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ESTIMATION OF THE POROUS MEDIUM PROPERTIES BY THE PULSE FIELD GRADIENT NMR METHOD
N К Dvoyashkin
Almetyevsk Oil Institute ul Lenma, 2, Almetyevsk, Tatarstan, Russia
Abstract
The time dependence of the effective self diffusion coefficient D(t) was experimentally studied by the pulse field gradient (PFG) NMR method for a liquid (water and tridecane) m a medium with solid obstacles represented by spherical particles of organic glass The possibilities of the PFG NMR method were demonstrated with respect to estimation of geometrical characteristics of a porous medium, such as the ratio Si /Vo between the surface area and the volume of the pore space
Over the last years, porous media and diffusion of liquids m them have been studied by the pulse NMR PFG method [1-5]
In this study, using the theoretical approaches described in [3-5], we would like to reveal the potential of the PFG NMR method m the experimental determination of some geometrical and transport parameters characterizing the pore space, such as the surface area pore volume ratio Si / V0
We analyzed a set of randomly packed organic glass particles of exactly spherical shape with the average radius ro«0,07 mm as porous media Tridecane (chemically pure grade) and distilled water were used as the model liquids The volume fraction of water and tridecane in the sample was tp/f0,21 The packing of particles was regarded as random, therefore, the porosity Ф of all samples was assumed to be ~3S% [6] The self-diffusion coefficient (SDC) of liquid molecules were measured by PFG NMR method Techniques tor sample preparation and measurements were described earlier [1,2]
The attenuation of the spin echo amplitude owing to the self-diffusion process A(g') (where g - is the PFG intensity) were not exponential in all cases, therefore, A(g'J curves were quantitatively analyzed via the introduction of the effective SDC D(t), which was derived from initial slope of A(g:) [1,2]
The measurements were conducted m the T=278-303K temperature range The diffusion time interval t was 1-350ms As a result of the measurements, the dependence of SDC on the diffusion time t and was shown to be nonlinear (Fig 1) For all system studied, the resultant С curves were of a similar type The form of the D(t) curves m Fig 1 indicates that, firstly, spatial obstacles set limits on the diffusion motion, and secondly, the porous medium should be regarded as an entity interconnected pores m this case [7]
a
As was shown in [3,4,7], one may obtain information on the structural features of the pore space by studying the time dependence of the SDC in a medium with limitations Analytical expressions were proposed for D(t) of a liquid m porous medium to characterize observations of the self-diffusion process (studied by the PFG NMR method [3-5] during three time intervals short-time, long-time, and mtermediate-time regimes We will consider only shor-time regime in this work
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