Segmented Permanent Magnet Arrangement for NMR Logging Tool
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Solution Overview
Problem
Nuclear magnetic resonance (NMR) logging tools face challenges in reducing attractive forces with magnetic steel casings, particularly in smaller wellbores, which prevents them from dropping down the wellbore and affects measurement accuracy due to the need for large permanent magnets that generate high attractive forces.
Innovation Solution
A three-part permanent magnet arrangement is used in the NMR logging tool, with magnets positioned at different longitudinal sections to minimize attractive forces while maintaining a uniform static magnetic field and effective pre-polarization within the antenna window, allowing the tool to operate in wellbores of various sizes and deviations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large permanent magnet is used to generate a strong static magnetic field for NMR measurements, then measurement precision is improved, but attractive force with magnetic steel casing increases causing the tool to stick in the wellbore
Solution Approach 1:
The permanent magnet is divided into multiple segments (first permanent magnet, second permanent magnet, third permanent magnet) positioned at different longitudinal locations. This segmentation allows the magnetic field to be distributed along the tool length, reducing the concentrated attractive force on the casing while maintaining the overall field strength needed for NMR measurements.
Solution Approach 2:
The patent transitions from a single-point magnetic field source to a distributed linear array of magnets along the longitudinal axis. This dimensional change from point to line distribution reduces the peak attractive force density while maintaining total magnetic field exposure for the formation, enabling the tool to pass through cased holes without sticking.
2Ease of operation
If a pad is added to reduce magnetic attractive force, then the tool can pass through casing, but device complexity and bulk increase making it unsuitable for smaller wellbores
Solution Approach 1:
The patent removes the separate pad component traditionally used to reduce magnetic attraction and instead integrates the attraction-reduction function directly into the permanent magnet structure itself. The segmented magnet arrangement inherently reduces attractive force without requiring an additional mechanical pad, simplifying the overall tool design.
Solution Approach 2:
The patent combines multiple functions into the permanent magnet structure: it generates the static magnetic field for NMR measurements and simultaneously reduces attractive force on the casing through its segmented arrangement. This eliminates the need for separate components and reduces overall tool complexity.
3Power
If a single large permanent magnet is used, then a strong static magnetic field is generated, but the tool cannot operate in wellbores of various sizes and deviations
Solution Approach 1:
The patent creates a flexible, adaptable magnetic field configuration by using multiple adjustable magnet segments that can be positioned at different longitudinal locations. This dynamic arrangement allows the tool to adapt to various wellbore sizes and deviations while maintaining adequate magnetic field strength for NMR measurements in different geological formations.
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 arrangement reduces undesirable attractive forces, enabling the NMR tool to pass through wellbores of any size and deviation without becoming stuck, while providing accurate measurements by ensuring proper pre-polarization and maintaining a uniform magnetic field for effective logging operations.
Implementation Method 1
a first permanent magnet disposed at a first longitudinal position within the NMR logging tool... a second permanent magnet disposed at a second longitudinal position within the NMR logging tool, the second permanent magnet located between the first permanent magnet and a third permanent magnet... to provide a static magnetic field within an antenna window
Implementation Method 2
NMR tools generally include a large permanent magnet used to impose a static magnetic field downhole to preferentially align certain nuclei in the formation and thereby produce a bulk magnetization
Implementation Method 3
The precessing nuclei generate a detectable radio frequency signal that can be used to measure statistical distributions of T1 and T2
Data Source
AI summary
A nuclear magnetic resonance (NMR) logging tool for use in wellbore logging operations is provided. The NMR logging tool features a three-part permanent magnet arrangement that reduces the attractive force between the NMR tool and casing, allowing the NMR tool to be lowered through wellbores of any size. Additionally, the disclosed arrangement of permanent magnets in the NMR tool may facilitate an effective pre-polarization of portions of the formation at different logging speeds. Further, the disclosed arrangement of permanent magnets in the NMR tool may provide a uniform or substantially uniform static magnetic field within the antenna window of the radio frequency (RF) antenna used to perform the measurements. The second magnet may be positioned between the first and third magnets in a longitudinal direction along the NMR tool, and a radio frequency (RF) antenna may be disposed adjacent the first magnet.