NMR Logging Tool Motion Error Reduction via Pulse Sub-sequences
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Solution Overview
Problem
Downhole NMR sensors face challenges in accurately measuring subsurface formations due to lateral motion and vibration during logging operations, leading to distortion of NMR data and inability to acquire reliable transversal NMR relaxation signals, especially in LWD and MWD applications.
Innovation Solution
The solution involves selecting a flip angle for refocusing RF pulses based on estimated lateral motion errors, using at least two Carr Purcell Meiboom Gill (CPMG) sequences with different flip angles, and setting the flip angle of refocusing pulses equal to or smaller than 90 degrees to reduce motion effects and improve signal-to-noise ratio (SNR).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of stationary object
If higher power RF pulses are used to increase the radial extent of the sensitivity area, then the sensitivity volume is increased, but the power consumption increases and may exceed available downhole power
Solution Approach 1:
The patent changes the parameter of RF pulse duration (length) to achieve broader bandwidth excitation. By using longer duration RF pulses instead of higher power short pulses, the system increases the radial extent of the sensitivity area while maintaining power consumption within available downhole power limits.
2Measurement precision
If conventional NMR logging is performed without accounting for lateral motion, then the measurement process is simple, but the NMR data is distorted and unreliable
Solution Approach 1:
The patent applies preliminary action by determining the flip angle of refocusing RF pulses based on estimated lateral motion errors before acquiring NMR data. This pre-calculation of the appropriate flip angle compensates for expected motion effects, allowing accurate NMR measurements even in the presence of lateral motion without requiring complex real-time correction systems.
3Measurement precision
If the flip angle of refocusing RF pulses is set to reduce motion effects, then motion-induced errors are reduced, but the signal-to-noise ratio may be affected
Solution Approach 1:
The patent optimizes the flip angle parameter by calculating it as a function of the product of the gradient magnitude and the maximum possible amplitude of lateral displacement. This optimized flip angle simultaneously reduces motion-induced errors and maintains an acceptable signal-to-noise ratio, achieving a balance between motion compensation and signal quality.
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 and reliability of NMR measurements by reducing motion-induced errors, allowing for more precise formation evaluation and increased SNR, even in conditions with significant lateral displacement.
Implementation Method 1
nuclear magnetic resonance (NMR) logging for formation evaluation
Implementation Method 2
a coil assembly that generates radio frequency (RF) control signals and detects magnetic resonance phenomena in the subsurface material
Data Source
AI summary
In some embodiments, a method includes generating at least a first pulse sub-sequence comprising a number of refocusing pulses at a first flip angle and generating at least a second pulse sub-sequence comprising a number of refocusing pulses at a second, lower flip angle. The method further includes detecting, by a nuclear magnetic resonance (NMR) sensor of a logging tool positioned in a wellbore formed in a subsurface formation, NMR spin-echo signals generated in response to at least the first pulse sub-sequence to acquire a first dataset of transversal NMR relaxation data, detecting, by the NMR sensor, NMR spin-echo signals generated in response to at least the second pulse sub-sequence to acquire a second dataset of transversal NMR relaxation data, and determining a property of the subsurface formation based on the first and second datasets of transversal NMR relaxation data.