Nuclear magnetic resonance, as an extensively recognized technique, is utilized to measure the pore size distribution of shales. However, the pore size distribution interpreted by nuclear magnetic resonance noticeably relies on the surface relaxivity (SR) of shales, which is influenced by many factors, including the measurement approach and mineralogical compositions. This paper chiefly deals with two approaches to measuring the surface relaxivity of shales based on a one-dimensional relaxation model. In continuing, the effects of mineralogical compositions and measurement approaches on the surface relaxivity of shales are methodically assessed. The surface relaxivity of shales is measured to be 1.34–2.59 μm/s by the spin echo approach and 1.40–2.44 μm/s based on the Carr–Purcell–Meiboom–Gill approach. Compared with the nitrogen adsorption approach, the suggested methods are more suitable for measuring the SR of shales. The achieved results reveal that the shale SR is negatively correlated with quartz and calcite and is positively correlated with pyrite and clay. In particular, the clay content seems to be the most vital mineral factor in determining the SR of shales.

Surface relaxivity is the most significant parameter that converts transverse relaxation time into pore size in porous media. However, the value of surface relaxivity is difficult to obtain. This paper focuses on measuring surface relaxivity to evaluate permeability of two types of sandstones (loose sandstone and tight sandstone), combining the high-speed centrifugation with nuclear magnetic resonance test. The results reveal the surface relaxivities for loose sandstones (1.89-2.20 μm/s) are larger than that for tight sandstones (0.69-0.87 μm/s). The measured surface relaxivity is consistent with that from the method of monolayer water adsorption. During centrifugation, the variation of water content is analyzed, including the pore with different size categories. Based on the measured surface relaxivity, the Schlumberger doll research model is used to estimate the absolute permeability of sandstones.