Impact of Paramagnetic Minerals on NMR-Converted Pore Size Distributions in Permian Carynginia Shales

Pore size distribution (PSD) is a fundamental petrophysical parameter for shale formation evaluation. Nuclear magnetic resonance (NMR), performing as a widely acknowledged technique, directly measures transverse relaxation time (T₂), which can be converted into PSD via surface relaxivity (SR). Technically, SR is utilized as a constant value in the entire formation, nevertheless, the laboratory calculated SRs revealed that they are likely to vary with mineralogy and can be influenced by Fe-bearing paramagnetic minerals, which could further affect NMR-converted pore structure properties. This study was performed on Permian Carynginia shale samples to compare the NMR-converted PSD with that measured by mercury injection capillary pressure (MICP). The surface relaxivity was calculated from the logarithmic mean T₂ value (T_2,lm) based on NMR measurement and the surface to volume ratio (SVR) based on low-pressure nitrogen gas adsorption (LP-N₂-GA). The results show that Fe-bearing paramagnetic mineral contents are linear positively correlated with SR values, which were calculated to range between 0.08 and 0.32 μm/s in our tested samples. The paramagnetic mineral of higher content expedites the NMR T₂ surface relaxation rate, leading to the divergent shifts in NMR- converted PSD curves.