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4 Oileld Review
Mining Heat
Heat emanating from the Earths core could replace a substantial percentage of the
energy currently produced by burning gas, oil and coal for electricity generation. The
Earths heat is an inexhaustible resource whose use creates almost no greenhouse
gas emissions. It is, in short, a nearly perfect solution to the worlds energy needs.
But before the world can take advantage of this abundant supply of heat, there are
daunting economic and technological hurdles to clear.
Craig Beasley
Rio de Janeiro, Brazil
Bertrand du Castel
Tom Zimmerman
Sugar Land, Texas, USA
Robert Lestz
Keita Yoshioka
Chevron Energy Technology Company
Houston, Texas
Amy Long
Singapore
Susan Juch Lutz
Salt Lake City, Utah, USA
Kenneth Riedel
Chevron Geothermal Indonesia LtdJakarta, Indonesia
Mike Sheppard
Cambridge, England
Sanjaya Sood
Houston, Texas
Oilfeld ReviewWinter 2009/2010: 21, no. 4.Copyright 2010 Schlumberger.
For help in preparation o this article, thanks to Mo Cordes,Houston; and Stephen Hallinan, Milan, Italy.
GeoFrame and TerraTek are marks o Schlumberger.
The mechanics o harvesting the Earths natural
subsurace heat seem to be amiliar petroleum
engineering tasks: drill and complete wells and
produce fuids rom wells landed in targeted or-
mations beneath the surace. But the prize in
geothermal energy production is not fuids. It is
heat. So while there is considerable potential or
technology transer rom the oil and gas upstream
businessdrilling rigs, bits, pressure control
and other basic practices and technologiesthe
specics o hydrocarbon and geothermal energy
production diverge.
For example, ultrahigh temperature repre-
sents an obvious problem in bringing oil industry
technology to bear on geothermal exploration and
production: It renders useless the sophisticated
tools and sensors that are dependent on pressure-
tight seals and electronics. The industry, however,
is continually overcoming temperature limita-
tions. In reality, the accurate characterization o
geothermal reservoirs is a more undamental
obstacle to realizing the ull energy potential rom
the Earths heat. Constructing geothermal reser-
voir models and simulations using seismic surveys
and logging data will require more innovation than
adaptation such as increases in hardware temper-
ature tolerances.
Still, the comparison between heat and
hydrocarbon exploitation remains compelling.
Many o the geothermal wells currently eeding
power plants have been constructed by oileld
workers using essentially traditional drilling and
completion equipment and techniques. Today,
those eorts have resulted in geothermal or,
more accurately, hydrothermal elds that eed
power plants producing about 10,000 megawatts
(MW) o electricity in 24 countries (below).1
1. Blodgett L and Slack K (eds): Geothermal 101: Basics oGeothermal Energy Production and Use. Washington, DC:Geothermal Energy Association (2009), http://www.geo-energy.org/publications/reports/Geo101_Final_Feb_15.pd (accessed August 1, 2009).
>Potential hydrothermal resources. The rst major hydrothermal developments were located in areaswith high tectonic activity marked by volcanoes, geysers, hot springs and large hot-water reservoirs.These resources are relatively shallow and oten fow to the surace naturally. A large portion opotential resources, given here in megawatts, is made up o enhanced geothermal systems (EGS) and iscontingent on technological development.
138
3,291
530
1,390
2,850 30,000 100,000
392 5,800
38,000
9,000
42,000
923 10,000
14,000
Potential hydrothermal resources
Installed hydrothermal capacity
Potential hydrothermal capacity
Potential capacity using EGS in the USA alone
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6 Oilfeld Review
Hydrothermal energy is a specic orm o
geothermal resource. Characterized by high
temperature, high permeability and rock that
contains large volumes o water, it is oten ound
at relatively shallow depths. Without stimulus, or
aided only by high-temperature electrical sub-
mersible pumps, these ormations can deliver
superheated water or steam to the surace
through large-diameter production wells. The
steam, or hot water fashed into steam at the sur-ace, is unneled to drive turbines that generate
electricity. Such ormations exist in relatively ew
places around the world. Hydrothermal reser-
voirs are ound predominantly in areas o high
tectonic activity where hot-water reservoirs are
abundant and pressured, such as in the area o
the Pacic Ocean known as the Ring o Fire.
Most ormations around the world that have
the requisite water and permeability do not have
sucient heat to be considered geothermal energy
sources. But there are others with deep, high-
temperature zones that lack only sucient water
or permeability, and it is these that may hold the
most promise as uture sources o geothermal
energy. The solution to tapping such widely avail-
able heat resources is through enhanced, or engi-
neered, geothermal systems (EGS).
Put simply, EGS projects create or sustain
geothermal reservoirs. In cases o low permeabil-
ity, the ormation may be hydraulically ractured.
Formations with little or no liquid or without a
sucient recharge source may be supplied with
water through injection wells. Today, engineers
and geophysicists are bringing techniques or
EGS to high-temperature dry reservoirs at depths
o 3 to 10 km [10,000 to 33,000 t] below the sur-
ace. At these depths, the rock is hot enough to
convert water to superheated steam.
These hot dry rock (HDR) systems are a unique
type o EGS, characterized by very hot basement
ormations with extremely low permeability. They
require hydraulic racturing to connect water-
injection wells to water-production wells.
