Laboratory Study of the Effect of Grain Size, Mineralogy and Fluid on Nuclear Magnetic Resonance T <sub>2</sub> Distribution

Abstract

Nuclear magnetic resonance (NMR) is used to investigate key reservoir parameters, such as porosity and permeability. We focus here on unconsolidated sediments to calibrate NMR measurements in near surface environments and improve our understanding of porosity and permeability controls in sediments. Further, the effects of grain size and fluids on NMR relaxation time are poorly understood. We present here NMR T2 data interpretations by considering the effects of grain size, mineralogy, and fluid type in unconsolidated geologic materials. We find a linear relationship between grain size and the T2LM in water-saturated quartz, gypsum, and volcanic sands. Grain size and T2LM appear uncorrelated in oil saturated quartz and gypsum sands. Furthermore, T2LM is low in oil-saturated quartz sand, gypsum sand and volcanic sand which leads to an underestimation of NMR permeability. The relaxation times in gypsum, quartz sand, gypsum sand is much larger than in clay minerals and indicates large pore volumes that can easily be assessed using NMR. The relaxation times also depend on the grain size, as well as amount and distribution of paramagnetic impurities present. Our result provides important calibrations to improve NMR data interpretation for estimation of pore size distribution, porosity, and permeability in unconsolidated formations. This study can also be used as a baseline to study synthetic samples to investigate the effect of clay minerals and gypsum on NMR measurements.