Heat generation in a geothermal reservoir is continuously supplied by radioactive decay in or below the reservoir. It is expressed in uW/m3 (microWatts per cubic meter). Normal values range from undetectable to 10 uW/m3. A typical geothermal well can produce a few to more than 10 megaWatts of power. That's enough to cover the base load electricity demand of about 1000 homes without creating any significant greenhouse gases.
The shallowest and most economic hot reservoirs are associated with volcanoes, dormant or otherwise. However, geothermal reservoirs can occur in sedimentary, metamorphic, as well as igneous rocks. Modern petrophysical logs and analysis methods have no problem handling these different types of reservoirs.
The properties of heat and heat transfer
are not usually part of a petrophysicist's lexicon. The table at the
right covers some of the basic terms and units of measurement.
Source: GSC Open File 5906.
Unconventional geothermal reservoirs are often called Enhanced (or Engineered) Geothermal Systems (EGS) or "hot, dry rock" reservoirs. They require hydraulic fracturing and horizontal wells to obtain a flow path through which water can be circulated in a closed loop.
Types of power plants using geothermal energy
operating today are.
As usual, there is some confusing terminology. Low temperature geothermal energy, more properly called geothermal heating using geothermal heat pumps (GHP's), exists everywhere, but should not be confused with the "deep -- hot" category. The industry also uses the term "ground water" to mean the water in the geothermal reservoir, not to be confuse with the more common usage as the near-surface potable water that is used by humans and agriculture.
The usual oilfield terms of resources, reserves, proven, probable, and possible have the same meanings. Reservoir volume is the total rock volume (km3) and net thickness replaces the concept of net pay.
IN CANADA AND USA
In the USA, geothermal power plants are currently operating in six states: Alaska, California, Hawaii, Idaho, Nevada, and Utah. The electric power generation potential from identified geothermal systems is 9.0 Gigawatts-electric (GWe), distributed over 13 states.
US states that produce geothermal energy (USGS image)
This is about 25% of USA's renewable energy (2008) but less than 1% of all electricity demand in the USA. Only 2.5 GWe have been developed and are on-line.
Slightly dated information for USA can be found on the USGS Geothermal Energy website.
California has more than half of the US geothermal production due to proximity to both sources and customers. Many good sources in the world are not close to electricity demand or power grid infrastructure, so are not economic today.
LOG ANALYSIS IN geothermal WELLS
Resistivity image log in a
fractured granite, with
Standard oilfield logging tools can survive 300F (150C) for short periods and hostile environment logging tools are good to 500F (260C). Such tools have been available since 1981 (but the USGS website about logging geothermal wells seems to be unaware of this). Resistivity and porosity logs are available for the high temperature range, but some specialty logs, such as acoustic and resistivity imaging, may not reach 500F yet. Technology is always on the move, so check with service companies for current availability. Purpose-built tools have also been used and logs of these may be found in project files.
There are numerous problems associated with petrophysical analysis of logs for any purpose, and geothermal wells are no exception. Poor borehole condition, high temperature, and unusual lithology are well known issues, even in the oil and gas industry.
Unfortunately, a DOE report written in 1979, based on the logging technology of the early 1970's, is still widely distributed and still believed even by USGS professionals. See "Geothermal Well Log Interpretation Midterm Report" by S. K. Sanyal, L. E. Wells, R. E. Bickham, 1979, LA-7693-MS Informal Report UC-66e. Sadly, the SPWLA Geothermal Log Interpretation Handbook dates from 1982 so it too is not much help to 21st century petrophysicists.
Most 1970's era complaints have long been resolved over the 45 years since the logs reported upon were run. Modern computer software, digital logging tools, new understanding of multi-mineral models, better knowledge of tool responses, realistic estimates of measurement accuracy, higher temperature and pressure ratings, statistically based calibration to ground truth, and 45 years of published works from 1000's of practitioners have solved a lot of the uncertainty concerns.
To perform a competent petrophysical analysis in a geothermal well, as for any well, we need a good set of digitized well logs, sample descriptions, core data (if any), and some basic well location and directional information. We can then use the standard deterministic or probabilistic models described in other Chapters of this Handbook. Review the Chapters on tight oil, tight gas, fractured reservoirs, igneous and metamorphic reservoirs, and lithology models.
The minimum log suite would include resistivity, shear and compressional sonic, neutron, density, photo-electric, spectral gamma ray, acoustic and/or resistivity image logs, where temperature limitations can be met. A temperature profile and some time lapse bottom hole temperatures are essential. If the well can flow, spinner surveys can be run to assess flow rates.
Deliverables expected are
rock mineralogy, porosity, water resistivity, matrix
intensity, fracture aperture, fracture porosity, fracture
orientation and dip angle, and rock mechanical properties,
such as shear and bulk modulus, Young's modulus, Poisson's
ratio, and Biot's constant. Since logs respond only to
minerals, the initial log analysis model will generate the
mineral composition of igneous rocks (eg. quartz, feldspar,
mica, etc and not generic rock types such as granite or
diorite). If needed, the minerals can be composed into rock
types for comparison to sample descriptions.
LOGGING EXAMPLES IN
EXAMPLE 2: Fracture identification at Coso, CA
From "Comparison Of Acoustic And Electrical Image Logs From The Coso Geothermal Field, Ca" by Nicholas C. Davatzes and Steve Hickman, USGS, 2005.
EXAMPLE 3: Spinner Survey, Geysers Field, CA
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