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. This webpage version is the copyrighted intellectual property of the author.

Do not copy or distribute in any form without explicit permission.

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

      1: IF Sw < 0
      2: THEN Sw = 0
      3: AND SwNEG = SwNEG + 1
      4: IF Sw > 1.0
      5: THEN Sw = 1.0
      6: AND SwPOS = SwPOS + 1

WHERE:
  Sw = water saturation from any method (fractional)
  SwNEG = counter for number of negative saturation values
  SwPOS = counter for number of saturation values over 1.0

COMMENTS:
Only the bulk 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.

RECOMMENDED PARAMETERS: None.

NUMERICAL EXAMPLE: None.

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

SwSMO - 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)

COMMENTS:
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%).

RECOMMENDED PARAMETERS: None.

NUMERICAL EXAMPLE: None.
 

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