Page 147 - The Geological Interpretation of Well Logs

- THE NEUTRON LOG -

and ZDL of Western Atlas (Table 10.4). In these combi- deflection opposite the limestone. Tiree other zones are
nation tools, the neutron sonde is positioned above the included in the pit, one of pure water, one of Austin Chalk
density sonde, thus logging the formation first, since the (porosity 26%) and one of Carthage Marble (porosity
gamma bombardment used by the density tool (Chapter 1.9%) (Belknap ef ai., 1959). These standards enable
9} could affect the neutron readings. neutron tools to be calibrated and the neutron log read-
ings standardized. However, recent evaluations of the test
Units of measurement
pit (Butler and Clayton, 1984) suggest that it is open to
Most neutron logs are now plotted on a standardized
errors and that a series of synthetic formations would
anthmetic scale of neutron (or limestone) porosity units.
offer a more reliable calibration standard than complex
Previously, each logging company used different units for
natural ones.
its own neutron tool, but it was shown (Archie, 1950) that
there is a consistent relationship between neutron-tool Log format
values and porosity in clean limestones (Figure 10.6). The neutron log is generally plotted across tracks 2 and 3
This value represents real porosity only under standard (Figure 10.8). The units, as discussed above, are neutron
conditions in clean limestones, but to find the real porosity units from empirical calibration. That is, neutron
porosity in other lithologies, the neutron-log value can be porosity units represent real porosity in clean limestones
converted by using tables or empirical calibrations (see but only in clean limestones. The most common scale is
Quantitative uses below). from 45% (to the left) to -15% neutron porosity units. A
An API standard unit does exist for neutron tools and ratio may be used instead of a % making these figures .45
is defined, as it is for the other nuclear tools, in the test to -.15 neutron porosity units.
pit in the grounds of the University of Houston, Texas. Since the neutron is generally run combined with the
This pit contains a 6 ft zone of Indiana limestone with density in one tool, a combined neutron-density Jog
a porosity of 19% and it is fresh-water wet (Figure heading has become standard. This generally shows the
10.7). One neutron API unit is 1/1000 of the difference scales for both the neutron and the density standardised
between the instrument zero with no radiation and the log for clean Jimestone matrix (see Section 10.7 Neutron-

THE NEUTRON CALIBRATION FACILITY
° °
3/8" steal deck plate
e4 ce and
Qo o 4 |

a ° |
2& - water

UNITS = ° | | | — ¢orrugaled pipe
|
— concrate
ARBITRARY limasione (1.9% porosity)
(Carthage marble)
in blocks
- » °o \ Indiana limasione

RESPONSE Ausiin limesione
{19% porosity)
ia blocks
{26% porosily)

TOOL ° ae | concrete
in blocks
NEYTRON a z sss ID 148” larger than
| |
dla. af holes In limestone

INDIVIDUAL LIMESTONE BLOCK
——

§
jp-e——
4+
ro
NIB

1
3 10 20 30 40
POROSITY %
Figure 10.6 Graph showing the consistent relationship Figure 10.7 The API nuclear log calibration facility at the
between neutron tool response and total limestone porosity: University of Houston, Texas, USA. (From Belknap e7 al.
Devonian, Crossett Limestone, Texas. (Re-drawn from 1959).
Archie, 1950).

137