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SPE-171342-MS
Improved Gas Lift Valve Performance using a Modified Design for
GLVSeat
F. Elldakli and M. Y. Soliman, Texas Tech University; M. Shahri,
Halliburton; H.W. Winkler and T. Gamadi,Texas Tech University
Copyright 2014, Society of Petroleum Engineers
This paper was prepared for presentation at the SPE Artificial
Lift Conference & Exhibition-North America held in Houston,
Texas, USA, 68 October 2014.
This paper was selected for presentation by an SPE program
committee following review of information contained in an abstract
submitted by the author(s). Contentsof the paper have not been
reviewed by the Society of Petroleum Engineers and are subject to
correction by the author(s). The material does not necessarily
reflectany position of the Society of Petroleum Engineers, its
officers, or members. Electronic reproduction, distribution, or
storage of any part of this paper without the writtenconsent of the
Society of Petroleum Engineers is prohibited. Permission to
reproduce in print is restricted to an abstract of not more than
300 words; illustrations maynot be copied. The abstract must
contain conspicuous acknowledgment of SPE copyright.
Abstract
Each gas lift valve (GLV) is a variable orifice until a fully
open port area is attained (under maximum stemtravel). As the ball
(stem) moves away from the ball/seat contact area, the area open to
flow increases untilthe flow area upstream to the port area equals
or exceeds the fully open port area.
Laboratory gas dynamic throughput testing indicates that each
injection-operated GLV often does notopen fully in actual
operation, mainly because of the bellows stacking phenomena. As a
result, the stemforms a restriction upstream to the flow path.
Therefore, actual flow through the GLV can be less thanexpected.
This paper addresses such issues and recommends a simple but
effective solution. A modifieddesign for the GLV seat was created
to help reduce the required stem travel to generate a flow area
equalto the port area.
Theoretical calculations confirm the actual gas dynamic
measurements and show that the minimumstem travel for the modified
design improves from 5 to 58% compared to using a
conventionalsharp-edged seat. This improvement should have a
significant impact on GLV performance. The modifiedseats for all
different ports sizes were manufactured and tested using a
benchmark valve test. Theexperiments showed that for the same stem
travel, the new design has a larger flowing area than that ofthe
sharp-edged seat. This paper details the new design, theoretical
calculations, and experimental results.
IntroductionGenerally, a gas lift is a simple, flexible, and
reliable artificial lift system with the ability to cover a
widerange of production rates. Gas lift systems are a closed
rotative system empowered by high-pressured gas.Therefore, the
surface facilities required to perform this application consist of
a compressing unit andsupplemental source of lean gas. A GLV,
however, is a backpressure regulator (Winkler 1987). The
entireprocess is used to reduc the wellbore fluid pressure gradient
by supplementing gas through an externalsource to withdraw more
liquid from the reservoir under higher drawdown. Many parameters
affect thegas lift system design, such as a change in the wellhead
and bottomhole pressures (BHPs), produced fluidtype, and
productivity index of the reservoir. As these parameters change,
the gas injection pressurechanges.
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A GLV basically regulates the pressure on its upstream side to
its downstream. While the upstreampressure is higher than the
dom-charged pressure, the GLV remains open. Therefore, calibrating
eachGLV to achieve the best performance at the wellbore is vital to
the artificial lift cycle of each well.Because a GLV consists of
many moveable mechanical compartments, achieving synergy between
all ofthose compartments should result in the best performance.
The objective of this study was to optimize the GLV performance
by measuring dynamic gasthroughput performance of each GLV using a
modified seat design. Before 1940, differential GLVs werecommon. In
those valves, the operation of the valve dependes on the
differential pressure between theinjection gas in the casing and
the fluid in the tubing. King (1940) invented the first GLV with
agas-charged bellows assembly. With that, gas lifting of lower BHP
wells became possible by means of acontrolled pressure change of
the surface injection.
Overall, a gas lift is a forgiving method of enhanced
production, in other words, even a poor gas liftdesign can increase
production. To achieve a higher ramp in fluid production rate using
gas lift, however,a more sophisticated design of each compartment
of the system is required.
Because a gas lift is a system of multiple compartments, a
comprehensive redesign may be nessary toincrease production rate.
The architectural design of each GLV is as important as the depth
of installationand number of GLVs used in each installation. Some
faulty designs may result in installing dozens ofGLVs to unload
well that does not require even one to be installed. Therefore, it
is very important tounderstand the system and not rush its
development.
