Utilisation of Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 473

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the fault is achieved by directional drilling, especially when the orientation and
dip of the fault zone at a certain depth is not precisely known. Directional drilling
also enhances the chance for intersecting a greater number of individual fractures
composing the fault zone.
In most cases, active pumping by using centrifugal pumps in the production
well will be required to achieve the desired flow rates.
Crystalline bedrock. Although the crystalline bedrock at great depth is not com-
pletely impermeable due to the presence of open fissures, fractures, or faults its
overall permeability is generally too low for geothermal power production. The
basic concept of the Hot-Dry-Rock (HDR) technology thus consists of creating
large fracture surfaces to connect at least two wells. During operation cold water
is injected in one of the wells and heated up by the rock temperature, while circu-
lating through the fracture system. It is then produced in the second well. To pre-
vent boiling an overpressure is maintained in the geothermal loop. Steam for
power generation is produced in the secondary loop using heat transferred from
the primary loop via a heat exchanger. Depending on drilling depth (usually above
5,000 m) and temperature (usually above 150 °C) a doublet system of commercial
size will operate at flow rates between 30 and 100 l/s with an installed electric
capacity of 2 to 10 MW. To ensure a technical lifetime of at least 20 years a dis-
tance of approximately 1,000 m between the two wells at depth and a total frac-
ture surface of 5 to 10 km 2 is required.
Due to the high flow velocities in the fractures especially near the injection and
the production well the flow impedance of the fracture system is important for the
performance of the system. For energetic and economic reasons the flow imped-
ance of the system (difference between inlet and outlet pressure divided by the
outlet flow rate) should not exceed 0.1 MPa s/l.
Fluid losses in the fracture system are another important factor for the opera-
tion of Hot-Dry-Rock (HDR) systems. Fluid losses above 10 % of the circulation
flow are not tolerable over prolonged time periods. This is not only true for the
very large volumes of freshwater lost in that way, but also for the additional
pumping power required for the make-up water. The problem of water losses can
be avoided or minimised by active pumping in the production well. In this way
the fluid losses in the high-pressure zone on the injection side are compensated by
a gain of fluid in the low-pressure zone on the production side.
In international projects performed during the last decades various types of
Hot-Dry-Rock (HDR) systems have been proposed and tested (Fig. 10.14).
− The basic model ("Los Alamos concept", Fig. 10.14) consists of a doublet of
deviated wells connected by planar parallel fractures. These fractures are cre-
ated by injecting large quantities of water at high pressure into insolated inter-
vals of the boreholes. This technique is known as hydraulic fracturing. Orienta-
tion and extent of the fractures is determined by monitoring the seismicity in-