Page 26 - Characterization and Properties of Petroleum Fractions - M.R. Riazi

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6 CHARACTERIZATION AND PROPERTIES OF PETROLEUM FRACTIONS
Reservoir fluid type TABLE 1.1—Types and characteristics of various reservoir fluids. API gravity of STO a
CH 4 , mol%
C 6+ , mol%
GOR, scf/stb
Black oil <1000 ≤50 ≥30 <40
Volatile oil 1000–3000 50–70 10–30 40–45
Gas condensate 3000–50 000 70–85 3–10 ≥45
Wet gas ≥50 000 ≥75 <3 >50
Dry gas ≥10 0000 ≥90 <1 No liquid
a API gravity of stock tank oil (STO) produced at the surface facilities at standard conditions (289 K and 1 atm).
produced at SC in standard cubic feet (scf) to the amount of properties outside the corresponding limits mentioned ear-
liquid oil produced at the SC in stock tank barrel (stb). Other lier. Determination of a type of reservoir fluid by the above
units of GOR are discussed in Section 1.7.23 and its calcula- rule of thumb based on the GOR, API gravity of stock tank
tion is discussed in Chapter 9. Generally, reservoir fluids are oil, or its color is not possible for all fluids. A more accu-
categorized into four or five types (their characteristics are rate method of determining the type of a reservoir fluid is
given in Table 1.1). These five fluids in the direction of in- based on the phase behavior calculations, its critical point,
creasing GOR are black oil, volatile oil, gas condensate, wet and shape of the phase diagram which will be discussed in
gas, and dry gas. Chapters 5 and 9. In general, oils produced from wet gas,
If a gas after surface separator, under SC, does not pro- gas condensate, volatile oil, and black oil increase in spe-
duce any liquid oil, it is called dry gas. A natural gas that after cific gravity (decrease in API gravity and quality) in the same
production at the surface facilities can produce a little liquid order. Here quality of oil indicates lower carbon, sulfur, nitro-
oil is called wet gas. The word wet does not mean that the gen, and metal contents which correspond to higher heating
gas is wet with water, but refers to the hydrocarbon liquids value. Liquids from black oils are viscous and black in color,
that condense at surface conditions. In dry gases no liquid while the liquids from gas condensates or wet gases are clear
hydrocarbon is formed at the surface conditions. However, and colorless. Volatile oils produce fluids brown with some
both dry and wet gases are in the category of natural gases. red/green color liquid. Wet gas contains less methane than a
Volatile oils have also been called high-shrinkage crude oil and dry gas does, but a larger fraction of C 2 –C 6 components. Ob-
near-critical oils, since the reservoir temperature and pressure viously the main difference between these reservoir fluids is
are very close to the critical point of such oils, but the critical their respective composition. An example of composition of
temperature is always greater than the reservoir temperature different reservoir fluids is given in Table 1.2.
[11]. Gases and gas condensate fluids have critical tempera- In Table 1.2, C 7+ refers to all hydrocarbons having seven
tures less than the reservoir temperature. Black oils contain or higher carbon atoms and is called heptane-plus fraction,
heavier compounds and therefore the API gravity of stock while C 6 refers to a group of all hydrocarbons with six car-
tank oil is generally lower than 40 and the GOR is less than bon atoms (hexanes) that exist in the fluid. M 7+ and SG 7+ are
1000 scf/stb. The specifications given in Table 1.1 for various the molecular weight and specific gravity at 15.5 C (60 F) for
◦
◦
reservoir fluids, especially at the boundaries between differ- the C 7+ fraction of the mixture, respectively. It should be re-
ent types, are arbitrary and vary from one source to another alized that molecular weight and specific gravity of the whole
[9, 11]. It is possible to have a reservoir fluid type that has reservoir fluid are less than the corresponding values for the
TABLE 1.2—Composition (mol%) and properties of various reservoir fluids and a crude oil.
Component Dry gas a Wet gas b Gas condensate c Volatile oil d Black oil e Crude oil f
CO 2 3.70 0.00 0.18 1.19 0.09 0.00
N 2 0.30 0.00 0.13 0.51 2.09 0.00
H 2 S 0.00 0.00 0.00 0.00 1.89 0.00
C 1 96.00 82.28 61.92 45.21 29.18 0.00
C 2 0.00 9.52 14.08 7.09 13.60 0.19
C 3 0.00 4.64 8.35 4.61 9.20 1.88
iC 4 0.00 0.64 0.97 1.69 0.95 0.62
nC 4 0.00 0.96 3.41 2.81 4.30 3.92
iC 5 0.00 0.35 0.84 1.55 1.38 2.11
nC 5 0.00 0.29 1.48 2.01 2.60 4.46
C 6 0.00 0.29 1.79 4.42 4.32 8.59
0.00 1.01 6.85 28.91 30.40 78.23
C 7+
Total 100.00 100.00 100.00 100.00 100.00 100.00
GOR (scf/stb) . . . 69917 4428 1011 855 . . .
. . . 113 143 190 209.8 266
M 7+
SG 7+ (at 15.5 C) . . . 0.794 0.795 0.8142 0.844 0.895
◦
46.7 46.5 42.1 36.1 26.6
API 7+
a Gas sample from Salt Lake, Utah [12].
b Wet gas data from McCain [11].
c Gas condensate sample from Samson County, Texas (M. B. Standing, personal notes, Department of Petroleum
Engineering, Norwegian Institute of Technology, Trondheim, Norway, 1974).
d Volatile oil sample from Raleigh Field, Smith County, Mississipi (M. B. Standing, personal notes, Department of
Petroleum Engineering, Norwegian Institute of Technology, Trondheim, Norway, 1974).
e Black oil sample from M. Ghuraiba, M.Sc. Thesis, Kuwait University, Kuwait, 2000.
f A crude oil sample produced at stock tank conditions.

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