petrophysics · permeability
Permeability — Winland R35
log(R35) = 0.732 + 0.588·log(k) − 0.864·log(φ×100)
click formula to derive ↑
Inputs
fraction
μm
Description
Winland's R35 equation is an empirical correlation between porosity, air permeability, and the 35th percentile pore throat radius from mercury injection capillary pressure (MICP). It is the foundation of the Pittman and Kolodzie pore-throat-based flow unit classification system used widely in tight to conventional reservoir characterization.
Variables
| Symbol | Unit | Description |
|---|---|---|
| k | mD | Permeability |
| R35 class | — | Flow Unit Class |
| φ | fraction | Effective porosity of the sample or interval. Winland's equation was calibrated to core plug porosity in fraction units, converted to percent (×100) inside the formula. |
| R35 | μm | 35th percentile mercury injection pore throat radius in microns. Typical values: < 0.1 μm (nano-pore tight gas), 0.1–0.5 μm (tight oil), 0.5–2 μm (moderate quality), 2–10 μm (good), > 10 μm (very good). Used to classify pore-throat-based flow units. |
Assumptions
- Rock is a sandstone or carbonate with inter-granular or inter-crystalline pore types — fracture-dominated or vuggy porosity systems violate the MICP-permeability relationship
- Porosity and permeability are from the same rock sample or representative interval average
Limitations
- Empirical calibration from a 1970s well-log dataset — use with caution in ultra-tight plays (k < 0.001 mD) where the calibration range was not well-represented
- R35 from logs (not directly measured); must be estimated from porosity-permeability core data and then projected across uncored intervals via Winland
Use Cases
- → Flow unit classification in Permian Basin tight oil: Map R35-predicted flow units from log-derived porosity and Timur/Wyllie permeability across lateral wells to identify sweet spots and optimize perforation interval selection.
- → Capillary pressure prediction for Sw−height modeling: R35 drives the pore throat distribution; use it to estimate the displacement pressure and capillary pressure curve shape needed to compute Sw as a function of height above free water level.
- → Core plug quality screening: Plot measured MICP R35 vs. porosity on a log-log scale; identify Winland isoperm lines (k = 0.01, 0.1, 1, 10, 100 mD) to quickly classify each plug into a flow unit tier.
Related Calculations
Region Notes
Permian Basin
Wolfcamp tight oil: R35 typically 0.05–0.5 μm in productive intervals, translating to k = 0.001–0.1 mD at φ = 8–12%. Flow units above R35 = 0.3 μm (k > 0.01 mD) are the primary stimulation targets.
Gulf of Mexico
High-quality GOM Miocene sands often have R35 = 3–20 μm at φ = 25–35%, giving k = 100–1000 mD — macro to mega-pore flow units with excellent productivity.
Anadarko Basin
Hunton carbonate: R35 0.5–5 μm in vuggy dolomite; note Winland was not calibrated to vuggy carbonates so treat k estimates as order-of-magnitude.
Eagle Ford
Tight oil: R35 < 0.1 μm in matrix, placing most of the rock in nano-pore class (k < 0.001 mD); production relies entirely on natural and hydraulic fractures.
Global
Always validate Winland k against conventional core permeability over the cored interval before using it to predict permeability in uncored wells.
References
Primary source
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