jeo term o metre

7/28/2019 Jeo Term o Metre

1/91

GEOCHEMISTRY OF

GEOTHERMAL SYSTEMS


2/91

WATER CHEMISTRYChemical composition of waters is expressed in termsof major anion and cation contents.

Major Cations: Na+, K+, Ca++, Mg++

Major Anions: HCO3- (or CO3

=), Cl-, SO4=

HCO3- dominant in neutral conditions

CO3= dominant in alkaline (pH>8) conditions

H2CO3 dominant in acidic conditionsAlso dissolved silica (SiO2) in neutral form

as a major constituent

Mino r con sti t uen ts: B , F, Li, Sr, ...


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WATER CHEMISTRYconcentration of chemical constituents areexpressed in units of

mg/l (ppm=parts per million)(mg/l is the preferred unit)

Molality

Molality = no. of moles / kg of solvent

No.of moles = (mg/l*10-3)/ formula weight


4/91

WATER CHEMISTRYErrors associated with water analyses are expressed in

terms ofCBE (Charge Balance Error)

CBE (%) = ( z x mc - z x ma ) / (z x mc + z x ma )* 100where,

mc is the molality of cation

ma is the molality of anion

z is the charge

If CBE 5%, the results are appropriate to use in any kind ofinterpretation


5/91

The constituents encountered in

geothermal fluidsTRACERS

Chemically inert, non-reactive, conservative constituents

(once added to the fluid phase, remain unchanged allowingtheir origins to be traced back to their source component -used to infer about the source characteristics)

e.g. He, Ar (noble gases), Cl, B, Li, Rb, Cs, N2

GEOINDICATORS

Chemically reactive, non-conservative species

(respond to changes in environment - used to infer about thephysico-chemical processes during the ascent of water tosurface, also used in geothermometry applications)

e.g. Na, K, Mg, Ca, SiO2


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In this chapter, the main emphasiswill be placed on the use of waterchemistry in the determination of :

underground (reservoir) temperatures :

geothermometers

boiling and mixing relations (subsurfacephysico-chemical processes)

WATER CHEMISTRY


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HYDROTHERMAL

REACTIONSThe composition of geothermal fluids are controlledby : temperature-dependent reactions between

minerals and fluids

The factors affecting the formation of hydrothermalminerals are:

temperature pressure rock type permeability fluid composition duration of activity


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The effect of rock type --- most pronounced atlow temperatures & insignificant above 280CAbove 280C and at least as high as 350C, thetypical stable mineral assemblages (in activegeothermal systems) are independent of rocktype and include

ALBITE, K-FELDSPAR, CHLORITE, Fe-EPIDOTE, CALCITE,QUARTZ, ILLITE & PYRITE

At lower temperatures, ZEOLITES and CLAYMINERALS are found.

At low permeabilities equilibrium between rocksand fluids is seldom achieved.

When permeabilities are relatively high and waterresidence times are long (months to years), water& rock should reach chemical equilibrium.


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At equilibrium, ratios of cations in solution are controlled by temperature-dependent exchange reactions such as:

NaAlSi3O8 (albite) + K+ = KAlSi3O8 (K-felds.) + Na+

Keq. = Na+ / K+

Hydrogen ion activity (pH) is controlled by hydrolysis reactions, such as :

3 KAlSi3O8 (K-felds.) + 2 H+ = K Al3Si3O10(OH)2 (K-mica)+ 6SiO2 + 2 K

+

Keq. = K+ / H+where,

Keq. = equilibrium constant,

square brackets indicate activities of dissolved species (activity is unity

for pure solid phases)


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ESTIMATION OF

RESERVOIR

TEMPERATURESThe evaluation of the

reservoir temperatures forgeothermal systems is made

in terms ofGEOTHERMOMETRY

APPLICATIONS


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GEOTHERMOMETRY

APPLICATIONS


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GEOTHERMOMETRY

APPLICATIONSOne of the major tools for theexploration & development

of geothermal resources


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GEOTHERMOMETRY

estimation of reservoir (subsurface)temperatures

usingChemical & isotopic compositionof

surface dischargesfrom

wellsand/or

natural springs/fumaroles


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GEOTHERMOMETERS

CHEMICAL GEOTHERMOMETERSutilize the chemical composition

silica and major cation contents of waterdischargesgas concentrations or relative

