such effects have been observed.