(213d) Application of the SAFT-VR Density Functional Theory to the Prediction of Interfacial Properties of Mixtures of Relevance to Reservoir Engineering

Interfacial phenomena of
complex mixtures play a crucial role in a number of industrial applications. In
particular, they are of key importance in oil-reservoir engineering. The
exploitation of oil reserves involves several complicated stages of extraction,
where the presence of vapour-liquid and liquid-liquid equilibria have a marked
effect on the operating conditions. The optimization of these conditions not
only depends on the bulk phase behaviour, but also on the interfacial
properties such as the interfacial tension between the phases, the interface
profiles, the interfacial thickness, and the surface adsorption (the relative
enrichment of one compound over another).

The SAFT-VR DFT formalism, which has
recently been extended to mixtures [F. Llovell et al., J. Chem. Phys. 133,
024704 (2010)], is applied to carefully selected binary mixtures that are representative
of the types of system relevant to enhanced oil recovery and carbon dioxide
sequestration processes. The approach is based on the statistical associating
fluid theory for attractive potentials of variable range (SAFT-VR) and includes
a specific DFT treatment, where the short range interactions are treated at the
local level and the long-range interactions are treated with an appropriate
perturbative contribution. The correlations between the fluid particles are
taken into account with an average correlation function following the ideas of
Toxvaerd [S. Toxvaerd, J. Chem. Phys. 64, 2863 (1976)]. Binary mixtures of
short and long chain n-alkanes, carbon dioxide + n-alkanes, water + carbon
dioxide and water + n-alkanes are investigated over wide ranges of temperature
and pressure. Molecular parameters for each pure compound are estimated from
the bulk vapour-liquid equilibrium data alone, and the unlike binary dispersive
energy parameter is adjusted to provide the best overall description of the
phase equilibrium properties of the mixtures. No additional parameters are
required in the implementation of the SAFT-VR DFT theory. Predicted
vapour-liquid and liquid-liquid interfacial tensions are found to be in
quantitative agreement with experimental data in most cases. Density profiles which
are characterised by noteworthy adsorption features are also presented. The
fraction of free hydrogen bonds in the neighbourhood of the interface is also
reported for the aqueous mixtures studied.