Quantifying pore size distribution by nuclear magnetic resonance method in tight sandstones: Comparison between water-wet and oil-wet saturated fluids

Nuclear magnetic resonance (NMR) is regarded as an efficient method for determining pore size distribution (PSD), but only the T 2 spectrum is acquired. The converting factor between the T 2 spectrum and PSD plays a key role in this conversion. Some previous publications researched the converting factors, but there exists tremendous difference, and further research is still necessary. In this study, NMR, high-speed centrifugation, and contact angle measurement are conducted for tight sandstones extracted from the Upper Paleozoic Shihezi Formation to derive the converting factor. The contact angle of water is very different among samples, while that of oil for all samples nearly approaches to 0. Combining high-speed centrifugation and contact angle measurement, the critical pore size is calculated by the Washburn equation, while the T 2 critical value can be obtained by the differential amplitude between the T 2 spectra of saturated and centrifugated states. The converting factor is subsequently obtained through the ratio between the critical pore size and the T 2 critical value. The PSD characterized from oil is similar among samples, which is caused by their similar properties, while the PSD characterized from water is extremely various among samples, indicating oil is preferred as saturated liquid in tight sandstones. When oil as testing fluid, the converting factors between T 2 value and pore radius distribute from 17.6 nm/ms to 56.3 nm/ms, with an average of 28.1 nm/ms. The average pore sizes distribute from 296.4 nm to 850.2 nm, with an average of 565.2 nm, forming a positive relationship with porosity. Our findings are conducive to appraising the PSD acquired from NMR. • Comparison with water, oil as saturated liquid is preferable for calculating converting factors in tight sandstones. • The converting factors distribute from 17.6 nm/ms to 56.3 nm/ms, with an average of 28.1 nm/ms. • The average pore sizes distribute from 296.4 nm to 850.2 nm, with an average of 565.2 nm.