THE NATURE OF PETROLEUM
Petroleum may be defined as a naturally occurring, complex
mixture of hydrocarbons which may be either gas, liquid or
solid, depending upon its own unique composition and the
pressure and temperature at which it is confined.
The principal hydrocarbon series found in petroleum are:
i.
Paraffins (also called saturated hydrocarbons or
alkanes) which have a general formula CnH2n+2 .
crude oils contain some paraffins.
All
The first few
members of this series are:
Abbreviation
Formula
C1
CH4
C2
C2H6
C3
C3H8
C4
C4H10
iC4
C4H10
Chemical Structure
H
H C H
H
H H
H C C H
H H
H H H
H C C C H
H H H
H H H H
H C C C C H
H H H H
H H H
H C C C H
H
H
H
C H
H
Name
Methane
Ethane
Propane
Normal Butane
Iso-Butane
1
ii.
Cycloparaffins (naphthenes)
The general formula is CnH2n. These compounds have
a
ring
structure,
the
simplest
member
being
cyclopropane. Typical members of this group are:
C3H6
C4H8
iii.
H H
C
H
H
C C
H
H
Cyclopropane
Cyclobutane
H
H
C
H
H
C H
H
Aromatics (benzene series)
The general formula is CnH2n-6 . These compounds are
chemically active and contain the benzene ring. The
simplest member is:
H
C
C6H6
Benzene
H C
H C
C H
C H
C
H
2
In
addition
to
hydrocarbons,
petroleum
may
contain
numerous impurities such as:
Carbon dioxide (CO2)
Hydrogen sulphide (H2S)
and other complex compounds of nitrogen, sulphur and
oxygen
So it become obvious that precise chemical analysis of
petroleum is impossible.
Consequently, petroleum is often classified as either
paraffin based, asphalt base or mixed base crude.
A paraffin based crude is an oil whose chief components are
paraffins, and which, when completely distilled, leaves a solid
residue of wax.
An asphalt base crude is an oil composed primarily of cyclic
compound (mostly naphthenes) which, when distilled leaves a
solid residue of asphalt.
3
Properties of Liquid Petroleum
The most widely used indicator of a crude oil’s worth to the
produces is its API (American Petroleum Institute) gravity.
API Gravity (degrees) 
141.5
 131 .5
S. G
Normally, the price which a producer receives for his oil
depends on its gravity, the less dense oils (higher API
gravity) being the most valuable.
Lighter oils contain higher percentages of the more valuable
products such as gasoline.
The surface or tank oil as finally sold by the producer is not
the same liquid which existed underground.
A reservoir oil contains in solution some components which
may be gases at standard temperature and pressure. Their
solubility is due to the high pressure and
temperature
existing underground.
4
As oil is produced (brought from underground to the
surface), the pressure is decreased until it reaches at
atmospheric conditions in the stock tanks.
This pressure reductions causes certain changes in the
reservoir fluid properties:
(a)
Some of the volatile fractions vaporize, causing
(b) the liquid volume to shrink
(c)
the liquid viscosity to increase.
A number of fundamental ideas must be thoroughly defined:
1. Bubble Point Pressure, Pb
The bubble point pressure is also known as saturation
pressure. It is the pressure at which the first gas is
liberated from the reservoir temperature.
2. Formation Volume Factor, Bo
It is the reservoir volume occupied per volume of tank
oil (oil reduced to standard conditions – 14.7 psia and
600F) and its dissolved gas.
5
The value of Bo is always greater than 1.0 because of:
(a)
thermal expansion [shown as VT in Figure 1(D)].
(b) swelling as gas is dissolved at the higher pressures
[shown by increasing values of Vro in Figure 1 (A),
(B), (C), (D)].
Note that Bo increases as the pressure is
decreased from Pi to Pb due to liquid’s expansion.
6
3. The solution gas oil ratio, Rs
It is denoted as GOR in the industry. It is the number
of standard cubic feet of gas dissolved per barrel of
tank oil (SCF/STB)
4. The oil viscosity (o) behaviour shown in Figure 2.
Figure 2
(a)
Viscosity decreases as pressure is reduced from
Pi to Pb due to the liquid’s expansion. This is due
to
the
greater
intermolecular
freedom
of
motion, and internal friction is reduced.
(b) Viscosity increases with pressure reduction
below Pb because the low viscosity fractions are
lost; this more than compensates for the effect
of liquid expansion.
7
Gaseous Petroleum (Natural Gas)
Natural gas is a highly valuable product and is produced from
three classes of wells:
1. From wells where
the dominant product is oil (oil
wells).
