核磁共振全直径岩心分析仪磁体的研制

采用核磁共振进行石油岩心分析，可以从一块岩样中得到孔隙度(总孔隙度、有效孔隙度、粘土束缚水孔隙度等)、自由流体指数(可动流体百分数)、孔径分布以及渗透率等多种参数，具有无损检测、一机多参数、一样多参数的显优点．我国油田以陆相沉积油田为主，其主要特点是储层存在严重的非均质性，所以采用核磁共振全直径岩心分析非常有必要性，和核磁共振标准岩心分析同样重要，磁体是核磁共振岩心分析仪的核心部件，这种磁体相对体积较小，有效气隙大，重量轻．在设计磁体时，首先要根据经验初步确定磁体类型和结构尺寸，然后再用有限元方法进行计算，得出准确的尺寸，装配完的磁体通过无源匀场使磁体的均匀度达到要求，磁场的均匀度和FID(free induction decay)信号衰减程度密切相关，FID衰减的越快，说明磁体均匀度越差，磁体经调试完毕后，使用煤油和四氯化碳模拟孔隙介质组成的标准样做FID测试，通过磁体的测试，可知磁体的均匀度、磁场场强的温度稳定性、磁体的总体稳定性良好，完全可以给核磁共振全直径岩心分析仪营造一个合适的匀场环境。

Design and Making of NMR Magnet for Core of Whole Diameter

Petroleum core analysis by NMR can get porosity (total porosity, available porosity, porosity of irreducible water etc.), free fluid index (mobile fluid index), pore distribution, permeability and many other parameters. It can nondestructively detect and has a lot of parameters with only one machine and one core. Chinese oil field is mainly continental deposit. The majority of formation core is heterogeneous. So it is necessary to analyze by NMR spectrometer for core of whole diameter and is important as well. It is suitable for core to have fractures and big holes. Standard core analysis usually does not have big holes or fractures. Therefore the analysis of whole diameter core reflects a real formation.Magnet is one of the main parts of NMR core analyzer, Which includes super conduction magnet, iron-core magnet, and permanent magnet. The first two types can make high magnetic field, but the cost of manufacturing and running is big. Magnetic field in NMR core analyzer is about 0.1T. Thus permanent magnet can give the field well and on the other hand the making cost and the running cost is small. The magnet in the paper has a small volume and wide gas gap. Otherwise it is light. The disadvantage is that its magnetic circuit is open. So it has much magnetic leak. Considering manufacturing cost and characteristics of this type of magnet, NbFeB is a good choice. At first, Finite Element Method computes the structure of magnet. Because of heterogeneous magnet block, iron machinery processing error and assembling error, uniformity are not fit for running of NMR. So core region must be shimmed. When homogeneity of core region is below 200 10~(-6), the instrument just can run. Shimming of magnetic field is completed by adding iron. The iron bar or plate of proper shape and size are added to the pole of the magnet in different locations. Some harmonic components helpless to the main magnetic field will be counteracted by iron. The evaluation criterion in the paper to homogeneity of magnetic field is widely used by many manufacturing plants. So it standardizes uniformity of NMR magnet. When magnet is shimmed to about 800 10~(-6), it is found that the iron bars on pole plate affect B distribution along the axis of central magnetic core region. By experiment, a proper height of iron bars is designed.Homogeneity of magnetic field has a close relation to FID (free induction decay) of NMR signal. The more rapidly FID signal decays, the worse homogeneity is. After the magnet is created, an experiment is carried out. The standard sample consists of mixed kerosene and CCl-4, which can simulate the real core porosity. Magnet is unstable in NMR instrument. Its magnetic parameters affect facility very much. Accordingly, the integral testing is necessary. A set of standard samples is tested at first. After 24 H continuous running, testing again. Comparing the two results examined the magnet quality. By testing, we find homogeneity of magnetic field, magnet stability in temperature, integral stability are good. Magnet can create a proper a uniformity region for NMR core analyzer. Core analysis indicates the design is satisfying. In conclusion, adopting permanent magnet as component of NMR core analyzer is a economic and simple method.