DERIVING NMR SURFACE RELAXIVITY AND PORE SIZE DISTRIBUTIONS BY NMR RELAXATION EXPERIMENTS ON PARTIALLY DE-SATURATED ROCKS

Abstract

Nuclear magnetic resonance (NMR) is a method used over a wide field of geophysical applications e.g. petrophysics, logging or hydrogeology, to non-destructively determine transport and storage properties of rocks and soils. Measured NMR amplitudes directly correspond to the rock's fluid (water, oil) content, whereas the NMR relaxation rates (longitudinal/transversal decay time T1, T2) can be used to derive pore sizes and permeability as they are related to physiochemical properties of the rock-fluid interface, i.e. surface relaxivity. This parameter, however, has to be determined and calibrated prior to estimating pore sizes from NMR measurements. Methods typically used to derive surface relaxivity to calibrate NMR pore sizes comprise mercury injection, pulsed field gradients (PFG-NMR) or grains size analysis. In this study we present an alternative approach to estimating NMR surface relaxivity jointly with pore size distributions using NMR relaxation data of partially de-saturated rocks. In this technique, a conventional (T1 or T2) NMR relaxation experiment is performed twice: firstly on the fully saturated sample and secondly on the partly de-saturated sample (e.g. by centrifuge, air pressure or suction) at a known differential pressure. Subsequently, the pore (capillary) radius distribution and surface relaxivity are derived by jointly optimizing the NMR relaxation data and the Young-Laplace equation. The method has been tested and validated with NMR data measured on various samples (porous filter ceramics) of well-known petrophysical properties, i.e. permeability, porosity, Spor, pore size distributions. Inversion results are in a good agreement with the known properties of the samples. The presented approach can therefore be applied to non-destructively estimate surface relaxivity and calibrate NMR pore sizes distributions using standard (1D) NMR relaxation experiments.