Other prospective ormations contain permea-
bility and water but are not hot enough or geo-
thermal applications. To exploit these resources,
less ambitious concepts are being advanced
through binary power plants. These plants usewater that is below the boiling point to heat a sec-
ond fuid with a boiling point that is below that o
water. The vaporized second fuid is unneled to
turbines to generate electricity(let).2
This article ocuses on hydrothermal and HDR
technology. The state o EGS technology is dis-
cussed through preparations or an EGS-expansion
project in Nevada, USA, a case history rom
>Geothermal power plants. Dry-steam power plants are the most basicstyle o geothermal power plants (top). Steam piped rom a hydrothermalreservoir directly enters turbines to generate electricity. As the steam coolsand condenses, the water is gathered and injected back into the reservoirwhere it is reheated as it travels through the ormation to the production well.Flash-steam plants (middle) use hot water that is below the boiling point whileat reservoir pressure but that fashes to steam at lower surace pressures.Binary power plants (bottom) use a closed system to exploit even coolerreservoirs whose water temperatures are less than 150C [302F]. Water fowsor is pumped to the surace and enters a heat exchanger where it brings asecond fuid, in this case isobutane, to its boiling point, which must be belowthat o water. The second fuid expands into a gaseous vapor that then powerselectricity-generating turbines. This fuid may be circulated through the heatexchanger or reuse rather than being disposed o and, because the waterdoes not come into contact with the power generator, maintenance costs areusually lower than with dry-steam or fash-steam hydrothermal plants.
Production well Geothermal zoneSteam
Turbine
Generator
Dry-Steam Power Plant
Condenser
Water
Air
Air andwater vapor
Coolingtower
Air
Water
Injection well
Water
Production well Geothermal zone
Waste brine
Turbine
Generator
Flash-Steam Power Plant
Condenser
Air
Air andwater vapor
Direct heatuses
Coolingtower
Air
Water
Injection well
WaterSteam
Steam
Brine
Production well Geothermal zone
Turbine
Generator
Binary Power Plant
Condenser
Air
Air andwater vapor
CoolingtowerAir
Injection well
Isobutane
Heat exchanger
Hot brine
Pump
Cool brineWaterWater
Isobutane
vapor
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Indonesia and lessons learned rom the original
HDR project located in the southwest o the
United States.
The High Cost of Deep Heat
The upside potential o geothermal energy may
be enormous. In 2008, world electricity con-
sumption was 2 terawatt years. The heat fux
continuously fowing rom the Earths core is
equivalent to about 44 terawatt years.3
Thesenumbers are astronomical o course, but i only a
small percentage o this potential were to be
tapped, it would easily supply most o the worlds
energy demands. Most geothermal resources are
also truly renewable in that the same fuids can
be reheated, produced, injected and recycled
throughout the lie o the reservoir.
Besides the technological questions are nan-
cial ones that persist in the ace o otherwise posi-
tive investment actors (above right). Geothermal
projects, with ew exceptions, require a signi-
cantly higher initial capital outlay than do oil and
gas, solar, wind and biomass projects. The risk is
also higher, and the current experience with
return on investment in geothermal installations
is discouraging. For example, a 50-MW hydrother-
mal project is estimated to yield an initial rate o
return o less than 11% and a prot-to-investment
(P/I) ratio o 0.8. By comparison, a large oil and gas
project typically yields an initial rate o return o
nearly 16% and a P/I o 1.5.4
These poor nancial results are partially a
refection o geography. Areas with avorable
hydrothermal conditions tend to be sparsely
populated and ar rom large electricity markets.
Financial results are also hampered by the di-
culty inherent in drilling and developing these
ormations. Geothermal resources are ound in
much harder and hotter rock than those or
which petroleum and mining industry bits are
designed, so drilling is slower and more costly. To
be economic, geothermal wells must accommo-
date relatively large fow volumes, and thereore
wellbore diameters must be greater than those o
most oil and gas wells. This adds considerably to
well construction costs. The extreme temperature
o geothermal environments orces operators to
choose high-priced premium products or suchthings as cements, drilling fuids and tubulars.
While in recent decades the oil industry
has greatly rened drilling and reservoir man-
agement ecienciesconsequently reducing
costsit has oten done so through such elec-
tronics-based innovations as logging while drill-
ing and subsurace monitoring. These tools are
currently restricted to temperatures below about
175C [350F] and are not available or use in
high-temperature geothermal wells.
Finding and Defning
With the exception o some blind deep, high-
temperature systems, the search or hydrother-
mal ormations is made relatively easy by hot
springs and umaroles that are visible at the sur-
ace.5 Additionally, many hydrothermal elds are
in deep sedimentary basins where oil and gas
drilling and, more importantly, data collection
have already occurred.
The geologic setting or hydrothermal reser-
voirs varies. The reservoirs in the largest elds
contain a wide range o rocks, including quartz-
ite, shale, volcanic rock and granite. Most o
these reservoirs are identied not by lithology
but by heat fow. They are convection systems in
which hot water rises rom depth and is trapped
in reservoirs whose caprocks have been ormed
by the mixing o upwelling geothermal fuids with
local groundwaters and by precipitation o car-
bonate and clay minerals.
