Szilas A.P. Production and transport of oil and gas, Gathering and Transportation
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laboratory analysis may be drained through four-way valve
1OC.
The rest
of
the
sample is discharged, by suitably turning valve
IOC’,
from tank
10
into the dclivci-y
pump, where
it
joins the main liquid stream. The
unit
is provided with auxiliary
equipment that will shut
off
oil
flow in the case
of
malfunctions,
if
the oil stream
passing through the unit deviates from prescriptions,
or
if
the daily
or
monthly limit
ofoil
to
be delivered has been attained. Transfer capacity ofthcse portable units is
32
-
1300 m3/d; their weight
is
4.2-
21
kN.
If
the central LACT station receives crude from several leases, each sort ofcrudc is
fed to a separate LACT unit. Some units, however, are designed
so
as
to
be able
to
handle several inflows. Designing, installing and operation of
LACT
systems as well
as requirements concerning accuracy of measurement and safety are treated
in
API
Std 2502.
6.7.4.
Design
of
field integrated
oil
production
systems
(FIOPS)*
Any system is the complex
of
interactive elements. Its determination is always
arbitrary since no system exists upon which external forces
or
other systems do not
exercise an impact. These impacts, however, in several cases can be considered as
boundary conditions. Even the perception
of
the cooperation regularities
of
a
relatively small number
of
elements may be useful. The mathematical modelling of
the operation
of
small systems facilitates the best possible arrangement according
to
the requirements, and the determination
of
the optimum operating parameters
of
the
so
created complex, respectively. Unifying the smaller systems results in a new,
larger, generally more complex system. The common operation of the original
subsystems can produce new possibilities
for
a higher level optimization. This
system enlarging procedure is synergistic
if
the economic gain obtained by the
optimization
of
the enlarged system is greater than the sum of the gains of the
subsystems optimized separately (Szilas 1980).
For
the time being the literature
discusses only relatively smaller systems of
oil
producing equipment.
A
system like
this may consist
of
the reservoir in the drainage area of
one
well,
one
producing well,
the flowline a7d separator and their chokes, these elements making up the
productior;
pat/,
or
“producrion
spoke”.
Its modelling
for
gasless oil is discussed
in
Section 2.2, wh le for gassy oil it is treated in Section 2.3. The Petroleum Engineering
Department
of
;he Miskolc Technical University for Heavy Industry studies the laws
of
a production system the main elements
of
which are hydrocarbon-bearing
reservoirs, the total number
of
the wells, and the gathering, treating and
transporting equipment on the surface. In order
to
simulate this production system
the solution
of
the partial problems, discussed bvlow, is necessary.
6
7
011.
AND
GAS
GATHERIN<;
141
(a) The design and location
of
optimum well producing systems
(i)
The producing rate, composition, and pressure of the fluid inflow from the
reservoir into the well is changing with time.
For
a time the production from a well
is
going on by self flowing. Later on, the production rate, prescribed by the reservoir
engineering project, can be usually achieved by artificial lifting equipment. Both the
end
of
the flowing period, and the technically proper
lift
method that ensures the
0.5
'
I
t
300
1,month
20
100
ta
Fig.
6.7
-
20.
Production
and
economic characteristics
of
a
key-well
(S7ilas
1983)
most economical production
in
the total remaining production
Ilk,
or
in
a
givcn
period, must be determined well by well. Such relations and computer programs
have been developed by the Petroleum Engineering Department of the Miskolc
Technical L'niversity for Heavy Industry. With the help of these relations and
programs, on the basis of the reservoir engineering project. from wcll to well, and
year by year, the specification of the technically silltable lifting method can be
determined, including its operating point
with
the minimum operational cost, and
also its value related to the production start timepoint.
Figure
(5.7-20,
in
the upper
half of the diagram, shows the expected values of the characteristics valid for one
well (key well)
in
the course of the production life on the basis of the reservoir
engineering project, where
y,,
is the liquid production rate,
R
the gas oil ratio. and
p,,,(,
the wellhead pressure and
pM,r
the flowing bottom hole pressure. On the basis of
the above data the lower section
of
the diagramme indicates that how the initial
discounted production cost changes in case
of
optimally designed rod pumping
(I),
and
in
case of continuous gas lifting
(11).
