NMR Saturation Pulse Sequences for Axial Motion Compensation
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Solution Overview
Problem
Nuclear Magnetic Resonance (NMR) logging tools face challenges in effectively saturating nuclear spins during axial tool motion, leading to inaccurate Dual Wait Time (DTW) and T1 saturation recovery measurements, especially at high rates of penetration (ROP) greater than 20 m/h, due to the mismatch between the saturated region and the region where NMR echoes are generated.
Innovation Solution
The use of a downhole assembly with a magnet arrangement and RF coils that generate a static and RF field, employing a first pulse sequence for saturation and a second sequence to generate signals only from the saturated region, while allowing for axial tool movement between the sequences, and a computer-readable medium to determine formation properties like T1 and T2 using signals acquired during axial motion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If axial tool motion is allowed during NMR saturation, then productivity is improved, but measurement precision deteriorates due to mismatch between saturated region and echo generation region
Solution Approach 1:
The patent applies dynamics by making the saturation pulse frequency可调 (adjustable) to match the Larmor frequency at different axial positions during tool motion. The system dynamically adjusts the RF pulse frequency based on the expected axial displacement, ensuring continuous resonance condition throughout the saturation period even as the tool moves through the formation.
Solution Approach 2:
The patent changes the frequency parameter of the saturation RF pulses to compensate for axial tool motion. By adjusting the pulse frequency to match the varying Larmor frequency experienced during motion, the system maintains effective saturation across the moving tool's measurement zone, resolving the mismatch between saturated and echo-generation regions.
2Reliability
If saturation pulse sequence is applied for extended period, then saturation completeness is improved, but tool movement increases causing region mismatch
Solution Approach 1:
The patent changes the frequency parameter of saturation pulses to adapt to tool motion during the extended saturation period. This allows the saturation sequence to run longer for complete saturation while compensating for axial displacement through frequency adjustment, preventing region mismatch despite increased tool movement.
Solution Approach 2:
The system dynamically adjusts pulse frequency during extended saturation sequences to account for continuous tool motion. This dynamic adaptation enables reliable complete saturation without requiring strict control of axial displacement, as the frequency tracking compensates for the tool's movement through the formation.
3Adaptability or versatility
If wide-band saturation pulses are used, then insensitivity to lateral tool motion is improved, but axial motion problem remains unresolved
Solution Approach 1:
The patent applies parameter changes by adjusting the frequency of saturation pulses to track the Larmor frequency variations caused by axial tool motion. This frequency adaptation specifically addresses axial motion compensation, complementing the wide-band approach's lateral motion insensitivity and resolving the remaining axial motion problem.
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 ensures effective saturation and accurate measurement of formation properties even at high ROP, improving the reliability of NMR logging by maintaining signal integrity during tool movement, thereby enhancing the determination of hydrocarbon presence and rock formation characteristics.
Implementation Method 1
The NMR tools generate a uniform or near uniform static magnetic field in a region of interest surrounding the wellbore
Implementation Method 2
The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality. Over time the nuclear spins align themselves along an externally applied magnetic field
Implementation Method 3
This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field, which tips the spins with resonant frequency within the bandwidth of the oscillating magnetic field away from the static field direction
Implementation Method 4
The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality
Implementation Method 5
The receiving coil is designed so that a voltage is induced by the precessing spins
Implementation Method 6
At the same time, the spins return to the equilibrium direction (i.e., aligned with the static field) according to an exponential decay time known as the spin-lattice relaxation time or T1
Implementation Method 7
After tipping, the spins precess around the static field at a particular frequency known as the Larmor frequency ω0
Data Source
AI summary
Saturation pulse sequences are designed to ensure complete saturation of nuclear spins for dual wait time measurements and saturation recovery measurements in the case of axial motion of a downhole NMR logging tool. Frequency and/or phase modulation may be used. An auxiliary saturation coil may be used.