GEOCHEMISTRY OF GEOTHERMAL SYSTEMS

1
GEOCHEMISTRY OFGEOTHERMAL SYSTEMS
2
WATER CHEMISTRY

• Chemical composition of waters is expressed in
  terms of 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 (pHgt8) conditions
• H2CO3 ? dominant in acidic conditions
• Also dissolved silica (SiO2) in neutral form
• as a major constituent
• Minor constituents B, F, Li, Sr, ...

3
WATER CHEMISTRY

• concentration of chemical constituents are
  expressed in units of
• mg/l (ppmparts per million)
• (mg/l is the preferred unit)
• Molality
• Molality no. of moles / kg of solvent
• No.of moles (mg/l10-3) / formula weight

4
WATER CHEMISTRY

• Errors associated with water analyses are
  expressed in terms of CBE (Charge Balance Error)
• CBE () ?( ?z x mc - ?z x ma ) / (?z x mc
  ?z x ma )? 100
• where,
• 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 of interpretation

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The constituents encountered in geothermal fluids

• TRACERS
• Chemically inert, non-reactive, conservative
  constituents
• (once added to the fluid phase, remain unchanged
  allowing their 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 the physico-chemical processes during
  the ascent of water to surface, also used in
  geothermometry applications)
• e.g. Na, K, Mg, Ca, SiO2

6
WATER CHEMISTRY

• In this chapter, the main emphasis will be placed
  on the use of water chemistry in the
  determination of
• underground (reservoir) temperatures
  geothermometers
• boiling and mixing relations (subsurface
  physico-chemical processes)

7
HYDROTHERMAL REACTIONS

• The composition of geothermal fluids are
  controlled by temperature-dependent reactions
  between minerals and fluids
• The factors affecting the formation of
  hydrothermal minerals are
• temperature
• pressure
• rock type
• permeability
• fluid composition
• duration of activity

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• The effect of rock type --- most pronounced at
  low temperatures insignificant above 280?C
• Above 280?C and at least as high as 350?C, the
  typical stable mineral assemblages (in active
  geothermal systems) are independent of rock type
  and include
• ALBITE, K-FELDSPAR, CHLORITE, Fe-EPIDOTE,
  CALCITE, QUARTZ, ILLITE PYRITE
• At lower temperatures, ZEOLITES and CLAY
  MINERALS are found.
• At low permeabilities equilibrium between rocks
  and fluids is seldom achieved.
• When permeabilities are relatively high and water
  residence 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 TEMPERATURES

• The evaluation of the reservoir temperatures for
  geothermal systems is made in terms of
  GEOTHERMOMETRY APPLICATIONS

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GEOTHERMOMETRY APPLICATIONS
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GEOTHERMOMETRY APPLICATIONS

• One of the major tools for the ?
• exploration development
• of geothermal resources

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GEOTHERMOMETRY

• estimation of reservoir (subsurface) temperatures
• using
• Chemical isotopic composition of surface
  discharges from
• wells and/or
• natural springs/fumaroles

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GEOTHERMOMETERS

• CHEMICAL GEOTHERMOMETERS
• utilize the chemical composition
• silica and major cation contents of water
  discharges
• gas concentrations or relative abundances of
  gaseous components in steam discharges
• ISOTOPIC GEOTHERMOMETERS
• based 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 GEOTHEROMOMETERS

• PART I. Basic Principles Types

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BASIC PRINCIPLES

• Chemical Geothermometers are
• developed on the basis of temperature dependent
  chemical equilibrium between the water and the
  minerals at the deep reservoir conditions
• based on the assumption that the water preserves
  its chemical composition during its ascent from
  the reservoir to the surface

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BASIC PRINCIPLES

• Studies of well discharge chemistry and
  alteration mineralogy
• the presence of equilibrium in several geothermal
  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 be affected
  by processes such as
• cooling
• mixing with waters from different reservoirs.

