Environmental Engineering Reference
In-Depth Information
Once the wells have been completed and the hydraulic conditions have been
confirmed by a performance test, production and injection wells are provided with
casings for the production and injection of the geothermal fluid. They are an inte-
gral part of the anti-corrosion system of the entire geothermal fluid circuit. Com-
monly inner-coated steel or plastic tubes are used.
Once a production pump has been installed below the water level (established
during production) the well is provided with inert gas (like Nitrogen) to prevent
corrosion.
Testing and modelling. The evaluation of the hydrodynamic properties of geo-
thermal reservoirs is referred to as a "test". In order to evaluate the properties of
geothermal reservoirs by specific tests, beginning with the drilling phase, the pro-
duction rate, the permeability of the reservoir, the temperature and pressure of the
geothermal fluid produced from the reservoir, the chemical properties and the gas
content of the geothermal fluid as well as the rock stability need to be determined.
Experience with geothermal heating stations in Northeast Germany has shown
that for flow rates between 50 and 100 m 3 /h for production and injection wells,
sandstone reservoirs (porous rock reservoirs) with a minimum useful porosity of
20 to 25 %, a minimum permeability of 0.5 to 1.0 µm 2 and an effective minimum
depth of 20 m are required.
The possible injection index (i.e. the defined injection pressure for a related
geothermal fluid volume flow) can be derived from geo-scientific investigations
of the reservoir rock and the performed production tests. Injection tests which
always bear the risk of damaging the reservoir can thus be avoided /10-7/.
The reservoir layers to be tested are chosen on the basis of well measurements
and experiments carried out on reservoir rocks removed during drilling. From
each of these layers a certain quantity of geothermal fluid is extracted. On the
basis of the obtained results the most promising reservoir layer to be used for the
production is chosen. Once the reservoir has been determined, it will be conclu-
sively evaluated with regard to its possible production and injection volume.
The data and parameters obtained during the drilling phase as well as by re-
gional geologic investigations allow for the elaboration of models describing the
hydrothermal and thermodynamic behaviour of the geothermal reservoir during
the production of geothermal heat by such a geothermal heating station. The most
important information is the expected water level within the production well (to
confirm the technical feasibility of the production volume flow) and the time scale
for temperature depletion of the reservoir, with a view proving the required tech-
nical lifetime of the geothermal heating station.
Numerous numeric models are available to simulate the reservoir operation.
The required modelling depth does not only depend on the dynamics investigated,
but is rather determined by the specific knowledge of the reservoir, which is
mostly quite limited, especially during the planning phase of such a geothermal
heating station. Extensive knowledge on the reservoir can only be gained by open-
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