Szilas A.P. Production and transport of oil and gas, Gathering and Transportation
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60
6
GATtILRING AND SLPARATION
01
011
ANI)
<)A5
Component
I
H
2o
n-C,
_____
~
nt
M
n
Pp""
I
.72:
kg
k
g/mole
hll'a
2
3
4
5
6
I
.0
18
0.0555
02x5
0.4')
10.0
12
0.
I39
0.7
IS
1.23
0.1945
~
~-
~
Exarnple6.4-3
(after Maddox 1963). What is the dew point at
1.72
bar prcssurc
of vapour composed of one
kg
H,O
and
10
kg
n-pentane; which component
will
condense first? The calculation results are given
in
7uhlc
6.4
-
6.
On
cooling
from
a higher temperature, the system
will
first attain 80.9'C. and hcncc, water
will
condense at the dew point.
6.4.2.
Factors affecting recovery
in
the separator*
(a)
Separator pressure
Figure 6.4-
7
shows liquid recovery from a wellstream characterized
in
7uhk
6.4
-
7.
It
reveals that, e.g. at 70 bars pressure, one million
m3
of
wcll fluid rclcasc
100
m3 liquid
C,,
. The curves show that, on increasing separator pressure, all
components will at first condense to an increasing extent. The increment
in
condensate due
to
unity increase
in
pressure is highest
in
the case
of
methane and
Table
6.4
-
7.
Component
c,
C2
c,
i-C,
n-C,
i-C,
n-C,
C6
C,
+
Total:
z,
0.71
in
0.
I
503
0.0478
0.0
1
04
0,0343
0.0096
0.01
14
00092
0.0152
1
,oooo
After
Campbell
(1955)
and Whinery and Campbell
(1958).
6.4
SEPARATION
OF
OIL
AND
GAS
61
grows less as the molecular weight increases. The curves
of
components heavier than
butane exhibit, however, peaks at which the liquid recovery is maximum for a given
temperature, being reduced on further pressure increase by retrograde vaporization.
This phenomenon is
of
considerable interest as regards the production of a liquid
stable at atmospheric pressure. Graph
1
in Fig.
6.4-8
is
a plot
v.
separator pressure
of
liquid yield at a separator temperature
of
27
"C;
Graph
2
represents the volume
of
stock-tank oil stable at atmospheric pressure and
38
"C
temperature, likewise v.
separator pressure. The volume of stock-tank oil, composed largely
of
pentane and
heavier components, will increase up to a pressure about
of41
bars to decrease again
at higher pressures.
A
volume of liquid equal to the ordinate difference between the
p,
bars
Fig. 6.4
~
7. Influence
of
pressure upon
liquid
recovery orcomponents at
T=
27
'
C, after Campbell
(1955)
700
-
9,
q
'"3
0
20
30
40
50
60
70
80
90
100
110
120
p.
bars
Fig. 6.4-
8.
Influence
of
pressure
upon
liquid
recovery
of
separator at
T=
27 "C, after Campbell
(1955)
62
6
GATHERING AND SFPARATION
OF
011,
AND
GAS
two graphs will evaporate
in
the tank at the given atmospheric temperature. Other
well fluids separated at other temperature will give curves displaced against those
given
in
the Figure, but their essential features
will
be same.
(b)
Separator temperature
Figure
6.4-Y
gives yield curves for various hydrocarbons out of a given
wellstream. for a pressure of
28
bars and various temperatures. The lower the
temperature, the less
will
be the increment
in
C,
and
C,
yield due to further decrease
of temperature. The maximum recovery of
C,
+
is at about
10
C.
-
Graph
I
in
Fiy.
6.4
-
I0
represents the total relative liquid yield of the separator; Graph
2
represents
the relative yield of stable liquid, capable of storage at atmospheric pressure: both
v.
separator temperature. Stable-liquid recovery is seen no to increase below about
-
10°C.
If
the primary aim is to recover a maximum amount of stable liquid.
temperatures between
-
1
and
-
10
'C
will
be preferred. Let us point out that at
temperatures below
+
10
'C,
the pentane and hexane content of the stable product
will
increase and this
will
result
in
a decrease of its gravity.