abundances of gaseous components insteam discharges

ISOTOPIC GEOTHERMOMETERSbased on the isotope exchange reactions

between various phases (water, gas,mineral) in geothermal systems


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Focus of the Course

CHEMICAL GEOTHERMOMETERS

As applied to water discharges

PART I. Basic Principles & Types

PART II. Examples/Problems


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CHEMICAL

GEOTHEROMOMETERSPART I.Basic Principles &Types


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BASIC PRINCIPLES

Chemical Geothermometers are

developed on the basis oftemperature

dependent chemical equilibriumbetween the water and the minerals atthe deep reservoir conditions

based on the assumption that the water

preserves its chemical compositionduring its ascent from the reservoir tothe surface


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BASIC PRINCIPLES

Studies of well discharge chemistry and

alteration mineralogy

the presence of equilibrium in severalgeothermal fields

the assumption of equilibrium is valid


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BASIC PRINCIPLES

Assumption of the preservation of water

chemistry may not always hold

Because the water composition may beaffected by processes such as

cooling

mixing with waters from different reservoirs.


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BASIC PRINCIPLES

Cooling during ascent fromreservoir to surface:

CONDUCTIVE

ADIABATIC


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BASIC PRINCIPLES

CONDUCTIVE Cooling

Heat loss while travelling through cooler

rocks

ADIABATIC Cooling

Boiling because of decreasing hydrostatichead


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BASIC PRINCIPLES

Conductive coolingdoes not by itselfchange the

composition of the water

but may affect its degree of saturationwith respect to several minerals

thus, it may bring about a modification

in the chemical composition of thewaterby mineral dissolution orprecipitation


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BASIC PRINCIPLES

Adiabatic cooling (Cooling byboiling)

causes changes in the composition ofascending water

these changes include

degassing, and hencethe increase in the solute content as a

result of steam loss.


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BASIC PRINCIPLES

MIXINGaffects chemical composition

since the solubility of most of thecompounds in waters increases withincreasing temperature, mixing withcold groundwaterresults in thedilution

of geothermal water


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Geothermometry applications are not

simply inserting values into specific

geothermometry equations.Interpretation of temperatures obtained

from geothermometry equations requires

a sound understanding of the chemicalprocesses involved in geothermal

systems.

The main task of geochemist is to verifyor disprove the validity of assumptions

made in using specific geothermometers

in specific fields.


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TYPES OF CHEMICALGEOTHERMOMETERS

SILICA GEOTHERMOMETERS

CATION GEOTHERMOMETERS(Alkali Geothermometers)


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based on the

experimentally determined

temperature dependent

variation in the solubility of silica in water

Since silica can occur in various forms in

geothermal fields (such as quartz,

crystobalite, chalcedony, amorphous silica)different silica geothermometers have been

developed by different workers



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SILICA GEOTHERMOMETERSGeothermometer Equation Reference

Quartz-no steam loss T = 1309 / (5.19log C) - 273.15 Fournier (1977)

Quartz-maximum

steam loss at 100 oC

T = 1522 / (5.75 - log C) - 273.15 Fournier (1977)

Quartz T = 42.198 + 0.28831C - 3.6686 x 10-4 C2 +

3.1665 x 10-7 C3 + 77.034 log C

Fournier and

Potter (1982)

Quartz T = 53.500 + 0.11236C - 0.5559 x 10-4 C2 +

0.1772 x 10-7 C3 + 88.390 log C

Arnorsson

(1985) based on

Fournier and

Potter (1982)

Chalcedony T = 1032 / (4.69 - log C) - 273.15 Fournier (1977)

Chalcedony T = 1112 / (4.91 - log C) - 273.15 Arnorsson et al.

