Geothermometer Calculations for Geothermal Assessment

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Geothermometer Calculations for Geothermal
Assessment

• Marshall J. Reed Robert H. Mariner
• U.S. Geological Survey
• Menlo Park, California

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Geothermal Resource Assessment
The Quantitative Estimate of Thermal Energy in a
Geothermal Reservoir Depends on a Knowledge of
Volume, Temperature, Density Heat Capacity
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Geothermal Reservoir Temperature
The Temperature of a Geothermal Reservoir can be
Measured Directly or it can be Estimated by
Chemical Geothermometer Calculations Based on
Water Chemistry

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Silica Solubility as a Function of Temperature
Quartz green Chalcedony red Giggenbach
blue approximation
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Cation Geothermometers

• Based on Mineral Reactions as a Function of
  Temperature
• Geothermometers were Calibrated from Chemical
  Analyses of Water at Known Temperatures in Wells
• Geothermometers were Calibrated with Dilute,
  Sodium-Chloride Type Waters at Near-Neutral pH
• Differences in Chemistry between Calibration
  Waters and New Geothermal Systems Requires Extra
  Caution in Application of Geothermometers
• Injection from Power Plant Operations will
  Disrupt and Reset Geothermometers

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Temperatures Calculated with Na-K-Ca-Mg
Geothermometer
Systems listed in USGS Circ. 790
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Temperatures Calculated with K-Mg Geothermometer
Systems listed in USGS Circ. 790
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Check of Analytical Reliability

• Each Ion in the Chemical Analysis is Converted to
  its
• Equivalent Concentration using the Equation
• (analytical concentration)
• Equivalent Concentration
• (mol. wt.) (ionic charge)
• Example for Ca
• (100 mg/L)
• 4.99 mEq/L
• (40.08 g/mol) (2 equivalents/mol)

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Check of Analytical Reliability

• The Total Equivalent Concentration (in mEq/L) of
  all
• Cations S cations, and the Total Concentration
  of all
• Anions S anions
• 100 S cations - S anions
• Charge Balance Error ()
• 0.5 (S cations S anions)