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Study of the dynamics, optimization and selection of intermittent
gas-lift methods—a comprehensive model
Odair G. Santos a , Sergio N. Bordalo b,*, Francisco J.S. Alhanati c
a PETROBRAS, Brazil b Department of Petroleum Engineering, State University of Campinas, SP Caixa Postal 6052, 13083-970 Campinas, SP, Brazil
cCFER, Brazil
Abstract
The gas-lift is a widely used method of artificial lift, for which there are different design options. There are empirical rules of
thumb to choose between the continuous gas-lift (CGL) and the intermittent gas-lift (IGL), but little exists in the literature for
the selection among the different intermittent gas-lift designs. Furthermore, computer simulators for these processes usually are
not in the public domain. This work presents a numerical model to study the behavior of the Conventional IGL, the IGL with
chamber (IGLC), the IGL with plunger (IGLP) and the IGL with pig. Simulations are presented under various reservoir
conditions, for different settings of the operation’s parameters. The model’s results can aid the engineer in the determination of
the optimum values of the parameters for each design option and in the choice of the most adequate IGL design for a particular
well. D 2001 Published by Elsevier Science B.V.
Keywords: Gas-lift; Petroleum production; Computer simulation; Optimization
1. Introduction
Artificial lift is used in petroleum production when
the energy of the reservoir is not enough to sustain the
flow of oil in the well up to the surface with satisfac-
tory economic return. Selection of the proper artificial-
lift method is critical to the long-term profitability of the oil well; a poor choice will lead to low production
and high operating costs. There is very little margin for
error when one is designing lift systems for petroleum
fields. However, proper selection of the best method
still is based on past experience, strong opinions, fa-
miliarity of operating personnel with the equipment,
preferences of company experts that favor some meth-
od, and unsubstantiated technical myths. There is a
strong need for reliable procedures of selection and
design. Computer programs that simulate the operation
of lift systems are an important part of such procedures
and the main concern of the present work.
The gas-lift is a widely used method of artificial lift,where gas is injected in the production well providing
energy to the flow. In some instances, the continuous
injection of gas, named continuous gas-lift (CGL), is
not efficient, and the intermittent (periodical) injection
of gas, named intermittent gas-lift (IGL), becomes the
more economical alternative. Different design options
are available to implement the IGL: conventional IGL,
IGL with plunger (IGLP), IGL with chamber (IGLC)
and IGL with pig, also known as pig-lift (PL). There
are some empirical, though questionable, rules of
0920-4105/01/$ - see front matter D 2001 Published by Elsevier Science B.V.
P I I : S 0 9 2 0 - 4 1 0 5 ( 0 1 ) 0 0 1 6 4 - 4
* Corresponding author. Fax: +55-19-2894916.
E-mail address: [email protected] (S.N. Bordalo).
www.elsevier.com/locate/jpetscieng
Journal of Petroleum Science and Engineering 32 (2001) 231–248
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thumb to choose between the CGL and IGL (Table 1),
but little exists in the literature for the selection among
the different IGL designs. Furthermore, there are no
public models or comparative studies of the dynamicsof all the IGL process cycles that are consistent for all
designs (there are computer codes that are property of
private or government companies, and that were
developed for one design option only). The engineer
needs a tool to determine the performance of the
various designs under certain field conditions, to tune
the operational parameters to their optimum values,
and to choose the best IGL method. The purpose of
this paper is to provide such a tool to rationalize these
tasks.
This paper introduces a computer model to simulate
the conventional IGL, IGLP, IGLC and PL. All the
IGL methods work in cycles, and each cycle is made of
stages the follow transient flow processes, conse-
quently, the task of simulation is non-trivial. Besides,
vertical two-phase flow of oil and gas occurs in the
well, which adds to the complexity of the computa-
tions. In the IGLP system, a solid plunger separates the
oil and gas flowing in the well, to prevent fallback of
oil. In the IGLC system, a chamber is used to accu-
mulate the oil at the bottom of the well, reducing the
back-pressure against the reservoir formation. In the
PL system, a foam-pig separates the oil and gas flow-ing in the well to prevent fallback of oil, and a double
column is used inside the well.
The computer simulation employs sets of time-
dependent differential equations that govrn the vari-
ous phases of each IGL cycle. Although each IGL
design has its own idiosyncrasy, they also share some
common structure and processes which are modeled
and dealt with in a way that is consistent across all
designs.
Examples are presented for specific conditions of
the petroleum reservoir, and for optimization of the
operational parameters. The performance of the differ-
ent IGL designs are compared on the basis of an eco-
omic criterion — with each design operating at its
optimum point.
1.1. Literature review
Clegg et al. (1993) presented an extensive over-
view of artificial lift design considerations, compari-
son of methods and their normal operating conditions.
Chacın (1994) discussed the state of the art of the de-
sign of IGL methods, presented a simplified algorithm
for the calculation of the production rate, and a proce-
dure to select the best IGL method—according to his
criteria, the one with the greater ratio of produced oil
volume to gas injected volume. Brown and Jessen
(1962), Brill et al. (1967), and Neely et al. (1974) did
some experimental work on specific field installations
of conventional IGL, establishing empirical rules for
the setting of the operational parameters. Although
they provide useful guidelines, those rules lack in gen-
erality. White (1963) developed the first simple math-
ematical relationships for the conventional IGL and
did experiments on laboratory installations. Machado
(1988) developed a mechanistic model coupling phys-
ical principles and empirical correlations to calculate
some variables of the IGL system. Liao (1991) ob-tained theoretical results that showed good agreement
with Brown, Brill and Neely. White (1982) conducted
tests with and without a plunger to demonstrate the
reduction of liquid fallback in the plunger case. Mower
et al. (1985) used different plungers to study the effect
of plunger geometry on the fallback. Chacın et al.
(1992) developed a mechanistic model, introducing
the empirical findings of Mower and Lea into the IGLP
model. Brown described the advantages of IGL with
chamber for reservoirs with low static head and low
productivity index, and provided a simplified proce-dure to estimate the average flow pressure at the bot-
tom of the well. Winkler and Camp (1956) applied the
IGL with chamber to reservoirs of low static head but
high productivity index. Berdeja and Mariaco (1971)
discussed the principles that should be applied to the
analysis of the IGL with chamber, optimization of the
method, and listed results of field cases. Acevedo and
Cordero (1991) presented field experiments showing
an increase of oil production and decrease of gas con-
sumption. The pig-lift was developed in one petroleum
Table 1
Practical criteria for selection of continuous (CGL) and intermittent
(IGL) gas-lift
Static head (hs/ H w) Productivity index (PI, m3/day MPa)
High
( ! 20)
Medium
(5 to 20)
Low
(V5)
High ( ! 0.7) CGL CGL/IGL IGL
Medium (0.4 to 0.7) CGL/IGL CGL/IGL IGL
Low (V0.4) IGL IGL IGL
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company somewhat recently, consequently, very few
publications exist. Lima and Cardoso (1993) devel-
oped a simulator to study the potentiality of this new
technique. Lima (1996) presented the working princi- ples of the pig-lift, and argued in favor of its advan-
tages over the other IGL methods.
