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@PHDTHESIS{Jacops:748491,
author = {Jacops, Elke},
othercontributors = {Littke, Ralf and Swennen, Rudy and Busch, Andreas},
title = {{D}evelopment and application of an innovative method for
studying the diffusion of dissolved gases in porous
saturated media},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2018-229435},
pages = {1 Online-Ressource (186 Seiten) : Illustrationen},
year = {2018},
note = {Cotutelle-Dissertation. - Veröffentlicht auf dem
Publikationsserver der RWTH Aachen University 2020;
Dissertation, Rheinisch-Westfälische Technische Hochschule
Aachen, 2018. - Dissertation, KU Leuven, 2018},
abstract = {Many countries consider clay-based materials for the safe
disposal of high- and intermediate level radioactive waste,
either because of the choice of argillaceous formations to
host the repository, or as a component in the engineered
barrier system. The clays under consideration have a high
capacity to retain radionuclides (strong fixation to clay),
limited water flow by means of low permeability and high
self-sealing properties by capillary sealing efficiency. In
Belgium, Boom Clay is considered as a potential host rock.
Within a geological repository, the production of gas is
unavoidable whereby the dominant process is anaerobic
corrosion of metals producing hydrogen. In a first stage,
the generated gas will dissolve in the porewater and
dissipate by diffusion. If the rate of gas generation is
larger than the diffusive flux, a free gas phase will form
which might have negative effects on the performance of the
barriers. In order to obtain a reliable estimate about the
balance between gas generation and gas dissipation, sound
diffusion coefficients for dissolved gases are essential.
Our first goal was to develop a suitable technique to
measure diffusion coefficients of dissolved gases. Besides
diffusion coefficients for a variety of dissolved gases (He,
Ne, Ar, CH4, C2H6 and Xe), also diffusion coefficients for
hydrogen are needed. It is well known that experiments with
hydrogen often suffer from experimental problems such as
microbial conversion of H2 into CH4. Therefore, it was
necessary to find a way to avoid/minimize this microbial
activity during diffusion experiments. It is known that
diffusion coefficients in free water (D0) depend on the size
of the molecule. More specifically, the size dependency of
D0 can be described by an exponential function. In this
study, it was observed that an exponential relationship was
also found for the effective diffusion coefficient (Deff) as
a function of molecule size, which confirmed that molecule
size also influences diffusion of molecules in porous
materials. A third objective was to investigate whether
similar exponential relationships could be found for other
clayey materials, which finally could be used to estimate
diffusion coefficients based on the size of the diffusing
molecule. As dissolved gases are considered to be
conservative tracers, they are useful for the assessment of
the transport properties and pore structure by means of
their diffusion coefficients. Thus, these diffusion
coefficients would also depend on the petrophysical
properties of the material, characterising its pore
structure. Hence, the last goal of this research study was
to investigate the influence of different petrophysical
properties on the diffusive behaviour of dissolved gases,
thus, allowing coupling between measured petrophysical
parameters, Deff and molecule size. In order to answer these
questions, an innovative method was developed to measure the
diffusion coefficient of dissolved gases using the double
through-diffusion methodology. This allowed to measure the
diffusion coefficients of two dissolved gases in a single
experiment with a high precision. When using hydrogen, a
complex sterilisation procedure combining heat
sterilisation, gamma irradiation, gas filtration and the use
of a microbial inhibitor was developed, which eliminated
microbiological disturbances. By using this procedure, for
the first time, reliable and accurate diffusion coefficients
for dissolved hydrogen were obtained for three different
samples of the Boom Clay. The obtained diffusion
coefficients enable a more precise assessment of the
problems related to H2 production/dissipation in a
repository environment. To investigate the relationship
between the molecule size and their diffusion coefficients
in more detail, diffusion experiments with gases of
different sizes and HTO were performed on different
clay-rich / argillaceous samples (Boom Clay, Eigenbilzen
Sands, Callovo-Oxfordian Clay, Opalinus Clay and bentonite).
Similar to the relationship between D0 and molecular size,
for all samples under investigation, a reliable relationship
between the molecular size and effective diffusion
coefficient was obtained which can be described by an
exponential function. The difference in distance between the
Deff and D0 curves relates to the geometrical factor (G ~
D0/Deff). This geometric factor provides information on how
the porous network influences diffusing molecules and
account for the tortuosity and constrictivity of the sample.
For the samples of the Boom Clay and the Eigenbilzen Sands,
the exponential coefficient is very similar to the D0
relationship. Similar exponential coefficients indicate that
the geometric factor will be quasi constant when the size of
the diffusing molecule increases. This matches with the
experimental results, where the difference in G between the
smallest and the largest molecule is less than 3. However,
for the other clayey samples (COX, OPA and bentonite), the
exponential factors differ from the one of the D0
relationship, hence G varies strongly with the size of the
diffusing molecule, which is also experimentally observed.
In literature, diffusion coefficients are often estimated by
using a constant value of G for a certain sample or
formation (often derived from diffusion experiments with
HTO). Based on the data presented in this work, one can
conclude that this approach is not always correct and it can
lead to a substantial overestimation of the diffusion
coefficient. Therefore, we propose an alternative method to
estimate diffusion coefficients of dissolved gases, based on
the exponential relationship that has been observed on a
large set of diversified samples. By measuring
experimentally the effective diffusion coefficient of two
unreactive, dissolved gases possessing a different size, one
can determine the exponential function and as a consequence,
one can derive the diffusion coefficients of other dissolved
gases (with a size in between the two measured gases) based
on their size. When using this approach for one of our
samples, the predicted and measured diffusion coefficients
differ by less than $30\%,$ which is deemed satisfactory for
predictive gas dissipation calculations. In order to
investigate how the transport properties of a dissolved gas
molecule can be linked to the petrophyiscal and
petrographical properties of a clay-rich sample, the main
focus was on clay-dominated Boom Clay samples (Putte and
Terhagen Member) and more sandy Eigenbilzen Sands. For these
samples, a detailed petrophysical analysis has been
performed. Diffusivity and hydraulic conductivity of the
Boom Clay and Eigenbilzen Sands are very different.
Petrophysical analysis showed large differences in
mineralogy and grain size distribution: samples of the Boom
Clay are rich in clay minerals and contain a large weight
percent’s (> $67\%)$ clay fraction (< 2µm), while the
samples of the Eigenbilzen Sands are rich in detrital quartz
and contain a large (> $43\%)$ sand fraction (> 62 µm).
These differences in composition are also reflected by their
microstructure. The Boom Clay samples are characterised by a
clay supported matrix with some homogeneously distributed
quartz grains; pores are not visible by the techniques used
(< 16 µm). Likely, the pores are mainly located in the clay
matrix and are very small (< 250 nm). These observations are
in line with previous studies. In contrast, samples of the
Eigenbilzen Sands contain large amounts of quartz, a
heterogeneous distribution of clay phases and interparticle
porosity adjacent to the quartz grains. The pores are still
partly located in the clay matrix, but there is also an
important fraction of larger pores (> 250 nm) which allows
enhanced transport of dissolved gases and water. Hence, a
clear link was found between the transport properties and
the petrophysical/petrographical properties of the samples.},
cin = {532410 / 530000},
ddc = {550},
cid = {$I:(DE-82)532410_20140620$ / $I:(DE-82)530000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2018-229435},
url = {https://publications.rwth-aachen.de/record/748491},
}
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