Pulsed Neutron Logging with Late-Gate Spectra for Borehole Background
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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 spectrum measurements that isolate the formation signal by distinguishing between borehole and formation capture regimes, thereby reducing borehole signal contamination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If continuous neutron radiation is used for gamma ray spectroscopy measurements, then measurement productivity is improved, but borehole capture background interference increases
Solution Approach 1:
The patent applies periodic pulsed neutron radiation instead of continuous radiation. The neutron source emits neutrons in periodic pulses with specific duty cycles (e.g., 10-90% duty cycle), creating distinct on-periods for neutron emission and off-periods for signal detection. This periodic action allows separation of borehole capture signals (which occur primarily during and immediately after pulse emission) from formation capture signals (which dominate during the off-period), thereby reducing borehole background interference while maintaining measurement productivity through efficient use of pulse sequences.
2Measurement precision
If time-gated energy spectrum measurements are performed during the off-period, then borehole signal contamination is reduced, but measurement time increases
Solution Approach 1:
The patent performs preliminary signal separation by gating measurements specifically during the off-period of the pulsed neutron cycle, before borehole capture signals can contaminate the formation signal. By pre-defining time gates that exclude the borehole-dominated early period and focus on the formation-dominated later period, the method achieves signal purification without requiring additional measurement passes or extended total measurement time.
Solution Approach 2:
The patent dynamically adjusts measurement parameters including duty cycle (10-90%), pulse width, and gate timing to optimize the balance between borehole signal rejection and measurement efficiency. The dynamic gating strategy allows flexible adaptation of measurement windows based on specific formation and borehole conditions, maximizing signal quality while minimizing total measurement time through intelligent parameter selection rather than fixed, conservative timing.
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 enhances the accuracy of elemental yield and concentration measurements by minimizing borehole signal interference, leading to more reliable geochemical logging results.
Implementation Method 1
A pulsed neutron generator is used to produce sharp and narrow neutron pulses
Implementation Method 2
neutron-induced gamma ray spectroscopy
Implementation Method 3
gamma ray scintillator detectors
Implementation Method 4
borehole and formation capture regimes
Implementation Method 5
selecting a decay window in which the one or more measurements are performed
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.