GRRM/How To/Execution/Temperature

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Applying the Methodology to Geothermal Projects




Execution Indices for Estimated Temperature
Subsurface Temperature Probe | Thermal Gradient Hole | Cation Geothermometry| Isotope Geothermometry | Silica Phase Geothermometry | Heat Flow

The following tables provide the details to select the appropriate execution index values for the GRRM attribute temperature. For each technique used to estimate temperature, select the 1 - 5 index value of the category that most closely reflects the understanding of how the analytical method and technique were followed that collected the project's data.

Note: if more than 1 technique is used, there will be as many execution index values as techniques, at minimum. If multiple sources of data are used (such as literature searches and independent sampling), there will be an execution index value for each source.

Subsurface Temperature Probe Thermal Gradient Hole Geothermometry: cation
5
  • Probe allowed to equilibrate
  • Cuttings and/or geophysics confirms measurement within the reservoir (i.e. downhole alteration mineralogy consistent with reading)
  • Analytical quality of results can be shown to be high (based on sampling replication and instrument calibration logs).
  • Reconnaissance completed in ordered grids and transects along known faults.
  • Geothermometer and other geochemical data confirm TGH resource temperature.
  • Results are from stabilized logs, not initial readings.
  • Evaluated for mixing and boiling through multiple iterations of different chemical relationships.
  • Interpreted in combination with high-quality data on other physical parameters (pH, dissolved gases)
  • The sample well does not allow mixing with cold, shallow waters
  • Use of Na-K-Ca-Mg, K-Mg, and/or Li-Mg systems to corroborate other geothermometers
  • Cation and anion balance shows minimal gap in vast majority of samples.
4
  • Probe allowed to equilibrate
  • Cuttings and/or geophysics have not confirmed measurement within the reservoir (i.e. downhole alteration mineralogy not consistent with readings)
  • Reconnaissance completed in either ordered grids or transects across faults.
  • Geothermometers and/or other geochemical data constrain extrapolated TGH resource temperature
  • Results are from stabilized logs, not initial readings
  • Appropriate corrections are made to determine end members when fluid is known to have mixed with other water sources.
  • Appropriate selection of reaction systems:
    • Na-K: highest temperatures in deep wells
    • Na-K-Ca: separate equations for <100 °C and >100 °C
  • Use of Na-K-Ca-Mg, K-Mg, and/or Li-Mg systems to corroborate other geothermometers.
3
  • Probe not allowed to equilibrate
  • Cuttings and/or geophysics have not confirmed measurement within the reservoir
  • Linear extrapolation suggests anomalously high temperatures in comparison to nearby locations.
  • Minimal probes placed given complexity of underlying geology.
  • Results are from initial readings, with minimal or no time between drilling for “equilibrated” logs.
  • No (or limited) corrections made even if:
    • fluid is known to have mixed with other water sources, or
    • partial pressures of CO2 and calcite precipitation are significant
  • Application without any recognition of the appropriate geologic setting (e.g. use of magmatic-volcanic systems in the Basin and Range)
  • Application to bicarbonate or acid sulfate stream waters that are derived from steam heating of near-surface waters and interaction with geothermal gases, and where the fluid cation chemistry does not reflect equilibrium with minerals at reservoir conditions.
  • Cation and anion balance shows significant gap in majority of samples, without a functional explanation.
2
  • Results taken from previous third-party studies of the area (either literature or contractors) with little or limited information on survey methods, replication, or error.
1
  • Assumed from studies of analogous geothermal settings, or extrapolated from studies of nearby areas.
Geothermometry: isotope Geothermometry: SiO2 phases Heat flow - 2m probes
5
  • Multiple isotope systems (hydrogen, carbon, oxygen, and/or sulphur) provide narrowly constrained temperature.
  • Analytical quality of results can be shown to be high (based on standards measured, sample replication, and calibration logs).
  • Steam and water discharge are both collected without air contamination.
  • Corrections for the following effects:
    • pH effects on silica solubility when pH > 9.
    • Salinity effects corrected for waters higher than seawater
    • Mixing /dilution effects with other sources (groundwater or surface)
    • Precipitation losses after sampling
  • Concentrations plotted against enthalpy to confirm
  • Appropriate phase selection: (<180 °C, chalcedony or quartz; 200 - 300 °C, quartz)
  • Analytical quality of results can be shown to be high (based on standards measured, sample replication, and calibration logs).
  • Reconnaissance completed in ordered grids and transects across known faults and/or other structural features.
  • Use of high-quality probe (such as thermocouple) with measurements of instrument error.
  • Use in areas where the groundwater aquifer is not above the geothermal system.
  • Results with significant standard deviations (outliers) eliminated prior to extrapolation to equilibrium.
4
  • Some, but not all of multiple isotope systems (hydrogen, carbon, oxygen, and/or sulphur) provide similar temperatures .
  • Corrections can be made for mixing /dilution effects with other water sources (groundwater or surface) - particularly relevant for oxidation of H2S.
  • All of the data correction best practices listed above.
  • Not plotted against enthalpy
  • Appropriate phase selection: (<180 °C, chalcedony or quartz; 200 - 300 °C, quartz)
  • Reconnaissance completed in either ordered grids or transects across faults.
  • Use of standard quality probes (thermistor or thermocouple) with measurements of instrument error.
  • Use in areas where the groundwater aquifer location has not been determined in respect to the geothermal system.
3
  • Multiple isotope systems (hydrogen, carbon, oxygen, and/or sulphur) provide narrowly constrained temperature.
  • Mixing /dilution effects with other water sources are not well understood or corrected for.
  • Steam and water discharge are not separated completely, or have evidence of air contamination.
  • Not enough information available to implement data correction best practices (e.g. erroneous pH, not enough information to identify dissolved silica or proportion of steam separated).
  • Significant, unexplained differences in sample results.
  • Not plotted against enthalpy.
  • Possibly inappropriate phase selection: (chalcedony at near or >180 °C, or quartz at near or >300 °C).
  • Use in areas where groundwater aquifer overlies the geothermal system.
  • Incomplete penetration due to sediment stiffness or unconsolidation.
  • Minimal probes placed without consideration of underlying geology.
  • Samples taken after likely reservoir dilution/mixing (i.e. spring snow melt, recent irrigation) and extrapolated to depth.
2
  • Results taken from previous third-party studies of the area (either literature or contractors) with little or limited information on survey methods, replication, or error.
1
  • Assumed from studies of analogous geothermal settings, or extrapolated from studies of nearby areas.