Magnet Design Of NMR Analyzer For Core Of Whole Diameter And Application In Oilfield

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 other many parameters. It can nondestructively detect and has a lot of parameter from only one machine and one core. Chinese oil field is mainly continental deposit. A majority of formation core is heterogeneous. So it is necessary to analyze by NMR spectrometer for core of whole diameter and is important as such. It is suitable for core having fracture and big holes. There is a great deal of secondary pores (fractures and dissolved pores) in carbonic acid rock. So it is more accurate.Magnet is one of the main parts of NMR core analyzer. Magnets include 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 higher. 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 low. The magnet in the paper has a small volume and wide gas gap. Otherwise it is light. At first, Finite Element Method computes the structure of magnet. After assembly of NMR magnet, because of heterogeneous magnet bar, iron machinery processing error and assembling error, uniformity can't fit running of NMR. So core region must be shimmed. When homogeneity of core region is below 200ppm, the instrument just can run. Shimming of magnetic field is completed by added iron. The proper shape and size iron bar or plate adds to the pole of the magnet in different location. Some harmonic component helpless to main magnetic field will be counteracting by iron. Homogeneous magnetic field is necessary in NMR magnet. In general shimming magnetic field is by experience. So it's difficult to shim. It depends on experiences frequently. Artificial Neural Network based on the ANN toolbox of Matlab is put forward to apply in the process. Magnet shimmed by ANN has a nice uniformity and can meet the condition of NMR core Analyzer well.Magnet is the unstable factor in quality of NMR instrument. Its magnetic parameters effect facility very much. Accordingly, the integral testing is necessary. Homogeneity of magnetic field has a closed relation to FID (Free induction decay) of NMR signal. The more rapidly does FID signal decay, the worse is homogeneity. After the magnet is created, an experiment is made. The standard sample consists of mixed kerosene and CCl₄, which can simulate the real core porosity. By testing, homogeneity of magnetic field, magnet stability in temperature, integral stability is good. Magnet can create a proper a uniformity region for NMR core analyzer. Core analysis indicates design is satisfying.After NMR testing, result of porosity, permeability, and mobile fluid percent from NMR cuttings analyzer is fit well with that of NMR logging at the same deep plot. On some oil field where NMR logging can't be developed, the NMR cuttings analyzer is a good substitute. In some aspect, the analyzer reduces cost of exploration and enriches the exploration method.