petrophysics · formation water
Rw Temperature Correction
Rw₂ = Rw₁ × (T₁ + 21.54) / (T₂ + 21.54)
click formula to derive ↑
Inputs
Ω·m
°F
°F
Description
Applies the Arps relationship to correct a water resistivity measurement from one temperature to another. Rw must always be corrected to formation (bottom-hole) temperature before being used as Rw in the Archie water saturation equation.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Rw₂ | Ω·m | Rw at T₂ |
| Rw₁ | Ω·m | Known water resistivity at the reference temperature T₁ — often from a produced-water sample, SP log analysis, or a regional water-resistivity catalog. |
| T₁ | °F | Temperature at which Rw₁ was measured or reported — often surface or lab temperature (~75°F / 24°C). |
| T₂ | °F | Target temperature to correct Rw to — normally the formation (bottom hole) temperature used in the Archie equation. |
Assumptions
- Water composition is dominated by NaCl — the Arps relationship is calibrated for sodium chloride brines and is less accurate for unusual ionic compositions
- T₁ is the actual temperature at which Rw₁ was measured, not an assumed or default surface temperature
- T₂ is representative of true formation temperature at the depth of interest (use the BHT calculator if only a gradient is known)
Limitations
- Accuracy degrades for very high salinities (>100,000 ppm) and at temperature extremes outside the typical 20-150°C calibration range
- Does not account for dissolved gas or non-NaCl ionic species (e.g., high CaCl₂ or KCl brines) which shift the temperature-resistivity relationship
- Errors in T₂ propagate directly into Sw — always cross-check T₂ against a calibrated geothermal gradient or measured BHT
Use Cases
- → SP-derived Rw correction to BHT: Correct an Rw value estimated from the SP log at a shallow reference temperature to formation temperature for use in Archie Sw.
- → Rw catalog normalization: Normalize water resistivity values from regional Rw catalogs (often reported at 75°F) to the temperature of the zone of interest.
- → Sw QC check: Cross-check whether an assumed Rw value is consistent with the produced-water salinity for the field.
Related Calculations
Region Notes
Permian Basin
Delaware Mountain Group brines are commonly very saline (Rw at 75°F often 0.02-0.04 Ω·m); correcting to BHT of ~180-200°F typically reduces Rw by roughly half.
Gulf of Mexico
Lower Tertiary reservoirs at high BHT (>250°F) can see Rw drop to a third or less of its surface-measured value — temperature correction is critical for Sw accuracy in these HPHT settings.
North Sea
Many Brent Group reservoirs use Rw catalogs referenced to 20°C (68°F); ensure the reference temperature in the catalog matches T₁ used here before correcting to BHT.
References
Primary source
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