Molecular simulation applied to fluid properties in the oil and gas industry

In order to illustrate the use of molecular simulation methods in the oil and gas industry, three typical fields of application are considered. The first of these is the prediction of fluid phase equilibria by Gibbs ensemble simulation, exemplified here by the systematic determination of full phase diagrams (including liquid-vapour critical points) for binary systems of hydrogen sulphide and various hydrocarbons (n-alkanes with 2-20 carbon atoms, benzene, ethylbenzene, cyclohexane). The second type of application is the adsorption equilibria in zeolites. For these applications, the comprehensive modelling of the interplay of hydrocarbon adsorption with polar compounds (alkanethiols, water) and cation mobility is a still a challenge for simulation methods. However encouraging results are shown when the different contributions to interaction energy (dispersion-repulsion, electrostatic, polarization) are separately addressed when simulating the adsorption of n-alkanes in sodium faujasites. The third field considered is the determination of transport properties, particularly viscosity, from equilibrium molecular dynamics (MID). Although they describe rather well the influence of pressure, temperature and carbon number, existing force fields for n-alkanes are found to underestimate the viscosity of liquids. By a specific modification of the torsion potential which does not alter the equilibrium conformations, the viscosity is significantly improved without compromising phase equilibrium properties. A brief outline of other current and future applications of molecular simulation in the oil and gas industry is given in conclusion.