Neutron log

Report
BASIC WELL LOGGING ANALYSIS –
NEUTRON LOG
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Hsieh, Bieng-Zih
Fall 2009
NEUTRON LOG

Neutron logs are porosity logs that measure the hydrogen
ion concentration (氫離子濃度) in a formation.

In clean formations (i.e. shale-free) where the porosity is
filled with water or oil, the neutron log measures liquidfilled porosity.
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NEUTRON LOG (CONT.)

Neutrons are created from a chemical source in the
neutron logging tool.

The chemical source may be a mixture of americium (鋂)
and beryllium (鈹) which will continuously emit neutrons.

These neutrons collide with the nuclei (原子核) of the
formation material, and result in a neutron losing some of
its energy.
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NEUTRON LOG (CONT.)

Because the hydrogen atom is almost equal in mass to the
neutron, maximum energy loss occurs when the neutron
collides with a hydrogen atom.

Therefore, the maximum amount of energy loss is a
function of a formation’s hydrogen concentration.

Because hydrogen in a porous formation is concentrated
in the fluid-filled pores, energy loss can be related to the
formation’s porosity.
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中子井測 (NEUTRON LOG)
•
•
•
中子井測是向地層放射
中子以偵測地層孔隙率。
中子以高速撞擊地層內
其他原子核,減速達到
熱動力速度(Thermal
velocity)時即被擄獲,
未被擄獲的中子到達接
收器後,即被探測針的
中子接收器測定並記錄
之。
中子井測之讀值可以表
示地層之孔隙率。
GAS EFFECT

Whenever pores are filled with gas rather than oil or
water, neutron porosity will be lowered.

This occurs because there is less concentration of
hydrogen in gas compared to oil or water.

A lowering of neutron porosity by gas is called gas effect.
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NEUTRON LOG (CONT.)

Neutron log response vary, depending on: (1)
differences in detector types, (2) spacing between source
and detector, and (3) lithology— i.e. sandstone,
limestone, and dolomite.

These variations in response can be compensated for by
using the appropriate charts.

A geologist should remember that neutron logs (unlike all other
logs) must be interpreted from the specific chart designed for
a specific log (i.e. Schlumberger charts for Schlumberger logs
and Dresser Atlas charts for Dresser Atlas logs).
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NEUTRON LOG (CONT.)

The first modern neutron log was the Sidewall Neutron
Log (側壁式中子井測).

The Sidewall Neutron Log has both the source and detector in
a pad which is pushed against the side of the borehole.

The most modern of the neutron logs is a Compensated
Neutron Log (補償式中子井測) which has a neutron
source and two detectors.

The advantage of Compensated Neutron logs over
Sidewall Neutron logs is that they are less affected by
borehole irregularities.
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NEUTRON LOG (CONT.)

Both the Sidewall and Compensated Neutron logs can be
recorded in apparent limestone, sandstone, or dolomite
porosity units.

If a formation is limestone and the neutron log is
recorded in apparent limestone porosity units, apparent
porosity is equal to true porosity.

However, when the lithology for a formation is sandstone
or dolomite, apparent limestone porosity must be
corrected to true porosity by using the appropriate chart.
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CHART FOR CORRECTING CNL FOR LITHOLOGY
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NEUTRON POROSITY FOR SHALY SANDS
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NEUTRON POROSITY FOR SHALY SANDS

After the volume of shale (Vsh) is determined, it can be
used to correct the porosity log for shale effect. The
formula for correcting the density log for volume of shale
is (Schlumberger, 1975):
Where,
 ФN-sh = neutron porosity corrected for shale
 ФN
= neutron porosity uncorrected for shale
 ФN clay= neutron porosity of adjacent shale
 Vsh
= volume of shale

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COMBINATION NEUTRON-DENSITY LOG
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COMBINATION NEUTRON-DENSITY LOG

The Combination Neutron-Density Log is a combination
porosity log.

Besides its use as a porosity device, it is also used to
determine lithology and to detect gas-bearing zones.
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GAS EFFECT

Gas effect is created by gas in the pores. Gas in the
pores causes the density log to record too high a porosity
(i.e. gas is lighter than oil or water), and causes the
neutron log to record too low a porosity (i.e. gas has a
lower concentration of hydrogen atoms than oil or water).

Where an increase in density porosity occurs along with a
decrease in neutron porosity in a gas-bearing zone, it is
called gas effect.

The effect of gas on the Neutron-Density Log is a very
important log response because it helps a geologist to
detect gas-bearing zones.
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TRUE POROSITY -- NEUTRON-DENSITY POROSITY

The method of determining neutron-density porosity is:
N  D 
N  D 
 N  D
2
2
For gas-bearing formation
2
 N  D
For oil- or water-bearing formation
2
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HOMEWORK #5 -- NEUTRON LOG (CNL)
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HOMEWORK #5 – NEUTRON LOG
CNL
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HOMEWORK #5 – NEUTRON LOG
Depth
7600
7610
7620
….
….
NPHI
ФN
Vsh
ФN-sh
Information:
The formation is Sandstone and
the neutron log is recorded in
apparent limestone porosity
units
….
….
….
….
….
7840
7850
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CHART FOR CORRECTING CNL FOR LITHOLOGY
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HOMEWORK #5 – NEUTRON-DENSITY LOG
Depth
ФN
ФN-sh
ФD
ФD-sh
ФN-D
ФN-D-sh
7600
7610
7620
N  D 
 N 2  D 2
2
….
….
….
N  Dsh 
N sh 2  Dsh 2
2
….
….
….
….
7840
7850
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