Page 180 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

P2: KVU/KXT
QC: —/—
T1: IML
P1: KVU/KXT
June 22, 2007
AT029-Manual-v7.cls
AT029-Manual
AT029-04
160 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
4.2 GENERALIZED CORRELATIONS
FOR PSEUDOCRITICAL PROPERTIES 21:30 natural gas. However, for gas condensate systems simple cor-
relations in terms of specific gravity have been proposed in
OF NATURAL GASES AND GAS the following forms similar to the above correlations and are
CONDENSATE SYSTEMS usually used by reservoir engineers [10]:
(4.3) T pc = 103.9 + 183.3SG g − 39.7SG 2
Natural gas is a mixture of light hydrocarbon gases rich in g
methane. Methane content of natural gases is usually above (4.4) P pc = 48.66 + 3.56SG g − 0.77SG 2
g
75% with C 7+ fraction less than 1%. If mole fraction of H 2 Sin These equations give higher critical temperature and lower
a natural gas is less than 4 × 10 −6 (4 ppm on gas volume basis) critical pressure than do the equations for natural gases since
it is called “sweet” gas (Section 1.7.15). A sample composition gas condensate samples contain heavier compounds. Because
of a natural gas is given in Table 1.2. Dry gases contain no
C 7+ and have more than 90 mol% methane. The main differ- of the greater variation in carbon number, the equations for
ence between natural gas and other reservoir fluids is that the gas condensate systems are much less accurate than those
amount of C 7+ or even C 6+ in the mixture is quite low and the for natural gas systems. For this reason properties of gas con-
main components are light paraffinic hydrocarbons. Proper- densate systems may be estimated more accurately from the
ties of pure light hydrocarbons are given in Tables 2.1 and 2.2. distribution models presented in Section 4.5.
The C 6+ or C 7+ fraction of a mixture should be treated as an Equations (4.1)–(4.4) proposed for pseudocritical proper-
undefined fraction and its properties may be determined from ties of natural gas and gas condensate systems are based
the correlations given in Chapter 2. If the detailed composi- on the assumption that mixtures contain only hydrocarbon
tion of a natural gas is known the best method of characteriza- compounds. However, these reservoir fluids generally contain
tion is through Eq. (3.44) with composition in terms of mole components such as carbon dioxide (CO 2 ), hydrogen sulfide
fraction (x i ) for calculation of pseudocritical properties, acen- (H 2 S), or nitrogen (N 2 ). Presence of such compounds affects
tric factor, and molecular weight of the mixture. Although the properties of the gas mixture. For such cases, correc-
the Key’s mixing rule is not the most accurate mixing rule tions are added to the calculated pseudocritical properties
from Eqs. (4.1)–(4.4). Corrections proposed by Wichert and
for pseudocritical properties of mixtures, but for natural gas Aziz [12] and Carr et al. [13] are recommended for the effects
systems that mainly contain methane it can be used with rea- of nonhydrocarbons on properties of natural gases [8]. The
sonable accuracy. More advanced mixing rules are discussed
in Chapter 5. Once the basic characterization parameters for method of Carr et al. for adjustment of calculated T pc and P pc
is given as follows:
the mixture are determined various physical properties can
be estimated from appropriate methods. (4.5) T c
pc = T pc − 44.44y CO 2 + 72.22y H 2 S − 138.89y N 2
The second approach is to consider the mixture as a single c
pseudocomponent with known specific gravity. This method (4.6) P pc = P pc + 30.3369y CO 2 + 41.368y H 2 S − 11.721y N 2
c
c
is particularly useful when the exact composition of the mix- where T pc and P pc are the adjusted (corrected) pseudocriti-
ture is not known. There are a number of empirical correla- cal temperature and pressure in kelvin and bar, respectively.
tions in the literature to estimate basic properties of natural y CO 2 , y H 2 S and y N 2 are the mole fractions of CO 2 ,H 2 S, and N 2 ,
gases from their specific gravity. Some of these methods are respectively. T pc and P pc are unadjusted pseudocritical tem-
summarized below. perature and pressure in kelvin and bar, respectively. These
In cases that the composition of a natural gas is unknown unadjusted properties may be calculated from Eqs. (4.1) and
Brown presented a simple graphical method to estimate pseu- (4.2) for a natural gas. The following example shows calcula-
docritical temperature and pressure from gas specific gravity tion of pseudocritical properties for a natural gas sample.
(SG g ) as shown by Ahmed [10]. Standing [11] converted the
graphical methods into the following correlations for estima- Example 4.1—A natural gas has the following composition in
tion of T pc and P pc of natural gases free of CO 2 and H 2 S: mol%: H 2 S 1.2%, N 2 0.2%; CO 2 1%, C 1 90%, C 2 4.8%, C 3 1.7%,
iC 4 0.4, nC 4 0.5%, iC 5 0.1, nC 5 0.1%.
(4.1) T pc = 93.3 + 180.6SG g − 6.94SG 2 g
a. Calculate T c , P c , ω, and M using properties of pure com-
(4.2) P pc = 46.66 + 1.03SG g − 2.58SG 2 g pounds.
b. Calculate the gas specific gravity.
where T pc and P pc are the pseudocritical temperature and c. Calculate T c and P c using Eqs. (4.1) and (4.2) and SG cal-
pressure in kelvin and bar, respectively. SG g is defined in
Eq. (2.6). This method is particularly useful when the exact culated from Part (b). --`,```,`,``````,`,````,```,,-`-`,,`,,`,`,,`---
composition of the mixture is not available. This method pro- d. Adjust T c and P c for the effects of nonhydrocarbon com-
vides acceptable results since nearly 90% of the mixture is pounds present in the gas.
methane and the mixture is close to a pure component. There-
fore, assumption of a single pseudocomponent is quite rea- Solution—Values of M, T c , P c , and ω for pure components
sonable without significant difference with detailed composi- present in the gas mixture can be obtained from Table 2.1.
tional analysis. Application of these equations to wet gases is These values as well as calculated values of M, T c , P c , and ω
less accurate. for the mixture based on Eq. (3.44) are given in Table 4.4. The
Another type of reservoir fluids that are in gaseous phase calculated values of M, T c , P c , and ω as shown in Table 4.4 are:
◦
under reservoir conditions are gas condensate systems. Com- M = 18.17, T pc =−68.24 C, P pc = 46.74 bar, and ω = 0.0234.
position of a gas condensate sample is given in Table 1.2. Its This method should be used for gases with SG g > 0.75 [10].
C 7+ content is more than that of natural gases and it is about a. Equation (2.6) can be used to calculate gas specific gravity:
few percent, while its methane content is less than that of SG g = 18.17/28.96 = 0.6274.

Copyright ASTM International
Provided by IHS Markit under license with ASTM Licensee=International Dealers Demo/2222333001, User=Anggiansah, Erick
No reproduction or networking permitted without license from IHS Not for Resale, 08/26/2021 21:56:35 MDT