(1983)

Alpha-Cristobalite T = 1000 / (4.78 - log C) - 273.15 Fournier (1977)

Opal-CT

(Beta-Cristobalite)

T = 781 / (4.51 - log C) - 273.15 Fournier (1977)

Amorphous silica T = 731 / (4.52 - log C) - 273.15 Fournier (1977)


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The followings should be considered :

temperature rangein which the equations are

valideffects ofsteam separation

possibleprecipitation of silica before sample collection

(during the travel of fluid to surface, due to silica oversaturation)

after sample collection

(due to improper preservation of sample)

effects ofpH on solubility of silica

possiblemixingof hot water with cold water


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Temperature Range

silica geothermometers are valid for

temperature ranges up to 250 Cabove 250C, the equations departdrastically from the experimentally

determined solubility curves


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Temperature Range

Fig.1.Solubility ofquartz (curve A)and amorphous silica (curve C) asa function of temperature at thevapour pressure of the solution.Curve B shows the amount of silicathat would be in solution after aninitially quartz-saturated solution

cooled adiabatically to 100 Cwithout any precipitation of silica(from Fournier and Rowe, 1966, andTruesdell and Fournier, 1976).

At low T (C)qtz less solubleamorph. silica more soluble

Silica solubility is controlled byamorphous silicaat low T (C)quartz at high T (C)


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Effects of Steam SeparationBoiling Steam Separationvolume of residual liquid

Concentration in liquid

Temperature Estimate

e.g.T = 1309 / (5.19log C) - 273.15C = SiO2 in ppmincrease in C (SiO2 in water > SiO2 in reservoir)decrease in denominator of the equationincrease in T

for boiling springs

boiling-corrected geothermometers(i.e. Quartz-max. steam loss)

SiO2liquid V1

(1)

V2 V1

liquid V2SiO2 (2)

steam


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Silica Precipitation

SiO2

Temperature Estimate

e.g.T = 1309 / (5.19log C) - 273.15

C = SiO2 in ppm

decrease in C (SiO2 in water < SiO2 in reservoir)increase in denominator

decrease in T


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Effect of pHFig. 2.Calculated effect of pHupon thesolubility of quartz at various temperaturesfrom 25 C to 300 C , using experimentaldata of Seward (1974). The dashed curveshows the pH required at varioustemperatures to achieve a 10% increase inquartz solubility compared to the solubilityat pH=7.0 (from Fournier, 1981).

pH Dissolved SiO2 (for pH>7.6)Temperature Estimate

e.g.

T = 1309 / (5.19 log C) - 273.15C = SiO2 in ppm

increase in C

decrease in denominator of the equation

increase in T

S C G O O S


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Effect of Mixing

Hot-Water High SiO2 contentCold-Water Low SiO2 content

(Temperature Silica solubility)

Mixing (of hot-water with cold-water)TemperatureSiO2 Temperature Estimate

e.g.T = 1309 / (5.19 log C) - 273.15C = SiO2 in ppm

decrease in C

increase in denominator of the equation

decrease in T


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Process Reservoir Temperature

Steam Separation Overestimated

Silica Precipitation

UnderestimatedIncrease in pH Overestimated

Mixing with cold water Underestimated

CATION GEOTHERMOMETERS


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(Alkali Geothermometers)

based on the partitioning of alkalies betweensolid and liquid phases

e.g. K+ + Na-feldspar = Na+ + K-feldspar

majority of are empirically developed

geothermometers

Na/K geothermometer Na-K-Ca geothermometer

Na-K-Ca-Mg geothermometer

Others(Na-Li, K-Mg, ..)


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Na/K Geothermometer

Fig.3. Na/K atomic ratios ofwell discharges plotted atmeasured downholetemperatures. Curve A isthe least square fit of thedata points above 80 C.Curve B is anotherempirical curve (fromTruesdell, 1976). Curves C

and D show theapproximate locations ofthe low albite-microclineand high albite-sanidinelines derived fromthermodynamic data (from

Fournier, 1981).



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Na/K Geothermometer

Geotherm. Equations Reference

Na-K T=[855.6/(0.857+log(Na/K))]-273.15 Truesdell (1976)

Na-K T=[833/(0.780+log(Na/K))]-273.15 Tonani (1980)

Na-K T=[933/(0.993+log (Na/K))]-273.15

(25-250 oC)

Arnorsson et al.

(1983)

Na-K T=[1319/(1.699+log(Na/K))]-273.15

(250-350 oC)

Arnorsson et al.

