The porosity and nuclear magnetic characteristics of artificial samples with the equal grain size

Relevance. The nuclear magnetic resonance relaxometry is a fairly new method for well logging and laboratory investigations. It is applied for determining petrophysical properties, e.g., porosity, permeability, water- and oil-saturation. However, the nuclear magnetic resonance relaxometry data interpretation rests on the representation of the pore space as a set of spheres of various diameters that fundamentally does not correspond to the structure of granular rocks. Aim. To examine an influence of grain size on the measurements results of nuclear magnetic resonance relaxometry porosity and compare with other conventional methods. Objects. Artificial samples made with various sand and quartz glass beads fractions. Methods. Water-saturation, gas-volumetric and nuclear magnetic resonance relaxometry were applied to determine porosity. X-ray diffraction method was used to clarify the mineralogical composition; to measure the grain size a laser particle size analyzer was used, and the grain shape was investigated in polished specimens. Results. The paper describes the results of porosity measurements with different methods for artificial samples. We observed an effect of underestimated porosity acquired with nuclear magnetic resonance relaxometry method for consolidated and unconsolidated samples with grain size less than 0.160 mm. The investigation of sand composition with the X-ray diffraction method and additional assessment of the particles shape and size led to conclude that these factors are not the cause of the underestimated porosity effect. Additional statistical samples and using of the other nuclear magnetic resonance relaxometer presented the reparability of previous results. Based on the research results, it was assumed that the cause of the underestimated porosity effect is the complex shape and increased specific surface area of the samples, which grows as the particle size decreases.