2. From wells where the gas itself is the principal product
(gas wells).
3. As gas from condensate wells.
Condensate wells
produce from reservoirs in which the hydrocarbons
(gas and liquid) originally existed as a single fluid (or
phase), the reservoir temperature and pressure being
above the critical point of the hydrocarbon mixture.
Each natural gas. Like each crude oil, is a unique mixture of
hydrocarbons.
All natural gas composed primarily of the
light members of the paraffin series and are predominantly
methane.
8
Numerous impurities are found in petroleum gases, some of
the more common being carbon dioxide (CO2), hydrogen
sulphide (H2S), water vapour, nitrogen and helium.
These impurities detract from the value of a natural gas by
raising the costs of processing it to pipeline and consumer
standards.
There are a number of definitions which can be presented
here:
1. Wet gas: A natural gas is said to be wet if it contains
an appreciable amount of natural gasoline content as
determined by standard tests
2. GPM: The natural gasoline content of a gas expressed
in gallons per thousand standard cubic feet (MCF).
Gases having a GPM of 1 to 2 are wet, while gas with a
GPM of 0.2 would be considered somewhat dry.
3. Sour gas: Natural gas containing hydrogen sulphide.
4. Sweet gas:
Natural gas containing no hydrogen
sulphide.
9
5. Gas gravity: The ratio of the density of a gas to the
density of air at standard conditions.
6. Standard conditions: 14.7 psia and 600F.
The Gas Law
The gas law as applied to the behaviour of natural gas is most
commonly stated as
PV = znRT
Where
. . . . . . . . . . . . . . . . . . (1)
P = absolute pressure
V = volume
z = deviation (also called compressibility)
factor to account for the difference
between actual and ideal gas volumes.
n = number of mols
R = gas constant
T = absolute temperature
The value of R is dependent on the system of units used as
given in the Table below.
P
atmospheres
atmospheres
mm mercury
gm per sq cm
lb per sq inch
lb per sq ft
atmospheres
V
cc
litres
cc
cc
cu ft
cu ft
cu ft
T
O
K
O
K
O
K
O
K
O
R
O
R
O
R
R
82.1
0.0821
62369.
8.315
10.7
1545.
0.730
10
Equation 1 may be rewritten as:
PV  z
W
RT
M
where
. . . . . . . . . . . . . . . . . . . (2)
W/M = n
W = total weight of gas
M = molecular weight of gas
V zRT

W
M
Hence,
P
or
v
zRT
PM
where v =V/W = specific volume of the gas
also
1
PM
=ρ=
v
zRT
where
 = gas density
Another useful expression relating to PVT behaviour of a
constant number of mols of gas is :
P1V1 P2V2
=
= nR = Cons tan t
z1T1 z2T2
11
Determination of z
The values of z for natural gas mixtures have been
experimental
correlated
as
functions
of
pressure,
temperature and composition.
In preparing a correlation for hydrocarbon mixture, the
ratio of actual pressure and temperature to the molal
average
critical
or
pseudo-critical
temperature (Tpc) have been used.
pressure
(Ppc)
and
These ratio are called
reduced pressures (Pr) and reduced temperatures (Tr).
i.e.
reduced pressure, Pr =
P
Ppc
reduced temperature, Tr =
T
Tpc
Figure 3 is a correlation of z as a function of these
quantities.
The chemical analysis of a natural gas is not always available,
an alternative method is by using Figure 4 and Figure 5.
12
13
14
15
Sample Problems:
1. Given the analysis in the table below of a natural gas
produced from an oil well, compute (a) the gas gravity,
and (b) the pseudo-critical temperature and pressure.
Component in the natural gas
methane
ethane
propane
Iso-butane
n-butane
Iso-pentane
n-pentane
Hexanes
Heptane +
Mol %
79.05
10.85
4.61
1.28
2.04
0.21
0.34
0.84
0.78
Also given the physical properties of light paraffin
hydrocarbons and miscellaneous compounds (Table 1)
Compound
Abbreviation
Name
of formula
C1
Methane
C2
Ethane
C3
Propane
iC4
iso-Butane
nC4
normal-Butane
iC5
iso-Pentane
nC5
normal-Pentane
nC6
normal-Hexane
nC7
normal-Heptane
nC8
normal-Octane
nC9
normal-Nonane
nC10
normal-Decane
Air
N2
Nitrogen
O2
Oxygen
CO2
Carbon Dioxide
H 2S
Hydrogen Sulphide
H 2O
Water
Molecular
weight
16.04
30.07
44.09
58.12
58.12
72.15
72.15
86.17
100.2
114.2
128.3
142.3
28.97
28.02
32.00
44.01
34.08
18.02
Critical
pressure,
psia
673
709
618
530
551
482
485
434
397
370
335
312
547
492
732
1072
1306
3206
Critical
temperature,
o
Rankine
344
550
666
733
766
830
847
915
973
1025
1073
1115
239
227
278
548
673
1165
16
Molecular weight of C7+ = 140
Specific gravity of C7+ = 0.85
2. What volume will 100 lb of the above gas occupy at P =
3000 psig, T = 1700F?