Thereore, the search or a commercial near-
surace hydrothermal reservoir is based on iden-
tiying tectonic activity, heat source, heat fow,
water recharge and outfow o deep fuids to the
surace. Permeability is typically characterizedby a network o ractures or active aults held
open by local in situ stresses.
The hunt or a hydrothermal reservoir begins
with an assessment o available regional data on
heat fow, seismic activity, thermal springs and
characteristic surcial elemental signature
rom remote sensing and imaging. Geophysical
geologic and geochemical techniques that can
provide inormation on the size, depth and shape
o deep geological structures are then pu
into eect.
Subsurace temperature measurements are
the most direct method or ascertaining the
existence o a hydrothermal system. Thermal
gradient holes can be as shallow as a ew meters
but to exclude surace-temperature eects the
preerence is or a depth o more than 100 m
[330 t]. Temperature surveys can delimit areas
o enhanced thermal gradientsa basic require
ment or geothermal systems. In volcanic terrains
high-temperature rocks may occur at relatively
shallow depths, and it is likely that a heat source
is present. In systems o deep circulation, high
temperatures indicate thin continental crust, high
rates o heat fow and deep permeable aults that
transmit mantle heat close to the surace.
Hydrothermal reservoirs require high tempera
tures and eective permeability, which is oered
by coherent rocks capable o supporting open rac
ture systems. These rocks have a relatively resis
tive signature. The associated clay-rich caprockshowever, have low resistivity. The resistivity con
trast at the base o the caprock, which can be
2. First Successul Coproduction o Geothermal Power atan Oil Well, JPT Online(October 21, 2008), http://www.spe.org/jpt/2008/10/frst-successul-coproduction-geothermal-oil-well/ (accessed July 14, 2009).
3. Pollack HN, Hurter SJ and Johnson JR: Heat Flow romthe Earths Interior: Analysis o the Global Data Set,Reviews of Geophysics31, no. 3 (August 1993): 267280.
>Alternative energy comparative value. Among renewable energy sources, geothermal energyis one o the most attractive based on the capacity actorthe percentage o energy actuallyproduced by a plant compared with its potential output when operated continually at ull capacity.It also compares avorably with other alternative energy sources when dierent metrics are used.(Capacity actor data rom Kagel A: A Handbook on the Externalities, Employment, and Economicsof Geothermal Energy. Washington, DC: Geothermal Energy Association, 2006.)
RenewableEnergy Sources
CapacityFactor, %
Reliabilityof Supply
EnvironmentalImpact
MainApplication
Geothermal 86 to 95 Continuous andreliable
Minimal landusage
Electricitygeneration
Hydroelectric 30 to 35 Intermittent, dependenton weather
Impacts due todam construction
Electricitygeneration
Wind 25 to 40 Intermittent, dependenton weather
Unsightly for large-scale generation
Electricitygeneration (limited)
Solar 24 to 33 Intermittent, dependenton weather
Unsightly for large-scale generation
Electricitygeneration (limited)
Biomass 83 Reliable Minimal (noncombustiblematerial handling)
Transportation,heating
4. Long A: Improving the Economics o GeothermalDevelopment Through an Oil and Gas IndustryApproach, Schlumberger white paper, www.slb.com/media/services/consulting/business/thermal_dev.pd(accessed September 15, 2009).
5. A umarole is a vent or opening in the Earths surace throughwhich steam, hydrogen sulfde or other gases escape.
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8 Oileld Review
determined through magnetotelluric (MT) mea-
surements, can provide an indication o geother-
mal prospectivity.6 MT has become a standard
method or mapping the caprock geometry con-
straining geothermal reservoirs.
I wells have been drilled in an area, many o the
parameters measured indirectly rom the surace
can be obtained directly rom well log data. These
logs can highlight regions o porosity, saline fuid
saturation and temperature variations, which may
indicate the presence o hydrothermal reservoirs.
Since these resources may be ound in rac-
tured, tectonically stressed areas, their presence
is oten marked by microseismic events that also
serve as a guide to drilling into the ractured
rocks once other avorable geothermal condi-
tions are established. By recording a relatively
large number o these events over weeks ormonths and calculating their epicenters, seis-
mologists can determine the location and orien-
tation o ractures.
Seismic refection and seismic reraction sur-
veys have been used only sparingly in geothermal
exploration. Although obtaining reraction pro-
les requires a considerable eort at depths o
5 to 10 km [16,400 to 33,000 t], standard seismic
refection surveys oten yield useul results in
these areas. During geothermal exploration,
gravity surveys are used to dene lateral density
variations associated with a magmatic heat
source in volcanic-hosted systems or with ault
blocks buried beneath sedimentary cover in sys-
tems o deep circulation. But their main value is
in dening changes in groundwater level and in
monitoring o subsidence and injection, which
are directly related to the resources ability to
recharge itsel. By correlating the surveys and
weather, it may be possible to dene the relation-
ship between data rom a gravity survey and the
precipitation that produces changes in shallow
groundwater levels. When corrected or this
eect, gravity changes show how much o the
water mass discharged to the atmosphere is
replaced by natural infow.7
The Concept
The most common approaches to geothermal
exploration include anomaly hunting, anomaly
stacking and conceptual modeling. Mathematical
velocity models are routinely used to predict the
depth to a ormation o interest, and physical
models can be used to simulate rock layers.