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BASIC PRINCIPLES

• Cooling during ascent from reservoir 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 hydrostatic head

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BASIC PRINCIPLES

• Conductive cooling
• does not by itself change the composition of the
  water
• but may affect its degree of saturation with
  respect to several minerals
• thus, it may bring about a modification in the
  chemical composition of the water by mineral
  dissolution or precipitation

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BASIC PRINCIPLES

• Adiabatic cooling (Cooling by boiling)
• causes changes in the composition of ascending
  water
• these changes include
• degassing, and hence
• the increase in the solute content as a result of
  steam loss.

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BASIC PRINCIPLES

• MIXING
• affects chemical composition
• since the solubility of most of the compounds in
  waters increases with increasing temperature,
  mixing with cold groundwater results in the
  dilution 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 chemical processes involved
  in geothermal systems.
• The main task of geochemist is to verify or
  disprove the validity of assumptions made in
  using specific geothermometers in specific fields.

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TYPES OF CHEMICAL GEOTHERMOMETERS

• 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

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SILICA GEOTHERMOMETERS
Geothermometer Equation Reference
Quartz-no steam loss T 1309 / (5.19 log 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 range in which the equations are
  valid
• effects of steam separation
• possible precipitation 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 of pH on solubility of silica
• possible mixing of hot water with cold water

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SILICA GEOTHERMOMETERS

• Temperature Range
• silica geothermometers are valid for temperature
  ranges up to 250 ?C
• above 250?C, the equations depart drastically
  from the experimentally determined solubility
  curves

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SILICA GEOTHERMOMETERSTemperature Range

• Fig.1. Solubility of quartz (curve A) and
  amorphous silica (curve C) as a function of
  temperature at the vapour pressure of the
  solution. Curve B shows the amount of silica that
  would be in solution after an initially
  quartz-saturated solution cooled adiabatically to
  100 ?C without any precipitation of silica (from
  Fournier and Rowe, 1966, and Truesdell and
  Fournier, 1976).
• At low T (?C) ?
• qtz less soluble
• amorph. silica more soluble
• Silica solubility is controlled by amorphous
  silica at low T (?C) quartz at high T (?C)

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SILICA GEOTHERMOMETERSEffects of Steam Separation

• Boiling ? Steam Separation
• volume of residual liquid?
• Concentration in liquid?
• Temperature Estimate?
• e.g.
• T 1309 / (5.19 log C) - 273.15
• C SiO2 in ppm
• increase in C (SiO2 in water gt SiO2 in reservoir)
  
• decrease in denominator of the equation
• increase in T
• for boiling springs
• boiling-corrected geothermometers
• (i.e. Quartz-max. steam loss)

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SILICA GEOTHERMOMETERSSilica Precipitation

• SiO2 ?
• Temperature Estimate?
• e.g.
• T 1309 / (5.19 log C) - 273.15
• C SiO2 in ppm
• decrease in C (SiO2 in water lt SiO2 in reservoir)
  
• increase in denominator
• decrease in T

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SILICA GEOTHERMOMETERSEffect of pH

• Fig. 2. Calculated effect of pH upon the
  solubility of quartz at various temperatures from
  25 ?C to 300 ?C , using experimental data of
  Seward (1974). The dashed curve shows the pH
  required at various temperatures to achieve a 10
  increase in quartz solubility compared to the
  solubility at pH7.0 (from Fournier, 1981).
• pH ?
• Dissolved SiO2 ? (for pHgt7.6)
• Temperature Estimate?
• e.g.
• T 1309 / (5.19 log C) - 273.15
• C SiO2 in ppm
• increase in C
• decrease in denominator of the equation
• increase in T

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SILICA GEOTHERMOMETERSEffect of Mixing

• Hot-Water ? High SiO2 content
• Cold-Water ? Low SiO2 content
• (Temperature ? Silica solubility ?)
• Mixing (of hot-water with cold-water)
• Temperature?
• SiO2 ?
• Temperature Estimate ?
• e.g.
• T 1309 / (5.19 log C) - 273.15
• C 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 ? Underestimated
• Increase in pH ? Overestimated
• Mixing with cold water ? Underestimated

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CATION GEOTHERMOMETERS (Alkali Geothermometers)

• based on the partitioning of alkalies between
  solid 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, ..)

