Time-Gated Neutron Spectra for Borehole Background Removal
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Solution Overview
Problem
Existing geochemical logging tools face challenges in reducing borehole gamma ray background interference during neutron-induced gamma ray spectroscopy, which complicates the analysis of formation elemental composition.
Innovation Solution
A pulsed neutron generator is used to produce sharp and narrow neutron pulses, allowing for time-gated energy spectra measurement that isolates the formation signal by timing the neutron bursts to minimize borehole interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If continuous neutron emission is used for gamma ray spectroscopy measurements, then sufficient neutron flux is available for measurement, but borehole gamma ray background interference increases
Solution Approach 1:
The patent applies periodic action by using pulsed neutron emission instead of continuous emission. The neutron source emits neutrons in periodic pulses with specific duty cycles (e.g., 10-50% duty cycle where emission occurs for 10-50 microseconds followed by idle time), allowing the borehole background to decay between pulses while maintaining sufficient average neutron flux for formation analysis.
Solution Approach 2:
The patent applies preliminary action by implementing a decay window period immediately following each neutron pulse during which no new neutrons are emitted. This decay window (e.g., 100-1000 microseconds duration) allows the borehole gamma ray background to decay before the next pulse, proactively reducing interference before measurements are taken during the subsequent pulse cycle.
2Object-affected harmful factors
If pulsed neutron emission with decay windows is used, then borehole background interference is reduced, but measurement time increases
Solution Approach 1:
The patent optimizes the periodic pulse cycle duration and duty cycle to balance background reduction with measurement efficiency. By adjusting parameters such as pulse width (e.g., 100 microseconds), idle time (e.g., 900 microseconds), and duty cycle (e.g., 10%), the system achieves effective borehole background suppression while maintaining practical measurement speeds for logging operations.
Solution Approach 2:
The patent applies parameter changes by dynamically adjusting neutron pulse characteristics including pulse width, pulse repetition frequency, and duty cycle based on formation properties and borehole conditions. These parameter optimizations allow the system to achieve adequate background suppression with minimal measurement time loss by adapting to different geological scenarios.
Data Source
AI summary
A method and systems for taking downhole measurements. The method may include selecting a pulsing scheme for taking one or more measurements using a pulsed neutron logging tool, selecting a neutron burst width for the pulsing scheme based at least in part on a neutron tube utilized by the pulsed neutron logging tool to form at least in part a neutron burst train, and selecting a decay window in which the one or more measurements are performed by the pulsed neutron logging tool. The method may further include selecting a starting time for a late gate in which the one or measurements are taken, disposing the pulsed neutron logging tool into a borehole, performing the neutron burst train with the pulsed neutron logging tool, and performing the one or more measurements with the pulsed neutron logging tool during the late gate.