Theory Behind Neutron Logging

Theory Behind Neutron Logging

In neutron logging, three processes are essential:


1- Neutron Emission:

  • Neutron tools emit high-energy (4.5?MeV) neutrons from a radioactive source comprising an alpha emitter (e.g., radium, plutonium, or americium) and beryllium-9.
  • Alpha particles interact with beryllium-9, producing carbon-12, fast neutrons, and gamma rays.

2- Neutron Scattering:

  • Fast neutrons undergo elastic scattering with atomic nuclei, losing energy and slowing down.
  • Energy loss is most efficient when colliding with nuclei of similar mass, notably hydrogen.
  • Neutrons slow from fast (>0.5?MeV) to thermal energies (<0.1?eV) through interactions, primarily with hydrogen.


Energy loss per collision depends on the target nucleus's relative mass and the scattering cross section.


Efficiency of hydrogen, silicon, and oxygen atoms in slowing down fast neutrons in a clean sandstone (φ?=?0.15).


3- Neutron Absorption:

  • Thermal and epithermal neutrons are absorbed by formation nuclei.
  • Hydrogen and chlorine are significant neutron absorbers in formations.
  • Neutron absorption leads to gamma-ray emission, detectable by some neutron logging tools.


Neutron Logs and Their Main Applications

Neutron logs are primarily used for:

  • Porosity Determination: Often combined with the density tool.
  • Gas Detection: In conjunction with density or sonic tools.
  • Shale Volume Estimation: Alongside the density tool.
  • Lithology Indication: Using both neutron and density or sonic logs.
  • Formation Fluid Typing
  • Applicable in Open and Cased Hole Logging


The amount of energy lost at each collision depends on the relative mass of the target nucleus and the scattering cross section.

?? Note: Hydrogen plays a crucial role in slowing down neutrons due to its similar size, causing significant energy loss during collisions.

Types of Neutron Logging Tools

There are three main types of neutron tools:

1- Gamma Ray/Neutron Tool (GNT):

  • Equipped with a neutron source and a single detector sensitive to high-energy capture gamma rays and thermal neutrons.
  • Can be run in both open and cased holes, typically centered.
  • Tool diameters: 3-3/8 inches for open holes; 1-11/16 or 2 inches for cased holes.
  • Source-to-detector spacing ranges from 15.5 to 19.5 inches, depending on the manufacturer.

2- Sidewall Neutron Porosity Tool (SNP):

  • Designed exclusively for open holes.
  • Features a source and a single detector with a 16-inch spacing.
  • Mounted on a skid pressed against the borehole wall to reduce mud and mudcake effects.
  • Detector sensitive to epithermal neutrons, unaffected by chlorine in high-salinity muds and formation fluids.


3- Compensated Neutron Log (CNL):

  • Sensitive to thermal neutrons with two detectors at 15 and 25 inches from the source; affected by the chlorine effect.
  • Run eccentered in the hole using an arm to press against the borehole wall, minimizing mud type influence.


Factors Affecting Neutron Logs

1- Chlorine Effect:

  • Thermal neutron tools measure neutrons and gamma rays from neutron capture.
  • Hydrogen and chlorine significantly contribute to neutron absorption.
  • High chloride content in drilling mud or formation fluids reduces the detected neutron flux, leading to overestimated porosity.

2- Shale Effect:

  • Shales contain clays with bound water, increasing hydrogen content despite low porosity.
  • Neutron logs in shales read higher apparent porosity than actual.

3- Gas Effect:

  • Gas-filled porosity lowers the hydrogen density.
  • Neutron tools interpret gas zones as lower hydrogen content, resulting in abnormally low porosity readings in gas-bearing formations.


Accelerator Porosity Sonde

This tool combines responses from multiple detectors to compensate for lithology and matrix density effects:

1- Near-to-Far Measurement:

  • Exhibits greater sensitivity to shale and gas effects.
  • Provides a response similar to conventional compensated neutron tools.

2- Near-to-Array Measurement:

  • Used to determine formation porosity.
  • Offers a vertical resolution of 1 foot.

3- Epithermal Array Detectors:

  • Monitor and correct for tool standoff effects.

4- Thermal Detector:

  • Determines porosity by detecting neutrons rather than gamma rays, unlike conventional pulsed neutron tools.



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