Page 145 - The Geological Interpretation of Well Logs

- THE NEUTRON LOG -

Sem
}=-—- 23cm Table 10.3 Thermal neutron capture cross-sections of some
elements, (note the values for H and C1 as well as B and Gd).
water
Element Capture cross-section, cross-section
barns
atomic weight
=16 oO Cc 0.0034 0.00028
N=132
0.33
H
0.33
0.00027
0.000017
limestane 0% Na 0.53 0.023
Mg
0.063
0.0027
0.0085
Al
Si .23 0.0057
16
Figure 10.3 Schematic trajectories of a neutron in a Lime-
cl 33.2 0.94
stone with no porosity and pure water. The slowing down
K 2.10 0.084
length in the limestone is far greater than in water (modified
from Ellis, 1987), Ca 0.43 0.011
B 759 70.3
Table 10.2 Neutron slowing-down parameters (*number of
Gd 49,000 312
collisions involved during change from 4.2 Mev to | eV).
(from Ellis 1987). Cd (shield
material) 2,450 21.9
Moderator *no. of collisions

~ cross-section/atomic weight gives a guide to the effect of
H 14.5 a thermal absorber relative to the volume of formation it
Cc 91.3 occupies.
0 121 - Cadmium is separated on the list as it is rarely encountered
Ca 305 naturally but is used in tool construction as a thermal]
H,O 15.8 absorber.
from Dunlap and Coates, 1988.
Limestone 20% b 29.7
Limestone 0% 132
cross-section of the absorbing nuclei of the formation,

which is a measure of how effective it is at capturing
penetration capabilities but after a few microseconds and
neutrons. Gadolinium and boron have large thermal
successive collisions (100 or so), the original fast neu-
neutron cross-sections but are quite rare: chlorine,
trons have slowed down through epithermic to thermic
common in saline formation fluids, has a moderate cross-
levels with about 0.25 eV of energy and a velocity of
section (Table 10.3).
around 0.22 cm/sec (Figure (0.2). To reduce a neutron
As far as logging is concerned, the dominant effect on
from 2 MeV (2200 cm/sec) to 0.025 eV (0.22 cm/sec)
neutrons during the collision and scattering phase, is the
requires 18 (elastic) collisions with hydrogen nuclei
mass of the (formation) nuclei, hydrogen dominating: the
but 257 (non-elastic) collisions with silicon and 368 with
dominant effect during the absorption phase is the capture
calcium nuclei (Serra, 1979). Expressed in another way,
cross-section of the thermal] neutron absorbers, the effect
elastic collision with hydrogen can take all a neutron’s
of hydrogen being much less marked (Table 10.3).
energy but in non-elastic collisions with heavier elements,
the energy reduction is typically around 10% to 25%: the
10.3 Tools
effect of hydrogen is seen as dominant (Table 10-2).
At the lower energy, thermic levels, the neutron is The neutron tool today generally consists of a fast neutron
thought of as diffusing, rather than having a velocity. For source and two detectors (Figure 10.4). The source
example, in a vacuum, a thermal neutron will diffuse bombards the formation with neutrons and the detectors
randomly for 13 minutes, but in earth materials the time measure their loss of energy as they pass through it.
varies: 5 psec in rock salt, 450 «sec in a limestone with- Toot sources are mostly chemical, such as plutonium-
out porosity and 900 wsec in a quartzite (Serra, 1979). beryllium (PuBe) or americum-beryllium (AmBe), which
The period of diffusion comes to an end as the neutrons produce fast neutrons with a peak energy level around
undergo absorption interactions. That is, they are cap- 4 Mey. These are the most common. Infrequently, high
tured by other nuclei which then change energy state and, energy neutrons at up to 14 MeV are produced using
mostly, become unstable. For example, some nuclei, on accelerometers, in which the neutrons are created by
capturing a neutron, spontaneously de-excite and emit bombarding a target with charged particles.
gamma rays of capture, the so-called n,y capture radiation Historically, the first neutron tools consisted of a
(an effect used in pulsed neutron logging) (Figure 10.2). source and just a single detector but these were quite
The rapidity of neutron absorption depends on the capture affected by borehole environment and most tools now
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