Although the present model is inspired on Liao’s
work on the conventional IGL, it includes several orig-
inal elements that make it quite different and complete,
such as the liquid film and the fallback; moreover, it
has the generality necessary to deal with all IGL de-
signs.
2. IGL systems
2.1. The conventional IGL
Fig. 1a illustrates the conventional IGL cycle and
its stages. The motor valve controls the injection of gas
following a timer program. The gas-lift valve is set to
open at a certain pressure in the casing; at this point the
tubing is already loaded with a column a liquid. The
gas elevates the liquid slug, leaving behind a film of
liquid— the fallback. At some point, the motor valve is
closed. The slug is produced at the surface, and after
that, the gas is produced also; some liquid is produced
by the dragging of the liquid film and droplets dis- persed in the gas. The gas-lift valve closes and the
decompression of the tubing begins. Finally, the res-
ervoir pressure feeds the bottom of the well, reloading
the system for the next cycle.
The main parameters of the system are shown in
Table 2, with their values for the simulation run of the
case-example for this paper. The following operating
parameters were tuned to find the optimum economic
daily rate of produced oil volume to injected gas vol-
ume: V gi — injected gas volume per cycle; l si — initial
length of the liquid slug; P co —casing pressure to open
the gas-lift valve. Besides, the values of the reservoir
static pressure P r and reservoir productivity index PI
were varied to study the suitability of the IGL.
2.2. The IGL with plunger
Fig. 1b illustrates the IGLP cycle and its stages. The
scheme is similar to the IGL, except that, in this case,
the gas pushes the plunger up elevating the liquid slug.
Fig. 1. (a) The conventional intermittent gas-lift cycle. (b) The intermittent gas-lift with plunger cycle. (c) The intermittent gas-lift with chamber
cycle. (d) The intermittent gas-lift with pig cycle.
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The fallback of liquid is admitted to be null for an ideal
plunger (no leakage). During the reloading of the well
for the next cycle, the plunger plummets back to its
initial position.
For the sake of comparison with the IGL, the same
well/reservoir system described in Table 2 was consi-
dered. Also, the same operating parameters, as in the
case of the IGL, were adjusted for the optimization of
the cycle: V gi, I si, P co.
2.3. The IGL with chamber
Fig. 1c illustrates the IGLC cycle and its stages. The
scheme is similar to the IGL, except that, when the gas-
lift valve opens, first the gas pushes the liquid from the
chamber into the tubing, then it elevates the liquid
slug. Fallback is also observed in the IGLC. During
reloading, the reservoir fluid is accumulated in thechamber, while in the IGL, the fluid fills the tubing.
Again, the well/reservoir system of Table 2 was
used to define the work conditions. The operating pa-
rameters adjusted for optimization are V gi and l si.
2.4. The pig-lift
Fig. 1d illustrates the PL cycle and its stages. As
with the other cycles, a motor valve controls the in-
jection of gas following a timer program, but, in this
case, a double-tubing is loaded with columns of liquid.The gas pushes the pig, transferring the liquid to one
leg, and then elevating the liquid slug. The fallback of
liquid is admitted to be null for an ideal pig. The slug is
produced at the surface, and after that, the gas is pro-
duced also. The motor valve is closed, and decom-
pression of the tubing begins. Finally, the reservoir
pressure feeds the bottom of the well, reloading both
legs of the tubing for the next cycle while the pig sinks
to its ready position. The functions of the tubing legs
are exchanged at every cycle.
Once again, the simulations were run for the well/
reservoir system of Table 2. The only operating para-
meter adjusted for optimization of the pig-lift is l si.
3. IGL model
3.1. System of equations
Table 3 shows the basic equations used in the simu-
lation of all the IGL methods. The equations are de-
rived from fundamental mass and momentum balances
applied to subsystems of the IGL (more details are
found in Santos, 1997, including the closure equa-
tions—equations of state, flow rate equations, valve’s
equation, geometric relationships, two-phase flow cor-
relations, head-loss correlations). Basic-equation b1
simulates the pressurization of the casing, B-equation b3 simulates the gas mass inside the tubing, B-equa-
tion b5 simulates the liquid film left behind by the slug,
B-equation b7 simulates the fallback, B-equations b8
and b9 simulate the slug elevation and B-equation b13,
in each of its forms, simulates the loading of the slug
for each IGL method.B-equation b3.1 simulates the
transfer stage of the IGLC, while B-equations b16.1
and b16.2 simulate the transfer stage of the pig-lift. For
each stage of the cycle, a system of non-linear time-
differential equations (Eq. (1)) was formed following
prescription in Table 4. The systems were solved withthe aid of commercial software of mathematical tools.
Xmvar
i¼1
ani
d X i
dt ¼ bn; n ¼ 1::meq : ð1Þ
3.2. Simulation
Fig. 2 shows examples of the simulation output for
the case-example described in Table 2, with a reser-
voir pressure P r = 8.35 MPa and a productivity index
Table 2
System parameters for the simulation of the case example
H w well depth 1500 m P wh well-head pressure 0.7 MPa co oil sp. gravitya 0.825
Dc casing diameter 5 1/2 in. P gi compressor pressure 7.0 MPa cw water sp. gravitya 1.07
Dt tubing diameter 2 3/8 in. T s surface temperature 27 °C dg gas rel. density b 0.7
H gv depth of gl-valve 1480 m aT temperature gradient 0.031 K/m uw water volume fraction 50%
a At standard conditions, relative to standard water. b At standard conditions, relative to standard air.
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Table 3
Basic equations for the IGL, IGLC, IGLP and PL
Variables—A: code variable; A; flow area; D: code variable; F ; flow switch; H: code variable; P: code variable; P ; pressure; Q: code variable; T ;
temperature; V : volume; Z : real gas factor; e: head-loss; f : friction factor; g : gravity; h: height; q: standard flow rate; t : time; v : velocity; w:
weight; y: film thickness; q: density.
