Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 163

Finding Interpretation Parameters

In order to perform a quantitative interpretation of a pulsed neutron log, certain
parameters need to be known. These are:

ʣ matrix
ʣ water
ʣ hydrocarbon (oil and/or gas)
ʣ shale

We will now explore various methods of finding these parameters.

Sigma Water
ʣ w is a simple function of the water salinity (ppm NaCl) and temperature (see
Fig. 11.24 ). If the salinity is not known, then a 100 % water-bearing section can give
us the required data. Note that if S w = 100, the basic equation reduces to:

( ) +

SSfSf
Log

= 1- ,

ma w

which can be rewritten to give:

( )

S SS f
f

- 1 -

log a

=

w

Thus, provided ʣ ma and ˕ are known, ʣ w can be back calculated directly. Note that
this method is similar to the R wa technique used with open-hole logs. For that reason,
the derived value of ʣ w is referred to as ʣ wa or sigma water apparent . If an extensive
water-bearing interval has been logged, a graphical method can provide a “double
whammy,” both ʣ ma and ʣ w from one plot. If pairs of values of ʣ and ˕ are plotted
on a graph such as the one given in Fig. 11.25 , all points at S w = 100 % will fall on a
straight line connecting S

( ) ( )

at f Sf = 0 % andat = 100 % .

ma w

Question #11.6. Σ w
Given: Formation water salinity = 230,000 ppm NaCl.
Temperature = 200 °F.
Find ʣ w = cu.

Question #11.7. Σ w
Plot ʣ vs. ˕ for the following log readings:

Level ʣ (cu) ˕ (%)

1 17 28

2 21 32

3 23 26

4 27 28

5 21 13