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BASIC WELL LOGGING ANALYSIS –

NEUTRON LOG

Hsieh, Bieng-Zih

Fall 2009

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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 liquid-filled 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.

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。

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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:

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22DN

DN

2DN

DN

For gas-bearing formation

For oil- or water-bearing formation

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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

NPHI ФN Vsh ФN-sh

7600

7610

7620

….

….

….

….

….

….

….

7840

7850

Information:

The formation is Sandstone and the neutron log is recorded in apparent limestone porosity units

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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

….

….

….

….

….

….

….

7840

7850

2

22DN

DN

2

22shDshN

shDN