Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 441

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ing up the reservoir and evaluating the discussed tests. This knowledge will also
have a major impact on the question of which model should be chosen.
Geological faults, interactions with neighbouring reservoirs and stratifications
within the reservoir form significant framework conditions and require detailed
modelling. This can only be provided by a local discretisation of these variables
on the basis of numerical simulation. Especially for the evaluation of long-term
behaviour, complicated numerical models are required, which allow for consid-
eration of the existing complex geological circumstances.
To date, the 3D models CFEST (Coupled Flow, Energy and Solute Transport)
and TOUGH (Transport of Unsaturated Groundwater and Heat) are recognised as
the most appropriate models to simulate geothermal-related problems. If there is
only very little knowledge available on the reservoir during the planning phase, or
in the case of a homogenous reservoir with an even parameter distribution, the 2D
model CAGRA (Computer-Aided Geothermal Reservoir Assistant) is also suit-
able /10-8/.

Secondary cycle Secondary cycle

Productio n wel l

Injection well

Production well

GHS GHS

Injection well

Minimum
distance in the reservoir Minimum distance in the reservoir

Secondary cycle Secondary cycle

Production well

Production

GHS GHS

Injection well Injection

Fig. 10.4 Possibilities of tapping geothermal fluids (GHS - geothermal heating station)

Design of the downhole system. Since the geothermal fluid circuit is responsible
for the major share of the investment for a geothermal heating station, investiga-
tions aim at finding a more cost-effective solution than the two-well system
(Fig. 10.4). The aboveground pipeline connection can be kept to a minimum, if
the production and injection well are drilled as divergent boreholes from a single
location.
If several reservoirs are found at one site, a simultaneous exploitation is gener-
ally possible. Such a system layout can minimise the distance between the produc-