Curves
KkumI
and
KkumII
show the
cumulated production costs.
It
is also shown that before time
t,
rod pumping. while
after
it
gas lifting is the moreeconomical. The total cost for the whole production life
in
case
of
rod pumping is smaller by
AK
than
it
is
in
case
of
continuous gas lifting.
(ii)
Considering the succession of the starting of the artificial lifting
in
the
individual wells and also their production life, the location project must
be
designed.
Fig.
6.7-21
shows a project prepared for a small field, operating
13
wells, as an
example (Szilas
1979).
It
is supposed that during the period
of
artificial production
two types
of
lifting methods are applied, i.e. the method of rod pumping and
6
7
011.
AND
GAS
GATHERING
143
continuous gas lifting.
f,
line connects the moments of the beginning
of
the
production of the wells, while
t,
connects the moments of ceasing of the flowing
production. From this moment on up
to
t,
production method
I
(rod pumping), and
then method
I1
(gas lifting)
is
the more economic. The numbers, given in the line of
the single wells, tell us on which well, and when new production units must be
mounted,
or
where they must be re-mounted
if
the production becomes
unprofitable.
Similar
projects, developed for key wells should not be considered as
rigid specifications. but only as estimations, since the details
will
be altered
in
the
practice. This method helps
us
to determine that for the planned production of the
wells when, what type and size and how many production units are necessary; what
well service capacity must be turned for the installation; and how large is the
expectable production cost. The application of the method shows that even
in
case
of an oil field of average size (operating
50-
100
wells) savings
in
the order of
31
million can be achieved, as compared to traditional practice.
(b)
Location
of
the units gathering, treating and
transporting
fluid
streams
from
wells
The most favourable type and parameters of the well centres and central
gathering station are those which take into account the rate and quality
of
the well
streams, and the parameters and requirements of the production and utilization
must be selected. The most favourable place
of
the well centres and central gathering
station. and, moreover, the most advantageous trace of the flow lines and other
transport lines belonging to the station, must be determined. Among the technically
feasible solutions generally the versions requiring the lowest investment cost are
considered as optimums. The planning method elaborated and programmed for
computer by the Petroleum Engineering Department of the Miskolc Technical
University for Heavy Industry is based upon dynamic programming (Gergely and
Pancze
1982).
It takes into consideration the well coordinates determined by the
reservoir engineering project, the specifications concerning flow lines and transport
lines, the type of the well centres, the terrain conditions
of
the oil field, the prohibited
territories, the crossing of public utilities, and every significant cost components.
The principle
of
optimization is as follows:
The number of the well centres to be set up should be
n,,
the number of wells
linked to the
iIh
well centre is
nWti,,
the flowline cost of the
jIh
well
is
Kf(i,j,.
The
investment cost of the transport line connecting the
Ph
well centre with the central
gathering station
is
marked with The investment cost
of
other pipelines (gas
gathering, water transporting pipes, etc.) should
be
K,(,,,,
and their number is
n,.
The
component of the investment cost, independent of the number of the wells belonging
to the gathering centres is
K,,
while the cost depending on the number
of
the wells is
Ku(i).
The investment cost of the surface gathering, treating and transporting devices
is thus
134
6
GATHERING
AND
SEPARATION
OF
OIL
AND
<;AS
i
f
r
=
1
0
n
jl
I
z:
3
v)
U
0
0
1
I
J
145
"
..
1
0
1000
2000
'b
3000
coo0
5ooo
1
(m)
Fig.
6.7-23.
Well centres and pipe line traces
of
the optimally located gathering system. aftcr
Siilas
(
1983)
The flow chart
of
the computation is shown on
Fig.
6.7-22.
Thc traccs
of
thc
optimum project determined by the above method that considers the coordinates
of
143
wells are shown by
Fig.
6.7-23.
By applying this design method,
in
the case
examined, about
30
percent
of
the investment costs could be saved, as compared to
the traditional location design procedures.
