Determination of pore size distribution and hydraulic properties using nuclear magnetic resonance relaxometry: A comparative study of laboratory methods

In this study, we evaluate the feasibility of using nuclear magnetic resonance (NMR) relaxometry measurements to characterize pore size distribution and hydraulic properties in four porous samples with different texture and composition. We compare NMR with two classical techniques based on water retention and mercury intrusion measurements. Both T₂ and T ₁ NMR relaxation measurements at 6.47 MHz were carried out for three saturated model samples (medium sand, fine sand, and a homogenous sand/kaolin clay mixture) and one saturated natural silt loam soil. Cumulative pore size distribution functions and mean pore diameters were calculated assuming average surface relaxivity parameters and a cylindrical capillary model of the pores. The mean pore diameters derived from T₂ and T₁ distributions as well as the cumulative pore size distribution functions agree satisfactorily with those derived from mercury intrusion and retention curves. The observed deviations are due to limitations of each method, sample preparation, and sample composition. To evaluate the influence of the variations observed in the hydraulic properties of the samples, the pore size distribution functions were scaled back to water retention functions, and the van Genuchten hydraulic parameters were estimated by inversion using the RETC software. The comparison shows that both T₂ and T₁ NMR relaxation measurements can be used to estimate pore size distribution and mean pore diameter, as well as the retention function and corresponding hydraulic properties.