Molecular Modelling of Pore Fluids in Clays
Recent advances in statistical mechanical molecular modelling of clay-fluid interactions will be discussed, with emphasis on empirical potential based Monte Carlo and molecular dynamics simulations. These methods will be illustrated by reference to the st
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Department ofPhysics and Astronomy, University College London, Gower Street, London WClE 6BT, UK. Email: [email protected] http://www.cmmp.ucl.ac.uk/-nts
Abstract. Recent advances in statistical mechanical molecular modelling of clay-fluid interactions will be discussed, with emphasis on empirical potential based Monte Carlo and molecular dynamics simulations. These methods will be illustrated by reference to the structure and dynamics of ions and solvent at hydrated clay surfaces. The aim of this research is to provide molecular scale insight into fluid dependent processes, for example; clay swelling, solute transport through clays, and clay compaction during burial. Models for the interparticle interactions will be reviewed, with particular reference to the various strategies used to represent clay-fluid systems. The general principles of Monte Carlo and molecular dynamics computer simulation will then be described. A number of specific issues arise when these techniques are applied to the properties of confined fluids: longrange interactions, system size limitations, boundary conditions, choice of thermodynamic ensemble, and statistical sampling. Throughout, we will compare and contrast recent computer simulations of clay-fluid systems with experimental data, and draw generallessons from these examples.
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Introducdon
The swelling clay minerals, for example smectite and vermiculite, are layer-type aluminosilicates that are widespread in geologic deposits, soils, and sedimentary rocks (Brindley & Brown, 1980). They are comprised of stacks of negatively charged mica-like sheets, which are held together by charge balancing interlayer counterions, for example sodium or calcium. Since these counterions have a strong tendency to hydrate, water molecules and other polar molecules can be intercalated between the clay layers. This creates an interlayer ionic solution, which causes the clay to swell. Typically, the expanding clay passes through three discrete hydration states, and is then govemed by Ionger-range electrical double layer interactions (Sposito 1982, Newman 1987). Detailed knowledge of clay swelling and interlayer fluids is essential if we are to understand many important natural and industrial processes. For example, clay hydration/dehydration plays a key role in subsurface fluid migration, darnage to buildings, and oil-well collapse (North, 1990). In addition, the interlayer region of clays provides an ideal environment in which to study the fundamental properties of2-dimensional pore-fluids. Statistical mechanical computer simulations provide a direct link between the microscopic properties of particles, such as their masses and interactions, and the observable properties of
B. Loret et al. (eds.), Chemo-Mechanical Couplings in Porous Media Geomechanics and Biomechanics © Springer-Verlag Wien 2004
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N. Skipper
macroscopic systems, such as the energetics, average structure, and transpoft coefficients (figure 1; Allen and Tildesley, 1987; Frenkel and Smit, 1996). Such computational techniques hav
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