Research On Coupling Effect And Decoupling Method Of NMR Coil Array

Nuclear magnetic resonance logging can accurately and effectively obtain formation petrophysical information and perform quantitative identification of fluid identification.It has unique advantages and has become an important part of geophysical logging.Nowadays,in order to deal with the spatial heterogeneity of complex oil and gas reservoirs,the use of coil array for azimuth detection has become one of the development trends of nuclear magnetic resonance logging.In order to realize the three-dimensional detection of the formation in the circumferential,radial and axial directions,it is necessary to use the coil array matched with the magnet to perform circumferential scan.In the testing process of the probe array antenna of the logging tool,the coupling effect between the coil arrays on the antenna performance is obviously not negligible.In this paper,starting from the coupling phenomenon between array antennas,it is clear that the coupling between the coils comes from the inductive coupling between the coils;using Kirchhoff's law,impedance matrix analysis method,parity mode analysis method to inductive coupling between the coils Analysis;establish the initial coil,use electromagnetic simulation software to simulate the mainstream high-field MRI array antenna decoupling method under low field conditions,compare and evaluate,decoupling the design requirements of the low-field NMR logging coil array The method was analyzed for adaptability.Introduce a capacitive decoupling network to the low-field NMR array antenna.After decoupling,the isolation between the coils of the dual-coil array reaches-17.00 d B,the isolation between the adjacent coils of the four-coil array reaches-14.30 d B,and between non-adjacent coils The decoupling effect is obvious,and the isolation reaches-33.92 d B;the influence of the decoupling method on the antenna performance is analyzed and suggestions for improvement are proposed.