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

Abstract

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 (T2,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.