(c)
The production tetrahedron
From the enumerated production sectors the complete, above specified
production system may be constructed. In the knowledge
of
the characteristics
of
this system the optimum solution can be found. The basis
of
the solution is the
production tetrahedron shown on
Fig.
6.7-24
(Szilas
1983).
The sketch shows the
steps
of
principle and the linking
of
the sectors in the production system based on
the reservoir engineering project. Integral parts
of
the system design are first the
determination
of
the type and sizes
of
the optimum production units, then that
of
the gathering system; then the optimum location; and finally the determination
of
the optimum operating parameters. Interactions are indicated by arrows. Smaller
arrows mean a smaller dependence. In order to decrease the number
of
iterations
it
is more effective
to
carry out the design work along the
1-2 -3
-4
graphs, as
it
is
justified by the "big arrows". Reservoir engineering projects are considered as
10
146
h
<;AlHI:KINCi
ANI)
Slil'AKAIION
01-
011
ANI)
(;AS
independent, singular problems. The optimum system design must be determined
for each reservoir engineering project version, and of them the best should be
chosen.
The above method can be carried out
if
in
the time of designing
a
necessary
amount of data and information of proper accuracy is available. Obviously, the oil
field will be more
or
less fully known at the end of the production life only. The rate
of inaccuracy and insecurity of the information increases moving backwards
in
time.
These realities, however, can be complied
with
(Szilas 1982a). That is
why
Reservoir en ineering
pr0,ecP
Opt.
place
of
the
gotheriq
system
I""'-/
Fig.
6.7
-
24.
The
production tetrahedron. after
Szilas
(1983)
(i)
the preliminary design must be started
in
the very first period of the
development of the field. Being aware already of the initial information (well depth,
well stream quality, productivity, reservoir pressure) several possibilities may be
excluded;
(ii)
concerning the extension of the field under development the engineer should
obtain the possibly most accurate data at the earliest possible time from the
geologist;
(iii)
in order
to
facilitate the economical development, expansion
or
modification
of the production system, well centres and oil transfer stations must be applied that
are factory assembled, that can
be
joined by modular system and that, to a great
extent, consist
of
skid mounted units;
(iv)
it
should not
be
neglected that the amount and accuracy of the information
concerning the field,
in
the course of the production, increases. Such financial and
investment policy should be introduced that, in case of possible further savings,
facilitates the modification of the former projects during the production life.
CHAPTER
7
PIPELINE TRANSPORTATION
OF
OIL
7.1.
Pressure waves, waterhammer
Pressure wave may occur
in
pipelines transporting fluids
if
the velocity
of
the
flowing liquid suddenly changes. The pressure wave propagates by the sonic
velocity from the point ofa leak
or a device, initiating the change of pressurc
in
both,
or perhaps even more connecting pipeline branches. During propagation the wave’s
amplitude is reduced, damped. and perhaps
it
is even reflected. The pressure wave
may be of positive character when a sudden decrease
in
the velocity of the flowing
fluid may cause a rise
in
the pressure: and
it
may be also of negative character when
the sudden rise
in
velocity
of
the fluid may result
in
dropping
of
pressure.
7.1.1.
The reasons
of
the waterhammer phenomenon
and
its
mathematical representation
Let us suppose that the pressure wave is analyzed by the epxerimental device
represented by the sketch shown
in
Fiq.
7.1
-
1. The most important part of this
device is the horizontal pipeline, on the one end of which a tank of constant fluid
level,
h=p/py,
is mounted, while the other end is equipped
with
a gate wave. Let a
steady flow with rate
y
occur from the tank through the pipeline and the open gate
valve, that gives a flow velocity
u
in the pipe. Supposing that the viscosity of the fluid
is negligible, the pressure line within a Cartesian coordinate system for length
v.
pressure will be a straight line that is parallel to the abscissa. The liquid is slightly
compressible, i.e. this parameter corresponds to the properties ofa real fluid. Let
us
suppose that the gate valve is shut down in an infinitely short period of time, and the
development of the pressure wave is analyzed after this action.
The analysis of this phenomenon is facilitated by
Fig.