(1983)

Na-K T=[1217/(1.483+log(Na/K))]-273.15 Fournier (1979)

Na-K T=[1178/(1.470+log (Na/K))]-273.15 Nieva and Nieva

(1987)

Na-K T=[1390/(1.750+log(Na/K))]-273.15 Giggenbach

(1988)


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Na/K Geothermometer

gives good results for reservoir temperatures

above 180C.yields erraneous estimates for low

temperature waterstemperature-dependent exchange equilibrium

between feldspars and geothermal waters is not

attained at low temperatures andthe Na/K ratio in

these waters are governed by leaching rather thanchemical equilibrium

yields unusually high estimates for waters

having high calcium contents


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Na-K-Ca Geothermometer

Geotherm. Equations Reference

Na-K-Ca T=[1647/ (log (Na/K)+ (log (Ca/Na)+2.06)+ 2.47)]-273.15

a) iflogCa/Na)+2.06 < 0, use =1/3 and calculate TCb) iflogCa/Na)+2.06 > 0, use =4/3 and calculate TCc) if calculated T > 100C in (b), recalculate TC using =1/3

FournierandTruesdell(1973)



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Na-K-Ca GeothermometerWorks well for CO2-rich or Ca-rich environments provided

that calcite was not deposited after the water left thereservoir

in case ofcalcite precipitation

Ca 1647

T = --------------------------------------------------------- - 273.15log (Na/K)+ (log (Ca/Na)+2.06)+ 2.47

Decrease in Ca concentration (Ca in water < Ca in reservoir)

decrease in denominator of the equation

increase in T

For waters with high Mg contents, Na-K-Cageothermometer yields erraneous results. For these

waters, Mg correction is necessary



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Na-K-Ca-Mg Geothermometer

Geotherm. Equations ReferenceNa-K-Ca-Mg T = TNa-K-Ca - tMgoC

R = (Mg / Mg + 0.61Ca + 0.31K) x 100

if R from 1.5 to 5

tMgoC = -1.03 + 59.971 log R + 145.05 (log R)2 36711(log R)2/ T - 1.67 x 107 log R / T2if R from 5 to 50tMgoC=10.66-4.7415 log R+325.87(log R)2-1.032x105(log R)2/T-1.968x107(log R)3/T2

Note: Do not apply a Mg correction iftMgis negativeor R50, assume a temperature = measured springtemperature.

T is Na-K-Ca geothermometer temperature in Kelvin

Fournierand Potter(1979)



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Na-K-Ca-Mg Geothermometer

Fig. 4.Graph forestimating the

magnesium temperature

correction to be

subtracted from Na-K-Cacalculated temperature

(from Fournier, 1981)

R = (Mg/Mg + 0.61Ca + 0.31K)x100

G O G O


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UNDERGROUND MIXING OF

HOT AND COLD WATERSRecognition of Mixed WatersMixing of hot ascending waters with cold waters atshallow depths is common.

Mixing also occurs deep in hydrothermal systems.

The effects of mixing on geothermometers is alreadydiscussed in previous section.

Where all the waters reaching surface are mixed waters,recognition of mixing can be difficult.

The recognition of mixing is especially difficult if water-rock re-equilibration occurred after mixing (complete orpartial re-equilibration is more likely if the temperaturesafter mixing is well above 110 to 150 C, or if mixingtakes place in aquifers with long residence times).



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HOT AND COLD WATERS

Some indications of mixing are as follows:

systematic variations of spring compositions

and measured temperatures,

variations in oxygen or hydrogen isotopes,

variations in ratios of relatively *conservat ive

elementsthat do not precipitate from solution

during movement of water through rock (e.g.Cl/B ratios).

SILICA ENTHALPY MIXING


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SILICA-ENTHALPY MIXINGMODEL

Dissolved silica content of mixed waters can be usedto determine the temperature of hot-watercomponent .

Dissolved silica is plotted against enthalpy of liquidwater.

Although temperature is the measured property, andenthalphy is a derived property, enthalpy is used asa coordinate rather than temperature. This isbecause the combined heat contents of two watersare conserved when those waters are mixed, but thecombined temperatures are not.

The enthalpy values are obtained from steam tables.



