Publication History: This article is based on "Crain's Analyzing Unconventional Reservoirs" by E. R. (Ross) Crain, P.Eng., first published in 2006, and updated annually until 2016. This webpage version is the copyrighted intellectual property of the author. Do not copy or distribute in any form without explicit permission.

geothermal ENERGY Basics
High temperature geothermal reservoirs can provide heat that can be used
to generate electricity from steam turbines. Both high and low temperature geothermal systems can be used to provide space heating, domestic hot water, or process hot water. They are not hydrocarbon bearing reservoirs, but the petrophysical properties of the rocks are just as important as they are for the hydrocarbon case.

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.

 UnitS conversionS Energy - Joules (J)   1 Joule = 0.2338 Cal   1 Cal = 4.187 J   1 kWh (kiloWatt.hour) = 3.6 MJ   1 MWy (MegaWatt year) = 31.56 TJ   1 BTU (British thermal unit) = 1055 J   1 barrel of oil equivalent = 5.7 GJ   1 tonne of oil equivalent = 42 GJ   1 m3 of natural gas = 38 MJ Power - Watts (W)   1 W = 1 J/s   1 W = 3.412 BTU/Hr   1 kW (kiloWatt) = 1.341 horse-power Heat Flow - Watt per sq. metre (W/m2)   1 W/m2 = 0.2388 x10^-5 cal/cm2sec   1 cal/cm2 sec = 41.87 kW/m2 Geothermal gradient -  Kelvin/metre (K/m)   1 mK/m = 1 C/km   1 mK/m = 0.5486 x 10^-3 F/ft Thermal Conductivity - Watts/metre.Kelvin        (W/mK)   1 W/mK = 2.39 x103 cal/cm sec C Range: Coal = 0.3, Water = 0.6, Rocks = 1.5 to 4.0, Metals =  40 to 400 W/mK. Prefixes: SI Units   k  kilo     10^3      m milli     10^-3   M Mega 10^6        u micro   10^-6   G Giga   10^9       n nano     10^-9   T Tera   10^12      p pico      10^-12   P Peta   10^15   E Exa     10^18

geothermal RESERVOIR TYPES
There are two basic types of geothermal reservoirs:
1. "Conventional" -- hot, wet, porous, permeable, often fractured.
2.  "Unconventional" -- hot, dry, no porosity or permeability
no natural  fractures-

Conventional geothermal reservoirs are exploited by producing hot water or steam from the reservoir and disposing of the spent steam to the atmosphere or condensing and injecting it back to the reservoir. Typical oilfield practices are used to enhance production, such as hydraulic fracturing and horizontal wells, provided the temperature does not exceed the limits of available technology.

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.
1. Dry steam plants, which directly use geothermal steam to turn turbines;
2. Flash steam plants, which pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines.
3. Binary-cycle plants, which pass moderately hot geothermal water by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to vapor, which then drives the turbines.

Schematic diagram of geothermal energy system. The "hot rock" portion, shown in red, could be porous, permeable, and fractured, making a conventional geothermal reservoir. Or it could be tight and un-fractured -- subsequent drilling of horizontal wells and hydraulic stimulation could be used to exploit this type of unconventional geothermal reservoir. (USGS  image)

About 70% of known geothermal reservoirs are below the 150C temperature limit for conventional logging tools; most are below the 260C limit for hostile environment tools. (red = magmatic, blue = non-magmatic reservoirs).
The Geysers geothermal system in California reaches 656F (346C). (USGS image)

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.

geothermal ENERGY IN CANADA AND USA
The largest conventional geothermal power resources in Canada are located in British Columbia, Yukon, and Alberta. These regions also contain potential for Enhanced Geothermal Systems. The most advanced project exists as a test geothermal site in the Meager Mountain-Pebble Creek area of British Columbia, where some exploration wells reached 240 - 260C at depths between 400 to 800 meters. Other wells had much lower temperatures. Three directional wells were then drilled in the hotter areas. Each well was estimated to be capable of producing 4 to 9 MWe, but there has been no attempt at commercial production.

A good reference for the Canadian scene is "Review of National Geothermal Energy Program Phase 2 – Geothermal Potential of the Cordillera", by A. Jessop, 2008, GSC Open File 5906.

Geothermal map of Canada. Red colours show areas where hot water or hot rock reservoirs may be present. Blue indicates warm water possibilities. (GSC image)

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.

Geothermal energy map for USA. (SMU image)

LOG ANALYSIS IN geothermal WELLS
Well logging to assess reservoir properties of geothermal prospects is possible in most cases. Lithology, porosity, permeability, fracture intensity, temperature, borehole shape and stability, stress regime, and elastic moduli are typical results that can be calculated from well logs, Time lapse temperature logs are used to estimate stabilized geothermal well temperature. Casing and cement integrity logs ensure safe and permanent well completions.

true dip and direction on right side of the log

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 permeability, fracture 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.

Once mineralogy, porosity, and temperature are known, rock properties pertinent to the geothermal industry can be derived. Thermal conductivity, specific heat capacity, volumetric heat capacity, isobaric enthalpy change, and diffusivity are derived from empirical curve fits to measured rock property data published in the literature. From these results and the reservoir volume, a complete assessment of its potential as an economic energy source can be made. These calculations are best performed by experts in geothermal energy and are probbaly beyond the scope of petrophysical practice.

Fractured granite example: raw data curves in Tracks 1, 2, and 3 with effective porosity, water saturation, and matrix permeability in Tracks 4, 5, and 6. The mineral model calculated from the log analysis is in Track 7 and the rock type model calculated from the minerals using a ternary diagram is in Track 8. Basalt was triggered from high density or high PE or both. This is an oilfield example in a deep, hot pluton.

Fracture frequency, aperture, and porosity log in a fractured granite reservoir derived from a resistivity image log. The most accurate method is based on the measured resistivity curves on the image log. The pixel count method is much less accurate because of borehole erosion and breakouts.

LOGGING EXAMPLES IN geothermal WELLS
These examples are taken from the petrophysical literature, some date back to the early 1980's and may not represent the full capability of today's technology.

EXAMPLE 1: Temperature Logs From Meager Mountain, BC
From: "Review of National Geothermal Energy Program Phase 2 – Geothermal Potential of the Cordillera", by A. Jessop, 2008, GSC Open File 5906.

Temperature logs from a Canadian geothermal prospect in the Rocky
Mountains of B
C. (GSC image)"

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.

Comparison of acoustic image log and resistivity image log in a geothermal well.
(a) BHTV amplitude image, (b) BHTV travel time image, (c) FMS resistivity image,
(d) sketch of fractures, (e) fracture orientation, (f) core image.
Dark colours are fractures or borehole breakouts, light colours are unaltered rock.
Direction scale at top of each log is N - E - S - W - N.

Synthetic and processed logs based on BHTV and FMS logs to quantify fracture intensity in a
geothermal reservoir.

EXAMPLE 3: Spinner Survey, Geysers Field, CA
From: "Well Logging In Hostile Environments - A Status Report", by E. Frost and W. H. Fertl, CWLS, 1985

Gamma ray, caliper, spinner, temperature, and long spaced density (full bore, counts per second) logs in a Geysers well in California, 1985. Temperature is above 485F.

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