281176 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.