Page 76 - Geothermal Energy Renewable Energy and The Environment

60 Geothermal Energy: Renewable Energy and the Environment

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5

10

Depth (km) 15

20

25

30

35
–21 –19 –17 –15 –13 –11
2
Log permeability (m )
FIGUre 4.8 The variation of permeability with depth. For reference, the depth of the deepest drilled well in
the world is also portrayed. (From Manning, C. E. and Ingebritsen, S. E., Reviews of Geophysics, 37, 127–50,
1999.)

where C is an empirical constant that is characteristic of the material under consideration. The
values for a are strongly dependent upon the roughness, tortuosity, and other properties of the
c
dominant fracture set and must be estimated.
For geothermal resources at depths greater than about a kilometer, permeability is much more
difficult to estimate. A variety of studies have used empirical data and model results that have greatly
improved our understanding of how permeability changes with depth. In Figure 4.8 these results
are presented to a depth of 35 km (modified from Manning and Ingebritsen 1999). At depths less
than 5 km, the effective permeability can vary by up to six orders of magnitude, making it virtually
impossible to develop predictive models that are sufficiently precise to allow useful estimates of how
well a site may perform, without some reliable data set from sampling in the field. At deeper levels,
the range in uncertainty diminishes, primarily because the elevated temperatures and pressures are
sufficient to overcome the effects of heterogeneous distribution of the intrinsic rheological properties
of the rocks. As pressures increase, weak fractures are closed and pore space is reduced by compres-
sion, resulting in a more restricted range of permeability values.
Also shown in the figure is the depth reached by the deepest well yet drilled. This well was
drilled offshore Qatar in 2008, and reached a depth of more than 12,290 meters. The deepest
onshore borehole drilled reached 12,261 meters in the Kola peninsula. Geothermal wells generally
are shallower than 5 km, although the future development of Enhanced Geothermal Systems (EGS;
see Chapter 14) will likely achieve greater depths. Clearly, for the foreseeable future geothermal
applications will require field tests and measurements, rather than a priori estimates, in order to
establish the actual permeability that can be achieved at any given site.

hydroloGIc properTIes oF real GeoThermal sysTems

The fluid flow properties of real geothermal systems have been summarized by Björnsson and
Bodvarsson (1990) who surveyed published data on geothermal power plants. Their results
(Figure 4.9) highlight several key points regarding experience in the power generation industry.