Pulsed Neutron Decay Correction for Formation Sigma
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Solution Overview
Problem
Accurate and precise determination of formation thermal neutron capture cross section (sigma) from pulsed neutron well logging instruments is challenging due to wellbore decay contamination and thermal neutron diffusion, especially in conditions with high chloride content fluids and small formation sigma values, leading to dual exponential decay that is not always clearly identifiable.
Innovation Solution
A method is developed to determine the formation thermal neutron decay rate by calculating multiple apparent decay rates at different time windows post-neutron burst and applying a decay rate correction factor based on wellbore capture cross section and fluid parameters to isolate the formation thermal neutron decay rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If thermal neutron die-away measurements are used to determine formation sigma, then formation capture cross section can be measured, but wellbore decay contamination and thermal neutron diffusion cause the apparent decay constant to be unrepresentative of the intrinsic formation decay constant
Solution Approach 1:
The measurement time window is segmented into multiple intervals, with different time windows used to calculate separate apparent decay constants. This allows the method to capture different aspects of the neutron population decay behavior and apply appropriate corrections for wellbore contamination and diffusion effects at different stages of the decay process
Solution Approach 2:
The method changes the parameter of measurement timing by using multiple different time windows to calculate apparent decay constants. By varying the time window parameters and selecting optimal windows, the method can minimize the influence of wellbore decay contamination and thermal neutron diffusion while maximizing the representation of intrinsic formation decay characteristics
2Adaptability or versatility
If measurements are made in wellbores with high chloride content fluids and small formation sigma values, then dual exponential decay occurs, but the dual exponential decay is not always clearly identifiable in the neutron population data
Solution Approach 1:
Instead of requiring clear identification of dual exponential decay components, the method applies partial correction by using multiple time windows to capture different aspects of the decay behavior. This approach provides sufficient correction for wellbore effects even when the dual exponential nature is not clearly identifiable, allowing measurements to proceed across various conditions without requiring definitive identification of decay modes
3Ease of operation
If a single apparent decay constant is used to represent formation decay, then the measurement process is simplified, but the decay constant is not always representative of the intrinsic formation decay constant due to wellbore and diffusion effects
Solution Approach 1:
The measurement process is segmented into multiple steps where apparent decay constants are calculated from different time windows. This segmentation allows the method to maintain operational simplicity while improving accuracy, as the multi-window approach can be systematically applied without requiring complex real-time analysis or interpretation of decay modes
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 method effectively corrects for wellbore contamination and diffusion effects, providing accurate formation sigma measurements across various conditions with minimal error, even in crossover scenarios where formation and wellbore decays overlap.
Implementation Method 1
imparting at least one controlled duration burst of high energy neutrons into formations surrounding a wellbore includes determining a first apparent decay rate of thermal neutrons
Implementation Method 2
capture gamma rays resulting from imparting at least one controlled duration burst of high energy neutrons into formations
Implementation Method 3
The decay of the thermal neutron population after a 'burst' of high energy (in the one million electron volt and above energy range) neutrons from the PNG is close to exponential
Data Source
AI summary
A method for determining a formation thermal neutron decay rate from measurements of radiation resulting from at least one burst of high energy neutrons into formations surrounding a wellbore includes determining a first apparent neutron decay rate in a time window beginning at a first selected time after an end of the at least one burst, a second apparent decay rate from a time window beginning at a second selected time after the burst and a third apparent decay rate from a third selected time after the burst. The second time is later than the first time. A thermal neutron capture cross section of fluid in the wellbore is determined. A decay rate correction factor is determined based on the first and second apparent decay rates and a parameter indicative of the wellbore capture cross-section. The correction factor is applied to the third apparent decay rate to determine the formation thermal neutron decay rate.