3.
(a)
What is the density of a miscellaneous 0.90 gravity
gas at 2000 psia and 1500F?
(b) What is the specific volume at these conditions?
Solutions:
1.
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Component
Mol %
M. wt
(1)x(2)
Pc, psia
Tc, 0R
(1)x(4)
(1)x(5)
methane
79.05
16.04
12.7
673
344
531
272
ethane
10.85
30.07
3.26
709
550
77.0
59.7
propane
4.61
44.09
2.03
618
666
28.5
30.7
Iso-butane
1.28
58.12
0.74
530
733
6.8
9.4
n-butane
2.04
58.12
1.19
551
766
11.2
15.6
Iso-pentane
0.21
72.15
0.15
482
830
1.0
1.7
n-pentane
0.34
72.15
0.25
485
847
1.6
2.9
Hexanes
0.84
86.17
0.72
434
915
3.6
7.7
Heptane +
0.78
140.0
1.09
405
1172
3.2
9.2
664
409oK
100.00
22.13
17
(a)
Column (1) is given.
Column (2) is obtained from
Table 1, and the molecular weight of the mixture is
the total of column (3).
Gas Gravity = Gs =
density of gas at Std. Cond.
density of air at Std. Cond.
Since one mol of any gas occupies the same volume
at standard conditions (379 ft3).
Gs =
22.13 / 379 22.13
=
= 0.76
29 / 379
29
where 29 = molecular weight of air
(b) The C7+ fraction is itself a mixture, and its pseudocritical properties must be obtained from Figure 5.
The other values in column (4) and (5) are from
Table 1. The answer is in column (6) and (7):
Ppc = 664 psia
Tpc = 4090R
18
2.
V=
znRT
P
n = 100/22.1 = 4.52
T = 460 + 170 = 6300R
P = 3000 + 14.7 = 3015 psia
R = 10.7
Obtain z from Figure 3,
Reduced pressure, Pr =
P 3015
=
= 4.54
Ppc 664
Reduced temperature, Tr =
T 630
=
= 1.54
Tpc 409
From Figure 3, z = 0.81
and
3.
(a)
V=
(0.81)(4.52)(10 .7 )(630)
= 8.2 ft3
3015
Gas gravity = 0.9
T = 460 + 150 = 6100F
P = 2000 psia
From Figure 4:
Tpc = 4520F
Ppc = 658 psia
Tr = T/Tpc = 610/452 = 1.35
Pr = P/Ppc = 2000/658 = 3.04
From Figure 3:
z = 0.67
19
Then the density,
ρ=
(c)
PM
(2000)(29)(0.9)
=
= 11 .9 lb/ft 3
zRT (0.67 )(10 .7)(610 )
The specific volume,
v=
1
1
=
= 0.084 ft3 / lb
ρ 11 .9
Additional questions:
4.
A cylindrical tank contains miscellaneous 0.8 gravity gas
at 2500 psia and 1000F.
The volume of the tank is
10ft3.
(a)
How many mols of gas are in the tank?
(b) What standard volume of gas is this?
(c)
1000 SCF of gas is released from the tank. This
causes the temperature to fall to 600F. What is
the final tank pressure?
Solutions:
(a)
From question 3, z = 0.67
n=
PV
(2500)(10)
=
= 6.2
zRT (0.67)(10.7)(560)
20
(b) The standard volume of the gas
Vs =6.2 x 379 = 2350 SCF
or
PsVs PV
=
Ts
zT
Vs =
(c)
PV Ts
(2500)(10)(520)
×
=
= 2350 SCF
zT Ps (0.67)(560)(14.7)
Mols remaining = 6.2 P
100
= 3.6 mols
39
znRT
V
But z = function of P, therefore the solution requires
trial and error.
P (3.6)(10 .7 )(520)

 2000
z
10
Assume
(i)
P = 1500, then z = 0.581*
P/z = 2580 (high)
(ii) P = 1300, then z = 0.617*
P/z = 2100 (high)
(iii) P = 1250, then z = 0.627*
P/z = 1955  2000 (close enough)
final P = 1250 psia
21