Indexes—P: plunger/pig; Lx: leg-x; c: casing; ch: chamber; b: bottom; f: film; g: gas; h: head; i: injection valve; l: liquid; p: produced; r:
reservoir; s: surface; sc: standard condition; t: tubing; v: gas-lift valvep; w: well; c1: casing at inj-valve/injection leg top; c2: casing at gl-valve/
injection leg pig; c3: chamber at gl-valve; c4: chamber at gas–liq contact; t1: tub glV/orif./U-bott; t2: tub gas–slug contact; t3: tubing at slug-
top; t4: tub liquid column top; ts: tubing at surface.
V cdqgc
dt ¼ q
gscð F giqgi À F gvqgvÞ ð b1Þ
P c2 ¼ P c1 expð H gv=kÞ ð b2Þ
Achhch
dqgch
dt þ
d
dt ðqgt AgHgÞ
¼ qgscðQ À F gpqgpÞ ðb3Þ
Ach
d
dt
Àqgchðhch À hgÞ
Á¼ qgsc F gvqgv ð b3:1Þ
PA À PB ¼ f gt qgt
v 2gt
2
Hg
Dt
þ qgt g Hg ð b4Þ
P c3 À PD ¼ qgch g ðHgchÞ ð b4:1Þ
P c1 À P c2 À f gt qgt
v 2gt
2
Hg
Dt
þ qgt g Hg ¼ 0 ð b4:2Þ
P c1 À P gt1 À f gtqgL1
v 2gt
2
H t
Dt
þ q gL1 gH t ¼ 0 ð b4:3Þ
P gt1 À P t2 À f gtqgL2
v 2gt
2
hg
Dt
À qgL2 ghg ¼ 0 ð b4:4Þ
Hf
d Af
dt ¼ F rqr À qf ð b5Þ
P wb ¼ P t1 þ ql g ð H w À HoÞ ð b6Þ
Atv l À Agv gt ¼ Af v f ð b7Þ
dhl
dt ¼ fhl < Ho : v l; 0g ð b8Þ
dhg
dt ¼ fhg < Ho : v gt; 0g ð b9Þ
dhg
dt ¼ fhg > 0 : Àv gt; 0g ð b9:1Þ
ðHl À hgÞdv l
dt þ v 2l À 1 À
Ag
At
v 2f À
Ag
At
v 2gt
À1
ql
P t2 À PC Àwp
At
þ g ðHl À hgÞ
þ f lv 2l
2
Hl À hg
Dt
þ ewh ¼ 0 ðb10Þ
PC ¼ P wh þ qgts g ðHo À HlÞ ð b11Þ
Af
dv f
dt þ 2p
Dt
2À y
v f
d y
dt À
f g
8
qgt
ql
v 2gt
Hg
H t1
þf l
4v 2f p
Dt
2À
Af
H t1
P t1 À P wh
ql
þ gAf ¼ 0 ðb12Þ
d
dt ð Ach þ AgÞhlr ¼ ~ F rqr þ F f qf ð b13Þ
P t1 À P E ¼ ql ghlr ð b14Þ
v l At À v gt AG ¼ qr ð b15Þ
hg
dv gt
dt À
ðPF À P t1Þ
ql
þ f lv 2gt
2
hg
DÀ ghg ¼ 0 ð b16:1Þ
hl
dv l
dt þ v l
AG
At
v lAG
At
À 1
þ 2
qr
At
À
ð P t1 À P t3Þ
ql
þ f lv 2l2
hl
Dt
þ ghl ¼ 0 ðb16:2Þ
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Table 4
(a) Description of equation set for each stage of the IGL cycle
Stage Initial event Equation set
Injection injection valve opens: F gi = 1 [b1,b2] F gi = 1, F gv = 0.
Elevation gas-lift valve opens: F gv = 1 [b1,b2,b3,b4,b5,b6,b7,b8,b9,b10,b11]
F gi = 1, F gv = 1, F gp = 0, F r =1, PA= P t1, PB = P t2, PC = P t3, Hg = hg, Hf = hg, Hl = hl, qgt =qgt1, v gt = v b, qf = qfs.
Production slug reaches the surface: h1 = H gv [b1,b2,b3,b4,b5,b6,b7,b9,b10]
F gi = 1, F gv = 1, F gp = 0, F r =1, PA= P t1, PB = P t2, PC = P wh, Hg = hg, Hf = hg, Hl = H gv, q¯gt =qgt1, v gt = v b, qf = qfs.
Decompression I gas reaches the surface: hg = H gv [b1,b2,b3,b4,b5,b6,b12]
F gi={1;0}, F gv = 1, F gp = 1, F r =1, PA= P t1, PB = P wh, Hg = H gv, Hf = H gv, qgt = qgt1s, v gt = 0.5v gs, qf = qfs.
Decompression II gas-lift valve closes: F gv = 0 [b3,b4,b5,b6,b12,b13,b14]
F gv = 0, F gp = 1, F r = 0, F ˜ r = 1, F f =0, PA= P t4, PB = P wh, Hg = H gv À hlr , Hf = H gv À hlr , qgt = qgt4s, v gt =0.5v gs, qf = qfs.
Decompression III film flow reverses: v f = 0 [b3,b4,b5,b6,b13,b14]
F gv = 0, F gp = 1, F r = 0, F ˜ r = 1, F f =1, PA= P t4, PB = P wh, Hg = H gv À hlr , Hf = H gv, qgt = qgt4s, v gt =0.5v gs, qf = qfb.
Loading gas pressure relieved: v gs = 0 [b5,b6,b11,b13,b14]
F r
= 0, F ˜ r
= 1, F f
=1, PC
= P t4
, Hl
= hlr
, Hf
= H gv
, qf
= qfb
.
Cycle restarts liquid reloaded: hlr = l si *loop back*
(b) Description of equation set for each stage of the IGLP cycle
Stage Initial event Equation set
Injection injection valve opens: F gi = 1 [b1,b2]
F gi = 1, F gv = 0.
Elevation gas-lift valve opens: F gv = 1 [b1,b2,b3,b4,b6,b8,b9,b10,b11,b13,b14]
F gi = 1, F gv = 1, F gp =0, PA= P gt1, Pb = Pt2, Pc = P t3, Hg = hg, Hl = hl, qgt =qgt1, v gt = v l, F ˜ r = 1, F f = 0.
Production slug reaches the surface: hl = H gv [b1,b2,b3,b4,b6,b9,b10,b13,b14]
F gi = 1, F gv = 1, F gp =0, PA= P gtl, PB = P l2, Pc = P wh, Hg = hg, Hl = H gv, qgt = qgt1, v gt = v l, F ˜ r = 1, F f = 0.