38
CATION GEOTHERMOMETERSNa/K Geothermometer

• Fig.3. Na/K atomic ratios of well discharges
  plotted at measured downhole temperatures. Curve
  A is the least square fit of the data points
  above 80 ?C. Curve B is another empirical curve
  (from Truesdell, 1976). Curves C and D show the
  approximate locations of the low
  albite-microcline and high albite-sanidine lines
  derived from thermodynamic data (from Fournier,
  1981).

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CATION GEOTHERMOMETERSNa/K Geothermometer
Geotherm. Equations Reference
Na-K T855.6/(0.857log(Na/K))-273.15 Truesdell (1976)
Na-K T833/(0.780log(Na/K))-273.15 Tonani (1980)
Na-K T933/(0.993log (Na/K))-273.15 (25-250 oC) Arnorsson et al. (1983)
Na-K T1319/(1.699log(Na/K))-273.15 (250-350 oC) Arnorsson et al. (1983)
Na-K T1217/(1.483log(Na/K))-273.15 Fournier (1979)
Na-K T1178/(1.470log (Na/K))-273.15 Nieva and Nieva (1987)
Na-K T1390/(1.750log(Na/K))-273.15 Giggenbach (1988)
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CATION GEOTHERMOMETERSNa/K Geothermometer

• gives good results for reservoir temperatures
  above 180?C.
• yields erraneous estimates for low temperature
  waters
• temperature-dependent exchange equilibrium
  between feldspars and geothermal waters is not
  attained at low temperatures and the Na/K ratio
  in these waters are governed by leaching rather
  than chemical equilibrium
• yields unusually high estimates for waters having
  high calcium contents

41
CATION GEOTHERMOMETERSNa-K-Ca Geothermometer
Geotherm. Equations Reference
Na-K-Ca T1647/ (log (Na/K) ? (log (?Ca/Na)2.06) 2.47) -273.15 a) if ?log?Ca/Na)2.06? lt 0, use ?1/3 and calculate T?C b) if ?log?Ca/Na)2.06? gt 0, use ?4/3 and calculate T?C c) if calculated T gt 100?C in (b), recalculate T?C using ?1/3 Fournier and Truesdell (1973)
42
CATION GEOTHERMOMETERSNa-K-Ca Geothermometer

• Works well for CO2-rich or Ca-rich environments
  provided that calcite was not deposited after the
  water left the reservoir
• in case of calcite precipitation
• Ca ?
• 1647
• T --------------------------------------------
  ------------- - 273.15
• log (Na/K) ? (log (?Ca/Na)2.06) 2.47
• Decrease in Ca concentration (Ca in water lt Ca
  in reservoir)
• decrease in denominator of the equation
• increase in T
• For waters with high Mg contents, Na-K-Ca
  geothermometer yields erraneous results. For
  these waters, Mg correction is necessary

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CATION GEOTHERMOMETERSNa-K-Ca-Mg Geothermometer
Geotherm. Equations Reference
Na-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 / T2 if R from 5 to 50 ?tMgoC10.66-4.7415 log R325.87(log R)2-1.032x105(log R)2/T-1.968x107(log R)3/T2 Note Do not apply a Mg correction if ?tMg is negative or Rlt1.5. If Rgt50, assume a temperature measured spring temperature. T is Na-K-Ca geothermometer temperature in Kelvin Fournier and Potter (1979)
44
CATION GEOTHERMOMETERSNa-K-Ca-Mg Geothermometer