Conceptual models are hypothetical, bringing
together observed and inerred inormation to
identiy geothermal targets and predict reservoir
capacity. Such models are oten combined with
geostatistical and classical technologies such as
those employed or reservoir characterization.
Hydrothermal conceptual models combine
observed and inerred inormation to illustrate
reservoir fuid and rock properties and oten
include data captured through cation and gas
geochemistry. They also take into account MTresistivity interpreted in the context o basic
geology and hydrology and through mapping o
surcial hydrothermal alteration.8
The most important element o a hydrother-
mal conceptual model is a predicted natural-
state isotherm patternsolid lines drawn to
indicate temperature and depth across a subsur-
ace section. Though dicult to arrive at during
the exploration stage, case histories indicate it
can be done based on interpretation o the
geothermometrya technique that allows the
determination o subsurace temperature using a
combination o methods including the chemistry
o hot-springs fuids and distribution o hydro-
thermal alteration minerals at the surace.
Patterns o geophysical anomalies and resistivi-
ties and a general knowledge o the local geology,
hydrology and aulting or structural history may
also be used.
Hot water circulating in the Earths crust may
dissolve some o the rock through which it fows.
The amounts and proportions o these solutes in
the water are a direct unction o temperature. I
the water rises quickly rom the geothermal res-
ervoir to the surace, its chemical composition
does not change signicantly and it retains
an imprint o the subsurace temperature.
Subsurace temperatures calculated rom hot-
springs chemistry have been conrmed by direct
measurements made at the base o holes drilled
into hydrothermal systems.9
Geothermometry uses ionic and stable isotope
ratios in the water to determine the maximum
subsurace temperature(above let). Geochemical
and isotopic geothermometers developed over the
past two decades assume that two species or com-
pounds coexist within the geothermal reservoir
and that temperature is the main control on theirratio.10 They also assume that no change in
that ratio has occurred during the waters rise
to the surace.
Gas ratio geothermometers can also be used
to determine subsurace reservoir conditions. By
integrating these geochemical data with inor-
mation rom temperature-gradient wells and
structural mapping, engineers can build concep-
tual models that display fuid-fow patterns
>Subsurace temperature predictions. Temperatures measured in wells drilledinto hydrothermal systems are compared with temperatures calculated romgeothermometers beore drilling. The dashed line indicates the location wherepoints would plot i measured and calculated values agreed perectly. Pointsabove the line indicate calculated temperatures that were underestimated.(Adapted rom Dufeld and Sass, reerence 9.)
300
200
100
Subsurfacetemperature
measuredinwell,
C
Temperature calculated from chemical geothermometer, C
100 200 300
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Winter 2009/2010 9
within a hydrothermal reservoir as geological
cross sections and maps (right). An upward fow
o water creates an upward isotherm pattern and
indicates permeable rocks. When reservoir fow
is vertical, temperatures increase signicantly
with depth. In an outfow zone the fow is hori-
zontal and temperatures decrease with depth.11
Permeable zones have smaller temperature
gradients with depth than do impermeable ones
and generally display a convective isotherm pat-tern. In very low-permeability ormations, the
temperature gradient is steep and is easily seen
in a cross section as closely spaced isotherms
that reveal a conductive thermal regime. The gra-
dient helps determine the location o permeable
and impermeable zones.
Since low resistivity usually indicates
low-permeability conductive clays, MT surveys
may be used to locate the base o a geothermal
caprock and, indirectly, its high thermal
gradient. The dimensions o the reservoir can
then be mapped and used to identiy drilling
targets and prospective locations o production
and injection wells.
Enhancing Nature
The hydrothermal elds that are now online and
that were discovered through these techniques
and models represent the geothermal industrys
low-hanging ruit. The uture o geothermal
energy lies in more-complex systems that must
be coaxed into production and in recovering
more heat rom those already in existence
through EGS projects (right).
Similar to processes in oil and gas operations,
conceptual modeling may be used to plan and
execute EGS projects or hydrothermal reservoir
development. Using data gained rom years o
production to construct better models, engineers
can assess the potential response o these geo-
thermal elds to inll drilling, water injection
and other processes that help extend the eld
and improve reservoir eciency.
At Desert Peak near Fernley, Nevada, a geo-
thermal eld was discovered and dened in the
1970s and 1980s. It has been delivering power to
a double-fash power plant since 1986 and is typi-
cal o the deep-circulation, or ault-controlled,geothermal systems o the western USA.12 An EGS
project that would expand the operation through
hydraulic and chemical stimulation is under
study. The study will determine the distribution
o rock types, aults, alteration minerals and
mineralized ractures east o the existing hydro-
thermal eld to create a new structural model o
the eld.13
6. For more on MT: Brady J, Campbell T, Fenwick A, Ganz M,Sandberg SK, Buonora MPP, Rodrigues LF, Campbell C,Combee L, Ferster A, Umbach KE, Labruzzo T, Zerilli A,Nichols EA, Patmore S and Stilling J: ElectromagneticSounding or Hydrocarbons, Oilfeld Review21, no. 1(Spring 2009): 419.
7. Manzella A: Geophysical Methods in GeothermalExploration, Lecture notes. Pisa, Italy: Italian NationalResearch Council International Institute or GeothermalResearch, http://www.cec.uchile.cl/~cabierta/revista/12/articulos/pd/A_Manzella.pd (accessed August 10, 2009).
