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POSIVA Report 1996-22

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Name:

Helium Gas Methods for Rock Characteristics and Matrix Diffusion

Writer:

Juhani Hartikainen; Kari Hartikainen; Aimo Hautojärvi; Kalle Kuoppamäki; Jussi Timonen

Language:

English

Page count:

81

ISBN:

951-652-021-9; 1239-3096

Summary:

Working report: HELIUM GAS METHODS FOR ROCK CHARACTERISTICS AND MATRIX DIFFUSION



Matrix diffusion has been studied in the Department of Physics at the University of Jyväskylä (JYFL)
since 1991. Gas flow and gas diffusion methods have been developed for measuring the porosity, the
effective diffusion coefficient and the permeability coefficient of rock samples. Porosity and the effective
diffusion coefficient are both important factors in matrix diffusion, and they have traditionally been
determined by liquid phase experiments. In the gas phase it is possible to make these measurements in a
much shorter time scale. For example, through-diffusion experiments in the gas phase are now routinely
used in matrix diffusion studies. However, even in the gas phase it is possible to measure only relatively
small samples (core lengths of up to about 10 cm) with a reasonably short measuring time and a
reasonable accuracy. From experiments on small samples one cannot always reliably determine the
macroscopic characteristics of rock because of its local variations in a scale bigger than the sample size.
Notice however that even the lengths of samples of the order of 10 cm are beyond the scope of liquid
phase diffusion experiments.

Channel flow experiments on long samples of small-diameter drill cores have now been introduced to
make the determination of porosity and effective diffusion coefficient more representative. The technique
is suitable for scanning a large number of samples in a rather short time. An in situ -version of the method
and an specially designed in situ -equipment have also been developed and tested both in laboratory and in
situ -conditions. The in situ -equipment can be applied to ordinary drill holes.

The measured diffusion coefficients are those of He atoms diffusing through rock samples whose pore
space is filled with nitrogen. It is possible, however, to derive from these coefficients the corresponding
coefficients for He atoms diffusing through samples saturated with water: the diffusion equation is the
same in the two cases apart from the actual value of the diffusion coefficient. Helium result can be scaled
to typical (heavier) molecules in water saturated samples by using an approximative scaling factor
1/35000.

A possible source of error in our estimates of De(H2O) is that nitrogen may fill a pore space in the samples
which is different from that filled by water. We expect, however, that the fraction of pore space which
cannot be saturated with water is not large. Similarly, if non-inert molecules are being considered, other
effects such as sorption and anionic exclusion should also be taken into account.

Another technique that has recently been developed at JYFL is that for measuring the permeability
coefficient of rock samples. It is based on flow of helium through the sample caused by a pressure
gradient. This method allows for an easy and efficient determination of gas permeability coefficients by
using d'Arcy's law for compressible fluids. The hydraulic conductivities of the samples can be calculated
from their measured permeabilities.

A pycnometer type of method has also been developed at JYFL. This method provides an independent
way to measure the porosity. It is based on a very accurate determination of the pressure and temperature
of gas before and after it has expanded to the evacuated sample cell in which the pore space of the sample
is the only unknown volume.
HELIUM GAS METHODS FOR ROCK CHARACTERISTICS AND MATRIX DIFFUSION



Matrix diffusion has been studied in the Department of Physics at the University of Jyväskylä (JYFL)
since 1991. Gas flow and gas diffusion methods have been developed for measuring the porosity, the
effective diffusion coefficient and the permeability coefficient of rock samples. Porosity and the effective
diffusion coefficient are both important factors in matrix diffusion, and they have traditionally been
determined by liquid phase experiments. In the gas phase it is possible to make these measurements in a
much shorter time scale. For example, through-diffusion experiments in the gas phase are now routinely
used in matrix diffusion studies. However, even in the gas phase it is possible to measure only relatively
small samples (core lengths of up to about 10 cm) with a reasonably short measuring time and a
reasonable accuracy. From experiments on small samples one cannot always reliably determine the
macroscopic characteristics of rock because of its local variations in a scale bigger than the sample size.
Notice however that even the lengths of samples of the order of 10 cm are beyond the scope of liquid
phase diffusion experiments.

Channel flow experiments on long samples of small-diameter drill cores have now been introduced to
make the determination of porosity and effective diffusion coefficient more representative. The technique
is suitable for scanning a large number of samples in a rather short time. An in situ -version of the method
and an specially designed in situ -equipment have also been developed and tested both in laboratory and in
situ -conditions. The in situ -equipment can be applied to ordinary drill holes.

The measured diffusion coefficients are those of He atoms diffusing through rock samples whose pore
space is filled with nitrogen. It is possible, however, to derive from these coefficients the corresponding
coefficients for He atoms diffusing through samples saturated with water: the diffusion equation is the
same in the two cases apart from the actual value of the diffusion coefficient. Helium result can be scaled
to typical (heavier) molecules in water saturated samples by using an approximative scaling factor
1/35000.

A possible source of error in our estimates of De(H2O) is that nitrogen may fill a pore space in the samples
which is different from that filled by water. We expect, however, that the fraction of pore space which
cannot be saturated with water is not large. Similarly, if non-inert molecules are being considered, other
effects such as sorption and anionic exclusion should also be taken into account.

Another technique that has recently been developed at JYFL is that for measuring the permeability
coefficient of rock samples. It is based on flow of helium through the sample caused by a pressure
gradient. This method allows for an easy and efficient determination of gas permeability coefficients by
using d'Arcy's law for compressible fluids. The hydraulic conductivities of the samples can be calculated
from their measured permeabilities.

A pycnometer type of method has also been developed at JYFL. This method provides an independent
way to measure the porosity. It is based on a very accurate determination of the pressure and temperature
of gas before and after it has expanded to the evacuated sample cell in which the pore space of the sample
is the only unknown volume.

Keywords:

File(s):

Helium Gas Methods for Rock Characteristics and Matrix Diffusion (pdf) (1.3 MB)


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