Chapter 15. pdf

0.001
0.1
1
10
100
1000
0.01
10
10
10
10
10
8
7
5
6
4
Time
N
e
u
tr
o
n
 E
n
e
rg
y
 (
e
V
)
Approx. 1 ms
Emission
Time
Thermal Neutrons
Fast Neutrons
Epithermal Neutrons
Intermediate Neutrons
Neutron Tool Release Energy

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 153
15.2.3
Neutron Absorption
Thermal (and to some extent epithermal) neutrons can be absorbed by the nuclei of the formation
atoms. The efficiency of neutron absorption varies from element to element. The only elements which
exhibit significant neutron absorbing behaviour and exist is reasonable quantities in rocks are
hydrogen and chlorine. The process of neutron absorption by hydrogen and chlorine are
(15.2)
and
(15.3)
In neutron logging, some tools measure the epithermal neutrons, some the thermal neutrons and some
the gamma rays emitted when a neutron is absorbed.
15.2.4
 
Hydrogen Index
We can define a partial concentration of hydrogens per unit mass 
(C
H
)
mass
of a material as the mass of
hydrogen atoms in the material divided by the mass of all the atoms of all elements in the material.
(15.4)
where:
A
H
= the atomic mass of hydrogen atoms in the material
A
i
= the atomic mass of non-hydrogen element 
i
n
H
= the number of hydrogen atoms in a molecule of the material
n
i
= the number of non-hydrogen atoms of element 
i
in a molecule of the material
Note
:
 i 
is summed over every non-hydrogen element in the material.
Thus, for pure water (H
2
O), where the atomic mass of hydrogen is 1.0 and the atomic mass of oxygen
is 16.0, the partial concentration of hydrogen 
(C
H
)
mass
= (2
×
1.0)/(1
×
16.0 + 2
×
1.0) = 1/9.
It is simple to obtain the partial concentration of hydrogens per unit volume 
(C
H
)
vol
, by multiplying the