Decompression I gas reaches the surface: hg = H gv [b1,b2,b3,b4,b6,b13,b14]
F gi={1;0}, F gv = 1, F gp =1, PA= P gtl, PB = P wh, Hg = H gv, qgt = qgt1s, v gt =0.5v gs, F ˜ r = 1, F f = 0.
Decompression II gas-lift valve closes: F gv = 0 [b3,b4,b6,b13,b14] F gv = 0, F gp = 1, P A= P t4, PB = P wh, Hg = H gvÀ hlr , qgt = qgt4s, v gt =0.5v gs, F ˜ r = 1, F f = 0.
Loading gas pressure relieved: v gs = 0 [b6,b11,b13,b14]
PA= P t4, PC = P t4, Hl = hlr , F ˜ r = 1, F f = 0.
Cycle restarts liguid reloaded: hlr = l si *loop back*
(c) Description of equation set for each stage of the IGLC cycle
Stage Initial event Equation set
Injection injection valve opens: F gi = 1 [b1,b2]
F gi = 1, F gv = 0.
Transfer gas-lift valve opens: F gv =1 [b1,b2,b3.1,b4.1,b6,b8,b9.1,b11,b15,b16.1,b16.2]
F gi = 1, F gv =1, Pc = P t3, PD = Pc4, Hl = h1, Hgch = hch À hg, qgch = qgc34, v gt = v gch.
Elevation chamber unloaded: hg = 0 [b1,b2,b3,b4,b4.1,b5,b6,b7,b8,b9,b10,b11]
F gi = 1, F gv = 1, F gp = 0, F r = 1, PA= P t1, PB = P t2, PC = P t3, PD = P t1, Hg = hg, Hf = hg, Hl = hl, Hgch = hch, qgch = qgc35, qgt = qgt1,V gt = V b, qr = qfs.Production slug reaches the surface: ht = Htl [b1,b2,b3,b4,b4.1,b5,b6,b7,b9,b10]
F gi = 1, F gv = 1, F gp = 0, F r = 1, PA= P tl, PB = P t2, Pc = P wh, PD = P tl, Hg = hg, Hf = hg, Hl = H tl, qr = qfs, Hgch = hch, qgch = qgc35, qgt =qgt1, v gt = v b.
Decompression I gas reaches the surface: hg = H tl [b1,b2,b3,b4,b4.1,b5,b6,b12]
F gi={1;0}, F gv = 1, F gp = 1, F r =1, PA= P tl, PB = P wh, PD = P tl, Hg = H tl, Hf = H tl, Hgch = hch, qgch = qgc35, qgt = qgt1s, v gt =0.5v gs, qr = qfs.
Decompression II gas-lift valve closes: F gv = 0 [b3,b4,b4.1,b5,b6,b12,b13,b14]
F gv = 0, F gp = 1, F r = 1, F ˜ r = 1, F f =0, PA= Pt4, PB = P wh, Hg = H tl À hlr , Hf = H tl À hlr , qgch =qgc3, qgt = qgt4s, v gt =0.5v gs, qr = qfs.
Decompression III film flow reverse V f = 0 [b3,b4,b4.1,b5,b6,b13,b14]
F gv = 0, F gp = 1, F r = 0, F ˜ r = 1, F f =1, PA= Pt4, PB = P wh, Hg = H tl À hlr , Hf = H tl, qgch =qgc3, qgt = qgt4s, v gt = 0.5v gs, qf = qfb.
Loading gas pressure relieved: V gs = 0 [b5,b6,b11,b13,b14]
F r = 0, F ˜ r = 1, F f =1, PC = P t4, Hl = hlr , Hf = Htl, qf = qfb.
Cycle restarts Liquid reloaded: hlr = l si At /( At + Ach) * loop back *
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P I =1 0 m3/day MPa; this pressure corresponds to a
ratio of static head to well depth hs/ H w = 0.6. The gas-lift valve has a seat diameter d gv = 0.5 in. The opration
parameters of the IGL and IGLP tuned for optimiza-
tion, as mentioned in Section 2, are represented here
by the non-dimensional trio {l si/ hs, P to/ P co,V gi/ V gref },
defined as:
l si/ hs, ratio of the initial length of the liquid slug
to the reservoir static head; P to/ P co, ratio of tubing pressure to casing pres-
sure to open the gas-lift valve;
V gi/ V gref , ratio of the gas volume injected per cycle to the reference volume.
The reference volume is the required volume calcu-
lated by Liao’s formula. For the cases of the conven-
tional IGL and the IGL with plunger, shown,
respectively, in Fig. 2a and b, the operating parame-
ters are {0.5, 0.7, 0.8}. For the IGLC, the parameter
set reduces to {l si/ hs, V gi/ V gref }—the values used for
Fig. 2c are {0.1, 0.8}. In the case of the pig-lift, the
motor valve bean is 20/64U, and the only optimization
parameter is {l si/ hs}, set to {0.25} for the elaboration
of Fig. 2d.Except for the pig-lift, the first graph in each set of
plots displays the casing pressure, P cl, at the injection
point (well-head), showing the pressurization up to the
closing of the motor valve (pressure peak) followed by
depressurization while gas flows into the tubing until
the gas-lift valve closes. For the pig-lift, the injection
point is at the top of one of the tubing’s legs and a gas-
lift valve is not employed. The second graph shows the
pressure at the bottom of the well, P wb, which governs
the flow from the reservoir to the well (Eq. (2))—the
lower the average value of P wb, the higher the averagedischarge rate is. The last part of the curve represents
the loading of the slug.
qr ¼ PIð P r À P wbÞ: ð2Þ
The third graph shows the elevation and production
of the slug (and the transfer, in the case of the IGLC
and PL)— hl and hg are the positions of the top and
bottom of the slug, respectively; their difference is the
length of the slug, which diminishes when fallback
occurs.
(d) Description of equation set for each stage of the PL cycle
Stage Initial event Equation set
Injection and transfer Injection valve opens: F gi = 1 [b3,b4.2,b6,b8,b9.1,b11,b15,b16.1,b16.2] F gi = 1, F gp =0, PC = P t3, Hg = H t À hg, Hl = hl, qgt = qgc, v gt = v P.
Elevation Injection leg unloaded: hg = 0 [b3,b4.3,b4.4,b6,b8,b9,b10,b11,b13,b14]
F gi = 1, F gp =0, PA= P gtl, PC = P t3, Hg = H t + hg, Hl = hl, qgt =qcl, qgL1 =qcl, qgL2 =qcl, v gt = v l, F ˜ r = 1, F f = 0.
