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.
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.
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.
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 fault sand 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.
Resource Identification
Geological,
hydro-geological, geophysical, and geochemical techniques are used to
identify and quantify geothermal resources. Geological and
hydro-geological 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).
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:
- 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 carriedout 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 age othermal field may consist of 10–100 wells.
Drilling can account for 30–50% of a geothermal project’s total cost.
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 oftendrilled 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.
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