• Fig. 4. Graph for estimating the magnesium
  temperature correction to be subtracted from
  Na-K-Ca calculated temperature (from Fournier,
  1981)
• R (Mg/Mg 0.61Ca 0.31K)x100

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UNDERGROUND MIXING OF HOT AND COLD WATERS

• Recognition of Mixed Waters
• Mixing of hot ascending waters with cold waters
  at shallow depths is common.
• Mixing also occurs deep in hydrothermal systems.
• The effects of mixing on geothermometers is
  already discussed 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 or partial re-equilibration is
  more likely if the temperatures after mixing is
  well above 110 to 150 ?C, or if mixing takes
  place in aquifers with long residence times).

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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 conservative
  elements that do not precipitate from solution
  during movement of water through rock (e.g. Cl/B
  ratios).

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SILICA-ENTHALPY MIXING MODEL

• Dissolved silica content of mixed waters can be
  used to determine the temperature of hot-water
  component .
• Dissolved silica is plotted against enthalpy of
  liquid water.
• Although temperature is the measured property,
  and enthalphy is a derived property, enthalpy is
  used as a coordinate rather than temperature.
  This is because the combined heat contents of two
  waters are conserved when those waters are mixed,
  but the combined temperatures are not.
• The enthalpy values are obtained from steam
  tables.

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SILICA-ENTHALPY MIXING MODEL

• Fig. 5. Dissolved silica-enthalpy diagram
  showing procedure for calculating the initial
  enthalpy (and hence the reservoir temperature) of
  a high temperature water that has mixed with a
  low temperature water (from Fournier, 1981)

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SILICA-ENTHALPY MIXING MODEL

• A non-thermal component
• (cold water)
• B, D mixed, warm water
• springs
• C hot water component at
• reservoir conditions
• (assuming no steam
• separation before mixing)
• E hot water component at
• reservoir conditions
• (assuming steam separation
• before mixing)
• Boiling
• T 100 ?C
• Enthalpy 419 J/g

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SILICA-ENTHALPY MIXING MODELSteam Fraction did
not separate before mixing

• The sample points are plotted.
• A straight line is drawn from the point
  representing the non-thermal component of the
  mixed water (i.e. the point with the lowest
  temperature and the lowest silica content point
  A in Fig.), through the mixed water warm springs
  (points B and D in Fig.).
• The intersection of this line with the qtz
  solubility curve (point C in Fig.) gives the
  enthalpy of the hot-water component (at reservoir
  conditions).
• From the steam table, the temperature
  corresponding to this enthalpy value is obtained
  as the reservoir temperature of the hot-water
  component.

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SILICA-ENTHALPY MIXING MODELSteam separation
occurs before mixing

• The enthalpy at the boling temperature (100?C) is
  obtained from the steam tables (which is 419 j/g)
• A vertical line is drawn from the enthalpy value
  of 419 j/g
• From the inetrsection point of this line with the
  mixing line (Line AD), a horizantal line (DE) is
  drawn.
• The intersection of line DE with the solubility
  curve for maximum steam loss (point E) gives the
  enthalpy of the hot-water component.
• From the steam tables, the reservoir temperature
  of the hot-water component is determined.

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SILICA-ENTHALPY MIXING MODEL

• In order for the silica mixing model to give
  accurate results, it is vital that no conductive
  cooling occurred after mixing. If conductive
  cooling occurred after mixing, then the
  calculated temperatures 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 at the same
  silica concentrations, as conductive cooling will
  affect only the temperatures, not the silica
  contents)
• in this case, the intersection of mixing line
  with the quartz solubility curve will give lower
  enthalpy values (i.e lower temperatures) than
  that obtained in case of conductive cooling.
• in other words, the temperatures obtained in case
  of conductive cooling will be higher than the
  actual reservoir temperatures (i.e. if conductive
  cooling occurred after mixing, the temperatures
  will be overestimated)

