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.
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
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.