Geothermal systems are classified either as hydrothermal systems (low or high
enthalpy), which are encountered in specific locations associated with the presence
of fluids and permeability within the Earth, or as enhanced geothermal systems, which
can be engineered everywhere. The economics of the latter can be improved if they are
located within or close to the boundaries of a hydrothermal system.
The positive environmental impact of all geothermal plants is attributed to foregone
CO2 emissions, had the same quantity of energy been produced by fossil fuels.
However, there may be some CO2 emissions associated with hydrothermal systems, which
have been quantified as 15-20 times less than fossil fuels in case of heat production
from a low enthalpy geothermal field, or 2.5 times less than natural gas for a
typical high enthalpy geothermal field. The latter value can be further reduced by
one order of magnitude (10 times or so) in geothermal heat and power cogeneration
plants. Zero CO2 emissions are foreseen for enhanced geothermal systems.
Adverse environmental impact may occur from low enthalpy geothermal utilization,
associated with the chemistry of the geothermal fluid, which may include considerable
quantities of chloride, small quantities of boron, and traces of arsenic, ammonia,
mercury, or heavy metals, making it unsuitable for disposal to the surface. The
problem is effectively solved by reinjecting all the produced geothermal fluid to the
same deep reservoir it originated from. This practice also has benefits to the water
replenishment of the deep system, improving sustainability and the economic life of
the plant.
Other impact from long term low enthalpy geothermal utilization may be the dropping
of water level of near surface aquifers and the flow reduction or dry-up of nearby
springs and shallow water wells. The problem can be solved by effective reservoir
engineering and inevitably by donating deep wells to affected water users.
Apart from the above, exploitation of high enthalpy hydrothermal reservoirs, may
result in additional effects to local environment, associated with the chemistry of
the steam, which may contain non condensable gases, such as CO2, traces of H2S and
entrained silica or other dissolved solids in the liquid phase. These are effectively
alleviated by minimizing the steam losses from the geothermal plant (in fact steam
losses should be allowed only at the safety valves), and by conveying the
non-condensable gases to the cooling tower draft, where they are diluted by large
quantities of air. If these measures fail to limit the H2S concentration in the air
below the smelling threshold, then an H2S treatment plant should be installed.
Other problems associated with high enthalpy hydrothermal systems are:
- Micro-seismic activity: it is of small magnitude, only registered by the
seismographers and does not pose any threat to buildings or structures.
- Soil subsidence: it is of one or more meters magnitude and occurs in the centre of
the production zone after many years of exploitation and withdrawal of large
quantities of geothermal fluids. During the plant design phase no buildings,
pipelines or other structures should be placed there.
- Noise: can be severe. It is encountered by proper plant engineering and by placing
noise barriers if necessary.
- Damage to local flora: it occurs every time steam or hot geothermal fluids are
released on local plants, trees, etc. Compensations for damaged property should be
foreseen.
- Aesthetics: with proper design, the geothermal plant can be transformed to a
tourist attraction, rather than an ugly mass of metal.
- Heat pollution: when waste heat is released to local environment. It can be
encountered by reinjection, or by geothermal heat and power cogeneration.
Regarding enhanced geothermal systems, none of the above problems is foreseen, as
they are engineered by introducing surface water into earth and recycling through the
system.
However, as mentioned above, it pays to engineer enhanced geothermal systems within
or just outside the boundaries of a hydrothermal system in order to benefit from
bonuses in temperature, depth and limited natural permeability or fluid presence. In
that case some of the environmental consequences mentioned above may occur, but
probably in a much lesser extent and magnitude. But we cannot tell for sure what will
be the case, as no plants are in operation in such systems at present.
One problem that has been reported during engineering of enhanced geothermal
reservoirs, is the occurrence of micro-seismic activity, probably associated with the
hydraulic fracturing works. These micro-earthquakes have a magnitude around 2 in the
Richter scale and pose no danger to existing buildings or structures.
Whether they will also be present during exploitation, and how they will evolve
overtime, remains to be identified in the field, but we anticipate that this
micro-seismic activity should not be greater than the one encountered in hydrothermal
systems. The reaction of local people to a possible presence of micro-earthquakes
remains to be identified and managed.
In any case, in order to a-priori guarantee the success of a geothermal exploitation
scheme, close collaboration, cooperation and relations with the local community
should be established. That way the local community will maximize their benefit from
the geothermal plant, any misunderstanding and possible conflicts will be resolved
and the consent of the local community towards geothermal development in their region
will be achieved.

BRGM
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45060 Orléans cedex 02
France