neutron log

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1 Neutron Log Contents This topic will cover : 1. Introduction 2. When to Use Neutron Logs 3. Neutron Log measurement principle ; 4. Neutron Porosity ; 5. Effect Factors on Neutron Logs. 1. Introduction •Neutrons are produced by chemical sources and pulsed sources. Chemical neutron sources use a mixture of americium and beryllium and emit a constant stream of neutrons and gamma rays. •Pulsed sources are relatively harmless when not energized. They contain an electron accelerator and a target. When activated, the accelerator sends a short pulse of electrons into the target, which then emits a burst of neutrons. •The standard dual-spacing neutron tool use chemical sources, and Pulsed neutron sources are used for thermal neutron decay logging and C/O logs. . americium and beryllium . . . . thermal neutron decay logging C/O logs . 2. When to Use Neutron Logs Neutron logs can be used in any borehole, open or cased, fluid-filled or air- filled. The earliest neutron tools had a single detector, but almost all now have 2 detectors to reduce borehole effects. Neutron tools come in several varieties: Dual-Spacing Neutron tool (DSN) Dual-Spacing Epithermal Neutron tool (DSEN) Compensated Neutron tool (CNL)

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Page 1: Neutron Log

1

Neutron Log

Contents This topic will cover :

1. Introduction �����

2. When to Use Neutron Logs ��� �� ����� �� �� ����

3. Neutron Log measurement principle ; ���� ��� �� �� ����

4. Neutron Porosity ; ����� �� ����

5. Effect Factors on Neutron Logs. ����� ���� � �� �� �� ����

1. Introduction •Neutrons are produced by chemical sources and pulsed sources. Chemical

neutron sources use a mixture of americium and beryllium and emit a constant

stream of neutrons and gamma rays.

•Pulsed sources are relatively harmless when not energized. They contain an

electron accelerator and a target. When activated, the accelerator sends a short

pulse of electrons into the target, which then emits a burst of neutrons.

•The standard dual-spacing neutron tool use chemical sources, and Pulsed

neutron sources are used for thermal neutron decay logging and C/O logs.

� �� ������ ���� � ��� ��!� �"��# ��!�� �$�� . ��!� �� ���� �"��#� ������

&� �� americium and beryllium '& � �� ���� �� ���� ���� ��(�� ����.

� ��!�� �$��� �) ���� * �$ ����+ , ��#� � (�� . �)� -��.� �&+ �� /�#�0 1 �/�

2�)� . ����+ (�� �13 �4� �� ��$�� � !� �� ���� �#�5 ��0 2�6� 7 -8� �� �/�

'& � �9:� �� ���� ���� .

� ��� �� ��� ��+���� �"���� ����� ������ ��!�� �"��#� ��!�� ��$/�� �� ��/��

������ �9&� �&.� �� ���� - .� thermal neutron decay logging �9� ��� #�

�&+ �9�#�; C/O logs.

2. When to Use Neutron Logs Neutron logs can be used in any borehole, open or cased, fluid-filled or air-

filled. The earliest neutron tools had a single detector, but almost all now have

2 detectors to reduce borehole effects. Neutron tools come in several varieties:

Dual-Spacing Neutron tool (DSN)

Dual-Spacing Epithermal Neutron tool (DSEN)

Compensated Neutron tool (CNL)

Page 2: Neutron Log

2

�9� - "�� �� ������ �#� �� ����� �� -� "� <��:� ��# �� Cased 7 =�&�� �"��/��� ��

=�6��� . ���� �� ���� �"��� ���# ��#�� �� 2�(#� �.� Detector �#� �/� �� �/# �>

��#� �� ���(#� �&�� �?� "�� . ���� �� ���� ��?� ���@���� ���+:

��� �� ��� ��+���� �"����) DSN(

��� �� ��� ��+���� �"���� � .� )DSEN( ��� �� ���� �$���� )CNL(

The standard dual-spacing neutron tools

detect thermal neutrons, and must have

fluid in the borehole to operate

correctly. The epithermal neutron tools

can be run in both air-filled and fluid-

filled boreholes. Both types can be run

in open or cased holes. In open hole the

DSN is usually run combined with

gamma ray and density tools..

��� �� ��� ��+���� �/"���� �/����) DSN(

2(�# ���� ���� � .� D/9� �� ��/#

E���) �"�� �� "�� ��F�(@� �#(� G.! . ���

�� ��� ��+���� �"���� � .�) DSEN (�#�

�� �- 9 �� @# �� ��> �/"&���� =�6��/� ��

�"����� . @#� �� �+���) DSN & DSEN( �#� �� - 9� �� ��> �/.��:�� �� �/�&F��

Cased . �� ��> �.��:�� DSN - 9� ���+ H� ���� ��(� ���� gamma ����#�� density.

