Page 144 - The Geological Interpretation of Well Logs

- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -

Table 10.1 The principal uses of the neutron log.

Discipline Used for Knowing

Quantitative Petrophysics Porosity Matrix
Hydrogen index

Qualitative Petrophysics Identification of gas Lithclogy

Geology Lithology - shales Gross lithology
Evaporites Neutron evaporite values
Hydrated minerals
Volcanic and Calibration
intrusive rocks
General lithology Combined with density*

*using neutron log combined with density log on compatible scales.

Principal uses energy energy
values classes
Quanttatively, the neutron log is used to measure porosi-
(eV) oe
10? 100 3
ty. Qualitatively, it is an excellent discriminator between
=
107F Fre °
gas and oil. It can be used geologically to identify gross =

4 MeV
lithology, evaporites, hydrated minerals and volcanic . chemical § Ss
tocks. When combined with the density log on compatible 5 wer st source x
scales, it is one of the best subsurface lithology indicators a s 5
2 10°F
available (Table 10.1). € o
< A
3 10° 2
s a
o
10.2 Principles of measurement 2 3 10°F a
5
Neutrons are subatomic particles which have no electrical 10%
5
charge but whose mass is essentially equivalent to that g 5
-_= = 10+ Cc
of a hydrogen nucleus. They interact with matter in two a
itherma)
principal ways, by collision and absorption: collisions 3 sé z y epitherma nN "
are mainly at higher energy states, absorption occurs at 3B
ao o.1F
lower energy.
§ g 5 thermal eles “4
The lifetime of a free neutron is one of losing energy
? o 0.01 ="
2 ! 3
and can therefore be usefully described in terms of ener- su
ve 0.001}
gy state, namely fast, epithermic and thermic in order of
decreasing energy (Figure 10.2). The energy loss from
Detectors for
fast neutron energy levels through epithermic to the limit
epithermal neutrons
of thermic energy, is generally thought of as a loss of (2) thermal neutrons
velocity which occurs especially through elastic scatter- ¥ rays
ing, that is collisions with particles having the same mass Figure 10.2 Schematic diagram of a neutron life, showing the
as neutrons. For logging purposes this is mainly hydrogen energy degradation after emission.and the neutron tool detector
nuclei. Collision with other, heavier particles, called levels. (From Serra, 1979; Tittle, 1961; Owen, 1960).
inelastic scattering, does not result in significant energy
loss (Table 10.2). These two moderating reactions are length will be short compared to that in a hydrogen free
considered to cause the velocity loss over a certain environment (Figure 10.3, Table 10.2). Slowing-down
trajectory called the stowing-down length. The slowing- length is an important concept in logging as it is used to
down length is proportional to the root mean-square place detectors at an optimum distance from the tool’s
distance from the point of emission of high energy neutron source.
neutrons to the point at which they reach the lower Most logging tools use a chemical source producing fast
limit of epithermal energy levels. This distance can be neutrons. These have an initial energy of around 4 MeV
calculated from a knowledge of the combined capture (see Tools), which means that they have an initial velocity
cross-sections of the constituent elements of the material of approximately 2800 cm/usec (Figure 10.2). With this
traversed. In a hydrogen rich medium, slowing-down energy and velocity, the neutrons have considerable
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