petrophysics · water saturation
Irreducible Water Saturation (Swirr)
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Inputs
fraction
fraction
Description
Predicts the irreducible water saturation floor in an uncored zone or offset well by dividing a calibrated bulk volume water (BVW) constant — established from core data or a nearby cored well at the same rock type — by the zone's own porosity. This is the algebraic inverse of Bulk Volume Water (bvw), which instead computes BVW forward from a known φ and Sw; here φ and a pre-established BVW are used to forecast Swirr where no direct core or Sw measurement yet exists.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Swirr | fraction | Irreducible Water Saturation |
| BVWc | fraction | A constant bulk volume water value calibrated from core data or an offset well/analog field at irreducible water saturation — NOT computed here. Read this off a Pickett plot's flattest, lowest BVW trend, or from core SCAL, before using this calculator. |
| φ | fraction | Effective or total porosity at the depth of interest in the uncored zone being evaluated. |
Assumptions
- The calibrated BVW constant was established from core or log data in the same rock type/grain-size population as the zone being evaluated
- The zone being evaluated is genuinely near irreducible conditions (not a transition zone with mobile water)
- Porosity is from the same definition (effective vs. total) as the porosity used to calibrate the BVW constant
Limitations
- Only as reliable as the calibrated BVW constant — a poorly chosen or non-representative BVW propagates directly into Swirr
- Assumes rock type/pore geometry is uniform between the calibration point and the zone being evaluated; grain-size or sorting changes shift the true BVW constant
- Not a substitute for NMR-measured Swirr where a resistivity- or NMR-based Sw log is actually available — this is specifically a fallback for uncored, unlogged, or NMR-absent zones
Use Cases
- → Permeability estimation without NMR: Predict Swirr from a calibrated BVW constant and porosity, then feed it directly into perm_timur where no NMR log exists.
- → Offset-well analog screening: Apply a BVW constant calibrated from a cored offset well to a nearby uncored well's porosity log to forecast where it will sit at irreducible conditions.
- → Pickett plot calibration QC: Compare a Pickett-plot-read BVW constant's implied Swirr against core-measured Swirr (where available) to validate the constant before applying it elsewhere.
Related Calculations
Region Notes
Permian Basin
Wolfcamp tight sands/carbonates: a BVW constant of 0.03-0.06 (per bvw's own region notes) typically implies Swirr in the 0.20-0.35 range at 15-20% porosity — reasonable inputs for perm_timur when NMR is unavailable.
Gulf of Mexico
Higher-porosity unconsolidated sands with BVW constants of 0.08-0.12 imply lower Swirr (often <0.20) at typical 25-30% porosity — consistent with these reservoirs' generally higher permeability.
Global
Always re-derive the BVW constant locally from core or a genuinely analogous offset well — a constant borrowed from an unrelated field or rock type is the single largest source of error in this calculation.
References
Primary source
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