GRRM/How To/Character

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Applying the Methodology to Geothermal Projects




Character Indices
Temperature | Reservoir | Fluid Chemistry | Volume

The following tables provide the qualitative and quantitative content that differentiate character index values. For each attribute (temperature, volume, reservoir, and fluid chemistry), select the 1 - 5 index value of the category that most closely reflects the project data.

Character Index: Estimated Temperature
Value Range Description
5 >300°C High-temperature two-phase liquid dominated
OR high enthalpy vapor-dominated
4 230 - <300°C Two-phase liquid-dominated systems:
  • high T, high enthalpy
  • moderate T, moderate enthalpy
3 150 - <230°C Moderate to low temperature, moderate to low enthalpy liquid-only systems
2 90 - <150°C Low temperature systems
1 <90°C Very low temperature systems
Character Index: Reservoir
Value Range - Estimated Permeability Value Range - Alteration Value Range - Feature Distribution
5 102 - 104 darcy (e.g. permeable basalt, Karst limestone) Alteration creating porosity at reservoir depths Nearly uniform matrix permeability; fractures widely or consistently distributed
4 1 - <102 darcy (e.g. fractured igneous & metamorphic) Minimal alteration; mineral reaction rims Fractures moderately distributed; Mostly high matrix permeability
3 10-2- <1 darcy (e.g. sandstone, limestone) Some alteration; some secondary mineral replacement Fractures or faults inconsistently distributed
2 10-8 - <10-2 darcy (e.g. non-welded and welded tuffs) Significant alteration: extensive replacement in some areas/minerals Singular or few dominant faults
1 <10-8 darcy (e.g. consolidated unfractured igneous & metamorphic shale) Fully altered: all secondary minerals, all vesicles filled Singular or few dominant faults; little to no matrix permeability
Character Index: Fluid Chemistry
Value Range - pH (at STP) Value Range - Bicarbonate Value Range - Total Dissolved Solids
5 7.0 7.0 - 7.5 RSI <1,000 ppm
4 5.6 - <7 or >7 - 8.4 6.0 - <7.0 RSI 1,000 - <5,000 ppm
3 4.5 - <5.6 or >8.4 - 10 5.0 - <6.0 RSI 5,000 - <20,000 ppm
2 3 - <4.5 or >10-11 4.0 - <5.0 RSI 20,000 - 100,000 ppm
1 <<3 or >11 <4.0 RSI >100,000 ppm
Character Index: Estimated Volume
Value Range Description
5 >20 km3 Extremely Large
4 >10 - 20 km3 Very Large
3 >5 - 10 km3 Large
2 >2.5 - 5 km3 Moderate
1 <= 2.5 km3 Small


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