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Fig. 5.Dissolved silica-enthalpy diagram showing

procedure for calculating

the initial enthalpy (and

hence the reservoirtemperature) of a high

temperature water that has

mixed with a low

temperature water (from

Fournier, 1981)



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A = non-thermal component(cold water)

B, D = mixed, warm watersprings

C = hot water component atreservoir conditions(assumingno s team

separationbefore mix ing)

E = hot water component atreservoir conditions(assumingsteam separat ion

before mix ing)

Boi l ingT = 100CEnthalpy = 419 J/g(corresponds to D in the graph)

Enthalpy values (at corresponding temperatures)are found from Steam Table in Henley et al.(1984)

419 J/g(100 C)0


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SILICA-ENTHALPY MIXING MODELSteam Fraction did not separate before mixing

The sample points are plotted.

A straight line is drawn fromthe point representing thenon-thermal component of themixed water (i.e. the point withthe lowest temperature and

the lowest silica content =point A in Fig.), through themixed water warm springs(points B and D in Fig.).

The intersection of this linewith the qtz solubility curve(point C in Fig.) gives the

enthalpy of the hot-watercomponent (at reservoirconditions).

From the steam table, thetemperature corresponding tothis enthalpy value is obtainedas the reservoir temperature

of the hot-water component.

419 J/g(100 C)0


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SILICA-ENTHALPY MIXING MODELSteam separation occurs before mixing

The enthalpy at the bolingtemperature (100C) isobtained from the steamtables (which is 419 j/g)

A vertical line is drawn fromthe enthalpy value of 419 j/g

From the inetrsection point ofthis line with the mixing line(Line AD), a horizantal line(DE) is drawn.

The intersection of line DEwith the solubility curve formaximum steam loss (point E)

gives the enthalpy of the hot-water component.

From the steam tables, thereservoir temperature of the

hot-watercomponent isdetermined.

419 J/g(100 C)0



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In order for the silica mixing model to give accurate results, itis vital that no conductive cooling occurred after mixing. Ifconductive cooling occurred after mixing, then the calculatedtemperatures will be too high (overestimated temperatures).This is because:

the original points before conductive cooling should lie to the

right of the line AD (i.e. towards the higher enthalpy values atthe same silica concentrations, as conductive cooling willaffect only the temperatures, not the silica contents)

in this case, the intersection of mixing line with the quartzsolubility curve will give lower enthalpy values (i.e lowertemperatures) than that obtained in case of conductive

cooling.in other words, the temperatures obtained in case ofconductive cooling will be higher than the actual reservoirtemperatures (i.e. if conductive cooling occurred after mixing,the temperatures will be overestimated)



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Another requirement for the use of enthalpy-silicamodel is that no silica deposition occurred before orafter mixing. If silica deposition occurred, thetemperatures will be underestimated. This is because:

the original points before silica deposition should be

towards higher silica contents (at the same enthalpyvalues)

in this case, the intersection point of mixing line withthe silica solubility curve will have higher enthalpyvalues(higher temperatures) than that obtained in caseof silica deposition

in other words, the temperatures obtained in case of nosilica deposition will be higher than that in case ofsilica deposition (i.e. the temperatures will beunderestimated in case of silica deposition)

CHLORIDE ENTHALPY MIXING


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CHLORIDE-ENTHALPY MIXINGMODEL

Fig.6. Enthalpy-chloridediagram for waters from

Yellowstone National

Park. Small circles

indicate Geyser Hill-typewaters and smal dots

indicate Black Sand-type

waters (From Fournier,

1981).



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ESTIMATION OF RESERVOIR

TEMPERATURE

Geyser Hill-type Waters

A = maximum Cl content

B = minimum Cl contentC = minimum enthalpy at

the reservoir

Black Sand-type Waters

D = maximum Cl content

E = minimum Cl content

F = minimum enthalpy at

the reservoir

Enthalpy of steam at 100 C =2676 J/g

(Henley et al., 1984)



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ORIGIN OF WATERS

N = cold water component

C, F = hot water components

F is more dilute & slightlycooler than C

F can not be derived from Cby process of mixingbetween hot and cold water(point N), because any

mixture would lie on orclose to line CN.

C and F are probably bothrelated to a still higherenthalpy water such aspoint G or H.



