Page 63 - Well Logging and Formation Evaluation

Full Interpretation 53

expected, then they may also be applied with confidence in other wells
drilled using similar drilling parameters.

3.3 ARCHIE SATURATION

SCAL data measurements of cementation (m) and saturation (n) expo-
nents should be incorporated into the Archie model. In m measurements,
the plugs will have been flushed with a brine of an equivalent salinity to
that expected in the reservoir and the resistivity measured. By plotting the
logarithm of formation factor, given by log(F) = log(R o /R w), against
log(porosity), according to Archie:

log F) =- m *log( ) f (3.3.1)
(
Therefore, the gradient of the line gives m. Note that the higher the m
value used, the higher the water saturations, S w, that will be calculated,
and vica versa.
In n measurements, the plugs will have been flushed with brine, then
desaturated (either with air or kerosene) to yield measurements of true
resistivity, R t , vs S w. By plotting the logarithm of the resistivity index,
given by log(I) = log(R t /R o ), against log(S w), according to Archie:

(
log I ( ) =- n *log S w . ) (3.3.2)
Therefore, the gradient of the line gives n. Note that the higher the n value
used, the higher the S w that will be calculated, and vica versa. Values of
n that are anomalously high (above 2.5) may be indicative of a mixed or
oil-wet system and require further investigation. Low values of n corre-
spond to good-quality water-wet permeable rock.
Having set m and n, there is no longer complete freedom to choose
R w if one is required to calculate S w = 100% in known water sands. If
formation-water salinity is well known from produced water samples, one
is sometimes faced with a dilemma of whether to honor m or R w . In many
cases, the true cause of this discrepancy is actually an error in the por-
osity calculation. However, where the porosities are robust, one has to
make a choice whether to change m or R w. It is always worth looking again
closely at the cementation-exponent measurements to see how much
scatter in the data there is and whether or not the m value chosen is really
reliable. If the measurements do not come from the water leg at all, it is
possible that diagenetic effects in the reservoir mean that values from the