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GEOCHEMISTRY OF GEOTHERMAL SYSTEMS

GEOCHEMISTRY OF GEOTHERMAL SYSTEMS. WATER CHEMISTRY. Chemical composition of waters is expressed in terms of major anion and cation contents. Major Cations: Na + , K + , Ca ++ , Mg ++ Major Anions: HCO 3 - (or CO 3 = ), Cl - , SO 4 = HCO 3 -  dominant in neutral conditions

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GEOCHEMISTRY OF GEOTHERMAL SYSTEMS

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  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 (pH>8) 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 (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. 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

  5. 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

  8. 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.

  9. 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)

  10. ESTIMATION OF RESERVOIR TEMPERATURES The evaluation of the reservoir temperatures for geothermal systems is made in terms of GEOTHERMOMETRY APPLICATIONS

  11. GEOTHERMOMETRY APPLICATIONS

  12. GEOTHERMOMETRY APPLICATIONS • One of the major toolsfor the exploration & development of geothermal resources

  13. GEOTHERMOMETRY • estimation of reservoir (subsurface) temperatures • using • Chemical & isotopic compositionofsurface dischargesfrom • wellsand/or • natural springs/fumaroles

  14. 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

  15. Focus of the Course • CHEMICAL GEOTHERMOMETERS • As applied to water discharges PART I. Basic Principles & Types PART II. Examples/Problems

  16. CHEMICAL GEOTHEROMOMETERS PART I.Basic Principles &Types

  17. 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

  18. BASIC PRINCIPLES • Studies of well discharge chemistry and alteration mineralogy • the presence of equilibrium in several geothermal fields • the assumption of equilibrium is valid

  19. 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.

  20. BASIC PRINCIPLES • Cooling during ascent from reservoir to surface: • CONDUCTIVE • ADIABATIC

  21. BASIC PRINCIPLES CONDUCTIVE Cooling • Heat loss while travelling through cooler rocks ADIABATIC Cooling • Boiling because of decreasing hydrostatic head

  22. 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

  23. 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.

  24. BASIC PRINCIPLES MIXING • affects chemical composition • since the solubility of most of the compounds in waters increases with increasing temperature, mixing with cold groundwaterresults in thedilution of geothermal water

  25. 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.

  26. TYPES OF CHEMICAL GEOTHERMOMETERS • SILICA GEOTHERMOMETERS • CATION GEOTHERMOMETERS (Alkali Geothermometers)

  27. 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

  28. SILICA GEOTHERMOMETERS

  29. SILICA GEOTHERMOMETERS The followings should be considered : • temperature rangein which the equations are valid • effects of steam 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 of pH on solubility of silica • possiblemixingof hot water with cold water

  30. 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

  31. 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 silicaat low T (C)quartz at high T (C)

  32. 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 > SiO2 in reservoir) decrease in denominator of the equation increase in T • for boiling springs boiling-corrected geothermometers (i.e. Quartz-max. steam loss)

  33. 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 < SiO2 in reservoir) increase in denominator decrease in T

  34. SILICA GEOTHERMOMETERSEffect of pH Fig. 2.Calculated effect of pHupon 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 pH=7.0 (from Fournier, 1981). • pH  • Dissolved SiO2  (for pH>7.6) • 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

  35. 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

  36. SILICA GEOTHERMOMETERS Process Reservoir Temperature • Steam Separation  Overestimated • Silica Precipitation  Underestimated • Increase in pH  Overestimated • Mixing with cold water  Underestimated

  37. 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).

  39. CATION GEOTHERMOMETERSNa/K Geothermometer

  40. 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 andthe 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

  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 < 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

  43. CATION GEOTHERMOMETERSNa-K-Ca-Mg Geothermometer

  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

  45. 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).

  46. 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).

  47. 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.

  48. 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)

  49. 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 (corresponds to D in the graph) Enthalpy values (at corresponding temperatures) are found from Steam Table in Henley et al.(1984)

  50. 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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