Page 83 - Basic Well Log Analysis for Geologist

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must be interpreted from the specific chart designed fora Where:
specific log (i.e. Schlumberger charts for Schlumberger dy_p = neutron-density porosity
logs and Dresser Atlas charts for Dresser Atlas logs). The dx = neutron porosity (limestone units)
reason for this is that while other logs are calibrated in basic bp density porosity (limestone units)
physical units, neutron logs are not (Dresser Atlas, 1975).
If the neutron and density porosities from Figure 32 ata
The first modern neutron log was the Sidewall Neutron
depth of 9,324 ft are entered into the root mean square
Log. The Sidewall Neutron Log has both the source and
formula, we calculate a porosity of 6.2%. This calculated
detector in a pad which is pushed against the side of the
porosity value compares favorably with the value obtained
borehole. The most modern of the neutron logs is a
from the crossplot method.
Compensated Neutron Log which has a neutron source and
Whenever a Neutron-Density Log records a density
two detectors. The advantage of Compensated Neutron logs
porosity of less than 0.0 acommon value in anhydritic
over Sidewall Neutron logs is that they are less affected by
dolomite reservoirs (Fig. 32: depth 9,328 ft)—the following
borehole irregularities. Both the Sidewall and Compensated
formula should be used to determine neutron-density
Neutron logs can be recorded in apparent limestone,
porosity:
sandstone, or dolomite porosity units. If a formation is
limestone, and the neutron log is recorded in apparent dy py = PN ton.
N-D 9
limestone porosity units, apparent porosity is equal to true
porosity. However, when the lithology of formation is
a
Where:
sandstone or dolomite. apparent limestone porosity must be
dy_p = neutron-density porosity
corrected to true porosity by using the appropriate chart
dy = neutron porosity (limestone units)
(Fig. 30 for Sidewall Neutron Log: or Fig. 31 for
dp = density porosity (limestone units)
Compensated Neutron Log). The procedure ts identical for
each of the charts and is shown in Figures 30 and 31. Figure 35 1s a schematic illustration of how lithology
affects the Combination Gamma Ray Neutron-Density log.
Combination Neutron-Density Log The relationship between log responses on the Gamma Ray
Neutron-Density Log and rock type provides a powerful
The Combination Neutron-Density Log is a combination
tool for the subsurface geologist. By identifying rock type
porosity log. Besides its use as a porosity device, it is also
from logs, a geologist can construct facies maps.
used to determine lithology and to detect gas-bearing zones.
Figure 35 also illustrates the change in neutron-density
The Neutron-Density Log consists of neutron and density
response between an oil- or water-bearing sand and a
curves recorded in tracks #2 and #3 (example. Fig. 32).
gas-bearing sand. The oll- or water-bearing sand has a
and a caliper and gammit ray log in track #1. Both the
density log reading of four porosity units more than the
neutron and density curves are normally recorded in
neutron log. In contrast, the gas-bearing sand has a density
limestone porosity units with each division equal to either
reading of up to 10 porosity units more than the neutron log.
two percent or three percent porosity: however, sandstone
Where an increase in density porosity occurs along with a
and dolomite porosity units can also be recorded.
decrease in neutron porosity in a gas-bearing zone, it is
True porosity can be obtained by, first. reading appareut
called gas effect. Gas effect 1s created bv gas tn the pores.
limestone porosities from the neutron and density curves
Gas in the pores causes the density log to record too high a
(example: Fig. 32 at 9.324 ft, by = 8% and dy = 3.5%).
porosity (i.e. gas is lighter than oil or water), and causes the
Then. these values are crossplotted on a neutron-density neutron log to record too low a porosity (te. gas has a lower

porosity chart (Figs. 33 or 34) to find true porosity. In the concentration of hydrogen atoms than oil or water). The
example from Figures 32 and 34. the position of the effect of gas on the Neutron-Density Log is a very important
crossplotted neutron-density porosities at 9,324 ft (Fig. 34) log response because it helps a geologist to detect
indicates that the lithology is a limey dolomite and the gas-bearing zones.
porosity is 6%. Figure 36 is a schematic illustration of a Gamma Ray
Examination of the neutron-density porosity chart (Fig. Neutron-Density Log through several gas sands. It
34) reveals that the porosity values are only slightly affected illustrates how changes in porosity, invasion, hydrocarbon
by changes in lithology. Therefore, porosity from a density, and shale content alter the degree of gas effect
Neutron-Density Log can be calculated mathematically. The observed on the Neutron-Density Log.
alternate method of determining neutron-density porosity is
to use the root mean square formula.
34 ho + TSlight variations of this formula may be used in some areus. Also, some
for? + by 7 log analysts restrict the use of this formula to gas-bearing formadions, and
xy Vo 2”
use by_p = (by + bp)/2 in oil- or water-bearing tormations.

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