If
the primary aim is to
increase the total liquid yield, e.g.
in
order to recover from the gas as much propane
and butane as possible. then
it
may be preferable to lower the temperature even
below
-
20
"C.
Low-temperature separation. however, necessitates some special
equipment that requires a considerable throughput to make
it
pay out. Separatoi
temperatures higher than
30
C
are not be recommended as a rule.
15c
ql8
v
-6
,3
'C
;,
'0C
5c
10
5
Fig.
6.4
-
9.
Influence
of
temperature upon
liquid
recovery
of
components at
p
=
28
bars.
after Campbell
(1955)
6.4.
SEPARATION
OF
OIL
AND
GAS
63
Fig.
6.4
-
700
4
q
,66
m'
77
600
500
400
300
200
100
0
-20
-10
0
10
20
30
SO
50
1,
OC
10.
Influence
of
temperature upon liquid recovery. at
p=28
bars. after Campbell
(1955)
(c)
Composition
of
the wellstream
Figure
6.4
-
I/
is a plot of stable stock-tank oil yield
v.
tempeature, at two
different separator pressures, for each of the three wellstreams characterized in
Table
6.4-8.
All
three curves are seen to flatten out at temperatures below about
-
7
"C, and to become practically independent
of
temperature and pressure.
Recovery of stable stock-tank oil is governed below temperatures of
-
7
"C
largely
by wellstream composition. The factor having the most profound influence
on
stable-liquid yield is the molar ratio of
C,,
.
Table
6.4-
8
lists, in addition to the
compositions
of
the three wellstreams marked
A,
Band
C,
also the absolute values
300
I
63
0
bars
01
,
1-7
40
lo
*O
30T,
"C
-20
-10
0
Fig.
6.4-
11.
Influence
of
composition upon liquid recovery, after Campbell
(1955)
64
850
kgjrn'
?,
;/I
750
800-
-
+
-
1
1,.
'
LLI
*
1
-&
700
~
+
'
-
650
~-
2
_.*
c
2.
600
-
*
550,
1
1
11
1
1
6.
GATHERING AND SEPARATION
OF
OIL
AND GAS
Component
c,
cz
c3
i-C,
n-C,
i-C5
n-C,
c,
C-
*
Total:
91.
m'
~
calc.
4
m'
Table
6.4-
8.
A
0.71
18
0
1503
0.0478
0.0
104
0,0343
00396
001
I4
0.0092
0.0152
1
.m
0-
1
379
I
4
0.0358
1
4
265
I
.0
272
B
0.700
1
0
I503
0.0742
0.0
I04
00343
0.0096
0.01
14
0.0063
0.0034
I
~oooo
0.
I496
1.1
0.02
I
1
0.59
I53
0.58
152
C
0.7354
0.1
503
0.0
I20
0.0087
0.0273
0.0036
0g045
0.0
1
04
0.0478
1
.oooo
0.1
143
0.82
0.0627
I
.8
490
1.8
492
6.4.
SEPARATION
OF
OIL
AND
GAS
65
of
z3
+
and
z5
+
,
and their values related to the yield for wellstream
A.
The relative
stable-liquid yields of the three wellstreams are plotted
in
Fig.
6.4-
11.
The values at
-
7
"C,
marked in the Figure by dashed lines, have also been listed
in
the Table. The
yields related
to
the yield
of
fluid
A
are seen to agree pretty well
with
the relative
values of z5
+.
It is further apparent that
C,
+
is not representative at all of stable-
liquid yield. Stable-liquid yield to be expected on separation below
-
7
"C
can be
estimated by means of the formula
!!!
=8160z,+
-20.
q
6.4
-
14
The last row of Table 6.4-
8
lists the values furnished by this formula
for
the three
wellstreams
A,
B
and
C.
There is a fair agreement
with
the
q,/q
values read off
Diagram 6.4
-
1 1.
Relationships characterizing recovery above
-
7
"C
are more
complicated.
Figure
6.4-
12
is the plot of an empirical relationship furnishing the
separator pressure ensuring maximum total liquid yield
v.
well-fluid gravity, for the
temperature range between
21
and
27
"C.
