Thermodynamic Properties of Asphaltenes: A Predictive Approach Based on Computer Assisted Structure Elucidation and Atomistic Simulations

M. Diallo, T. Cagin, J. L. Faulon, and W. A. Goddard, III,

Asphalts and Asphaltenes II,
Development in Petroleum Science Series, 40 B, Chapter 5, pp 103-127
Eds. T.F. Yen and G. V. Chilingirian, Elsevier Science, Amsterdam 2000.

Abstract

Crude oil is a complex mixture of hydrocarbons and heteroatomic organic compounds of varying molecular weight and polarity. A common practice in the petroleum industry is to separate the crude oil into four chemically distinct fractions: saturates, aromatics, asphaltenes and resins. Asphaltenes are operationally defined as the non-colatile and polar fraction of petroleum that is insoluble in n-alkanes (i.e. n-pentane). Conversely resins are defined as the non-colatile and polar fraction of petroleum that is soluble in n-alkanes (i.e. n-pentane), and aromatic solvents (i.e. toluene), and insoluble in ethyl acetate. A commonly accepted view in the petroleum chemistry is that crude oil asphaltenes form micelles which are stabilized by adsorbed resins kept in solution by aromatics. Two key parameters that control the stability of asphaltene micelles in a crude oil are the ratio of aromatics to saturates and that of resins to asphaltenes. When these ratios decrease, asphaltene micelles will coalesce and form larger aggregates. The precipitation of asphaltene aggregates can cause such problems such as reservoir plugging and wettability reversal. The adsorption of asphaltene aggregates at oil-water interfaces has also been shown to cause the steric stabilization of (W/O) petroleum emulsions. Consequently, the oil industry needs quantitative tools and thermodynamic data to predict asphaltene aggregation and precipitation as a function of crude oil composition and reservoir temperature and pressure.

This chapter describes a new approach for predicting the thermodynamic properties of asphaltenes. This combines computer assisted structure elucidation (CASE) with atomistic simulations. To illustrate this approach, we use quantitative and qualitative structural data as input to a CASE program (SIGNATURE) to generate a sample of 10 model asphaltene structures for Saudi crude oil (Arab Berri). We then carry out molecular mechanics (MM) calculations and molecular dynamics (MD) simulations to estimate selected volumetric and thermal properties of the model structures. We find that the estimated values are in good agreement with the available experimental data.

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