Numerical simulations of gas transport in argillaceous rocks: A molecular dynamics and pore-scale simulation study

Owusu, Jerry Peprah (2023). Numerical simulations of gas transport in argillaceous rocks: A molecular dynamics and pore-scale simulation study (Unpublished). (Dissertation, University of Bern, Institute of Geological Sciences)

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This dissertation investigates the gas transport and clay behavior within the context of deep
geological disposal of nuclear waste. The repository for spent fuel and high-level waste can
generate substantial amounts of gas through processes such as anaerobic corrosion of carbon
steel, radiolysis of water, and radioactive decay in the waste. Likewise, gas production can
occur in low and intermediate-level waste repositories due to chemical degradation of organic
waste materials and corrosion of metals. If these gases cannot sufficiently escape from the
vicinity of the repository, a localized build-up of gas pressure could compromise the integrity
of the barriers and the safety design of the repository. Therefore, a thorough understanding of
gas transport mechanisms and processes is crucial for assessing the repository’s performance.
Diffusion is the primary mechanism governing solute and fluid transport in these clays due to
their low permeability. While experiments can provide valuable transport parameters for de-
signing the barrier materials, they may not fully capture the long-term evolution of transport
processes and specific subsurface conditions. Consequently, numerical and computer simu-
lations become indispensable for determining the transport mechanisms and exploring the
behavior of the system beyond the limits of experimental detection. These simulations offer
the opportunity to explain experimental results, probe scales, and processes that are below the
detection limit of experiments, and enhance our understanding of the transport mechanisms
involved.
Gas diffusion simulation in fully saturated Na-montmorillonite (Na-MMT) was performed and
the effects of pore size, gas species, and temperature were investigated. Classical molecular dy-
namics simulations were utilized to study the diffusion coefficients of various gases (CO2, H2,
CH4, He, Ar). The findings indicate that the diffusion coefficients are influenced by the pore
size, with H2 and He demonstrating higher mobility compared to Ar, CO2, and CH4. The be-
havior of gases is affected by the confinement and the structuring of water molecules near the
clay surface, as evidenced by density profiles and radial distribution functions. The obtained
diffusion coefficients for different gases and slit pore sizes were parameterized using a single
empirical relationship, enabling their application in macroscopic simulations of gas transport.
Considering the long-term desaturation and resaturation process, the study extends to simulate
gas diffusion in partially saturated Na-MMT and investigates the partitioning of gas molecules
between the gas-rich and water-rich phases. Classical molecular dynamics simulations were
employed to explore the impact of gas-filled pore widths, temperature, gas mean free path, gas
size, and gas molecular weights on diffusion coefficients and partitioning coefficients. The re-
sults demonstrate that the diffusion coefficient in the gas phase increases with larger gas-filled
pore widths and eventually converges asymptotically towards the diffusion coefficient in the
bulk state. Partitioning coefficients were found to be strongly dependent on temperature and
gas molecular weights. Furthermore, non-equilibrium molecular dynamics simulations were
conducted to investigate the mobility of gases in a pressure-driven flow within a partially sat-
urated Na-MMT mesopore. The results reveal the presence of slip boundary conditions at the
microscale, which challenges the assumptions made in continuum models. To predict the dif-
fusion coefficient and dynamic viscosity of the gas, a Bosanquet-type equation was developed
as a function of the average pore width, gas mean free path, geometric factor, and thickness of
the adsorbed water film.
Na-montmorillonite, being a swelling clay, undergoes changes in its swelling behavior when
exposed to different chemical species like gas due to variations in chemical potential. These
alterations can subsequently impact the hydraulic properties and transport mechanism of the
clay. Consequently, we investigated the influence of gas presence on the swelling pressure
of Na-MMT. To achieve this, classical molecular dynamics simulations were employed as a
methodology to examine the effect of gas on swelling pressure. The findings indicate that gas
molecules cause an increase in the swelling pressure of Na-montmorillonite, with an approx-
imate rise of 3 MPa. The specific behavior observed is influenced by factors such as the dry
density and the characteristics of the gas species. Additionally, the analysis includes a com-
prehensive exploration of structural transformations occurring within the clay interlayer, pro-
viding insights into the discrepancies observed between experimental and simulated curves,
particularly at high levels of compaction.
The thesis delves into pore-scale modeling to determine diffusion coefficients of water in com-
pacted porous smectite clay structures. This exploration is motivated by the limitations inher-
ent in conventional approaches used to obtain transport parameters, which tend to oversim-
plify the intricate porous nature of clay media by treating them as a continuum. This oversim-
plification neglects the behaviors occurring at smaller scales. To overcome this limitation, the
thesis employs various techniques such as random walk simulations, lattice Boltzmann mod-
eling, and large-scale molecular dynamics simulations to investigate transport mechanisms.
These advanced modeling techniques take into account local diffusivities within the represen-
tative elementary volume, allowing for a more accurate understanding of transport phenom-
ena. By considering local diffusivities, particularly near chemically reactive clay surfaces, this
approach sheds light on the significance of accurately comprehending transport phenomena
in porous materials. By overcoming the limitations of conventional approaches, the thesis
provides valuable insights into the diffusion coefficients of water within compacted porous
smectite clay structures.
This thesis offers a comprehensive exploration of gas transport and clay behavior, focusing on
their relevance to deep geological disposal of nuclear waste and energy storage. By establishing
connections between simulations conducted under fully saturated and partially saturated con-
ditions, examining the influence of gases on swelling pressure, and incorporating pore-scale
modeling, this research provides valuable insights into diffusion, swelling, and pore-scale pro-
cesses. These findings contribute to the development of effective barrier materials and enhance
our understanding of waste management strategies in complex geological environments. The
knowledge gained from this study has practical implications for improving the safety and effi-
ciency of deep geological disposal systems and advancing energy storage technologies.

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