(d)
Stage separation
In Sections (a),
(b)
and (c) we have analyzed the effects of temperature, pressure
and wellstream composition, tacitly assuming two-stage separation with one
separator and one stock-tank.
If
the number of separation stages is more than two,
Fig.
6.4
-
13.
5
Stable
26
!iiEEEG
25
2
3
4
5
P2
,
Ls
stock-tank
oil
recovery
in
three-stage separation, after
Campbell
(1955)
66
6
GATHERING AND SEPAKAIION
OF
OIL
AND GAS
this will improve the recovery
of
stable stock-tank oil but diminish the total-liquid
yield. Hence, evaporation
loss
in
open tanks
will
be less after stage separation.
Increasing the number
of
stages from two
to
three brings a considerable
improvement. According to an investigation on 13 wells, the recovery of stable
stock-tank oil was improved by
8
percent on an average (with a range from 3 to 22
percent). On increasing stage number from three to four, the improvement
in
recovery tends to be much less, and four-stage separation is not usually economical.
Figure
6.4-
13
shows what percentage of the liquid
4rl
discharged from the first
separator stage (composition characterized in
T'hlr
6.4
-
Y)
will collect as stable oil
(qr3)
in the stock tank as a function of second-stage pressure, provided first-stage
pressure is 27.6 bars and third-stage pressure is 1.01 bar. Liquids
A
and
B
passing
from the first-stage into the second derive from one and the same wellstream, but
first-stage temperature was
-
17.8 "C
for liquid
A
and 26.7
"C
for liquid
B,
resulting
Component
Table
6.4
-
9.
Z,
0.1
191
0.2 I29
0.1686
0.1779
0.0544
0066
I
0.0564
0.0948
00498
I
~oooo
27.6
-
17.8
658
0.0826
0.1060
0.0986
00403
0
1634
0.07
I
3
0.0924
0.1130
02324
(C)
0101R
00272
0.0348
0.02
I6
0,0276
0.0177
0.0220
0,7473;
27.6
26.7
27.6
26.7
293
I
131
in a lower
C5+
content for
A.
Figure
6.4-
13
and Table
6.4-9
once more reveal
stable-liquid recovery to be the better the higher the
C5+
content of the liquid
entering stage two. The Figure reveals further that stable stock-tank oil yield is
maximal in all three cases at the same, second-stage pressure
of
about
3.5
bars.
Pressure in the first separator stage is limited by the maximum possible wellhead
pressure or, more precisely, by the maximum incoming flowline pressure. Third-
stage pressure is either atmospheric or is determined by the design features of the
gas-gathering system. The only pressure that may
be
freely chosen within limits
in
6.4.
SEPARATION
OF
OIL
AND
GAS
67
order to achieve maximum stable stock-tank oil yield is, then, the second-stage
pressure.
The most favourable second-stage pressure can be determined either by in-plant
experiment, time-consuming equilibrium calculations,
or
approximative cal-
culations. Equilibrium calculations are to be carried out
for
numerous values
of
pressure, in the manner outlined
in
the foregoing section, which is a tedious
procedure. The methods of approximation are more rapid but less accurate. Several
such methods are known.
In
a rough approximation, second-stage pressure can be
calculated from the relationship
P2
=
JPI
P3
.
6.4-
15
The p2 furnished by this formula is usually higher than optimal.
A
more accurate result
for
a low-molecular-weight gas-liquid mix can be obtained
by a procedure which expresses second-stage pressure by one
of
two equations
(Whinery and Campbell
1958).
The first one is used when the relative molecular
.I
I
I
05
10
15
20
25
30
Mr
Fig.
6.4-
14.
Determining the
A
factor.
after Whinery and Campbell (1958)
weight
M,
of the well fluid, referred to air, is higher than
I,
and the second one when
it
is lower than
1.
The equations are for
M,>
1,
p2
=
16
Apy.686
+
2.96
x
10S(A
+
0057)
6.4- 16
and
for
M,
<
1,
p2
=
8
AP:”~’
+
5.75
x
1OS(A
+
0.028).
6.4-
17
The
A
factor is plotted v.
M,
in
Fig.
6.4
-
14.