Production slug reaches the surface: ht = H tl [b3,b4.3,b4.4,b6,b9,b10,b13,b14]
F gi = 1, F gp =0, PA= P gtl, PC = P wh ,, Hg = H t + hg, Hl = ht , qgt =qcl, qgL1=qcl, qgL2= qcl, v gt = v l, F ˜ r = 1, F f = 0.
Decompression gas reaches the surface: hg = H t [b3,b4.3,b4.4,b6,b13,b14]
F gi = 0, F gp =1, PA= P gtl, P t2 = P wh ,, Hg = 2 H t , hg = Ht , qgt = qgL, qgL1 = qLl, qgL2 = qL2, v gt =0.5v gs, F ˜ r = 1, F f = 0.
Loading gas pressure relieved: V gs = 0 [b6,b11,b13,b14]
PA= P t4, PC = P t4, Hl = hlr , F ˜ r = 1, F f = 0.
Cycle restarts Liquid reloaded: hlr = l si/2 * loop back *
(e) General settings
Parameter IGL IGLP IGLC PL
Ach 0 0 Ach 0
Ag At À Af At At À Af At
AG – – Ach At
D – – Dch Dt
Ho H gv H gv H t1 H t PE P t4 PA P t4 PAPF – P c4 P c2Q F gv qgv F gv qgv F gv qgv F gi qgiwP 0 wP 0 0
Table 4 (continued )
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In Table 5, the simulator results for the conventional
IGL are compared to Liao’s calculations, which were
tuned to the data of Brown, Brill and Neely. A satisfac-
tory degree of agreement is observed. Accurate labora-tory data is scarce for the gas-lift with plunger and the
pig-lift. Nevertheless, the order of magnitude of the
elevation time obtained in the simulations is equal to
that observed in the field, though values are only good
as 10%, to the best of the authors’ knowledge. The
velocity of the real plunger and pig would be influ-
enced by the fallback, because the slug weight would
decrease along the way up to the surface. Unfortunate-
ly, laboratory data is even more difficult to find for the
gas-lift with chamber. The order of magnitude of the
production gains, over the CGL and IGL, calculated in
the simulations are in the range that is observed in the
field, to the best of the authors’ knowledge.
3.3. Optimization
The immediate application of the simulator model
is the optimization of the lift operation, i.e., the
maximization of a ‘‘goal’’ by variation of the oper-
ation parameters. This ‘‘objective’’ function might be
the daily production (volume of oil) or daily profit
(difference between economic gain and cost) for
Fig. 2. (a) Sample of the simulator output for the IGL’s example. (b) Sample of the simulator output for the IGLP’s example. (c) Sample of the
simulator output for the IGLC’s example. (d) Sample of the simulator output for the PL’s example.
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instance. Here, the most economical point of oper-
ation—maximum daily profit— is taken up as the
criterion of comparison among the different IGL
methods. For the sake of example, market values in
Brazil, at the time of the development of this work,
were adopted to define the economic conditions,
leading to the constraint expressed by Eq. (3), relating
the increment in gas input, dQgas, to an increase in oil
output, dQoil, for an open system, all factors being
constant but the volume of gas injected.
dQgasdQoil
< 1000: ð3Þ
For the same reservoir case used for Fig. 2, the
most economical point was found to be {0.3, 0.7, 0.5}
for the IGL, {0.3, 0.6, 0.58} for the IGLP, {0.1, 0.49}
for the IGLC and {0.4} for the PL. Further simula-
tions yielded the optimum operating point for the IGL
methods, under various reservoir conditions (PI and
P r ), as shown in Table 6. Each cell of Table 6 gives the
daily liquid production Qlp and daily gas consumption
Fig. 2 (continued ).
Table 5
Comparison of Liao’s model with the present model
Variable Liao’s model Present model
Volume of liquid slug
reaching the surface (m3)
0.309 0.299
Time for elevation (s) 275 289
Average bottom-hole
flowing pressure (MPa)
3.4 3.8
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Qgi, with the parameter settings for the most econom-
ical point.
3.4. Selection
The results presented in the previous section are
useful for the adjustment of the operation, given dif-
ferent work conditions. Another important use of the
simulator model is the selection of the best IGL design.
It must be emphasized that other engineering factors
are also relevant for the final decision, and should be
considered in an integrated analysis.
For the purpose of comparison, once the optimum
operating point was determined for each method,
under different reservoir condition (PI and P r ), then,methods were ranked following the economic crite-
rion given by Eq. (3). The continuous gas-lift (CGL)
was included in the analysis too. The results are
presented in Table 7 with the best option placed at
the top of each cell, where the relative position of the
conventional IGL emphasized in gray, for reference.
In some of the cases shown on Table 7, when the
differences in gain are not too large, other design and
operation issues will determine which method to
choose (Clegg et al., 1993). If the selection criterion
of maximum oil production is adopted, the rankingwill certainly be modified.
For the cases studied, the conventional IGL per-
forms reasonably well, ranging from average to best
economic performance, usually outdoing the CGL.
