电缆核磁共振测井仪探测特性研究

磁场形状和分布对核磁共振测井仪探测特性有决定性的作用. 该文利用电磁场有限元方法,模拟分析了国际主流核磁共振测井仪MRIL-P、CMR、 MREx、以及MR Scanner等4种仪器的磁场分布,考察了探头结构参数对磁场分布的影响,并进一步分析了仪器探测特性及其影响因素,同时对探头磁场形状进行了优化匹配,确定了探头的结构设计,并对比分析了4种探头之间的优缺点. 结果表明,梯度磁场方案优于均匀磁场方案; 居中型探头结构相对简单,信号量大,但其测量信号受钻井液影响; 贴井壁型探头结构相对复杂,信号量较小,但其设计灵活,测量信号不受钻井液影响.     

核磁共振测井仪探头设计中的数值方法

核磁共振测井仪探头的优化设计能够增强仪器的探测特性,提高仪器的信噪比, 而探头设计中的数值方法对设计结果至关重要.本文利用电磁场有限元方法对贴井壁型核磁共振测井仪探头静磁场和射频场进行了2D和3D的数值模拟,深入分析了数值模型形状、模型尺寸、 单元形状对数值模拟结果的影响,并将有限元数值模拟结果与实测数据做了对比. 结果显示:数值模拟结果与实测数据符合.在设计核磁共振测井仪探头结构时, 选取与井眼形状一致的圆形模型,模型尺寸范围在10—15倍探头外径, 并采用三角形单元可以有效提高数值模拟方法精度,增强优化设计结果的可靠性.     

随钻核磁共振测井仪探测特性研究

随钻核磁共振测井是继电缆核磁共振测井之后的重要进步,具有直接探测原状地层孔隙度、束缚水、渗透率和流体饱和度等特点,受到广泛关注. 随钻核磁共振测井需要在更加恶劣的环境下作业,对探头尤其是对磁体有更高的要求. 该文研究随钻核磁共振测井的探测特性. 基于测量环境和工作方式,分析了随钻核磁共振测井静磁场设计中的特殊问题;利用有限元方法,分别模拟了MRIL-WD（哈里伯顿）、proVISION（斯伦贝谢）和MagTrak（贝克休斯） 3种随钻核磁共振测井仪的静磁场分布;根据数值模拟结果,提出可以抑制仪器随钻具运动（轴向转动、纵向钻进和径向振动）对测量的影响的随钻核磁共振测井静磁场方案,为自主随钻核磁共振测井仪器的设计、实现和优化提供了思路和注意事项.     

优化重聚脉冲提高梯度场核磁共振信号强度

缩短射频脉冲宽度, 有助于解决脉冲电力消耗大、样品吸收率高、信噪比低等极端条件核磁共振探测的关键问题. 本文首先分析射频脉冲角度对核磁共振自旋回波信号强度的影响机理, 基于Bloch方程推导了回波信号幅度与扳转角、重聚角的关系. 在特制核磁共振分析仪上采用变脉冲角度技术, 分别在均匀磁场和梯度磁场条件下实现对扳转角和重聚角与回波信号强度关系的数值模拟和实验测量. 结果表明, 梯度场中, 扳转角为90°、重聚角为140°的射频脉冲组合获得最大首波信号强度, 比180°脉冲对应的回波幅值提高13%, 能耗降低至78%. 选用该重聚角(140°) 优化设计饱和恢复脉冲序列探测流体的纵向弛豫时间T₁特性, 准确获得 T₁分布.该结果对于低电力供应、且对信噪比有较高要求的核磁共振测量, 如随钻核磁共振测井和在线核磁共振快速检测等, 具有重要意义. 关键词： 核磁共振 信号强度 重聚脉冲角度 Bloch方程     

径向非均匀磁场下的磁控管工作性能模拟

 为了改善磁控管的输出频谱,使模拟结构更接近实际情况,考虑了磁钢和极靴的实际尺寸。用MAFIA对磁控管内的磁系统进行建模和模拟。得到的磁场纵向分布有随着径向半径的增大而增大的趋势,这种径向不均匀性与实验测试结果一致。将非均匀的径向阶梯形变化磁场分布带入腔体热测计算,模拟得到的π模工作频率2.437 GHz,实际工作频率2.450 GHz,相对偏差0.5%,并在高频谐波的抑制上获得了输出频谱的显著改善,使微波炉磁控管具有更好的电磁防护和实际应用。     

径向非均匀磁场下的磁控管工作性能模拟

为了改善磁控管的输出频谱,使模拟结构更接近实际情况,考虑了磁钢和极靴的实际尺寸。用MAFIA对磁控管内的磁系统进行建模和模拟。得到的磁场纵向分布有随着径向半径的增大而增大的趋势,这种径向不均匀性与实验测试结果一致。将非均匀的径向阶梯形变化磁场分布带入腔体热测计算,模拟得到的π模工作频率2.437 GHz,实际工作频率2.450 GHz,相对偏差0.5%,并在高频谐波的抑制上获得了输出频谱的显著改善,使微波炉磁控管具有更好的电磁防护和实际应用。     

光纤模场分布的特性

基于MATLAB语言编程,数值模拟了单模光纤LP01模场光强度与归一化半径之间的分布特性,计算结果显示随着归一化工作频率的逐渐增大,单模光纤LP01模的模场更加集中于纤芯.研究结论为单模光纤的应用提供参考.     

