Chapter 15. pdf

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 150
15.
THE NEUTRON LOG
15.1 Introduction
The 
neutron
log is sensitive mainly to the amount
of hydrogen atoms in a formation. Its main use is
in the determination of the porosity of a formation.
The tool operates by bombarding the formation
with high energy neutrons. These neutrons
undergo scattering in the formation, losing energy
and producing high energy gamma rays. The
scattering reactions occur most efficiently with
hydrogen atoms. The resulting low energy
neutrons or gamma rays can be detected, and their
count rate is related to the amount of hydrogen
atoms in the formation.
In formations with a large amount of hydrogen
atoms, the neutrons are slowed down and absorbed
very quickly and in a short distance. The count
rate of slow neutrons or capture gamma rays is low
in the tool. Hence, the count rate will be low in
high porosity rocks.
In formations with a small amount of hydrogen
atoms, the neutrons are slowed down and absorbed
more slowly and travel further through the rock
before being absorbed. The count rate of slow
neutrons or capture gamma rays in the tool is
therefore higher. Hence, the count rate will be
higher in low porosity rocks.
Figure 15.1 shows a typical neutron tool.
Figure 15.1
A typical neutron tool.

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 151
15.2
 
Theory
In neutron logging there are three processes of interest: neutron emission, neutron scattering and
neutron absorption.
15.2.1