Determining Nuclear Magnetic Resonance Surface Relaxivity of Shales

Abstract

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.