8. Cumming W: Geothermal Resource Conceptual ModelsUsing Surace Exploration Data, Proceedings o theStanord University 34th Workshop on GeothermalReservoir Engineering, Stanord, Caliornia, USA(February 911, 2009).
9. Dueld WA and Sass JH: Geothermal EnergyCleanPower rom the Earths Heat, US Geological Survey,Circular 1249, http://pubs.usgs.gov/circ/2004/c1249/(accessed August 3, 2009).
> Isotherms rom geothermometry. Cation geothermometry data rom a umarole
and a chloride hot spring can be modeled using a geological interpretationto obtain a subsurace temperature prole. The hot spring is assumed to beclose to the top o the water table. Propylitic alteration transorms iron- andmagnesium-bearing minerals into chlorite, actinolite and epidote. (Adaptedrom Cumming, reerence 8.)
Acid sulfatefumarole
Smectite clays
Marine clays
Heat and gasfrom magma
Upflow infractures
Propyliticzone
Argillic zone
Zeolite-smectitezone
Unaltered
Chloridespring
212F
302F
392F
482F
572F
100C
150C
200C250C
300C
>Enhanced geothermal systems potential in the USA. Estimates or thepotential energy payout rom EGS resources at depths between 3 and 10 kmare more than 13 million exajoules (EJ). Recovery o even a small percentagewould be more than enough to supply all the electrical needs o the nation.[Adapted rom The Future o Geothermal Energy, http://geothermal.inel.gov/publications/uture_o_geothermal_energy.pd (accessed June 30, 2009.)]
Category of ResourceThermal Energy, in
Exajoules [1 EJ = 1018 J]
Conduction-dominated EGSSedimentary rock formationsCrystalline basement rock formationsSupercritical volcanic EGS
100,00013,300,00074,100
Hydrothermal 2,400 to 9,600
Coproduced fluids 0.0944 to 0.4510
10. A geothermometer is a mineral or group o mineralswhose composition, structure or inclusions are xedwithin known thermal limits under particular conditionso pressure and composition and whose presence thusdenotes a limit or a range or the temperature oormation o the host rock.
11. Cumming, reerence 8.12. A double-fash system uses brine separated rom
geothermal water beore it was fashed. The brine isfashed a second time at a lower pressure, and theresulting steam is used to drive a separate turbineor is sent to the high-pressure turbine through aseparate inlet.
13. Lutz SJ, Moore JN, Jones CG, Suemnicht GA andRobertson-Tait A: Geological and StructuralRelationships in the Desert Peak Geothermal System,Nevada: Implications or EGS Development,Proceedings o the Stanord University 34th Workshopon Geothermal Reservoir Engineering, Stanord,Caliornia (February 911, 2009).
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10 Oileld Review
The model proposed is based on analysis o
mud logs and cores and incorporates new data
rom three wells drilled in the production portion
o the eld. Two cross sections have been con-
structed based on correlations observed in these
three wells (above).
Researchers logged a candidate stimulus well,
27-15, adjacent to the current production area to
aid in evaluating lithologies and characterizing
stress and ractures. Gamma ray and caliper
data were recorded and borehole images were
also acquired. Features identied rom these
resistivity-contrastgenerated images include
bedding planes, lithologic contacts, oliations, con-
ductive mineral grains, drilling-induced ractures
and natural ractures.14
In combination with other petrologic and
petrographic studies incorporated into a
GeoFrame model, this imaging provided a more
complete understanding o the geological charac-
teristics o the well as a candidate or EGS.
Further rock mechanics testing conducted at theSchlumberger TerraTek Geomechanics Center o
Excellence in Salt Lake City, Utah, USA, will
characterize rock strengths and stress behavior
o potential reservoir rocks within the proposed
stimulation interval.
The researchers noted that the productive
portion o the Desert Peak geothermal eld lies
within an older structural horst bounded by north-
west-trending aults. The results o tracer tests
indicate that fuids injected into the production
area can cross into currently nonproductive areas
along younger northeast-trending aults. The sci-
entists were unable, however, to determine the
depth o the fuid transmissivity and whether the
basement ault served as a barrier or conduit to
geothermal fuids. Upcoming hydraulic and chemi-
cal stimulation experiments are expected to
increase permeability and fuid-racture connec-
tivity in this enhanced system.
Making the Good Better
The dominant tools o EGSreservoir model-
ing, drilling, hydraulic racturing and water
injectionare amiliar to petroleum engineers.
Unortunately, their use in geothermal applica-
tions is more than a matter o adapting them to
increased temperatures.
For example, in oil and gas ormations, both
induced and natural racturing are reasonably
well-understood concepts. But because oil sandsare ractured to increase fow in discrete strati-
graphic intervalsand the goal in a geothermal
resource is to maximize heat exchange in large
volumes o ractured crystalline rockthe oper-
ations dier greatly in their application. Whereas
traditional hydraulic racturing operations are
constrained predominantly by rock stresses and
boundary considerations, complex rock and fuid
interactions and heat transer must be consid-
ered when determining injection rates, pumping
times and injection temperatures or racturing
geothermal ormations.