Although the conventional IGL is not always the best
among the IGL methods, it may still be selected due to
its simplicity and reliability, especially when the gains
offered by the other methods are only marginal. Also,
it was determined that the ideal IGLP performs very
well for low-pressure reservoirs. For high-pressure
Table 6
Economic optimum
hs/ H w, P r (MPa) PI (m3/day MPa)
2.5 5 10 20(d) PL
0.2, 2.78 2.2 4.4 8.4 16.9
965 1750 2500 5312
60, 70, 41 60, 70, 41 60, 70, 50 40, 60, 58
0.4, 5.57 8.6 14.8 27.6 42.5
2518 3032 4886 6328
30, 70, 41 40, 70, 41 40, 70, 50 50, 70, 50
0.6, 8.35 14.0 25.9 48.0 80.0
2521 4136 6773 11,469
30, 70, 41 30, 70, 41 30, 70, 50 30, 60, 58
0.8, 11.13 19.4 36.8 67.9 106.0
2726 4705 8539 9868
30, 70, 41 30, 70, 41 30, 70, 50 40, 70, 41
(b) IGLP
0.2, 2.78 3.1 5.8 10.6 17.9
670 1273 2319 3912
50, 60, 70 50, 60, 70 50, 60, 70 50, 60, 70
0.4, 5.57 9.2 17.2 28.1 47.3
1765 3303 4491 8347
30, 60, 70 30, 60, 70 40, 70, 70 40, 60, 70
0.6, 8.35 14.8 27.7 48.1 70.5
2509 4708 7002 10,052
30, 60, 70 30, 60, 70 30, 60, 58 50, 70, 70
0.8, 11.13 20.2 37.9 67.3 100.0
2736 5122 9026 12,812
30, 60, 58 30, 60, 58 30, 60, 58 40, 65, 60
(c) IGLC
0.2, 2.78 4.4 7.1 13.5 24.6
745 1290 2389 4222
50, 70 30, 53 30, 53 30, 53
0.4, 5.57 9.6 17.8 32.9 54.8
1435 2634 5272 8578
20, 60 20, 60 20, 70 20, 70
0.6, 8.35 16.0 29.5 51.4 83.9
2391 4215 6891 10,252
10, 49 10, 49 10, 49 10, 49
0.8, 11.13 21.9 40.0 68.8 109.0
3107 5447 8804 12,620
10, 60 10, 60 10, 60 10, 60
(d) PL
0.2, 2.78 2.8 5.5 10.1 17.7
1200 2300 4248 7366
70 70 70 70
0.4, 5.57 7.4 13.7 23.9 38.2
1750 3239 5619 8812
60 60 60 60
0.6, 8.35 13.5 24.2 39.2 65.7
3031 5326 8371 16,142
40 40 40 30
Table 6 (continued )
hs/ H w, P r (MPa) PI (m3/day MPa)
2.5 5 10 20
(d) PL0.8, 11.13 21.4 39.5 65.4
5400 9603 14,764
25 25 25
Reading key for the table cells: Qlp (m3/day).
Qgi (m3/day).
l si/ hs, P to/ P co, V gi/ V gref (%, %, %).
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reservoirs with high PI, the ideal IGLP performance
drops below the conventional methods —IGL and
CGL. It must be noted that the conventional IGL
comes a close second to the ideal IGLP. These resultsindicate that reduction of the fallback may not be the
critical factor for the improvement of the performance
of intermittent gas-lift. In general, the IGLC performs
very well for all the reservoirs studied, with the
advantage that its performance does not degrade too
much, comparatively to the other methods, as the
reservoir pressure decreases. Since pressure depletion
will occur along the life of a reservoir, the IGLC
would be a good long-term choice. The IGLC fares
better than its competitors because its liquid produc-
tion compensates its gas consumption, even when thegas input is greater than that of the other methods,
especially the CGL. The installation of the IGLC is
very similar to the IGL, but the IGLC produces more
liquid and is more economical than the IGL. The ideal
pig-lift performs poorly for high-pressure reservoirs,
mainly because of its high gas consumption. For low-
pressure reservoirs, the ideal PL does better than the
CGL, but, again, it falls behind the other methods due
to the gas input, even though its liquid output is as
good as the IGLP and better than the IGL. It must be
noted that the PL installation is somewhat simpler
than the others considered here.
4. Dynamics
The engineer must dominate the subject of his
work, therefore it is not enough to compute the IGL’S
output; the relationships among the variables must be
understood as well. The simulator model is helpful in
assessing the measure of the influence of any operation
control parameter on the dependent variables of the
IGL. The following sections describe some of these
studies providing insight into the IGL’s dynamics.
4.1. Variables
The daily production rate QL is equivalent to the
average discharge per cycle—the ratio of the volume
of liquid produced at the surface per cycle, V Ls, to the
duration of one cycle t 1. These two variables are the
main concern of the present analysis. Some of the
parameters that affect them are: the injected volume of
gas per cycle, V gi; the gas-lift valve’s seat diameter,
d gv; the initial length of the liquid slug (load), l si; the
casing pressure ahead of the gas-lift valve at the time
of its opening, P co.Other important variables condition the behavior of
IGL systems too, but they usually do not offer ample
opportunity for control; as so they are assumed con-
stant in all of the foregoing discussion: (a) geometry
of the well—casing diameter, Dc, tubing diameter, Dt ,
well perforation depth, H w, tubing length, H t ; (b) fluid
properties — specific weights of oil, co, water, cw, and
gas dg, water volumetric fraction, /w, reservoir gas–
liquid ratio, RGL, viscosities of oil, lo, water, lw, and
gas, lg; (c) reservoir state—static pressure, P r , pro-
ductivity index, PI, geothermal gradient, aT; (d) sur-face facilities—well-head pressure, P wh, gas injection
pressure, P gi, surface temperature, T s. For simplicity,
H t was set equal to H w, and the depth of the gas-lift
valve, H gv, was also kept constant when comparing
the IGL methods.
For one type of gas-lift valve, all geometric param-
eters are known once the seat nominal diameter is
given—the diameter of the flow orifice, the open–
close characteristic curve, and the flow rate perform-
ance curve. The initial length of the liquid slug defines
Table 7
Selection rank for IGL methods (economic criteria)
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the load to be elevated, is determinant of the ‘‘counter-
pressure’’ in the tubing, and, specifically, of the tubing
pressure downstream of the gas-lift valve at the mo-
ment of its opening, P to. The load will be represented by the non-dimensional parameter L = l si/ hr , where hr is the height of the column of liquid equivalent to the
reservoir static pressure ( P r / qL g ), or the maximum
load. The opening pressure of the valve on the casing
side will be represented by the non-dimensional para-
meter R = P to/ P co. The closing pressure, P cc, becomes
implicitly defined. The total amount of gas injected
will be presented by non-dimensional parameter
V = V gi/ V gref , where V gref was originally defined in
the work of Liao (1991). V gi also establishes the dura-
tion of injection by the motor valve at the surface. Al-
ternatively, the minimum amount of the gas needed to
trigger the opening of the gas-lift, V g, valve may be
chosen as the reference volume, since its value is un-
ambiguously related to the parameters mentioned
above. The reservoir static head will be presented by
P = P r / P H, where P H = q gH w is the static head of the
well filled up with liquid (note also that P = hr / H w).
Finally, due to a lack of standards, the productivity
index of reference is arbitrarily set to PI* = 1 m3/day
MPa, in the definition of the non-dimensional index
I = PI/PI*. The study was conducted with the case of
Table 2, for 0.2 < P < 0.8 and 2.5 < I < 20, but all thegraphics presented here are for P = 0.6 and I = 10. For
the sake of comparison, the liquid volume fed by the
reservoir to the well, in the first cycle, is used as a
measure of discharge (Santos et al., 1998).