53
SILICA-ENTHALPY MIXING MODEL

• Another requirement for the use of
  enthalpy-silica model is that no silica
  deposition occurred before or after mixing. If
  silica deposition occurred, the temperatures will
  be underestimated. This is because
• the original points before silica deposition
  should be towards higher silica contents (at the
  same enthalpy values)
• in this case, the intersection point of mixing
  line with the silica solubility curve will have
  higher enthalpy values(higher temperatures) than
  that obtained in case of silica deposition
• in other words, the temperatures obtained in case
  of no silica deposition will be higher than that
  in case of silica deposition (i.e. the
  temperatures will be underestimated in case of
  silica deposition)

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CHLORIDE-ENTHALPY MIXING MODEL

• Fig.6. Enthalpy-chloride diagram for waters from
  Yellowstone National Park. Small circles indicate
  Geyser Hill-type waters and smal dots indicate
  Black Sand-type waters (From Fournier, 1981).

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CHLORIDE-ENTHALPY MIXING MODEL

• ESTIMATION OF RESERVOIR
• TEMPERATURE
• Geyser Hill-type Waters
• A maximum Cl content
• B minimum Cl content
• C 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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CHLORIDE-ENTHALPY MIXING MODEL

• ORIGIN OF WATERS
• N cold water component
• C, F hot water components
• F is more dilute slightly cooler than C
• F can not be derived from C by process of mixing
  between hot and cold water (point N), because any
  mixture would lie on or close to line CN.
• C and F are probably both related to a still
  higher enthalpy water such as point G or H.

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CHLORIDE-ENTHALPY MIXING MODEL

• ORIGIN OF WATERS
• water C could be related to water G by boiling
• water C could also be related to water H
• by conductive cooling
• water F could be related to water G or water H by
  mixing with cold water N

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ISOTOPES IN GEOTHERMAL EXPLORATION
DEVELOPMENT
61
ISOTOPE STUDIES IN GEOTHERMAL SYSTEMS

• At Exploration, Development and Exploitation
  Stages
• Most commonly used isotopes
• Hydrogen (1H, 2H D, 3H)
• Oxygen (18O, 16O)
• Sulphur (32S, 34S)
• Helium (3He, 4He)

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ISOTOPE STUDIES IN GEOTHERMAL 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 fluid
  composition
• H- O- Isotopes
• Sources of components (elements, compounds) in
  geothermal fluids
• He-Isotopes (volatile elements)

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Sources of Geothermal Fluids and
Physico-Chemical Processes

• STABLE
• H- O-ISOTOPES

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Sources of Geothermal Fluids Stable H-
O-Isotopes

• 1H 99.9852
• 2H (D) 0.0148
• D/H
• 16O 99.76
• 17O 0.04
• 18O 0.20
• 18O / 16O

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Sources of Geothermal Fluids Stable H-
O-Isotopes

• (D/H)sample-
  (D/H)standard
• ? D (?) ----------------------------------- x
  103
• (D/H)standard
• (18O/16O)sample-
  (18O/16O)standard
• ? 18O (?) --------------------------------------
  ------ x 103
• 
  (18O/16O)standard
• Standard Standard Mean Ocean Water
• SMOW

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Sources of Geothermal Fluids Stable H-
O-Isotopes

• (D/H)sample- (D/H)SMOW
• ? D (?) ----------------------------------- x
  103
• (D/H)SMOW
• (18O/16O)sample-
  (18O/16O)SMOW
• ? 18O (?) --------------------------------------
  ------ x 103
• (18O/16O)SMOW

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Sources of Geothermal Fluids Stable H-
O-Isotopes

• Sources of Natural Waters
• Meteoric Water (rain, snow)
• Sea Water
• Fossil Waters (trapped in sediments in sedimanary
  basins)
• Magmatic Waters
• Metamorphic Waters

69
Sources of Geothermal Fluids Stable H-
O-Isotopes
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Sources of Geothermal Fluids Stable H-
O-Isotopes
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Sources of Geothermal Fluids Stable H-
O-Isotopes
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Sources of Geothermal Fluids Stable H-
O-Isotopes
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Physico-Chemical ProcessesStable H- O-Isotopes

• Latitute ?
• dD? d18O?
• Altitute from Sea level ?
• dD? d18O?