In recent years, stimulation o oil-bearing or-
mations by racturing has become increasingly
sophisticated and ecient as the industry devel-
oped methods or modeling, plotting, tracking
and even controlling racture direction. But most
o these techniques rely heavily on electronicsensors placed downhole near the sandace
depth. Temperature limitations render these
devices useless in geothermal zones.
Still, oileld-style interventions are being
applied successully in many o the worlds larg-
est geothermal elds, which are typically the
highest temperature volcanic-hosted systems.
These operations are essentially EGS and include
such established projects as the Salak geother-
mal eld, operated by Chevron. The largest o its
kind in Indonesia, the Salak eld is located
within a protected orest about 60 km [37 mi]
south o Jakarta(next page, top right).
Chevron has maintained steam production
levels and optimized heat recovery at Salak
through inll drilling and water injection into
deep wells on the elds margins where permea-
bility is low. Through the use o tracers, chemical
and microseismic monitoring, and pressure-
temperature surveys o individual wells, Chevron
has been able to gauge the impact o its injection
strategy and to move injection wells arther rom
the elds center and closer to its edges. This
approach has simultaneously generated more area
or inll drilling and expanded the eld. It has
also allowed the company to convert several
injection wells into producers once the ormation
has thermally recovered.
More recently, geophysical data, including MT
and time-domain electromagnetic surveys on the
elds margins, have identied potential reser-
voir extensions to the west and north o the
proven area. To the west, the Cianten Caldera
exhibits a low-resistivity layer at depths similar
to those in the Salak reservoir, and microseismic
data show distinct depth distribution o the
proven reservoir through the western area.
Drilling results in the caldera indicated non-commercial temperatures. Ring dike intrusions
appeared to preclude fuid circulation rom the
proven reservoir. Geothermal reservoir boundar-
ies tend to be vague, and new wells oten encoun-
ter low-permeability but hot ormations that
must be stimulated to provide adequate injection
rates. The operator thereore began a long-term,
>One o two Desert Peak cross sections. This conceptual cross section o the geothermal feld showsthe stratigraphy and interpreted structure rom Well 29-1 in the south to Well 27-15 in the north. The keyeatures o this section are the gently dipping top o the basement rocks in the north, the presence oa pre-Tertiary 1 (PT-1) interval in Well 27-15 and the thick Tertiary section (green) in the southern wells.Faults and structural interpretations are based on lithologies and stratigraphic sequences encountered ineach well, and locations o lost circulation zones identifed rom well cuttings and well logs. Well 27-15 isthe candidate or hydraulic stimulation. (Adapted rom Lutz et al, reerence 13.)
2,500
Depth,
m
0
500
1,000
1,500
2,000
9,000
Depth,
ft
0
1,000
2,000
3,000
4,000
5,000
6,000
Well29-1
Truckee and Desert Peak Fms
ChloropagusFormation
Rhyolite (lower)
Rhyolite(lower)
Rhyolite(upper)
PT-2 (upper)
PT-2
(upper)
PT-1
PT-2 (upper)
Quartzite
PT-2 (lower)
PT-2 (lower)Dolomite
Dacite
Well27-15
7,000
8,000
Tr-J mudstone
Faults dashed where inferred
Lost circulation zone
0 1,000m
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Winter 2009/2010 11
massive cold-water injection program. This oper-
ation takes advantage o the extreme tempera-
ture dierences between the injectate and the
ormationmore than 149C [268F]and the
ormations relatively high coefcients o thermal
contraction to create ractures.
Three injection stimulations were conducted
on one low-permeability well in the Cianten
Caldera that lies within the boundaries o the
Salak concession. These stimulations includedinjection o about 9.8 million bbl [1.6 million m3]
o water. To evaluate the impact o these treat-
ments on injection perormance, the operator
used a modifed Hall plot and analysis that indi-
cated racture development within the ormation
(below right). Injectivity improvements were also
quantifed through periodic pressure-allo tests
and the creation o a geomechanical reservoir
simulation model calibrated against feld his-
tory.15 The fnal analysis concluded that injectiv-
ity had been increased signifcantly. Two
additional wells drilled in the area will undergo
the same type o stimulation to allow injection o
water produced rom the high-temperature core
o the reservoir.
The Great Heat Exchange
Hot dry rockHDRreservoirs represent par-
ticularly high-potential geothermal systems. The
total amount o heat that may be unlocked rom
these reservoirs worldwide through injection or
racturing has been estimated at 10 billion
quadsabout 800 times more than that esti-
mated or all hydrothermal sources and 300 times
that available rom hydrocarbon reserves.16
14. Kovac KM, Lutz SJ, Drakos PS, Byersdorer J andRobertson-Tait A: Borehole Image Analysis andGeological Interpretation o Selected Features in WellDP 27-15 at Desert Peak Nevada: Pre-StimulationEvaluation o an Enhanced Geothermal System,Proceedings of the Stanford University 34th Workshopon Geothermal Reservoir Engineering, Stanord,Caliornia (February 911, 2009).
15. Yoshioka K, Pasikki R, Suryata I and Riedel K: HydraulicStimulation Techniques Applied to Injection Wells at theSalak Geothermal Field, Indonesia, paper SPE 121184,presented at the SPE Western Regional Meeting,San Jose, Caliornia, USA, March 2426, 2009.