全开放式单边核磁共振技术研究及系统开发

为了在单边磁体产生的逸散磁场中实现核磁共振技术,信号检测深度能满足相关应用需求,该文分析了非均匀磁场下的核磁共振技术条件,确定射频场目标,设计了单边平面射频线圈和探头,搭建出一套单边核磁共振实验系统.探头具有短脉宽(90°脉宽2.5μs),系统死时间短(10μs)的特点,可实现15μs的半回波时间.应用该系统,在15.3 T/m的巨大梯度磁场下,实现接近5 mm厚度的信号检测,并利用单边磁体的自然梯度磁场,开展了恒定梯度磁场下的扩散测量方法探索.结果表明,该系统可满足需要一定探测深度的领域的应用需求.     

弱磁场测定实验

本文将介绍在核磁共振实验的基础上、试制流水式核磁共振磁强计并把它开设成一个弱磁场测定实验的情况.除了自制的探头和极化器之外,用的都是实验室的一般仪器.测到的最低磁场为0.0128高斯,相对误差为 1×10. 利用核磁共振测量磁场是各种测量磁场的方法中最为精确的一种.但由于NMR吸收信号的幅度随磁场的减弱急剧变弱,所以即使有低频探头也难测量低于300高斯的磁场. 六十年代国外就有人研制出流水式核磁共振磁强计[4],只用一个探头就可以测量从0.5高斯到1万高斯的磁场.1979年有人用这一技术测量弱磁场,最低测到毫高斯[5].我国于1980年也研…     

核磁共振显微术的DESIRE效应模拟(英文)

要在合理时间內进行微米等级解析度的扫描时,核磁共振波谱的信噪比一直是一个很大的限制. DESIRE效应利用扩散现象来增强信号强度及解析度的概念被提出后,信噪比在一维的情况下大约可以增加一个数量级.在先期的研究中,模拟外加磁场的效应通常是使用叠代数值计算,而扩散的效应则是利用有限微分法解Bloch-Torrey e-quation ,这样的模拟方法通常很耗时.该研究中,分别使用Shinnar-LeRoux演算法及基于旋积的扩散模拟方法来加速计算出外加磁场及扩散对于信号的影响,因此可以节省大量计算时间以进行复杂的计算.利用一维模拟结果预测在不同参数实验中解析度的改变和信号增加值,并找出最佳的参数值.而由模拟结果指出,Sinc3脉冲波形可以得到最佳的信号增加量及不错的解析度.     

Nuclear magnetic resonance in inhomogeneous magnetic fields

The response of the spin system has been investigated by numerical simulations in the case of a nuclear magnetic resonance (NMR) experiment performed in inhomogeneous static and radiofrequency fields. The particular case of the NMR-MOUSE was considered. The static field and the component of the radiofrequency field perpendicular to the static field were evaluated as well as the spatial distribution of the maximum NMR signal detected by the surface coil. The NMR response to various pulse sequences was evaluated numerically for the case of an ensemble of isolated spins (1/2). The behavior of the echo train in Carr-Purcell-like pulse sequences used for measurements of transverse relaxation and self-diffusion was simulated and compared with the experiment. The echo train is shown to behave qualitatively differently depending on the particular phase schemes used in these pulse sequences. Different echo trains are obtained, because of the different superposition of Hahn and stimulated echoes forming mixed echoes as a result of the spatial distribution of pulse flip angles. The superposition of Hahn and stimulated echoes originating from different spatial regions leads to distortions of the mixed echoes in intensity, shape, and phase. The volume selection produced by Carr-Purcell-like pulse sequences is also investigated for the NMR-MOUSE. The developed numerical simulation procedure is useful for understanding a variety of experiments performed with the NMR-MOUSE and for improving its performance. Copyright 2000 Academic Press.     

2D NMR技术在石油测井中的应用

近几年,2D NMR技术得到迅速发展,特别是在核磁共振测井领域. 该文将主要介绍2D NMR技术的脉冲序列、弛豫原理以及2D NMR技术在石油测井中应用. 2D NMR技术是在梯度场的作用下,利用一系列回波时间间隔不同的CPMG脉冲进行测量,利用二维的数学反演得到2D NMR. 2D NMR技术可以直接测量自扩散系数、弛豫时间、原油粘度、含油饱和度、可动水饱和度、孔隙度、渗透率等地层流体性质和岩石物性参数. 从2D NMR谱上,可以直观的区分油、气、水,判断储层润湿性,确定内部磁场梯度等. 2D NMR技术为识别流体类型提供了新方法.     

