Study On Low Field Nuclear Magnetic Resonance Response Characteristics Of Heterogeneous Rock Model

With the development of Magnetic Resonance Imaging（MRI）technology,it has been one of the important methods to study pore structure of porous media.Different from the T₂ distribution of the whole rock sample obtained by traditional Nuclear Magnetic Resonance（NMR）core analyzer,MRI can obtain the spatially-resolved T₂distribution of rock samples,which can better guide the NMR response characteristics of heterogeneous rock model.In this work,the characteristics of the induced magnetic field and internal gradient of different pore structures are analyzed by using the correlation between the internal magnetic field gradient and pore structures.And brief evaluate the heterogeneity of different pore structure models by G-B_zⁱ correlation map.To improve the accuracy of heterogeneous rock model evaluation,based on the transverse relaxation time T₂ which measured by NMR technology can effectively characterize the pore size distribution,the principle of random walk algorithm and phase encoding are used to simulate the attenuation signals in linear gradient magnetic fields of the periodic stacking model,the spatially-resolved T₂ distribution is obtained by inversion,the T₂ distribution variation on the z-axis has a good correspondence with the pore size variation characteristics of the heterogeneity model,and the validity of the method for studying heterogeneous rock model is also confirmed.Then,prepare some simple heterogeneous glass bead models,and measure them with phase-encoded pulse sequence.After inversing the data,the NMR spatial response characteristics of different heterogeneity models were observed.Combined with the response characteristics,the core samples were measured and analyzed,and the analysis results were consistent with the CT scan results.In addition,the method is better consistent with the logging curve.While improving the resolution,it also avoids the influence of instrument motion on the measurement.It has good practicability for studying on the low field NMR response characteristics of the heterogeneous rock model.