Page 100 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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80 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
for T c and P c as observed by some researchers [51, 85]. How-
to back-calculate the critical properties of the heaviest end of
ever, Eq. (2.42) and subsequently derived Eqs. (2.67) and error associated with critical properties of heavy fractions is
(2.68) have the internal consistency and can be used from the reservoir fluid from an EOS based on a measured physical
C 5 to C 50 although they are developed for hydrocarbons from property such as density or saturation pressure [51, 52, 70].
C 20 to C 50 Firoozabadi et al. [63, 64] have studied extensively the wax
The 1980 Riazi–Daubert correlations for T c and P c were and asphaltene precipitation in reservoir fluids. They ana-
generally used and recommended by many researchers for lyzed various methods of calculating critical properties of
light fractions (M < 300, carbon number < C 22 ). Yu et al. [84] heavy petroleum fractions and used Eq. (2.42) for the critical
used 12 different correlations to characterize the C 7+ plus properties and acentric factor of paraffins, naphthenes, and
fraction of several samples of heavy reservoir fluids and bi- aromatics, but they used Eq. (2.43) for the critical pressure of
tumens. Based on the results presented on gas-phase compo- various hydrocarbon groups with M > 300. Their evaluation
sition, GOR, and saturation pressure, Eqs. (2.63) and (2.64) was based on the calculation of the cloud point of different
showed better or equivalent predictions to other methods. oils. It is believed that fractions with molecular weight greater
Whitson [53] made a good analysis of correlations for the crit- than 800 (∼C 57 ) mainly contain aromatic hydrocarbons [63]
ical properties and their effects on characterization of reser- and therefore Eq. (2.42) with constants given in Table 2.6 for
voir fluids and suggested the use of Eqs. (2.63) and (2.64) for aromatics is an appropriate correlation to estimate the prop- --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
petroleum cuts up to C 25 . But later [51] based on his observa- erties of such fractions.
tion for phase behavior prediction of heavy reservoir fluids, More recently Jianzhong et al. [87] reviewed and evalu-
he recommended the use of Kesler–Lee or Twu for estimation ated various methods of estimation of critical properties of
of T c and P c of such fluids, while for estimation of critical petroleum and coal liquid fractions. Their work followed
volume he uses Eq. (2.98). Soreide [52] in an extensive eval- the work of Voulgaris et al. [88], who recommended use of
uation of various correlations for the estimation of critical Eq. (2.38) for estimation of critical properties for the purpose
properties recommends use of the API-TDB [2] method for of prediction of physical properties of petroleum fractions
estimation T c and P c (Eqs. (2.65) and (2.66)) but he recom- and coal liquids. They correctly concluded that complexity
mends Twu method for the critical volume. His recommenda- of correlations does not necessarily increase their accuracy.
tions are based on phase behavior calculations for 68 samples They evaluated Lee–Kesler, Riazi–Daubert, and Twu meth-
of North Sea reservoir fluids. In a recently published Hand- ods with more than 318 compounds (>C 5 ) including those
book of Reservoir Engineering [48], and calculations made on found in coal liquids with boiling point up to 418 C (785 F)
◦
◦
phase behavior of reservoir fluids [86], Eqs. (2.65), (2.66) have and specific gravity up to 1.175 [87]. They suggested that
been selected for the estimation of critical properties of unde- Eq. (2.38) is the most suitable and accurate relation especially
fined petroleum fractions. Another possibility to reduce the when the coefficients are modified. Based on their database,

Acentric Factor

FIG. 2.20—Prediction of acentric factor of pure hydrocarbons from Eq. (2.42).

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