Effects of magnetic field intensity and gradient on measurement results of core nuclear magnetic resonance T₂ spectrum

Nuclear magnetic resonance(NMR) logging instruments generally measure the NMR signal of reservoir fluid in a uniform field with a resonance frequency of 2 MHz or a gradient field with a resonance frequency of less than 1 MHz. For laboratory NMR core analyzers, in addition to the commonly used 2 MHz, the resonance frequency of 12 MHz or 21 MHz is often used for experimental measurement of tight reservoirs such as shale. In order to determine the effects of magnetic field intensity and gradient on the NMR measurement results, the sensitivity of the magnetic field intensity and gradient of the core experiments of glutenite and shale cores under water-saturatedstate was systematically studied, and the relationship between the T₂ spectrum shape, position, nuclear magnetic porosity, T₂ geometric mean and the magnetic field intensity and gradient of different rock samples was analyzed. The experimental results show that under the uniform field, glutenite samples are very sensitive to the change of magnetic field intensity, while shale samples are less sensitive. The existence of external gradient field will make the short relaxation information of glutenite and shale missing, resulting in that the NMR signal cannot be measured completely. When the NMR core experiment is used to calibrate the interpretation parameters of NMR logging, the experimental measurement results must be corrected if there is a large difference in the magnetic field intensity or gradient between the laboratory NMR core analyzer and the NMR logging instrument.