Neutron Capture Identification in Pulsed Neutron Logging
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Solution Overview
Problem
In oilfield logging, the use of radioisotope sources for density and neutron porosity measurements is hindered by regulatory issues and hazards, and the inability to separate inelastic and capture gamma-rays leads to contaminated signals, making precise porosity and gas saturation determination challenging.
Innovation Solution
Employing an electronic neutron generator to produce neutrons that generate gamma-rays through inelastic scattering, and using data processing to subtract thermal and epithermal capture backgrounds from total gamma-ray counts to isolate pure inelastic gamma-ray measurements, thereby separating formation density and hydrogen index signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If an electronic neutron generator is used to produce neutrons for gamma-ray measurements, then the hazards and regulatory issues associated with radioisotope sources are eliminated, but the ability to separate inelastic and capture gamma-rays becomes more difficult due to signal overlap
Solution Approach 1:
The electronic neutron generator emits neutrons in periodic bursts rather than continuously. By timing the gamma-ray detection to occur during specific intervals within each burst cycle, the method separates inelastic scattering signals (occurring immediately during the burst) from capture gamma-ray signals (occurring after the burst when neutron energy has decreased). This temporal gating approach resolves the signal overlap problem while maintaining the safety advantages of electronic neutron sources.
2Measurement precision
If neutron porosity tools use radioisotope sources to measure hydrogen index, then measurements can be obtained, but the sources cannot be shut off and are heavily regulated
Solution Approach 1:
The patent replaces the mechanical/chemical radioisotope source with an electronic neutron generator that uses electrical power to produce neutrons. This substitution allows the neutron source to be turned on and off by controlling the electrical power supply, eliminating the inability to shut off sources that characterizes radioisotope-based tools. The electronic system provides both measurement capability and operational flexibility.
3Measurement precision
If density tools use radioisotope sources to measure formation density, then density measurements can be obtained, but the sources pose safety hazards and regulatory challenges
Solution Approach 1:
The patent replaces the radioisotope gamma-ray source in density tools with an electronic neutron generator combined with gamma-ray detection. Instead of using a radioactive material that continuously emits gamma-rays, the electronic system uses controlled neutron emission followed by detection of resulting gamma-rays. This substitution eliminates the safety hazards and regulatory issues of radioisotope sources while maintaining the ability to measure formation density through inelastic scattering gamma-ray analysis.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for accurate determination of porosity and gas saturation by isolating independent signals, improving the precision of oilfield logging measurements and reducing contamination from hydrogen index responses.
Implementation Method 1
the electronic neutron generator may emit neutrons into a formation, which may in turn produce gamma-rays via inelastic scattering and neutron capture events
Implementation Method 2
After neutrons have decreased in energy below approximately 1 MeV, they typically have insufficient energy to inelastically scatter; however, they continue to lose energy by elastic scattering
Implementation Method 3
Neutrons which decrease in energy completely to thermal energy continue to elastically scatter at that energy, often for many hundreds of microseconds until they are captured and this may lead to the emission of one or more gamma-rays
Data Source
AI summary
The present disclosure is intended to overcome the problem of hydrogen contamination of the density signal. The approach is to compute the neutron capture portion of the total gamma ray counts and subtract it from the total counts resulting in a pure inelastic gamma ray measurement.