Page 173 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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m Mole fraction
mixture in terms of arbitrary number of subfractions (pseu-
docomponents) with known mole fraction, boiling point, spe-
N Naphthenic T1: IML 4. CHARACTERIZATION OF RESERVOIR FLUIDS AND CRUDE OILS 153
n Refers to SCN group with n carbon number cific gravity, and molecular weight. This approach is called
P Paraffinic pseudoization. The main objective of this chapter is to present
pc Pseudo-Critical methods of characterization of hydrocarbon-plus fractions,
T A distribution coefficient in Eq. (4.56) for which involves prediction of distribution of hydrocarbons in
boiling point the mixture and to represent the fluid in terms of several nar-
v Volume fraction row range subfractions. However, for natural gases and gas
w Weight fraction condensate fluids that are rich in low-molecular-weight hy-
o Value of a property at x c = 0 in Eq. (4.56) drocarbons simple relations have been proposed in the lit-
∞ Value of a property at M →∞ erature. In this chapter types of data available for reservoir
20 Value of a property at 20 C fluids and crude oils are discussed followed by characteriza-
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tion of natural gases. Then physical properties of single car-
Acronyms bon number (SCN) groups are presented. Three distribution
models for properties of hydrocarbon plus fractions are intro-
%AAD Average absolute deviation percentage defined duced and their application in characterization of reservoir
by Eq. (2.135) fluids is examined. Finally, the proposed methods are used to
API-TDB American Petroleum Institute—Technical calculate some properties of crude oils. Accuracy of charac-
Data Book terization of reservoir fluids largely depends on the distribu-
%D Absolute deviation percentage defined by tion model used to express component distribution as well as
Eq. (2.134) characterization methods of petroleum fractions discussed in
EOS Equation of state Chapter 2 to estimate properties of the narrow boiling range
GC Gas chromatography pseudocomponents.
KISR Kuwait Institute for Scientific Research
%MAD Maximum absolute deviation percentage
OGJ Oil & Gas Journal 4.1 SPECIFICATIONS OF RESERVOIR
PDF Probability density function FLUIDS AND CRUDE ASSAYS
PNA Paraffins naphthenes aromatics
RMS Root mean squares defined by Eq. (4.59) Characterization of a petroleum fluid requires input para-
RVP Reid vapor pressure meters that are determined from laboratory measurements.
2
RS R squared (R ), defined in Eq. (2.136) In this section types of data available for a reservoir fluid or
SCN Single carbon number a crude oil are presented. Availability of proper data leads to
TBP True boiling point appropriate characterization of a reservoir fluid or a crude oil.
VLE Vapor–liquid equilibrium
AS DISCUSSED IN CHAPTER 1, reservoir fluids are in the forms of 4.1.1 Laboratory Data for Reservoir Fluids
natural gases, gas condensates, volatile oils, and black oils. As
Data on composition of various reservoir fluids and a crude oil
shown in Table 1.1, these fluids contain hydrocarbons from C 1
to compounds with carbon number greater than 50. Composi- were shown in Table 1.1. Further data on composition of four
tion of a reservoir fluid is generally expressed in mol% of non- reservoir fluids from North Sea and South West Texas fields
hydrocarbon compounds (i.e., H 2 S, CO 2 ,N 2 ), C 1 ,C 2 ,C 3 , nC 4 , are given in Table 4.1. Data are produced from analysis of the
iC 4 , nC 5 , iC 5 ,C 6 , and C 7+ . The boiling range of reservoir fluids fluid by gas chromatography columns capable of separating
--`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
can be greater than 550 C(∼> 1000 F). Crude oil is produced hydrocarbons up to C 40 or C 45 . Composition of the mixture is
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by separating light gases from a reservoir fluid and bringing usually expressed in terms of mol% for pure hydrocarbons up
its condition to surface atmospheric pressure and tempera- to C 5 and for heavier hydrocarbons by single carbon number
ture. Therefore, crude oils are generally free from methane (SCN) groups up to C 30 or C 40 . However, detailed composition
and contain little ethane. The main difference between vari- is available for lower carbon numbers while all heavy hydro-
ous reservoir fluid and produced crude oil is in their composi- carbons are lumped into a single group called hydrocarbon-
tion, as shown in Table 1.1. Amount of methane reduces from plus fraction. For example in Table 4.1, data are given up to
natural gas to gas condensate, volatile oil, black oil, and crude C 9 for each SCN group while heavier compounds are grouped
oil while amount of heavier compounds (i.e., C 7+ ) increase in into a C 10+ fraction. It is customary in the petroleum indus-
the same direction. Characterization of reservoir fluids and try to lump the hydrocarbons heavier than heptane into a C 7+
crude oils mainly involves characterization of hydrocarbon- fraction. For this reason the mol% of C 7+ for the four mixtures
plus fractions generally expressed in terms of C 7+ fractions. is also presented in Table 4.1. For hydrocarbon-plus fractions
These fractions are completely different from petroleum frac- it is important to report a minimum of two characteristics.
tions discussed in Chapter 3. A C 7+ fraction of a crude oil has These two specifications are generally molecular weight and
a very wide boiling range in comparison with a petroleum specific gravity (or API gravity) shown by M 7+ and SG 7+ , re-
product and contains more complex and heavy compounds. spectively. In some cases a reservoir fluid is presented in terms
Usually the only information available for a C 7+ fraction is of true boiling point (TBP) of each SCN group except for the
the mole fraction, molecular weight, and specific gravity. plus fraction in which boiling point is not available. The plus
The characterization procedure involves how to present this fractions contain heavy compounds and for this reason their

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