Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 168

Sigma-Ratio Crossplot
If no open-hole logs are available one essential piece of information will be missing,
namely, the porosity. Fortunately there is a method for finding porosity from pulsed
neutron logs. It requires only the value of sigma and ratio, read directly from the log.
These two readings are then cross-plotted to give porosity. A side benefit is that the
plot also gives values of ʣ wa , the apparent capture cross section of the water.
Figure 11.29 gives an example of this type of chart.
Fig. 11.29 Determination of ˕ and ʣ wa from ʣ and ratio
It should be noted that many different charts are available form service compa-
nies that cover a multitude of different casing sizes and formation water salinities.
The particular chart used here for illustrative purposes was built for 5½″ casing and
water salinity >50 Kppm NaCl.
Question #11.10. ʣ-Ratio Crossplot
A log is run in 5½″ casing with an 8-5/8″ open-hole. The borehole fluid salinity is
80,000 ppm NaCl; ʣ Log = 20 cu and the ratio = 2.8.
(a) Find ˕ = ____________ %
(b) Find ʣ wa = ___________ cu