petrophysics · formation water
Formation Water Resistivity from SP
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Inputs
mV
Ω·m
°F
Description
Estimates formation water resistivity from the SP log's static deflection (SSP), independent of salinity or produced-water data — a genuinely distinct data source from rw_salinity. The SSP deflection is converted to an equivalent water resistivity (Rwe) via the standard SP relationship, then treated as an approximation of true Rw. Useful when no water sample or salinity catalog exists but a clean SP log is available.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Rw | Ω·m | Formation Water Resistivity |
| SSP | mV | Static SP deflection read from a thick, clean, permeable bed against an adjacent shale baseline — typically negative when the mud filtrate is fresher than the formation water. |
| Rmf | Ω·m | Mud filtrate resistivity, already corrected to formation temperature (use rw_temp if only a surface/mud-report value is available). Read from the log header or mud report. |
| T | °F | Formation (bottom hole) temperature at the depth of interest — typically BHT from the log header or a temperature-gradient estimate. |
Assumptions
- SSP is read from a genuinely clean, thick, permeable bed with a properly established shale baseline — not raw SP without shale-baseline correction
- Rmf is already corrected to formation temperature (use rw_temp first if only a surface-temperature value is available)
- Mud filtrate and formation water are both predominantly NaCl — the K constant and Rmfe approximation are calibrated for NaCl-type fluids
Limitations
- The Rwe≈Rw simplification used here is a documented approximation, most reliable for Rw above ~0.1 Ω·m at formation temperature; a full chart-based Rwe-to-Rw correction (needed for more saline/lower-Rw formations) is not modeled — disclosed here rather than fabricated, per this project's practice of flagging unverified chart steps instead of guessing at unpublished constants
- The Rmfe≈0.85×Rmf approximation degrades for saline mud systems (Rmf ≤ 0.1 Ω·m) where the full Schlumberger chart relationship is non-linear
- SP response can be suppressed or distorted in shaly/laminated sands, thin beds, or hydrocarbon-bearing zones — SSP should be read from the cleanest, thickest available bed
- No independently published numeric worked example with a stated Rw result was located during research to classify this as a published-tier vector; shipped as verified-tier, computed directly from the documented formula chain, consistent with this project's established Waxman-Smits/Newman precedent for disclosing rather than dropping a citation gap
Use Cases
- → Rw estimation without a water sample: Derive a working Rw for the Archie equation from the SP log alone when no produced-water sample or salinity catalog exists for the field.
- → Cross-check against salinity-derived Rw: Compare this SP-derived Rw against rw_salinity's output for the same zone — a genuinely independent data source (log response vs. water chemistry) that should agree within the method's known uncertainty.
- → New-field Rw screening: In an exploration well with no offset water data, use the SP log as the first available Rw estimate before core or production data becomes available.
Related Calculations
Region Notes
Gulf Coast
Classic thick, clean Gulf Coast sands with fresh mud filtrate give some of the most reliable SSP readings for this method — the setting the SP method was originally developed for.
Permian Basin
Thin, interbedded Wolfcamp/Spraberry sands often suppress SSP deflection relative to a true clean-bed reading — cross-check against rw_salinity rather than relying on SP alone in these settings.
Global
SP-derived Rw is most useful as a cross-check alongside, not a replacement for, salinity- or sample-based Rw — treat significant disagreement between methods as a flag to re-examine input log quality rather than averaging the two.
References
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