Presentations and Scales:

A Neutron log will present φN, the neutron porosity, on a linear scale in tracks

2 and 3. The Lithology must be assumed (usually limestone or sandstone) to

determine φN.

When run in combination with the density, φN is overlaid on ρb or φd and it is

important that they be presented in compatible scales. This will facilitate

Lithology identification and gas detection. The two primary overlays are the

compatible sandstone and compatible limestone presentations.

��� - "�� �� ������ I � φN����� �� ���� �� ���� �/ � . D/9 I /��

�9����&�) �)� ���+ 9. - 9 �� 9. �&� (��.�� φN.

Page 3: Neutron Log

3

����+ - 9� ��.�,�� H� ����#� density, φN D# �� H� ����# �&�#� ρb �� ����� �/���#�

φd �)� ��6� �� �6�� ��#� �$� �� �� ���� ������) ��9���( . 8)� 2�� �6� 2 /���

�&+ �9����&� 2(#� K�F� .��# �� ����; ��) I + 9.� �&� � �9���� I /+�

9.� - 9� �9����. A. Compatible Sandstone Scales

Neutron:

60 to 0

Density:

1.65 to 2.65 g/cm3;

Neutron:

45 to -15

Density:

1.90 to 2.90 g/cm3)

B. Compatible Limestone Scales

Neutron:

30 to -10

Density:

2.00 to 3.00 g/cm3;

Neutron:

45 to -15

Density:

1.95 to 2.95 g/cm3)

Page 4: Neutron Log

4

3. Neutron Log measurement principle ���� ��� �� � � ����

Neutron Log is another kind of radioactive logs . It uses chemical or pulsed

sources to produce fast neutrons .These neutrons radiate into formation &

collide with nuclei of the atoms they encounter. The process slows down fast

neutron to a thermal state, which ultimately captured by a nucleus with induces

gamma ray emitted. The thermal neutron count rates and induced gamma rays

can be recorded to estimate hydrogen content or matrix lithology.

������ � "�� �� ������ �) 1�� �L �� ������ � "�� �+��(5 . �)� �����/� ��!/�

�"��# �� �$�� ����� ���� ���� �� �� . *8) ���� ���� H(� �� ��#�� �� !/�� M�/��

� 8� ��� �6&���� .�&��� N �� ���� ���� �� �� ��0 ���.� � .� , ���� �� ��6�� ���

*��� H� O���� ��(1 ���� O.�� . D�� =�!.0 ���� ���� � .� ��(�� ���� P�.�� �#� ��

�9�� ���� -��.� �9� �6� �� matrix lithology. Neutron classification by its energy

Neutron Name Energy

high energy > 10 Mev

fast 10 Mev~10 Kev

intermediate 10 Kev~10 ev

epithermal 10 ev ~ 0.1 ev

thermal about 0.03ev

Fast neutrons (4 MeV)

epithermal neutrons (0.l to 10 eV)

thermal neutrons(0.03 eV )

� Thermal neutrons are easily absorbed by certain atomic nuclei.

Chemical Sources

•This is how a neutron produced by americium and beryllium chemical sources

.

Page 5: Neutron Log

5

•This chemical source is called AmBe source (americium and beryllium),it is

commonly used by thermal neutron logging today. The pulsed source is usually

a non radioactive Tritium target , which could emit high energy neutron when

impacted by high energy electron stream

*8) ��!�� �"��#� ���� AmBe source (americium and beryllium) , �)� �����/�

���+ ��� � ��� ��� - "�� ���� ������ � .� . ��!��� �$��� �) ���/+ 2�/) �/�

Tritium Q H(� 7 -8�� �#� �� O�� ��� �� ��� ���+ ����+ �� ! ��� ��� �#� ���+

��� �.

Neutron interacts with formation ������ ��� � ���� �� �� ����

•Right Fig. shows three types of interaction a neutron will interact with mass

nucleus . Fast neutrons are slowed down during this process . Thermal

neutrons’ count rates are sensitive to hydrogen content for the reason that

neutron collide with hydrogen nucleus will lost largest amount of energy .

�� � �&+ ��� G$� O@� 1��� �� �@+�:�� �� ���� 2�� �/+�:� H/� �/&�# ��/�� .

���� ���� �� �� N �� �@� *8) �&��� . D�� =�!.0 ���� ���� � .� ���. -�/�.��

�9� �6� E�8� D��� ���!� �� ���� H� ��� �9� �6� 2�� ��: ��# � �# �� ��� � .

