Neutron-Gamma Density Correction for Low Porosity Formations
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Solution Overview
Problem
Existing neutron-gamma density (NGD) measurements are less accurate in formations with low hydrogen index or low porosity and those containing heavy elements, due to inadequate accounting for fast neutron transport, especially when downhole tools lack optimal configurations or fast neutron detectors.
Innovation Solution
A downhole tool system with a neutron generator, neutron detectors, and gamma ray detectors, along with data processing circuitry that applies corrections to neutron and gamma ray count rates using a neutron transport correction function, estimates apparent porosity and fast neutron signals to determine accurate NGD measurements, even without a fast neutron detector or optimal detector spacing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional NGD measurement techniques are used without correction functions, then the measurement process is simple and fast, but the measurement precision deteriorates in low-porosity formations and formations with heavy elements
Solution Approach 1:
The patent applies preliminary action by pre-calculating and storing correction functions based on formation characteristics (porosity, hydrogen index, heavy element content) before actual measurement. During measurement, the system selects and applies the appropriate pre-computed correction function based on detected formation properties, avoiding real-time complex calculations while maintaining high accuracy across diverse formation types
Solution Approach 2:
The patent implements parameter changes by dynamically adjusting measurement parameters (neutron energy levels, detection thresholds, correction factors) based on the detected formation characteristics. The system modifies operational parameters adaptively to optimize measurement accuracy for specific formation conditions such as low porosity or heavy element presence
2Measurement precision
If downhole tools are equipped with optimal configurations including fast neutron detectors, then measurement precision improves, but device complexity and manufacturing cost increase
Solution Approach 1:
The patent applies copying by using detected neutron and gamma ray count rates to create a virtual representation of formation properties, then applying correction functions that replicate the effect of having optimal detector configurations. This allows standard tool configurations to achieve accuracy comparable to optimized configurations through software-based correction rather than hardware modification
Solution Approach 2:
The system compensates for suboptimal detector configurations by dynamically changing processing parameters and correction factors based on detected formation characteristics, effectively adapting the measurement process to achieve accuracy equivalent to optimal configurations without requiring optimal hardware
3Measurement precision
If detector spacing is optimized for each formation type, then measurement precision improves, but the ease of operation deteriorates due to need for formation characterization first
Solution Approach 1:
The patent performs preliminary formation characterization automatically before density measurement by analyzing neutron and gamma ray count rates to determine porosity, hydrogen index, and heavy element content. Based on these preliminary results, the system automatically selects the appropriate correction function, eliminating the need for manual formation identification and simplifying operator workflow while maintaining accuracy
Solution Approach 2:
The system implements feedback by using initial measurement data (neutron and gamma ray count rates) to automatically determine formation characteristics and adjust the measurement process accordingly. The feedback loop continuously refines the measurement by selecting appropriate correction functions based on detected formation properties, maintaining accuracy without requiring manual intervention
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
The system achieves accurate NGD measurements for a broad range of formations, including those with low hydrogen index, low porosity, and heavy elements, maintaining accuracy despite suboptimal tool configurations, by correcting for fast neutron transport and detector placement issues.
Implementation Method 1
emit neutrons into a formation such that some of the neutrons inelastically scatter off elements of the formation and generate inelastic gamma rays
Implementation Method 2
the gamma ray detectors may detect first and second count rates of inelastic gamma rays that Compton scatter through the formation to reach the downhole tool
Data Source
AI summary
Systems, methods, and devices are provided to determine an accurate neutron-gamma density (NGD) measurement for a broad range of formations, including low-hydrogen-index or low-porosity formations and formations with heavy elements. For example, such an NGD measurement may be obtained by emitting neutrons into a formation such that some of the neutrons inelastically scatter off elements of the formation and generate inelastic gamma rays. The neutrons and inelastic gamma rays that return to the downhole tool may be detected. Some characteristics of certain formations are believed to affect the fast neutron transport of the formations. Thus, if a formation has one or more of such characteristics, a correction may be applied to the count rate of neutrons, the count rate of inelastic gamma rays, or the neutron transport correction function, upon which the neutron-gamma density (NGD) may be determined.