A NEW METHOD LINKING NUCLEAR MAGNETIC RESONANCE RELAXATION DATA AND PORE SIZE IN CARBONATE

Low field nuclear magnetic resonance (NMR) has been a reliable method to characterize the porous media. The cross section shape of the capillaries in porous reservoir was generally simplified as a circular during the transition from relaxation time to pore size distribution (PSD). However, using a simple single shape factor (which relates the surface-area-to-volume ratio (S/V) to the radius (r) of the pores) always failed to derive an accurate PSD from the NMR T₂ distribution for carbonates. The integration of thin section observation and acoustic logging data analysis of carbonate rocks from various depositional and diagenetic settings in Tarim Basin allowed the quantification of the equivalent specific surface for different pore types. The NMR T₂ distribution was thus transformed to the pore size with specific factors. The number of peaks shown by the T₂ spectrum determined how many shape factors to use. The pore size distribution obtained by the new method existed consistency with the PSD from mercury intrusion capillary pressure measurements. The widely use of acoustic logging and the regular pore type distribution in carbonate reservoir make this method trackable.