Neutron slowing down and captured �� ����� � � ����

Element σ elastic Number of

Collisions to 0.03

ev

Thermal σ capture

H 20.0 18 0.30

C 4.8 115 0.00

N 10.0 130 1.88

O 4.1 150 0.00

Na 3.5 215 0.51

Al 1.7 251 0.23

Si 1.5 261 0.13

Cl 10.0 329 .631

Ca 9.5 371 0.43

Hydrogen is an excellent material for slowing down neutrons, and chlorine is

an excellent Material form capturing thermal neutrons.

�9� �6� ���� �K���� =� �5 ���� ���� , ��� �&# ���� �K���� �; ���� ���� � .� .

Page 6: Neutron Log

6

Thermal Neutron Detector ����� � � ���� �����

4. Neutron porosity ������ � � ����

Dual-spacing Neutron Tool (DSN) has two detectors to eliminate borehole

effects . The ratio of near counts / far counts are directly used to estimate

porosity .The counts of each detector has approximately logarithmic relation

ship as below:

Log Φ = C - KN

��� �� ��� ��+���� �"���� �����) DSN (�6� ���(#� R&��&� �� �?� "�� . ���� =�!/.0

D �� ��0 ���� ������ � (��� ���� ������ .=�!.0 �# 2�(#� �6� ��@+ ��� Q�� �� ��

��# *@+� :

where Φ is porosity ;

N is near or far counts ;

K,C is constants related to tool, borehole size

and lithology.

Apparent neutron porosity and true porosity �������� �� �!�� �������� ��"�"���

The DSN tools are calibrated by pure limestone

filled with fresh water .

��� �� ��� ��+���� �/"���� �/����) DSN ( � �/+

� ��� 9. - 9 ��� =�&� *����� ��8��.

NSSΦ = NLSΦ + 6%

NLSΦ = NLSΦ

NDSΦ = NLSΦ - 3%

Page 7: Neutron Log

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5. Effect factors on φN #�� $�� %&'��� ()� ������ � � ����

A. Environmental Corrections �����*��� ��+���� DSN corrections for environmental factors are small but many.

The largest corrections are for borehole diameter, dh, and borehole temperature,

T.

��..!� DSN ����&� �"�� � F! �#� � �# . G.!�� �#; �) �� "�� dh �9 ��

� . "�� T. Corrections to open hole φNLS may be determined by following Figure.

Corrections for cased hole neutron logs use a similar chart that also includes

casing and cement thickness

��..!�� ��S� �.��:�� φNLS �#� �� ��.� ����� ����� . ��..!�� �S� � ���:���

Cased �9&� ��� ���� ����� �� P��(� -8� ��$� /� case E��� ����;.

B. Shale Effects �&,� �����

The response of the neutron log to a mixture of clean matrix, porosity, and

shale may be written in the form:

���9�� �9� ��� ���� &�� �� clean matrix � ����� � � �#� �� D�# � �!��� : φN= φT + Vsh φNsh Here φN is the log reading,

φT is the true, clean matrix neutron porosity,

Page 8: Neutron Log

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and Vsh φNsh is the bulk volume of shale times the neutron response in the shale.

�9. ��9 � � D� $� �� ���9�� �� ���� �� � �.

C. Gas effects on φN �&,� -�.�� ()� ������ � �����

At gas-bearing formation the porosity is

filled by gas , it lowers the density of pore

fluid. But neutron tool will see large amount

of gas as smaller volume of fresh water (

sometime 10:1 porosity lost) ,which lead to

abnormally lower porosity at gas-bearing

formation .This effect is also called

“excavation effect ”.

�� ���#�� �&��.� K�F&� ������ ��#� �=�&�� K�F��� ,

�)� I:�� ����#� �"��� ����� . �#� ��� �� ��/��

��2 - � ��# K�F� � �#� �9.# F! �� =�/��

���� ���� ���� ��0 ����� �$:��� Q ���+) 8�( (

�� ���#�� �&��.� K�F&� . 8) �?/�� ���/ �$/�

“excavation effect ”. .

Summary

In this topic , we discussed :

1. Neutron Log measurement principle (from neutron production, interaction to

detection) ;

2. Neutron Porosity ;

3. Gas effect on neutron porosity .

Question to be answered :

1. Make a classification to neutron according to its energy . Is epithermal

neutron log sensitive to formation fluid salinity ?

2. How does gas affect neutron porosity ? How to use CNL ,DEN and SONIC

curves to find gas-bearing formation ?