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ORIGIN OF WATERS

water C could be relatedto water Gby boiling

water C could also berelated to water H

by conductive cooling

water F could be relatedto water G orwater Hby

mixing with cold water N

B ED

B


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steamsteam

C

GF

H

N

H

cold water reservoir

hot water reservoir

steamhot water

mixed water

residual liquid from boiling

hot water undergoingconductive cooling

mixed water undergoingconductive cooling

residual liquid undergoingconductive cooling


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60/91

ISOTOPESIN

GEOTHERMALEXPLORATION

& DEVELOPMENT

ISOTOPE STUDIES IN


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ISOTOPE STUDIES INGEOTHERMAL SYSTEMS

At Exploration, Development andExploitation Stages

Most commonly used isotopes

Hydrogen (1H, 2H =D, 3H)

Oxygen (18

O,16

O) Sulphur (32S, 34S)

Helium (3He, 4He)

ISOTOPE STUDIES IN


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ISOTOPE STUDIES INGEOTHERMAL SYSTEMS

Geothermal Fluids

Sources

Source of fluids(meteoric, magmatic, ..)

Physico-chemical processes affecting the fluid comosition

Water-rock interaction

Evaporation

Condensation

Source of components in fluids(mantle, crust,..)

Ages(time between recharge-discharge, recharge-sampling)

Temperatures(Geothermometry Applications)


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Sources of Geothermal Fluids


H- & O- Isotopes

Physico-chemical processes affecting the fluidcomposition

H- & O- Isotopes

Sources of components (elements,compounds) in geothermal fluids

He-Isotopes (volatile elements)


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Sources of Geothermal Fluids andPhysico-Chemical Processes

STABLE

H- & O-ISOTOPES



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StableH- & O-Isotopes1H = % 99.98522H (D) = % 0.0148

D/H

16O = % 99.7617O = % 0.0418O = % 0.2018O / 16O



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StableH- & O-Isotopes(D/H)sample- (D/H)standardD () = ----------------------------------- x 103

(D/H)standard

(18O/16O)sample- (18O/16O)standard18O () = -------------------------------------------- x 103

(18O/16O)standard

Standard = Standard Mean Ocean Water

= SMOW



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StableH- & O-Isotopes

(D/H)sample- (D/H)SMOWD () = ----------------------------------- x 103(D/H)SMOW

(18O/16O)sample- (18O/16O)SMOW18O () = -------------------------------------------- x 103

(18O/16O)SMOW



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Sources of Natural Waters:

1. Meteoric Water(rain, snow)2. Sea Water3. Fossil Waters (trapped in sediments in sedimanary basins)

4. Magmatic Waters5. Metamorphic Waters



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0

0

-40

-80

-120

10 20 30-10-20

O (per mil)18

D (per mil)

+

SMOW

Field ofFormationWaters

MagmaticWaters

Most igneousbiotites &hornblendes

MetamorphicWaters



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Ocean

Seepage

precipitation

evaporation

River

H, O1 16

H, O1 16

D, O18

D, O18

D, O18 H, O1 16

H, O1 16D, O18

D, O18

(D/H) < (D/H)vapor water

va or


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0

-40

-80

-120-12 -8 -4 0

del- O (per mil)18

+SMOW

Condensation

Evaporation

Water-RockInteraction



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18

MagmatikSular

0

-50

-100

-150-15 -10 -5 0 +5 +10

Larderello

The Geysers

Iceland

Niland

Lassen Park

Steamboat Kaynaklar

O (per mil)

D (per mil)

Physico-Chemical Processes:


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Stable H- & O-Isotopes

Latitute D 18OAltitute from Sea levelD 18O



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Aquifers recharged by precipitation from

lower altituteshigherD - 18OvaluesAquifers recharged by precipitation from

higher altituteslowerD - 18OvaluesMixing of waters from different aquifers



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Physico Chemical Processes:


Boiling and vapor separation D 18O in residual liquidPossible subsurface boiling as a

consequence of pressure decrease

(due to continuous exploitationfrom production wells)

M it i St di i


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Monitoring Studies in

Geothermal Exploitation

Aquifers recharged by

precipitation from

lower altituteshigherD - 18OAquifers recharged by

precipitation from

higher altituteslowerD - 18OBoiling and vapor

separation D 18O inresidual liquid

Any increase inD - 18Ovalues due to sudden pressure

drop in production wells

recharge from (other)aquifers fed by

precipitation from lower

altitutes

subsurface boiling andvapour separation

Monitoring Studies in


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g

Geothermal Exploitation

Monitoring of isotope composition ofgeothermal fluids during exploitation canlead to determination of, and thedevelopment of necessary precautionsagainst

