petrophysics · water saturation
Apparent Water Resistivity (Rwa)
click formula to derive ↑
Inputs
Ω·m
fraction
—
—
Description
A rapid quick-look hydrocarbon indicator: rearranges the Archie equation to solve for the water resistivity that WOULD explain the observed Rt if the formation were 100% water-saturated (Sw=1). Comparing this apparent Rwa to a known or estimated formation Rw flags likely hydrocarbon zones (Rwa >> Rw) before a full Sw calculation is run — a different workflow step from sw_archie, not a saturation output itself.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Rwa | Ω·m | Apparent Water Resistivity |
| Rt | Ω·m | True formation resistivity from a deep-reading resistivity curve, ideally corrected for invasion. |
| φ | fraction | Porosity at the depth of interest — effective or total, same curve used elsewhere in the Sw workflow. |
| m | — | Cementation exponent — same as Archie; 2.0 is the clean-sand default, calibrate from core when possible. |
| a | — | Tortuosity/cementation constant — same as Archie; 1.0 is the clean-sand default. |
Assumptions
- φ, m, and a are reasonably well known for the formation, independent of any Sw assumption
- Rt is corrected for invasion and represents true, undisturbed formation resistivity
- A reference Rw (from rw_salinity, rw_temp, sp_rw, or a water sample) is available to compare Rwa against — this calculator does not itself judge water vs. hydrocarbon, only computes Rwa
Limitations
- Not a saturation calculation — provides no Sw value, only a comparison basis; use sw_archie (or a shaly-sand model) for an actual Sw once Rw is confirmed
- In shaly sands, conductive clay lowers Rt independent of Sw, which can make a genuinely low-Sw (hydrocarbon) zone show a deceptively low Rwa — same limitation as clean-formation Archie Sw
- Sensitive to the same m/a choices as Archie Sw; miscalibrated exponents shift Rwa the same way they shift Sw
Use Cases
- → Rapid log-length hydrocarbon screening: Compute Rwa continuously down a well and flag intervals where it departs from the expected regional Rw, before committing to specific a/m/n values for a full Sw run.
- → Rw sanity check: In a known water-bearing interval, Rwa should closely match an independently derived Rw (from salinity or SP) — a mismatch signals a porosity, resistivity, or exponent problem worth resolving before trusting Sw elsewhere in the well.
- → Pay zone triage across multiple wells: Compare Rwa profiles across offset wells sharing a common Rw to quickly rank which intervals warrant a full Sw and net pay workup.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Spraberry: with Rw≈0.03-0.08 Ω·m at BHT, a computed Rwa above roughly 0.15-0.20 Ω·m is a strong quick-look hydrocarbon flag before running full Sw.
Gulf of Mexico
Unconsolidated Miocene/Pliocene sands: use the same a≈0.62, m≈2.15 set as sw_archie's regional guidance so Rwa stays directly comparable to locally derived Rw values.
Global
Rwa quick-look screening is only as useful as the reference Rw it's compared against — always establish Rw independently (salinity, SP, or water sample) for the field before relying on Rwa alone.
References
Primary source
Need geoscience support? BauerCalc is just one of the tools we build. If you're looking for independent expertise in well planning, geosteering, reservoir characterization, or opportunity evaluation, visit BauerSubsurface.com.