Optimization of nuclear magnetic resonance refocusing pulses to enhance signal intensity in gradient B0 field

Li, Xin; Xiao, Lizhi; Liu, Huabing; Zhang, Zongfu; Bao-Xin, Guo; Hui-Jun, Yu; Zong, Fangrong

doi:10.7498/aps.62.147602

Cited by 7 publications

(1 citation statement)

References 13 publications

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“…Suboptimal acquisition parameters in the pulse sequence can exacerbate these effects, leading to complications in the NMR response, introduction of image artifacts, and inaccuracies in the derived petrophysical parameters. However, the exact underlying mechanism remains unclear. − Therefore, it is imperative to discuss the influence of the pulse sequence properties used on NMR responses.…”

Section: Introductionmentioning

confidence: 99%

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T₂-y Method in Low-Field NMR

Liang,

Jia,

Xiao

et al. 2024

Energy Fuels

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The low-field nuclear magnetic resonance (LF NMR) technique proves valuable in determining porosity, permeability, pore size, and wettability through T 2 measurements in rocks. Recently, there has been growing interest in low-field nuclear magnetic resonance imaging (LF MRI) technology, which can provide sliced T 2 distributions and position profiles for the oil industry. However, there is a lack of detailed observation and discussion on the relevant application limitations and influencing factors. This work addresses this gap by presenting a comprehensive experiment and numerical investigation aimed at exploring T 2 -y maps for high-porosity sandstone and low-porosity dolomite cores using a phase-encoded T 2 imaging method. First, experiments were conducted to obtain sliced porosity and permeability for estimating rock heterogeneity along the core height. It was noted that T 2 components shorter than 0.3 ms were overlooked, leading to underestimated NMR porosity when comparing MRI-projected T 2 distributions with bulk T 2 distributions. Then, typical micropore modeling and magnetization evolution were employed to simulate and discuss factors, affecting the accuracy of the MRI T 2 spectra and image profiles. These factors include the off-resonance frequency caused by the external static magnetic field or internal field, the imaging-encoded time, the excitation pulse angle, the refocusing pulse angle, and gradient properties. The results showed that field inhomogeneity significantly influenced MRI-T 2 relaxation, particularly at high off-resonance frequencies. It was found that minimizing the image-encoded time is ideal for measuring short relaxation components. Additionally, the excitation pulse angle greatly impacted the amplitude of the T 2 distributions, whereas the refocusing pulse angle affected both the amplitude and the peak values of the T 2 spectra, especially in higher magnetic field inhomogeneity. Increasing gradient strength and duration were beneficial for imaging profiles but detrimental to the T 2 distribution and porosity. The investigation offers both practical guidance and theoretical insight for undertaking T 2 -y measurements in rocks, facilitating the optimization of the pulse sequence and the acquisition conditions, as well as the manipulation of the magnetization data.

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Section: Introductionmentioning

confidence: 99%

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T₂-y Method in Low-Field NMR

Liang,

Jia,

Xiao

et al. 2024

Energy Fuels

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Numerical investigating the low field NMR response of representative pores at different pulse sequence parameters

Fan

Liu

et al. 2021

Computers & Geosciences

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Optimization of shaped pulses for radio frequency excitation in NMR logging

Guang-hui

Xiao

Luo

et al. 2021

Review of Scientific Instruments

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The radio frequency (RF) excitation pulse of the nuclear magnetic resonance (NMR) logging tool can realize slice measurement by designing shaped pulses. In the case of a certain main magnetic field, the accuracy of the shaped pulse design has a very important impact on the signal-to-noise ratio (SNR) of the NMR signal and the measurement of the short relaxation signal. Hard pulse excitation will produce an undesirable infinite number of side lobes that may perturb the spins in unwanted regions. Soft pulse can achieve selective excitation and has a better slice profile and shorter energy release time while it is not conducive to the measurement of short relaxation signals. This article focuses on the design of shaped pulses in extreme downhole environments and analyzes the characteristics of the three shaped pulses in the two cases of equivalent bandwidth and equivalent pulse duration. At the same time, a kind of RF-shaped pulse transmitting circuit with phase difference control is realized. According to the pulse type optimization strategy, the appropriate shaped pulse is selected. When echo spacing (TE) >0.6 ms, the SNR can be increased to more than 12%. When TE is small, it will automatically switch to the hard pulse mode, which is good for short relaxation measurement.

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Optimization of nuclear magnetic resonance refocusing pulses to enhance signal intensity in gradient B0 field

Cited by 7 publications

References 13 publications

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T₂-y Method in Low-Field NMR

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T₂-y Method in Low-Field NMR

Numerical investigating the low field NMR response of representative pores at different pulse sequence parameters

Optimization of shaped pulses for radio frequency excitation in NMR logging

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Optimization of nuclear magnetic resonance refocusing pulses to enhance signal intensity in gradient B0 field

Cited by 7 publications

References 13 publications

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T2-y Method in Low-Field NMR

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T2-y Method in Low-Field NMR

Numerical investigating the low field NMR response of representative pores at different pulse sequence parameters

Optimization of shaped pulses for radio frequency excitation in NMR logging

Contact Info

Product

Resources

About

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T₂-y Method in Low-Field NMR

Application Limits and Influencing Factors in Characterization of Rock Cores Using the Phase-Encoded T₂-y Method in Low-Field NMR