Decrease in enthalpy due to start ofrecharge from cold, shallow aquifers, or

Scaling problems developed as a result ofsubsurface boiling

(S )


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(Scaling)

Vapour Separation

Volume of (residual) liquid Concentration of dissolved components

in liquid Liquid will become oversaturated

Component (calcite, silica, etc.) willprecipitate

Scaling


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Dating of Geothermal Fluids

3H- & 3He-ISOTOPES

D ti f G th l Fl id


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Dating of Geothermal Fluids

Time elapsed between Recharge-Discharge orRecharge-Sampling

points (subsurface residence residence

time)

3H method

3H-3He method

TRITIUM (3H)


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TRITIUM (3H)

3H = radioactive isotope of Hydrogene (with a short half-life)3H forms

Reaction of14N isotope (in the atmosphere) with cosmic rays

14

7N + n 31H + 126C Nuclear testing

3H concentration

Tritium Unit (TU) 1 TU = 1 atom 3H / 1018 atom H

3H 3He + Half-life = 12.26 year

Decay constant () = 0.056 y-1

3H D ti M th d


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3H Dating Method

3H concentration level in the atmosphere hasshown large changes

n between 1950s and 1960s (before andafter the nuclear testing)

Particularly in the northern hemisphere

Before 1953 : 5-25 TU

In 1963 : 3000 TU

3H D ti M th d


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3H Dating Method

3H-concentration in groundwater < 1.1 TU

Recharge by precipitations older than nuclear testing

3H-concentration in groundwater > 1.1 TU

Recharge by precipitations younger than nucleartesting

N=N0e-t 3H0 (before 1963) 10 TU

3H= 3H0e-t = 0.056 y-1t = 2003-1963 = 40 years

3H 1.1 TU

3H D ti M th d


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3H Dating Method

APPARENT AGE

3H= 3H0e-t

3H = measured at sampling point3H0 = measured at recharge point

(assumed to be the initial tritium concentration)

= 0.056 y-1t = apparent age

3H 3He


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H HeDating Method

3He = 3H03H (D = N0-N)

3H= 3H0 e-t (N =N0e -t)

3H0=3H et

3He = 3H et - 3H = 3H (et1)

t = 1/ * ln (3He/3H + 1)3He & 3H present-day concentrations measured in water sample


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Geothermometry Applications

Isotope Fractionation Temperature Dependent

Stable isotope compositions utilized in Reservoir Temperature estimation

Isotope geothermometers

Based on: isotope exchange reactions between phases

in natural systems

(phases: watre-gas, vapor-gas, water-mineral.....)

Assumes: reaction is at equilibrium at reservoir

conditions


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Isotope Geothermometers

12CO2 +13CH4 =

13CO2 +12CH4 (CO2 gas - methane gas)

CH3D + H2O = HDO + CH4 (methane gas water vapor)

HD + H2O = H2 + HDO (H2 gas water vapor)

S16O4

+ H2

18O = S18O4

+ H2

16O(dissolved sulphate-water)

1000 ln (SO4 H2O) = 2.88 x 106/T2 4.1(T = degree Kelvin = K)

I t G th t


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Isotope Geothermometers

Regarding the relation between mineralization

and hydrothermal activities

Mineral Isotope Geothermometers

Based on the isotopic equilibrium between

the coeval mineral pairs

Most commonly used isotopes: S-isotopes

S h (S) I t


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Suphur (S)- Isotopes

32S = 95.02 %33S = 0.75 %34S = 4.21 %36S = 0.02 %

(34S/32S)sample- (34S/32S)std.34S () = -------------------------------------------- x 103

(34S/32S)sample

Std.= CD

=S-isotope composition of troilite (FeS) phase in Canyon DiabloMeteorite

S-Isotope Geothermometer


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S-Isotope Geothermometer

34S = 34S(mineral 1) - 34S(mineral 2)34S = 34S= A (106/T2) + B

800 400 200 150 100 50Temperature C0


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Pyrite-Galena

0

4

8

12

4

8

0

4

2

Sphalerite-Galena

Pyrite-Sphalerite

jeo term o metre

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