A Probe Design Of Low Gradient For Nuclear Magnetic Resonance Logging While Drilling

With the continuous expansion of the exploration and development scale of complex reservoirs and unconventional reservoirs,the nuclear magnetic resonance(NMR)logging technology for high-angle and horizontal wells has been widely used.However,the strong vibration of the instrument during drilling reduces the signal-to-noise ratio(SNR)and produces an invalid signal.The low gradient static magnetic field will reduce the effect of radial vibration.In this paper,the "Inside-out" structure is the main magnet structure,and then the focusing magnet,the high magnetic permeability material and the combination of the two are added respectively to design four magnet schemes.To obtain the resonance frequency of 500 k Hz,the finite element method is used to sequentially change the thickness,height and spacing of the magnetic material to numerically simulate the four magnet schemes.Then the structure of the main magnet,the focusing magnet and the high magnetic permeability material is optimized.Finally the low gradient is achieved.For the solenoid structure matched by the four magnet schemes,the method is used to analyze the influence of the coil diameter and the number of turns on the electrical parameters of the antenna,including the antenna inductance,the internal resistance and the antenna Q value.With the coil diameter and number of turns determined,the influence of the coil diameter and spacing on the distribution of the RF magnetic field is further analyzed.According to the design indicators,the optimal parameters of the four probe schemes is determined by iterative optimization.For the four probe schemes,with the knowledge of spin dynamics,the factors affecting the detection characteristics are analyzed from five aspects,which are orthogonal matching effect,frequency deviation,sensitive area volume,signal strength and SNR.