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.

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

?? 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.