Page 143 - The Geological Interpretation of Well Logs

10

THE NEUTRON LOG

10.1 Generalities it bound water, water of crystallization or free pore-water.
This hydrogen richness is called the hydrogen index (#/)
The log
which is defined as the weight % hydrogen in the forma-
The neutron log provides a continuous record of a forma-
tion/wt % hydrogen in water, where H/ water = | (Table
tion’s reaction to fast neutron bombardment. It is quoted
10.8). However, the oilfield interest in water is as a pore
in terms of neutron porosity units, which are related to a
fluid filler and porosity indicator so that the neutron log
formation’s hydrogen index, an indication of its richness
fesponse is given directly in neutron porosity units.
in hydrogen.
Neutron porosity is real porosity in clean limestones, but
Formations modify neutrons rapidly when they contain
other lithologies require conversion factors. Since it is
abundant hydrogen nuclei, which in the geological
calibrated to limestones, the log is sometimes called the
context are supplied by water (HO). The log is therefore
Limestone Curve (Figure 10.1).
principally a measure of a formation’s water content, be
NEUTRON LOG
* Scale: neutron porosity unite %
60 48 36 24 12 O -12
fl li
SHALE
QUARTZITE $0
7-7
SANDSTONE ¢10% #(@N = 6.5%) |
— 7
¢o @N= 0% !
LIMESTONE
$10%
60 oN= 1.0)%
DOLOMITE
610% #(@N |
18.0%)
=
c---
SHALE
I
tee,
! gas
7 effect
SANDSTONE OIL |
620% — watER
#(oN = 16.0%) j
Sa
poorly
compacted
shales very variable
\ @N=75-25%
SHALE
an
N
\
N
compact ~ Sy
COAL a
+ bh +
SALT + + tb + + @N=~-3% {
-——
tee
SILL QGNEOUS) po! \
Cee SETAE
SoA
SHALE
Figure 10.1 The neutron log: some typical responses. The neutron log shows hydrogen index which is converted to neutron
porosity units.
* Porosity with fresh water and the Schlumberger CNL tool (cf. Figure 9.1 which is on a compatible scale of porosity).