Wall NMR in the weak ferromagnets YCrO3 and LuCrO3

Complex NMR spectra, consisting of a maximum of six resonance lines, were observed in the domain walls of YCrO₃ and LuCrO₃ and studied as a function of temperature and the in-plane magnetic field. The distribution of the NMR frequency across the domain wall was calculated taking into account the anisotropy of hyperfine fields and quadrupolar interactions. The calculated frequency distribution was used to simulate the shape of the NMR spectra in YCrO₃. The analysis of theoretical spectra in the centext of our experimental results and in particular the asymmetric behaviour of resonance lines in a magnetic field indicate the critical importance of the relaxation processes on the shape of NMR spectra. This conclusion was confirmed by an experiment in which the relaxation was simulated in YFeO₃. The NMR spectrum in LuCrO₃ can be explained by the same model.     

稠油储层核磁共振孔隙度校正方法

受原油粘度的影响,利用核磁共振（NMR）测井获取的稠油储层的测井NMR孔隙度远低于地层的真实孔隙度,给稠油储层评价和NMR测井技术的应用带来极大挑战.为提高稠油储层孔隙度计算精度,必须对测井NMR孔隙度进行稠油校正.本研究选取我国南海东部盆地某油田韩江组10块代表性岩心样品,分别开展了原始状态、饱含稠油、残余油和饱含水状态的NMR实验.研究了孔隙含油相对体积对岩心NMR孔隙度的影响,并建立了基于地层深测向电阻率分类的岩心NMR孔隙度校正模型.将基于实验结果建立的岩心NMR孔隙度校正模型推广到实际地层,对目标区域A14井稠油储层实测测井NMR孔隙度进行处理的结果表明：本文提出的方法能够有效地校正孔隙含稠油对实测测井NMR孔隙度的影响,得到地层的真实孔隙度;校正前、后的稠油储层测井NMR孔隙度与常规气驱法测量的岩心孔隙度之间的相对误差由11.19%降低到4.84%.     

岩心核磁共振实验对低孔低渗碳酸盐岩储层的适应性研究

低孔低渗碳酸盐岩储层矿物成分复杂、岩石骨架参数难以确定、储集空间类型多样、孔隙结构及孔渗关系复杂、常规测井曲线响应特征不明显,使其测井评价极其困难．本文利用核磁共振测井定量评价低孔低渗碳酸盐岩储层岩心的孔隙结构、计算储层参数．利用T₂谱分布曲线分析孔隙结构、计算T₂截止值;在此基础上计算岩心总孔隙度、有效孔隙度、束缚水孔隙度、渗透率等储层参数,并与常规岩心实验结果进行对比分析;最后,总结出核磁共振测井在低孔低渗储层中的应用优势与局限,为核磁共振测井评价模型的建立提供基础数据．     

An analytical methodology for magnetic field control in unilateral NMR

Traditionally, unilateral NMR systems such as the NMR-MOUSE have used the fringe field between two bar magnets joined with a yoke in a 'U' geometry. This allows NMR signals to be acquired from a sensitive volume displaced from the magnets, permitting large samples to be investigated. The drawback of this approach is that the static field (B0) generated in this configuration is inhomogeneous, and has a large, nonlinear, gradient. As a consequence, the sensitive volume of the instrument is both small and ill defined. Empirical redesign of the permanent magnet array producing the B0 field has yielded instruments with magnetic field topologies acceptable for varying applications. The drawback of current approaches is the lack of formalism in the control of B0. Rather than tailoring the magnet geometry to NMR investigations, measurements must be tailored to the available magnet geometry. In this work, we present a design procedure whereby the size, shape, field strength, homogeneity, and gradients in the sensitive spot of a unilateral NMR sensor can be controlled. Our design uses high permeability pole pieces, shaped according to the contours of an analytical expression, to control B0, allowing unilateral NMR instruments to be designed to generate a controlled static field topology. We discuss the approach in the context of previously published design techniques, and explain the advantages inherent in our strategy as compared to other optimization methods. We detail the design, simulation, and construction of a unilateral magnet array using our approach. It is shown that the fabricated array exhibits a B0 topology consistent with the design. The utility of the design is demonstrated in a sample nondestructive testing application. Our design methodology is general, and defines a class of unilateral permanent magnet arrays in which the strength and shape of B0 within the sensitive volume can be controlled.     

偶极子声波测井仪发射声系性能快速检测方法*

该文提出了一种针对偶极子声波测井仪发射声系性能进行快速检测的方法。通过实验测量方法,采用专门设计的简易装置测量偶极子声波测井仪发射声系激发的声场波形,并提取声场强度、声源主频以及空间分布对称性等特征参数。如果准确掌握了仪器处于完好状态时的这些特征参数,以此为参考依据与实验测量结果进行对比,通过设定可接受的相对偏差,就可完成对偶极子声波测井仪发射声系性能的快速评价。相对于现有的声波测井仪器检测方法,该方法具有高效、便捷的优势,便于在测井前线基地应用。