Neutron Log

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 (

   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.