Surface and Volume Diffusion of Water and Oil in Porous Media by Field Cycling Nuclear Relaxation and Pgse NMR
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SURFACE AND VOLUME DIFFUSION OF WATER AND OIL IN POROUS MEDIA BY FIELD CYCLING NUCLEAR RELAXATION AND PGSE NMR S. Godefroy1,2, J.-P. Korb1, D. Petit1, M. Fleury2 and R. G. Bryant3 1
Laboratoire de Physique de la Matière Condensée, UMR 7643 CNRS, École Polytechnique, 91128 Palaiseau, France 2 Institut Français du Pétrole, 92852 Rueil-Malmaison, France 3 Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 ABSTRACT The microdynamics of water and oil in macroporous media with SiO2 or CaCO3 surfaces has been probed at various temperatures by magnetic field-cycling measurements of the spin-lattice relaxation rates. These measurements and an original theory of surface diffusion allowed us to obtain surface dynamical parameters, such as a coefficient of surface affinity of the liquid molecules and the surface diffusion coefficient. The water surface diffusion coefficients are compared to the volume self-diffusion coefficients of water in pores, measured by PGSE method, the latter values being more than an order of magnitude higher than the surface ones. Complementary information on the nature of the solid-liquid interface was given by NMR chemical shift experiments at high magnetic field. INTRODUCTION The interest of Nuclear Magnetic Resonance for oil recovery comes mainly from the use of the technique for in-situ well logging applications and laboratory characterization of oil-bearing rocks [1]. The technique relies on the measurement of proton nuclear relaxation at low magnetic fields, and gives various petrophysical properties, such as porosity, saturation, permeability, pore size distribution and wettability. NMR proton relaxation times of fluids in pores are enhanced by dipolar interaction with the paramagnetic impurities at the pore surface [2]. However, a better interpretation of the data requires the understanding of the molecular surface dynamics. A question then arises: how is it possible to obtain structural and dynamical information on liquids at the pore surface by nuclear spin-relaxation methods? Probing directly the molecular dynamics at the pore surface by standard NMR relaxation methods is difficult, in particular because the fast exchange of the low fraction of molecules in the surface layer with the bulk averages the spectral properties. Measurements using standard techniques have been previously reported [3,4]. For example, surface dynamics were studied by pulsed-field gradient spin-echo (PGSE) technique on partially saturated samples [3]. Recently, non-standard nuclear magnetic relaxation dispersion (NMRD) experiments were proposed to point out the microdynamics of liquids at the surface of microporous media [5,6]. Our aim here is to understand the surface nuclear relaxation processes, and the dependence of the nuclear spin relaxation on the nature of the solid-liquid interface. We report NMR experiments performed on water and oil saturating homogeneous macroporous media representative of some oil-bearing rock surface properties, with SiO2 or CaCO3 surfaces. The longitudina
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