Publication History: This article is based on Chapter 8 of "The Log Analysis Handbook" by E. R. Crain, P.Eng., published by Pennwell Books 1986  Updated 2004, 2020. This webpage version is the copyrighted intellectual property of the author. Do not copy or distribute in any form without explicit permission.

Selection of WATER SATURATION MethodS
The answers for all the water saturation solutions will vary, and in some cases will be unreasonable or impossible to calculate due to lack of data. In order of preference, we would choose:

BEST MODEL -- Use whenever possible
1. Simandoux Model, reverts to Archie Model when Vsh = 0. Cznnot create negative  saturation answers.

OTHWER SATURATION MODELS --- Use only within  limitations
2. Archie Model. Best use is in clean sands or carbonates. Use Simandoux to handle both clean and shaly rocks.

3. Dual Water Model, reverts to Archie Model when Vsh = 0. Czn create negative  saturation answers.

4 Waxmzn - Smits Model. Requires CEC and Qv data  which is hard to find.

5. Buckle's  Method. Use when resistivity is missing.Also used to find irreducible water saturation SWir in water zones.

6. Many alternate models are covered in this Handbook.

Trim results betweem 0.0 and 1.0 and apply smoothing funvyion to results if desired.

Material Balance for Water Saturation
Water saturation results must be constrained by the following:

1: IF Sw < 0
2: THEN Sw = 0
3: IF Sw > 1.0
4: THEN Sw = 1.0

Where:
Sw = water saturation from any method (fractional)

Only the bbulk volume water (dual water) method could create a negative water saturation.

All methods can create values greater than 1.0 (100%). If too many points are over 1.0, water resistivity, shale resistivity, porosity, and shale volume parameters should be reviewed.

Schlumberger proposed a smoothing function to reduce statistical noise in saturation data at both the high and low ends of the spectrum.

SmoothSw - Water Saturation Smoothing
1: IF Sw > 0.75
2: THEN Sw = (1 - ((16 / 3) (1 - Sw) ^ 3) * (5 - 128 * (1 - Sw) ^ 3))
3: IF Sw < 0.25
4: THEN Sw = Sw + 0.04 * (1 - 4 * Sw) / (1 + 21 * Sw)

Otherwise Sw is unchanged.

Where:
Sw = water saturation from any method (fractional)

The smoothing function is not necessary for hand calculator work, especially if Sw data is rounded to the nearest 0.05 (or 5%). The function prevents water saturation from going below 0.04 (4%).

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