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GEOCHEMISTRY OF
GEOTHERMAL SYSTEMS
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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
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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
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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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SILICA GEOTHERMOMETERS
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
SILICA GEOTHERMOMETERS
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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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SILICA GEOTHERMOMETERS
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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SILICA GEOTHERMOMETERS
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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SILICA GEOTHERMOMETERS
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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SILICA GEOTHERMOMETERS
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 GEOTHERMOMETERS
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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SILICA GEOTHERMOMETERS
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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SILICA GEOTHERMOMETERS
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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SILICA GEOTHERMOMETERS
Process Reservoir Temperature
Steam Separation Overestimated
Silica Precipitation
UnderestimatedIncrease in pH Overestimated
Mixing with cold water Underestimated
CATION GEOTHERMOMETERS
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CATION GEOTHERMOMETERS
(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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CATION GEOTHERMOMETERS
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).
CATION GEOTHERMOMETERS
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CATION GEOTHERMOMETERS
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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CATION GEOTHERMOMETERS
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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CATION GEOTHERMOMETERS
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)
CATION GEOTHERMOMETERS
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CATION GEOTHERMOMETERS
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
CATION GEOTHERMOMETERS
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CATION GEOTHERMOMETERS
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)
CATION GEOTHERMOMETERS
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CATION GEOTHERMOMETERS
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).
UNDERGROUND MIXING OF
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UNDERGROUND MIXING OF
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.
SILICA ENTHALPY MIXING
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SILICA-ENTHALPY MIXINGMODEL
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)
SILICA ENTHALPY MIXING
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SILICA-ENTHALPY MIXINGMODEL
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
SILICA ENTHALPY MIXING
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SILICA-ENTHALPY MIXINGMODEL
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)
SILICA ENTHALPY MIXING
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SILICA-ENTHALPY MIXINGMODEL
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).
CHLORIDE ENTHALPY MIXING
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CHLORIDE-ENTHALPY MIXINGMODEL
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)
CHLORIDE ENTHALPY MIXING
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CHLORIDE-ENTHALPY MIXINGMODEL
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.
CHLORIDE ENTHALPY MIXING
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CHLORIDE-ENTHALPY MIXINGMODEL
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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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
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
Sources of Geothermal Fluids
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Sources of Geothermal Fluids
StableH- & O-Isotopes1H = % 99.98522H (D) = % 0.0148
D/H
16O = % 99.7617O = % 0.0418O = % 0.2018O / 16O
Sources of Geothermal Fluids
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Sources of Geothermal Fluids
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
Sources of Geothermal Fluids
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Sources of Geothermal Fluids
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
Sources of Geothermal Fluids
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Sources of Geothermal Fluids
StableH- & O-Isotopes
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
Sources of Geothermal Fluids
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Sources of Geothermal Fluids
StableH- & O-Isotopes
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
Sources of Geothermal Fluids
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Sources of Geothermal Fluids
StableH- & O-Isotopes
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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Sources of Geothermal Fluids
StableH- & O-Isotopes
0
-40
-80
-120-12 -8 -4 0
del- O (per mil)18
+SMOW
Condensation
Evaporation
Water-RockInteraction
Sources of Geothermal Fluids
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StableH- & O-Isotopes
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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Physico-Chemical Processes:
Stable H- & O-Isotopes
Latitute D 18OAltitute from Sea levelD 18O
Physico-Chemical Processes:
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Physico-Chemical Processes:
Stable H- & O-Isotopes
Aquifers recharged by precipitation from
lower altituteshigherD - 18OvaluesAquifers recharged by precipitation from
higher altituteslowerD - 18OvaluesMixing of waters from different aquifers
Physico-Chemical Processes:
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Physico Chemical Processes:
Stable H- & O-Isotopes
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