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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