Water Salinity Index Ratio Formula
Water Salinity Index Ratio calculates water saturation from salinity index for resistivity logs workflows in petrophysics.
How engineers use this formula
Use this formula when the listed inputs (C, C_a, phi, phi_a) 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 0.3 fraction.
35000
10500
0.2
0.06
Inputs
C
ppmWater Salinity
C_a
ppmApparent Water Salinity
phi
fractionPorosity
phi_a
fractionApparent Porosity
Outputs
S_w
Water Saturation from Salinity Index
S_w_phi
Water Saturation from Apparent Porosity Index
C_a
Apparent Water Salinity
C
Water Salinity
phi_a
Apparent Porosity
phi
Porosity
Source and review
reviewedTheory, Measurement, and Interpretation of Well Logs, Bassiouni, Z. (1994)
Bassiouni, Z. 1994. Theory, Measurement, and Interpretation of Well Logs. SPE Textbook Series Vol. 4, Chapter 9, Page 183.
Source