Emitted neutrons interact with matter through elastic scattering, where they lose some energy, or inelastic scattering, where the target nucleus absorbs neutrons and emits slower neutrons and gamma rays. The amount of energy lost depends on the mass of the nucleus collided with. Hydrogen causes the highest energy loss at 67% per collision due to its similar mass to neutrons. Heavier nuclei cause less energy loss. Neutron logs measure formation hydrogen content to indicate porosity and lithology. There are two main types - single neutron pad logs detecting epithermal neutrons over short intervals, and compensated neutron logs detecting thermal neutrons over deeper intervals. Neutron logs are used to determine porosity, delineate porous formations

2. Theory
• Emitted neutrons interact with matter by:
Elastic scattering
• Elastic collisions involving some loss of energy (small nucleus) and no
loss of energy(heavy nucleus)

3. Theory
• Emitted neutrons interact with matter by:
Inelastic scattering
• Inelastic scattering where target nucleus absorbs fast neutronsand
emits a slower neutron and gamma rays

4. Effect of target size on energy loss
• Energy lost by collisions is dependent on mass difference
• Hydrogen approximately same mass as neutron, thus highest loss of
energy (67%) per collision
• The heavier the nucleus, the less energy lost
Hydrogen content
– Water, oil HI = 1.0
– gas HI = 0.5 - 0.75
– shale HI = 0.17 - 0.42
– clean rock HI = 0.0

5. Principle of Operation
• Neutrons emitted from radioactive source
• Collide and lose energy (Billiard ball effect)
• Primarily dependent on hydrogen concentration or
index
• Detect either epithermal neutrons, thermal
neutrons, capture gamma rays or combination
• Thus, measures the formations ability to attenuate
the passage of neutrons

6. Types
• Two types of neutron logs are commonly used
1. Single Neutron Pad log (SNP)
2. Compensated Neutron Log (CNL)

7. Sidewall Neutron Porosity log (SNP)
• Tools:
Pad-type single detector.
• Detects:
Epithermal neutrons.
• Depth interval:
Shallow, 8”

8. Compensated Neutron Log (CNL)
• Tool:
Mandrel-type 2 detectors.
• Detects:
Thermal Neutrons
• Depth interval:
Deeper, 12”

9. Compensated Neutron Log (CNL)

10. Neutron log

11. Uses
1. Determination of porosity / Lithology.
2. Delineation of porous formations.
3. CNL can be used in cased hole

12. • Unit:
Neutron log is measured in count/minute
• Limitations:
1. In shale there is weak hydrogen bonding, so neutrons are captured
by hydrogen and loss of neutron occurred. This limitation are
overcome by knowledge of seismic log.
2. In case of gas in reservoir rock the gas molecules are too far from
one another so neutron are not lost shows low porosity then the
actual porosity. This limitation is overcome by the density log
identify the presence of gas.
3. In case of gypsum (CaSO4 .2H2O), water molecule are present in
gypsum. So neutron is captured by hydrogen show shows more
neutron log value but there is no fluid in the formation.

13. THANKS