16. Duchane D and Brown D: Hot Dry Rock (HDR)Geothermal Energy Research and Development atFenton Hill, New Mexico, GHC Bulletin(December2002), http://geoheat.oit.edu/bulletin/bull23-4/art4.pd
(accessed August 11, 2009).Quad is a short term or quadrillion and is a unit oenergy equal to 1015 BTU [1.055 1018 J]. It is theequivalent o about 180 million bbl o oil [28.6 million m 3].For reerence, the total 2001 US energy consumptionwas about 90 quads. The total HDR resource numberspublished by Duchane and Brown were calculated bysumming the thermal energy content o rock beneaththe Earths land masses at temperatures above 25C[77F] rom the surace to 10,000 m [33,000 t]. Whilethese numbers seem astronomical and do includeresources that are impractical to recover because theyare low temperature or are unreachable, they stillrepresent an enormous amount o energy.
>Salak eld, Indonesia.
A S I A
I N D O N E S I A
P H I L I P P I N E S
mi
0
0 100
100km
Salak
Darajat
Jakarta
I N D O N E S I A
>Evaluating injection perormance. A modied Hall plot provides a qualitativeindicator o injection perormance. The Hall integral (orange) is a straightline i the well skin actor does not change over time. A steeper slopeindicates some type o fow resistance, such as plugging or scaling, while
a shallower slope indicates ormation stimulation. In subtle cases, such asthis one in Salak eld, plotting the Hall derivative (blue) on the same scaleimproves the diagnosis. A derivative curve above the integral curve indicatesincreased resistance and below the integral curveas shown hereongoingstimulation. This analysis conrmed racture development during cold-waterinjection in the eld. (Adapted rom Yoshioka et al, reerence 15.)
0
5.0 x 104
1.0 x 105
1.5 x 105
2.0 x 105
2.5 x 105
Hallinte
gral
Cumulative injection, bbl
8.0 x 1066.0 x 1064.0 x 1062.0 x 1060
Hall integral
Hall derivative
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12 Oilfeld Review
Unlike hydrothermal EGS, there are, as yet,
no commercial HDR elds, so experience with
these systems has been conned primarily to
pilot projects. O particular importance to the
concept is an extended study at Fenton Hillthe
rst HDR projectthat began in the early 1970s.
The Fenton Hill HDR site is about 64 km [40 mi]
west o Los Alamos, New Mexico, USA. It includes
two conned reservoirs created in crystalline
rock at 2,800 and 3,500 m [9,200 and 11,480 t]
with reservoir temperatures o 195C and 235C
[383F and 455F], respectively. Flow tests were
conducted in each o the reservoirs or almost a
year. The project, conducted over a period o
about 25 years, ended in 1995.
HDR systems are essentially reservoir-
creation projects. One o the most important les-
sons learned at Fenton Hill is that it is nearly
impossible to connect two existing boreholes by
creating a hydraulic racture between them.
Reservoirs should thereore be created by stimu-
lating or creating ractures rom the initial bore-
hole and then accessing them by two production
boreholes (let).17
Work at Fenton Hill also advanced the caseor HDR elds by dening which critical actors
in their construction are controllable. For exam-
ple, the reservoirs size is a direct linear unction
o the amount o fuid injected into it (next page).
Similarly, temperature, injection pressure and
fow rate, production backpressure, and the num-
ber and placement o wells are all manageable
variables within HDR eld development.
While many o the technological questions
associated with HDR systems were answered
through the work at Fenton Hill, uncertainties
about reservoir creation remain. Although a rela-
tionship can be established between fuid volume
injected and resulting volume made available or
heat exchange, the ractured surace area within
that volume o rock is more dicult to quantiy.
One approach renders an order-o-magnitude
estimate o the rock volume required. This is
obtained by equating the heat fow rate rom the
reservoir with the change in stored thermal
energy, assuming uniorm extraction o heat
throughout the volume. The heat fow rate is a
unction o rock density, volume and heat capacity,
and the change in rock temperature over time.
A numerical simulation study by Sanyal and
Butler suggests the electrical power generation
rate achievable on a unit rock volume basis
is 26 MWe/km3 [106 MWe/mi
3].18 This power-
production correlation requires a volume o
roughly 0.19 km3 [0.05 mi3] to generate 5 MWe.
Such a cube would measure 575 m [1,886 t] on
each side, and the simulation is based on an
assumption o uniorm properties, including
permeability, within the stimulated region.
The study concluded that i constant pro-
duction is maintained, generation capacity is pri-
marily a unction o the stimulated rock volume.
Other considerations may include well congura-tion, number o wells within a reservoir volume,
reservoir mechanical properties, reservoir stress
state and natural racture eatures. These char-
acteristics collectively determine how the reser-
voir is best stimulated to create the requisite
volume and the fow paths necessary or eective
heat extraction.19
>The EGS concept as applied to HDR. Fractures are generated roman injection well (blue) drilled into a low-permeability reservoir o deepcrystalline rock. Production wells (red) are then drilled into the racturedzone. Injected water is heated as it fows rom the injection well to theproduction wells.
5
00to1,0
00m
500to1
,000m
4,0
00to
6,0
00m
Cry
stallin
ero
ck
s
Se
dim
e
nts
Productionwell
Injectionwell
Powergeneration
Heatdistribution
Stimulated
fracture
system
Cooling
Makeup waterreservoirHeat
exchangerCentralmonitoring
17. Brown DW: Hot Dry Rock Geothermal Energy:Important Lessons rom Fenton Hill, Proceedingsof the Stanford University 34th Workshop on GeothermalReservoir Engineering, Stanord, Caliornia (February911, 2009).
