PetrophysicsResistivity Logs
Water Saturation from Neutron Capture Cross Section Formula
Water Saturation from Neutron Capture Cross Section calculates water saturation for resistivity logs workflows in petrophysics.
How engineers use this formula
Use this formula when the listed inputs (Sigma_log, Sigma_ma, Sigma_w, Sigma_h, phi) are known and the assumptions behind the cited resistivity logs relationship match the engineering case being checked.
Assumptions
- Input values are representative for the well, reservoir, fluid, or equipment case being evaluated.
- The declared units match the field-unit constants used in the formula.
- The cited formula applies to the selected petroleum engineering workflow.
Limitations
- The calculation does not replace a full engineering model or operating procedure.
- Accuracy depends on the source correlation, assumptions, input quality, and unit consistency.
Common mistakes
- Mixing unit systems without converting the inputs.
- Using default example values as field recommendations.
- Applying the formula outside the source assumptions.
Default example
Using the default inputs, S_w equals 0.567677 fraction.
Sigma_logcapture units
25
Sigma_macapture units
12
Sigma_wcapture units
110
Sigma_hcapture units
20
phifraction
0.22
Inputs
Sigma_log
capture unitsMeasured Formation Capture Cross Section
Sigma_ma
capture unitsMatrix Capture Cross Section
Sigma_w
capture unitsWater Capture Cross Section
Sigma_h
capture unitsHydrocarbon Capture Cross Section
phi
fractionPorosity
Outputs
S_w
fraction
Water Saturation
Sigma_log
capture units
Measured Formation Capture Cross Section
Source and review
reviewedBassiouni, Z. 1994. Theory, Measurement, and Interpretation of Well Logs. SPE Textbook Series Vol. 4, Chapter 9, Page 181.
Source