Diffusion and Gas Flow Dynamics in Partially Saturated Smectites.

Area of Science:

• Geochemistry
• Materials Science
• Chemical Engineering

Background:

• Clays are crucial natural and engineered barriers for nuclear waste disposal.
• Gas generation from waste degradation requires migration through clay barriers to prevent pressure buildup.
• Gas migration in partially saturated clays involves movement through water films and gas-filled pores.

Purpose of the Study:

• To investigate gas molecule mobilities (He, H2, CO2, Ar, CH4) in Na-montmorillonite mesopores under varying saturation.
• To evaluate the hydrodynamic behavior of pore fluid in partially saturated clays.
• To understand gas partitioning between water-rich and gas-rich phases and its dependence on environmental factors.

Main Methods:

• Classical molecular dynamics (MD) simulations to determine gas diffusion coefficients and partitioning.
• Nonequilibrium molecular dynamics (NEMD) simulations to study gas mobility and viscosity in different dynamic regimes.
• Development of a Bosanquet-type equation to predict gas diffusion and viscosity based on pore characteristics.

Main Results:

• Gas diffusion in the gas phase increases with pore width, approaching bulk diffusion.
• Gas diffusion in water films remains constant in bulk-like phases but changes in very thin films.
• Gas partitioning is similar to bulk phases and depends on temperature and gas molecular weight; slip boundary conditions are necessary at the microscale.

Conclusions:

• Gas migration in clay barriers is complex, influenced by saturation, pore structure, and fluid-phase interactions.
• Molecular dynamics simulations provide critical insights into gas transport mechanisms at the nanoscale.
• The developed Bosanquet-type equation can aid in modeling gas transport in geological repositories.