4.2. The conventional IGL
4.2.1. Influence of the injected volume of gas, V gi , on
V Ls , t 1 and Q L
Fig. 3 displays results with d gv = 12.7 mm (1/2 in.),
L = 0.5 and R = 0.7. The simulation shows two inferior limits for V gi: the minimum value to open the valve
(6), and the minimum value for the slug to reach the
surface (5). One of the two becomes the operation
minimum value for each particular installation. Notice
in Fig. 3(1) that V Ls is smaller than V Li, the initial
volume of the liquid slug; that is due to the fallback,
the slug’s loss of liquid during its rising— DV L. Also,
it is observed that part of V Ls is provided by the
produced slug, and the rest comes from a fraction of
the fallback dragged by the gas during the decom-
pression phase. The volume of the produced slug
increases monotonically with V gi, but is non-linear,
and the rate of increase becomes very small for larger
values of V gi. The amount of liquid dragged by the gasis significant only for high values of V gi. These
observations are explained by the relationship between
V gi and the ascent velocity. Fig. 3(2) shows that, for the
same depth, the elevation period diminished as V giincreases; therefore the slug has less time to lose
liquid. Besides, the gas at high speed is more proficient
to drag the liquid film left behind by the slug.
Fig. 3(3) shows the moment that the gas-lift valve
closes (t vc), comparing it to the moments of arrival at
the surface of the top and bottom of the slug (t Le, t Lp),
which, respectively, define the elevation and produc-
tion phases. For low values of V gi, the valve closes
during elevation, thus the lift proceeds sustained by
the gas energy stored in the tubing. For high values of
V gi, the valve remains open during production, con-
tinuously supplying energy for maintenance of the
process, including the dragging of the liquid film,
when the valve stays open after the production phase.
The increase in V Ls, discussed above, does not mean
a gain in production rate, which is also dependent on
the period of the cycle, t 1, which, in turn, is the sum of
the periods of each phase. The period of the elevation
phase diminishes as V gi increases, but it tends to acertain limit (Fig. 3(4)); on the other hand, the decom-
pression per iod t gd À t Lp always increases as V giincreases (Fig. 3(4)), because there is more gas to be
purged from the tubing. This last effect becomes
predominant and t 1 increases. The combination of the
effects of V gi on V Ls and t 1 results in the behavior of the
discharge rate, QL, shown in Fig. 3(5)— for smaller V gi,
the increase in V Ls becomes an increase in QL, while for
larger V gi, the increase in t 1 translates into a decrease of
QL. Therefore, a ‘‘critical’’ value of V gi exists, deter-
mining the maximum daily flow rate (note that it is not at the same point where V Ls is at its maximum).
The maximum discharge is usually associated to
very large gas injection rates, consequently, the costs
may overcome the gains, and the point of maximum
daily profit will be different from the point of max-
imum daily flow rate. For the sake of the practice in the
field, it is important to emphasize the sensitivity of the
discharge rate with regard to V gi — with amplitudes of
the order of 15% to 20% of the median of the range of
QL, when 0.3 < V < 1.5, for the current examples.
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4.2.2. Influence of the initial length of the slug (load),
l si , on V Ls , t 1 and Q L
For a well-reservoir system (given P and I ) with
the R and V parameters set at fixed values, the larger the load L, the larger the volume of the produced
liquid. This happens because the distance to reach the
surface is smaller for the larger l si, while the average
speed does not change as much, if R and V are kept
constant; therefore, the travelling time is also smaller,
and there is less time for the slug to lose liquid during
the elevation phase (see Fig. 3(6), where the fallback is expressed as FB = 1 À V Lf / V Li). Surely, it takes more
time to produce the longer slug, which allows for
more penetration of the gas shot into the body of the
Fig. 3.(1)IGL: L = 0.5, R = 0.7.(2)IGL: L = 0.5, R = 0.7.(3)IGL: L = 0.5, R = 0.7.(4)IGL: L = 0.5, R = 0.7.(5)IGL: L = 0.5, R = 0.7.(6)IGL:V o = minimum
V o = minimum to open the valve. (7) IGL: V o = minimum to open the valve. (8) IGL: L = 0.5, V = V o. (9) IGL: L = 0.5, V = V o. (10) GLI: L = 0.5,
V = V o. (11) IGLC: L = 0.1. (12) IGLC: L = 0.1. (13) IGLC: L = 0.1; IGL: L = 0.5 (same hsi). (14) IGLP: L = 0.5, R = 0.7. (15) IGLP: L = 0.5,
R = 0.7. (16) IGLP: L = 0.5, R = 0.7. (17) Pig-lift.
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liquid, augmenting the fallback during the production
phase. Nevertheless, as the slug is produced, the liquid
mass in the tubing diminishes, causing an acceleration
of the slug that lessens the penetration; effectively, the
augmentation of the fallback during the production
phase is offset by its reduction during the elevation
phase. Apparently, it would be desirable to operate
with larger loads, but, then, the cycle’s period t 1would also increase, mainly due to the longer period
of recharge (Fig. 3(7)). Since the effects of L on V Lsand t 1 tug QL in opposite directions, one must use the
simulator to search for the ‘‘critical’’ value of L, for
which the discharge rate is maximum, or for the
‘‘economic optimum’’ value of L, different from its
‘‘critical’’ value, that maximizes the daily profit. The
sensitivity of QL to L is high—with amplitudes of the
order of 40% of the median of the range of QL, when
0.2 < L < 0.6, for the current examples.
Fig. 3 (continued ).
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4.2.3. Influence of the casing pressure at the opening
of the gas-lift valve, P co , on V Ls , t 1 and Q L
In this example, the valve’s seat diameter was set to
d gv = 12.7 mm (1/2 in.), while the load and the injection
were fixed, respectively, at L = 0.5 and V = V o (V o = V gv/
V gref , the minimum value required to open the valve).
Under such conditions, V Ls is greater for the smaller
values of R, i.e., larger P co (Fig. 3(8)). This occurs
because the higher the pressurization of the casing is,
the higher the rate of injection in the tubing will be,
yielding a greater speed of ascent which, in turn, bears a
larger V Ls, as argued back in Section 4.2.1. At the same
time, a larger P co implies more gas to open the gas-lift
valve, V gv, and, as seen before in Section 4.2.1, more
gas means more liquid at the surface. With regard to the
cycle’s period, the higher speed of the slug translates
Fig. 3 (continued ).
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into a shorter elevation period, but the larger the
amount of gas requires a longer period for decompres-
sion. The final balance is unfavorable, resulting in an
increase of t 1, as P co increases (decreasing R, Fig. 3(9)).Again, these effects exert contrary pulls on QL, creating
a ‘‘critical’’ point with respect to R, where QL has a
maximum value (Fig. 3(10)). The sensitivity of QL to R
is small, comparatively to the sensitivity associated to V
and L —with amplitudes of the order of 6% of the
median of the range of QL, when 0.5 < R < 0.75, for the
current examples.
