Chapter 15. pdf

15.3.3
Compensated Neutron Log (CNL)
This tool is designed to be sensitive to thermal neutrons, and is therefore affected by the chlorine
effect. It has two detectors situated 15 in and 25 in from the source. The detector further from the
source is larger to ensure that adequate count rates are observed. The critical measurement for this tool
is the difference in the thermal neutron population, which results from neutron capture and neutron
scattering. The tool readings are presented in limestone porosity units in the same way as the sidewall
neutron porosity tool. The CNL tool has a very strong source of neutrons to ensure that the measured
count rates are sufficiently high to obviate any significant errors associated with statistical fluctuations

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 157
in the count rate despite the longer source-detector spacing for this tool compared to the GNT and SNP
tools. The stronger source permits a deeper depth of investigation as well as allowing the tool to
operate in cased holes. The CNL tool is run eccentred in the hole by an arm which presses the tool
against the side of the borehole. This means that the tool is insensitive to the type of mud in the hole,
but implies that the readings are only for one portion of the borehole wall. The CNL log is shown in
Fig. 15.4.
Figure 15.4
The CNL tool (Schlumberger).
15.4 Log Presentation
The data from all tool types are recorded in tracks 2 and 3.
The data from the GNT tool is given in API units as shown in Fig. 15.5.
The data from the SNP tool is given in equivalent limestone porosity units, with the scale running
from approximately –10% to 30%, as shown in Fig. 15.6.
Data from the CNL tool is given in equivalent limestone porosity units, with the scale running from
approximately –10% to 30%, as shown in Fig. 15.7.