A Study of Rock Matrix Diffusion Properties by Electrical Conductivity Measurements
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ABSTRACT Traditional rock matrix diffusion experiments on crystalline rock are very time consuming due to the low porosity and extensive analysis requirements. Electrical conductivity measurements are, on the other hand, very fast and larger samples can be used than are practical in ordinary diffusion experiments. The effective diffusivity of a non-charged molecule is readily evaluated from the measurements, and influences from surface conductivity on diffusion of cations can be studied. A large number of samples of varying thickness can be measured within a short period, and the changes in transport properties with position in a rock core can be examined. In this study the formationfactor of a large number of Aspb diorite samples is determined by electrical conductivity measurements. The formation factor is a geometric factor defined as the ratio between the effective diffusivity of a non-charged molecule, to that of the same molecule in free liquid. The variation of this factor with position along a borecore and with sample length, and its coupling to the porosity of the sample is studied. Also the surface conductivity is studied. This was determined as the residual conductivity after leaching of the pore solution ions. The formation factor of most of the samples is in the range 1E-5 to 1E-4, with a mean value of about 5E-5. Even large samples (4-13 cm) give such values. The formation factor increases with increasing porosity and the change in both formation factor and porosity with position in the borecore can be large, even for samples close to each other. The surface conductivity increases with increasing formation factor for the various samples but the influence on the pore diffusion seems to be higher for samples of lower formation factor. This suggests that the relation between the pore surface area and the pore volume is larger for samples
of low formation factor.
INTRODUCTION The effective diffusivity of a non-charged molecule that migrates through the pore system of a rock sample deviates from the diffusivity of the molecule in a free solution by aformation factor, Ff. The formation factor takes into account the hindrance of the molecule by the solid rock material, and comprises the porosity (c), the tortuosity (t 2 ) and the constrictivity (6 1) of the pores: Ff=
(1)
2
The formation factor can be seen as a geometric constant for each piece of rock. Knowing the formation factor makes it possible to determine the effective pore diffusivity, Dpc, of other molecules if their diffusivity in free liquid, Dw, is known:
767 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
DPP = FfDw
(2)
In analogy with this, the formation factor can also be determined from electrical conductivity measurements. The electrical conductivity through the pores of a rock sample (Kp), saturated with a salt solution of a high ionic strength (>IM), is related to the formation factor and the conductivity of the free salt solution (Kw) [1]: Kp
Ff- =
(3)
Kw +
To determine the pore conductivity due only to
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