Comparison of low-field NMR and microfocus X-ray computed tomography in fractal characterization of pores in artificial cores

Studying the pore structure is very important for better understanding the characteristics of fluids storage and transport in rock reservoirs. To investigate the better way to characterize complex pore structures of rock reservoir, three artificial core samples with different permeabilities were investigated by Micro X-ray computed tomography (Micro-CT) and Nuclear Magnetic Resonance (NMR). A method for quantifying NMR pore structure is proposed based on the pore geometry derived from Micro-CT. The Pore Size/volume Distribution (PSD) from Micro-CT and NMR was compared. Moreover, some macroscopic physical properties of the cores were explained at the micro-scale based on the fractal analysis. Our results show that Micro-CT is a reliable and convenient way to interpret the NMR T-2 spectrum into real PSD for some brittle materials. The difference between 2D and 3D box-counting fractal dimensions of pores in artificial cores is less than one. The fractal dimensions of pores demonstrate a good negative correlation with the permeability. Moreover, the calculated fractal dimensions prove the existence of multiple fractal structure in artificial cores based on the NMR measurement. The fractal properties of PSD are mainly governed by the large pores in the tested cores. It was also found that the complexity of pore surface and fine pore structure has significant influence on heterogeneity. Finally, Micro-CT is proved to be ineffective in describing the fractal characteristics of a wide range of pore sizes due to limitation of resolution, which could be achieved by NMR more easily.