4.3. The IGL with chamber
In the case of the IGLC, the operation parameters
studied were V and L. In the following example, a pi-
lot valve was employed with seats of 7.9 mm (5/16 in.)
and 14.3 mm (9/16 in.), for the pilot and main sections,
respectively, and set to open at the pressure available at
the surface compression facility. For the IGLC, l si is
equal to the length of the accumulation chamber, de-
fining the slug load and the period of charge, just as in
the case of the conventional IGL. The difference is in
the height of the slug when lift begins, after the phase
of liquid transfer from the chamber into the tubing: for
the IGL, hsi = l si, while, for the IGLC, hsi = l si.( Dc/ Dt )2.
The casing pressure and the tubing counter pressureare determined by these variables.
The simulations have shown that the behavior of the
IGLC is very similar to that of the IGL, as presented in
the previous sections. The differences lie on the posi-
tions of the ‘‘critical’’ and ‘‘economic optimum’’
points. It is worthy of note that, at these points, the
discharge rate of the IGLC is consistently greater than
that of the IGL, while the values of L are lower, as well
as the fallback. Results are displayed in Fig. 3(11,12
and 13). The advantage obtained by the IGLC comes
from its low average well-bottom pressure.
4.4. The IGL with plunger
The ideal IGLP is a model in which leakage does
not occur around an ideal plunger, hence there is no
fallback — seemingly a desirable feature. Accordingly,
the produced volume of liquid is equal to the load, and
is not affected by V gi, P co or d gv. Still, there is min-
imum required value for V gi, dependent on L and R,
for the slug to reach the surface—for instance, with
d gv = 12.7 mm (1/2 in.), L = 0.5 and R = 0.7, V min= 0.6.
Notice that, under the same conditions, the conven-
tional IGL is able to produce with V = 0.4, consuming
less gas. For V above V min, the ideal IGLP’s V Ls islarger than the IGL’s, due to the absence of the fall-
back, but for even higher values of V , the IGL’s V Lsapproaches the same ideal value of the IGLP (Fig.
3(14)). The larger V Ls of the ideal IGLP is not neces-
sarily an advantage (Santos et al., 1998), because its
cycle’s period is adversely affected. The ideal IGLP
cycle is longer than the IGL cycle (Fig. 3(15)), result-
ing in discharge rates of the same order (Fig. 3(16)).
In some cases, the ‘‘economic optimum’’ belongs to
the IGL because the IGLP consumes more gas. The
IGLP’s cycle is longer because its slug remains larger
(no fallback): first, the slug’s ascension is slower due
to the greater opposition of friction (more length) and
gravity (more weight), resulting in a longer elevation
period; second, the production period is longer be-
cause the slug’s speed is lower and the slug’s length is
longer; third, the loading (recharging) phase is also
longer because only the reservoir contributes to it,
while, in the IGL case, part of the fallback returns to
the bottom of the well, completing the load sooner.
4.5. The IGL with pig
The following observations regard the ideal pig-
lift, where the ideal foam-pig does not allow any
leakage. Consequently, for the same load L, V Ls is
invariable as in the IGLP case. Besides, there is not a
gas-lift valve, so the only parameter left is the length
of the slug. Evidently, V Ls = V li, which is proportional
to L — there is no need for simulation here. But, as L
increases, so does the cycle’s period t 1 (Fig. 3(17)—
left), leading to lower discharge rates (Fig. 3(17)—
right). To shorten the cycle’s period, both legs of the
tubing are connected to the discharge port for simul-taneous exhaust of the gas thus reducing the decom-
pression period (Fig. 3(17), dotted lines).
5. Conclusion
This paper presents a study of the influence of the
operation parameters of intermittent gas-lift systems
on the performance of four alternative methods, car-
ried out through a new consistent full-transient simu-
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lation model, developed for this task and disclosed for
the public domain. Due to space limitations, only the
highlights of the study are discussed, and a few
examples are displayed. Nevertheless, important rela-
tionships between the variables of the process are
clarified, providing insight into the inner workings of
the IGL system. Running more cases with the simu-
lator, the engineer may further his understanding of
the system. The results show the sensitivity of the IGL
to the parameters of its design. Therefore, the selec-
tion of the best method, whatever criteria are consid-
ered, requires a careful analysis, as well as the
determination of each method’s optimum point of operation. The simulator is necessary to resolve these
complex tasks. A comparative study of the methods is
also presented. Again, the simulator proved to be an
invaluable tool. Although limited in scope, the study
reveals that ranking the IGL methods is not a trivial
job, and also challenges some unfounded myths. The
examples show the potential of the simulator to build
comparison tables that may be an aid for ballpark
assessment of the methods suitability and optimum
settings of the operating parameters; then, the simu-
lator can be used again to refine the project. Thesimulator may also be coupled to more sophisticated
computer-aided design and selection programs, as one
of the CAD&S’s processing modules.
Work is under way to improve details of the si-
mulator—either its physical or numerical models, but
what are the most needed, today, are carefully con-
trolled experimental works, with large-scale fully ins-
trumental apparatuses, and more accurate and reliable
field data. A laboratory rig will be designed to provide
a reliable database for a definitive validation of the si-
mulator. As it stands, numerical values appearing in
this article must be viewed with prudence; neverthe-
less, the qualitative observations on the variables’
relationships and the comparisons among the IGL
methods may still hold, since the same model assump-
tions and equations were applied to all methods. As the
work progresses, the authors pledge to keep the pet-
roleum engineering community up to date with further
developments.
In 1993, gas-lift installations reached over half of
about 100,000 wells in the USA, each delivering at
least 540 m3/year (3400 bbl/year), and accounted for
the majority of offshore wells (Clegg et al., 1993)Most of these gas-lift devices employed the CGL
method. Although few official data is available, it is
quite certain that gas-lift is largely used in Brazil also
(Fig. 4 illustrates two examples), and will continue to
be, as a good alternative for reservoirs of low pressure
or low productivity. Hence, proper selection and
optimization of IGLs are of paramount importance.
Acknowledgements
The authors wish to express their thanks to Mr.Attilio Triggia, MSc for valuable suggestions. Thanks
are also due to PETROBRAS, CEPETRO/UNICAMP
and the Dept. of Petroleum Engineering of FEM/UNI-
CAMP.
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