The procedure may
be
used
if
the third
stage is atmospheric, in which case the result is accurate
to
within
k5
percent.
68
-2
Component
I
6.
GATHERING
AND
SEPARATION
OF
OIL
AND
GAS
M
I
x2
2
3
Table
6.4
-
10.
I
6.0
I
30.07
44.09
58.12
72.15
86.
I
7
I
15.22
6.40
6.0
I
4.4
I
5.8
I
7.22
4.3
I
5.76
0.40
0.20
0.1
0
0.
I0
0.
I0
005
0.05
I
39.92
Example
6.4-4
(after Whinery and Campbell 1958).
Let the well fluid be
characterized by
Table
6.4-10.
Let first-stage pressure be 34.5 bars, and let the
third stage be atmospheric. What is the most favourable second-stage pressure'?
=
1.38
39.92
28.96
M,=
~
zc,
+
zc2
+
zc3
=
0.7, and
Fig.
6.4
-
14
yields
A
=
0.421. Since
M,
is
greater than
I,
we
take Eq. 6.4- 16;
p2=
16
x
0.421 (34.5
x
10')0'686+2.96
x
105(0.421
+0.057)=
=
3.47
x
lo5
N/m2
=
3.47 bars.
6.4.3.
Basic separator
types
The compositions of the liquid and gas discharged from a separator differ
somewhat from
those predicted by calculation. In addition to the fact that
calculation procedures based
on
auxiliary diagrams do not model real-life
conditions quite accurately, this is due to the circumstance that the liquid will
contain gas bubbles and the gas will contain mist.
Of
these,
it
is the liquid content of
the gas that may cause trouble (in the transportation system). Modern separators
are designed
so
that the mist content
of
the discharged gas
be
less than
0.1
g/m3.
In most gas-liquid separators, the elements ensuring separation may be divided in
three groups.
(i)
Means of rough separation. These serve
to
effect a first separation of
liquid and gas, mainly by centrifugal force and the force of gravity. They include the
inlet separating element, bames, liquid passages, and that part
of
the separator
between liquid surface and inlet (in vertical separators)
or
above the liquid level (in
horizontal and spherical separators).
(ii)
Mist extractor. In order
to
separate mist
not settled out by centrifugal force
or
gravity, a mist extractor is placed in the way of
the gas stream. The mist extractor may
be
of
the vane or coalescing-pack type, or
it
might
be
a hydrocyclone, often installed outside the separator body.
(iii)
Oil
6.4.
SEPARATION
OF
OIL
AND
GAS
69
collector. The liquids separated from the well-stream and condensed
in
the
separator collect
in
the bottom part of the separator vessel. Liquid level is
maintained within given limits by an LLC (liquid-level-control) device,
in
order to
prevent gas from entering the oil outlet, and liquid from rising up to the elements
mentioned above, and
to
ensure sufficient retention time for the gas bubbles to
break out of the liquid.
Shapewise, separators may be vertical cylindrical, monotube
or
dual-tube
horizontal,
or
spherical. In the sections below
I
shall describe typical examples for
each of these.
(a)
Vertical
separators
The wellstream enters the vertical or upright cylindrical separator through an
inlet installed at about two-thirds height. The inlet may be radial
or
tangential [(a)
and (b)
in
Fig.
6.4-
151.
Tangential inflow has the advantage that
it
exploits for
separating liquid and gas a centrifugal force which may exceed the force of gravity
Fig.
6.4
-
15.
Section
A-A
=@
Fig.
6.4
-
16
by up to two orders ofmagnitude: furthermore, the liquid will move in the separator
in
a spiral along the shell, while gas will rise in the central space along the separator
axis. This considerably reduces the likelihood of contamination with mist as against
the case of radial inflow. The inlet separating element shown in
Fig.
6.4-16,
a
development on the radial-inflow idea, has lately been adopted by numerous
makers
of
separators. Its use substantially promotes the separation of liquid and gas
directly after inflow.
Figure
6.4-
17
is a schematic diagram of a modern vertical
separator. The wellstream enters separator
2
through inlet
1
and the separating
element mentioned above. Liquid discharge is controlled by motor valve
5,