18. Sanyal SK and Butler SJ: An Analysis o PowerGeneration Prospects rom Enhanced GeothermalSystems, Proceedings of the Stanford University 34thWorkshop on Geothermal Reservoir Engineering,Stanord, Caliornia (February 911, 2009).
MWe stands or electrical megawatt.
19. Polsky Y, Capuano L Jr, Finger J, Huh M, Knudsen S,Mansure AJC, Raymond D and Swanson R: EnhancedGeothermal Systems (EGS) Well ConstructionTechnology Evaluation Report, Sandia ReportSAND2008-7866: Sandia National Laboratories,December 2008.
20. Polsky et al, reerence 19.
21. Kumano Y, Moriya H, Asanuma H, Wyborn D and
Niitsuma H: Spatial Distribution o CoherentMicroseismic Events at Cooper Basin, Australia,Expanded Abstracts, 76th SEG Annual Meeting andExhibition, New Orleans (October 16, 2006): 595599.
Microseismic multiplet analysis, based on a high-resolution relative hypocenter location technique, useswaveorm similarity to identiy events located ongeometrically or geophysically related structures.
22. Petty S, Bour DL, Livesay BJ, Baria R and Adair R:Synergies and Opportunities Between EGSDevelopment and Oileld Drilling Operations andProducers, paper SPE 121165, presented at theSPE Western Regional Meeting, San Jose, Caliornia,March 2426, 2009.
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Winter 2009/2010 13
Despite the progress being made on the tech-
nological aspects o HDR exploitation, commer-
cial viability o these prospects remains elusive
as a consequence o their depth and tempera-
ture. For example, commercial hydrothermal
well depths range rom less than 1 km to a rare
ew that reach about 4 km [13,000 t], such as the
EGS project in Soultz-sous-Forts, France. HDR
wells, because they are in crystalline basement
ormations, are typically much deeper. As a con-
sequence, HDR wells are likely to be character-
ized by varied lithology and the extensively
documented problems associated with deep drill-
ing and completion.20
The Gap
Owing to the obvious similarities between hydro-
carbon and heat mining, it is tempting to assume
that adapting the technology o the ormer to the
latter is a matter o ocus. Recent development o
tools or use in some applicationsHPHT oil and
gas wells, hydrothermal elds and steam-
foodingencourages such assumptions.Geothermal energy resources, however, dier
across the world, and the ease with which this
technology transer will take place is a unction
o those dierences. The highest grade o
resourcehydrothermalis shallow, permeable
and hot and has a natural water-recharge system.
The techniques and methods used to tap that
resource are and will continue to be amiliar to
oileld personnel.
Lower-grade resources that require interven-
tion in the orm o injection or racturing, or
whose temperatures are below the boiling point
o water, are also being produced at a prot
through the use o technology adapted rom the
petroleum industry. Coproduction is a current
technique that uses the hot water produced with
oil and gas to run binary plants, which in some
cases generate all the elds electricity needs.
But the real prize in geothermal energy pro-
duction will come once the technology required
or EGS and HDR reservoirs is widely available.
Despite current barriers to commerciality, HDR
projects do have an advantage over those or
conventional hydrothermal systems in that they
can be located near major electricity markets.
That they still require much technological
innovation, however, has created a tendency
among many o those best equipped to solve these
problemspetroleum industry proessionalstoabandon the notion o HDR developments in avor
o more immediate and amiliar pursuits.
With the prospects o large payos, there has
been progress on making HDR projects economi-
cally attractive, including the vital area o reser-
voir-creation monitoring and control. In the
Cooper basin o Australia, or example, geophysi-
cists recently applied microseismic multiplet
analysis to a dataset rom an HDR hydraulic rac
turing operation to help characterize the devel
oping racture system within the reservoir.21
The greatest potential or improving the eco
nomics or geothermal energy projects, as in any
high-risk, high-cost venture, is by risk reduction
through a better understanding o the subsur
ace. The unknowns that aect drilling and com
pletion risk, environmental impact, stimulation
and overall project success are all exacerbatedby a lack o knowledge about lithology, stres
regime, natural seismicity, preexisting aults and
ractures, and temperature at depth.22
Correcting these shortcomings will be a
matter o growth, but o a type with which the
E&P industry is long amiliar. It took the oshore
industry more than 50 years o lessons learned
between the rst well drilled in shallow water
just out o sight o land to routine placement o
wells in water depths o more than 3,000 m
[10,000 t] and hundreds o kilometers rom
shore. Moving rom shallow, high-grade hydro
thermal ormations to deep, hot dry rocks wil
require a similar evolution in technology, equip
ment and trained personnel. Given the prize
in the ong, however, it is certainly just a matte
o time. RvF
>Controlling reservoir size. During a massive hydraulic racture test atFenton Hill, a linear relationship was established between the seismicallyactive reservoir volume and the volume o injected fuid, as determinedrom microseismic event location data. (Adapted rom Duchane and Brown,reerence 16.)
0
20
40
60
Seism
icvolume,
1,0
00,0
00m
3
Volume of fluid injected, 1,000 m3
0 2010 30
80