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.
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
reservoirs can provide heat that can be used
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
Heat generation in a
geothermal reservoir is continuously supplied by radioactive decay
in or below the reservoir. It is expressed in uW/m3
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
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.
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
1 BTU (British thermal unit) =
1 barrel of oil equivalent =
1 tonne of oil equivalent = 42
natural gas = 38 MJ
Power - Watts (W)
1 W = 1 J/s
1 W = 3.412 BTU/Hr
1 kW (kiloWatt) = 1.341
Heat Flow - Watt per sq. metre (W/m
= 0.2388 x10^-5 cal/cm2sec
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
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
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
There are two basic types of
1. "Conventional" -- hot, wet, porous, permeable, often fractured.
2. "Unconventional" -- hot, dry, no porosity or
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.
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
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
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 =
The Geysers geothermal system in California reaches 656F (346C). (USGS
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
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
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
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
Resistivity image log in a
fractured granite, with
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
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
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
examples are taken from the petrophysical literature, some date back
to the early 1980's and may not represent the full capability of
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
Temperature logs from a Canadian geothermal prospect in the Rocky
Mountains of BC. (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
(a) BHTV amplitude image, (b) BHTV travel time image, (c) FMS
(d) sketch of fractures, (e) fracture orientation, (f) core image.
Dark colours are fractures or borehole breakouts, light colours are
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
EXAMPLE 3: Spinner Survey, Geysers Field, CA
From: "Well Logging In Hostile
Environments - A Status Report", by E. Frost and W. H. Fertl,
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.