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. Water Saturation from Ratio Method
When no porosity data is available, saturation can be obtained by comparing the shallow and deep resistivity logs. This formula is not shale corrected.

1: SWrt = SXO * ((RXO / RESD) / (RMF@FT / RW@FT)) ^ (1 / N)

Where:
N = saturation exponent (fractional)
RESD = resistivity reading on deep log (ohm-m)
RXO = resistivity reading on shallow log (ohm-m)
RMF@FT = mud filtrate resistivity (ohm-m)
RW@FT = formation water resistivity at same temperature as RMF (ohm-m)
SWrt = water saturation from ratio method (fractional)
SXO = estimated water saturation in invaded zone (fractional)

A common assumption is that Sxo = Sw^(1/5). Substituting this into equation 1 and assuming N = 2, we get:
2: SWrt = ((RXO / RESD) / (RMF@FT / RW@FT)) ^ (5/8) COMMENTS:
See below for a graphical solution to this formula. The method is a last resort to be used when no porosity log is available.

If there is a nearby water zone AND it has the same porosity as the pay zone, the Rmf/Rw ratio can be estimated from the Rxo/Rt ratio in the water zone, and used in equation 1 or 2, thus exact knowledge of Rw is not required. Graphical solution for Ratio method (with shale correction from SP) RECOMMENDED PARAMETERS:
N = 2.0
for water zone SXO = 1.00
for hydrocarbon zone with high porosity SXO = 0.60
for hydrocarbon zone with medium porosity SXO = 0.70
for hydrocarbon zone with low porosity SXO = 0.80
for heavy oil and tar sands, SXO = SW = 0.10 to 0.30

NUMERICAL EXAMPLE:
1. For Sand C:
RESD = 1.0 ohm-m
RESS = 2.0 ohm-m
RMF = 0.75 @ 25 degrees C
RMF@FT = 0.54 @ 43 degrees C
RW@FT = 0.036 @ 43 degrees C
N = 2.00
Swr = (2.0 / 1.0) / (0.54 / 0.036) ^ (1 / 2) = 0.36

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