7.1
-
2
in
which eight parts
are shown, arranged in three columns and four lines. The two outer columns denote
final states, while the column in the middle refers to moments when the pressure
from moves at about the halfway
of
the pipeline. Within each part two diagrams are
shown. The upper one represents the flow velocity in the function of the length. The
height of the ordinate is proportional to the velocity change
v,
-
0
=
Ac.
Flow occurs
only
in
the field shaded with horizontal lines. The direction of flow is indicated by
arrows. The lower diagram shows the pressure change in the function of length. The
ordinate height of the vertically shaded field’s territory is equivalent to the values of
148
7
PIPELINE
TRANSPORTATION
OF
OIL
dp
waterhammer, pressure jump, pressure difference that can
be
computed from
Eq.
7.1
-
1,
a height that can be of
a
positive sign above the central line marking the
p
tank pressure,
or
of
a
negative sign below it. At the moment, matching the “empty”
field
(a)-
1,
due to the sudden shut down
of
the gate walve
a
pressure jump
of
vertical face develops, the “face height”
of
which
is
+
dp.
The front, in the direction
marked by the great arrow, propagates towards the tank with acoustic velocity
w.
Fig.
(6)
-
1
shows that to the right
of
the wave front the liquid is already quiet and
here its pressure exceeds the hydrostatic pressure
of
the tank by
Ap,
while on the left-
hand side the liquid flows toward the closed gate walve.
Fig.
(c)
-
1
shows that final
situation when the pipeline is just filled with quiet liquid of(p
+
dp)
pressure. Due to
this excess pressure, from the moment suiting to empty field
(c)
-
2,
which is the same
as for
(c)-
1,
flow starts from the pipeline into the tank.
Fig.
(b)
-2
shows the
position
of
the front
in
a
t
time after the beginning
of
the outflow. It can be seen that
on
the left
of
the front the liquid is flowing towards the tank, its pressure here
decreases to the tank pressure.
Figure
(a)
-2
shows that final state when in the
complete pipe length fluid flows from the shut-down pipe end towards the tank, and
Fig.
7.1
-
I
--
-I-
-
3
7.
I.
PRESSURE
WAVES.
WATERHAMMER
149
the pressure of the liquid column in the pipeline equals
p.
No
inflow occurs from the
shutdown pipe end and that is why in the situation and moment of(u)
-
2,
that is the
same as in case of
(u)-3,
flow at the gate valve stops, and due to the impact of
“flowing away” the pressure of the fluid decreases by
Ap.
Figure
(h)
-3
shows the
position ofthe front after
t
time.
It
can be read
off
that on the right-hand side the flow
has already stopped, and its pressure is
(p-
LIP).
On the left-hand side the fluid still
flows towards the tank. and here its pressure is equal to the pressure of the tank,
p.
In
final situation
(c)-
3
the liquid is already in quiet
in
the total length of the pipeline,
and its pressure is smaller than the pressure
p
of the tank by
Ap.
Due to the occurring
pressure difference flow starts from the tank towards the gate valve again. The
situation after time
t
is shown by
Fig.
(h)
-4.
In this case the fluid, on the left-hand
side ofthe front, flows again in the original direction with tank pressure, while on the
right-hand side
it
is still in a quiet state, and here the pressure is smaller by
Ap.
In the
end position, shown by
Fig.
(a)
-4,
in the total length of the pipeline a fluid of
1‘
velocity and
p
pressure flows toward the shut-down gate valve. The first period of
0
pw
4
L/w
ib)
0
’+
(C)
Fig.
7.1
-3.
Change
of
the pressure in selected points
of
the system charactenzed by
Fig.
7.1
-
I
the wave motion is, then, over, and the second period similarly to the first one
begins.
Let us examine now how the pressure changes at certain selected points of the
tank-pipeline-valve system in the function
of
time. After Thielen (1972b) on
Fig.
7.1
-3
the
p-
t
relationship is shown at the closed gate valve (a), at the halfway of pipe-
length
(b),
and at the tank (c). The period of the pressure wave in
Figs
(a)
and
(b)
is
4Lfw.
L
is the distance between the tank and the gate valve. The amplitude of the