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Physico-Chemical ProcessesStable H- O-Isotopes

• Aquifers recharged by precipitation from lower
  altitutes ?higher dD - d18O values
• Aquifers recharged by precipitation from higher
  altitutes ?lower dD - d18O values
• Mixing of waters from different aquifers

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Physico-Chemical ProcessesStable H- O-Isotopes

• Boiling and vapor separation ?
• dD? d18O? in residual liquid
• Possible subsurface boiling as a consequence of
  pressure decrease (due to continuous exploitation
  from production wells)

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Monitoring Studies in Geothermal Exploitation

• Any increase in dD - d18O values ?
• due to sudden pressure drop in production
  wells
• ?recharge from (other) aquifers fed by
  precipitation from lower altitutes
• ?subsurface boiling and vapour separation

• Aquifers recharged by precipitation from lower
  altitutes ?higher dD - d18O
• Aquifers recharged by precipitation from higher
  altitutes ?lower dD - d18O
• Boiling and vapor separation ?
• dD? d18O? in residual liquid

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Monitoring Studies in Geothermal Exploitation

• Monitoring of isotope composition of geothermal
  fluids during exploitation can lead to
  determination of, and the development of
  necessary precautions against
• Decrease in enthalpy due to start of recharge
  from cold, shallow aquifers, or
• Scaling problems developed as a result of
  subsurface boiling

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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.) will
  precipitate
• Scaling

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Dating of Geothermal Fluids

• 3H- 3He-ISOTOPES

80
Dating of Geothermal Fluids

• Time elapsed between Recharge-Discharge or
  Recharge-Sampling points (subsurface residence
  residence time)
• 3H method
• 3H-3He method

81
TRITIUM (3H)

• 3H radioactive isotope of Hydrogene (with a
  short half-life)
• 3H forms
• Reaction of 14N isotope (in the atmosphere) with
  cosmic rays
• 147N 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

82
3H Dating Method

• 3H concentration level in the atmosphere has
  shown large changes
• In between 1950s and 1960s (before and after the
  nuclear testing)
• Particularly in the northern hemisphere
• Before 1953 5-25 TU
• In 1963 ?3000 TU

83
3H Dating Method

• 3H-concentration in groundwater lt 1.1 TU
• Recharge by precipitations older than nuclear
  testing
• 3H-concentration in groundwater gt 1.1 TU
• Recharge by precipitations younger than nuclear
  testing
• NN0e-?t
  3H0 (before 1963) ? 10 TU
• 3H 3H0e-?t
  ? 0.056 y-1
• t 2003-1963 40 years
• ? 3H ? 1.1 TU

84
3H Dating Method

• APPARENT AGE
• 3H 3H0e-?t
• 3H measured at sampling point
• 3H0 measured at recharge point
• (assumed to be the initial tritium
  concentration)
• ? 0.056 y-1
• t apparent age

85
3H 3He Dating Method

• 3He 3H0 3H (D N0-N)
• 3H 3H0 e-?t (N N0e -?t)
• 3H0 3H e?t
• 3He 3H e?t - 3H 3H (e?t 1)
• t 1/? ln (3He/3H 1)
• 3He 3H present-day concentrations measured
  in water sample

86
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

87
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 H218O S18O4 H216O (dissolved
sulphate-water)
?
1000 ln ? (SO4 H2O) 2.88 x 106/T2 4.1 (T
degree Kelvin K )
88
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

89
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 Diablo Meteorite
90
S-Isotope Geothermometer

• ?34S ?34S(mineral 1) - ?34S(mineral 2)
• ?34S ??34S A (106/T2) B

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