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Geothermal
Resources
Understanding geothermal energy begins with
an understanding of the source of this energy—the
earth’s internal heat. The Earth’s temperature
increases with depth, with the temperature
at the center reaching more than 4200 °C (7600
°F). A portion of this heat is a relic of
the planet’s formation about 4.5 billion years
ago, and a portion is generated by the continuing
decay of radioactive isotopes. Heat naturally
moves from hotter to cooler regions, so Earth’s
heat flows from its interior toward the surface.2
Because the geologic processes known as plate
tectonics, the Earth’s crust has been
broken into 12 huge plates that move apart
or push together at a rate of millimeters
per year. Where two plates collide, one plate
can thrust below the other, producing extraordinary
phenomena such as ocean trenches or strong
earthquakes. At great depth, just above the
down going plate, temperatures become high
enough to melt rock, forming magma.3 Because
magma is less dense than surrounding rocks,
it moves up toward the earth’s crust and carries
heat from below. Sometimes magma rises to
the surface through thin or fractured crust
as lava.
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Figure 1. Schematic of geothermal
power plant production and injection wells.
Source: U.S.
Department of Energy, http://www.eia.doe.gov/kids/renewable/geothermal.html. |
However, most magma remains below earth’s
crust and heats the surrounding rocks and
subterranean water. Some of this water comes
all the way up to the surface through faults
and cracks in the earth as hot springs
or geysers. When this rising hot
water and steam is trapped in permeable rocks
under a layer of impermeable rocks, it is
called a geothermal reservoir. These
reservoirs are sources of geothermal energy
that can potentially be tapped for electricity
generation or direct use. Figure 1 is a schematic
of a typical geothermal power plant showing
the location of magma and a geothermal reservoir.4
Here, the production well withdraws heated
geothermal fluid, and the injection well returns
cooled fluids to the reservoir.
Resource
Identification
Geological, hydrogeological, geophysical,
and geochemical techniques are used to identify
and quantify geothermal resources. Geological
and hydrogeological studies involve mapping
any hot springs or other surface thermal features
and the identification of favorable geological
structures. These studies are used to recommend
where production wells can be drilled with
the highest probability of tapping into the
geothermal resource. Geophysical surveys
are implemented to figure the shape, size,
depth and other important characteristics
of the deep geological structures by using
the following parameters: temperature (thermal
survey), electrical conductivity (electrical
and electromagnetic methods), propagation
velocity of elastic waves (seismic survey),
density (gravity survey), and magnetic susceptibility
(magnetic survey).5 Geochemical
surveys (including isotope geochemistry)
are a useful means of determining whether
the geothermal system is water or vapor-dominated,
of estimating the minimum temperature expected
at depth, of estimating the homogeneity of
the water supply and, of determining the source
of recharge water.
Geothermal exploration addresses at least
nine objectives:6
- Identification of geothermal phenomena
- Ascertaining that a useful geothermal production
field exists
- Estimation of the size of the resource
- Classification of the geothermal field
- Location of productive zones
- Determination of the heat content of the
fluids that will be discharged by the wells
in the geothermal field
- Compilation of a body of data against which
the results of future monitoring can be viewed
- Assessment of the pre-exploitation values
of environmentally sensitive parameters
- Determination of any characteristics that
might cause problems during field development
Drilling
Once potential geothermal resources have been
identified, exploratory drilling is carried
out to further quantify the resource. Because
of the high temperature and corrosive nature
of geothermal fluids, as well as the hard and
abrasive nature of reservoir rocks found in
geothermal environments, geothermal drilling
is much more difficult and expensive than conventional
petroleum drilling. Each geothermal well costs
$1–4 million to drill, and a geothermal
field may consist of 10–100 wells. Drilling
can account for 30–50% of a geothermal
project’s total cost.7 Typically, geothermal
wells are drilled to depths ranging from 200
to 1,500 meters depth for low- and medium-temperature
systems, and from 700 to 3,000 meters depth
for high-temperature systems. Wells can be drilled
vertically or at an angle. Wells are drilled
in a series of stages, with each stage being
of smaller diameter than the previous stage,
and each being secured by steel casings, which
are cemented in place before drilling the subsequent
stage. The final production sections of the
well use an uncemented perforated liner, allowing
the geothermal fluid to pass into the pipe.
The objectives of this phase are to prove the
existence of an exploitable resource and to
delineate the extent and the characteristics
of the resource. An exploratory drilling program
may include shallow temperature-gradient wells,
“slim-hole” exploration wells, and
production-sized exploration/production wells.
Temperature-gradient wells are often drilled
from 2–200 meters in depth with diameters
of 50–150 mm. Slim-hole exploration wells
are usually drilled from 200 to 3000 meters
in depth with bottom-hole diameters of 100 to
220 mm. The size and objective of the development
will determine the number and type of wells
to be included in exploratory drilling programs.8
Endnotes
2. Energy & Geoscience Institute at the
University of Utah. Geothermal Energy Brochure.
http://www.egi.utah.edu/geothermal/GeothermalBrochure.pdf;
accessed Sep 24, 2002.
3. Energy & Geoscience Institute at the
University of Utah. Geothermal Energy Brochure.
http://www.egi.utah.edu/geothermal/GeothermalBrochure.pdf;
accessed Sep 24, 2002.
4. U.S. Department of Energy, Energy Information
Administration: Geothermal Energy: Energy
from the Earth’s Core. http://www.eia.doe.gov/kids/renewable/geothermal.html.
5. Mary H. Dickson and Mario Fanelli Istitutodi
Geoscienze e georisorse, Pisa, Italy
http://iga.igg.cnr.it/documenti_igaenergia/Geothermal%20Energy.pdf;
Accessed Sep 30, 2002.
6. LUMB, J. T., Prospecting for geothermal
resources. In: Rybach, L. and Muffler, L.
J. P., eds. Geothermal Systems, Principles
and Case Histories, J. Wiley & Sons, New
York, 1981, pp. 77–108.
7. Energy & Geoscience Institute at the
University of Utah. http:www.egi.utah.edu/geothermal/brochure/Improving/improving.htm;
accessed Oct 1, 2002.
8. The World Bank Group. Rural and renewable
Energy page. http://www.worldbank.org/html/fpd/energy/geothermal/technology.htm>;
accessed Oct 1, 2002.
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