Conventional, carbon number compositional analysis techniques provide the weight or mole fraction of pure components up to C6- and carbon number pseudo-components that represent the C7+ fraction. These typically reported carbon number oil compositions cannot be used with asphaltene and/or wax phase behavior models and are not suitable for interpretation of experimental asphaltene and wax studies, because they do not provide direct phase compositional information on the waxes, resins, and asphaltenes. Waxes, resins, and asphaltenes are usually distributed among the heavier carbon number pseudo-components in these carbon number-type compositional analyses. Hence, new techniques and methodologies are required for analyzing hydrocarbon fluids for the amount and type of waxes, resins, and asphaltenes and for developing EOS oil characterizations suitable for predicting PVT, wax, and asphaltene phenomena. This will eliminate the traditional guess-work that is involved in the development of EOS oil characterizations of asphaltenic and/or waxy fluids.
A new, efficient, and more accurate technique for analyzing crude oil and its fractions to determine hydrocarbon group types, i.e., paraffins (including paraffin waxes), aromatics, resins, and asphaltenes is the PARA analysis. Distillation, solvent extraction, high performance liquid chromatography (HPLC), gel permeation chromatography (GPC), and other techniques are used to perform the oil analyses and obtain the compositional and structural data required for characterizing each hydrocarbon group with its individual pseudocomponents. The analyses. in addition to providing weight and/or mole percent for each fraction, provides a starting a priori oil characterization suitable for EOS type modeling methods. The starting oil characterization includes an experimentally assisted selection of pseudo-components for each hydrocarbon group type together with an initial estimate of their critical properties, molecular weight, and acentric factor. This paper discusses the PARA-Based (Paraffin-Aromatic-Resin-Asphaltene) EOS reservoir fluid characterization technique and presents an example application of PVT and wax phase behavior modeling. Equation of State (EOS) Approach -
An equation of state is a thermodynamic relation or function involving the measurable thermodynamic variables P, T,. This section gives a brief description of equations of state.
Origin of Equations of State. In the field of hydrocarbon thermodynamics it has been assumed that we deal with "simple compressible substances." For these substances, the only important (i.e., of significant magnitude) reversible work mode is by volume change (Pd work). The state postulate of thermodynamics says that:
The number of independently variable thermodynamic properties for a specified system is equal to the number of relevant reversible work modes plus one.
This means that in hydrocarbon systems, if two thermodynamic properties are specified, the state of the system is completely specified. That is, all the other thermodynamic properties are specified as well. Mathematically, this is shown as:
If two of the properties of a simple compressible substance are specified. then the third can be calculated from Eq. 1. Eq. 1 is therefore called an equation of state. The simplest P- -T EOS is that of the ideal gas.
A gas is ideal if its molecules occupy no volume and do not interact with each other. This is clearly not true for real gases but the ideal gas law has served us well over the years in many areas of endeavor.
On the basis of molecular arguments, to account for molecular volume and interaction, van der Waals, in 1983, proposed the following P- -T EOS as a modification to the ideal gas law of Eq. 2.