01 mining heat

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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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Winter 2009/2010 5


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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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Winter 2009/2010 7

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


9/10

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.


10/10

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

01 mining heat

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