Statement of the problemThe mechanics of a GLV are exclusively
based on pressure balance across the valve itself. Current GLVsare
all based on King (1940) and consist of a dome section, which is
charged with gas (usually nitrogen)at a certain pressure. On one
end, a dome seal is present for pressure charge and discharging
purposes.The dome section is attached to the bellows assembly. The
bellows assembly is helical in shape and actslike a spring. The
bellows are attached to the stem, which ends at the ball. All of
these mentioned sectionsmove as a single unit in each GLV. When the
GLV is closed, the ball is seated onto a sized port area. Asa rule,
in each GLV, the ball is 1/16-in. larger in diameter than each port
size. A check valve on thedownstream side of the port prevents the
backflow from either the tubing or casing to interfere with
oneanother.
The basic components that comprise a GLV are shown in Fig. 1.
The loading element can be anitrogen-charged bellows, a spring, or
a combination of both. Fig. 2 shows schematics of a benchmarkvalve.
Winkler (1987) originally came up with this design to measure the
gas throughput capacity of eachGLV. Winkler and Camp (1987)
implemented such a tool in field applications, with successful
results. Adetailed description of this valve can be found in API
RP11V2 (2001), ISO 17078-2 (2007), and Shahri(2011).
As gas is injected the casing pressure rises, and applies on the
bellows area. A fraction of the injectionpressure acts on the ball
pushing the stem down. This amount depends on the ball surface
area. In staticmode, the only pressure acting on the bellows is the
dome-charged pressure, which is reffered to as closingpressure. As
the injection gas pressure ramps up, the force on the bellows
increases, and when it passesthe bellows set charge pressure, the
GLV initially begins to open. Note that the GLV opening mechanismis
gradual. The stem travel in the actual GLV system is based on the
difference between the opening andclosing forces and the
bellows-assembly load rate. The equivalent port area for a
partially open valve isdefined by the lateral surface area of the
frustum of a right circular cone. The frustum area is
generatedbetween the ball surface and the valve seat as the valve
stem moves away from the seat. Therefore, a GLVis a variable
orifice until maximum stem travel or a fully open port area is
attained. Then it becomesequivalent to an orifice (pipe flow).
2 SPE-171342-MS
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Flowing area is one of the most important parameters in each
GLV. This area is generated by the stemmovement away from the seat.
This movement depends on the bellows efficiency. During the gas
liftoperation, the bellows is frequntally exposed to the injection
pressure (opening force) and dome-chargedpressure (closing force).
Therefore, the inner and outer convolutions of the bellows come in
contact oneanother. As a result, the bellows experiences stacking,
and the GLV stem does not travel a sufficientdistance to create a
flow area equal to the port area. Therefore, the GLVs ball and stem
form a restrictionin the flow path and the actual throughput flow
in the GLV becomes less than the theoretically calculatedvalue.
Modified DesignA modified design for the GLV that helps reduce
the stem travel required to generate a flow area equalto the port
area was achieved. The workflow in such a process begins with
calculating the minimum stemtravel for a modified seat and
comparing the results with current geometry (sharp-edged seat)
beforemanufacturing the new seat/port assembly. The new seat design
is easy to manufacture and is compatiblewith other GLVs. The
specifications for the modified GLV seat are controlled by changing
the angle ofthe taper.
Different angles are used with each port to create different
port top diameters, as shown in Fig. 3. Thetheoretical minimum stem
travel can be calculated using Eq. 1 which is similar to the
equation developedby Kulkarni (2005). The stem travel in Eq.1 is
function of the port bottom radius (rp), port top radius (rT),and
ball radius (rb).
(1)
The ball size for each port top diameter is used based on API
recommendations (ball OD Port topdiameter 1/16 in.). The ratio is
based on the ball/seat contact area. Table 1 shows the amount
oftravel required to achieve a flow passage equivelent to the port.
The calculations are basically the samefor a sharp-edged seat.
Figure 1Schematic of an injection pressure operated (IPO) GLV
andcomponents.
Figure 2Schematic of a benchmark valve and components
(fromWinkler 1987)
SPE-171342-MS 3
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Theoretical calculations show that the minimum stem travel for
the modified design improves the stemtravel from 5 to 58% compared
to using a conventional sharp-edged seat designs. This
improvementshould have a significant impact on GLV performance.
TestingIn each gas lift design, GLVs should be tested to assure
the proper amount of gas is passed to lift thepredicted volume of
liquid. To quantify the GLV mechanics and behavior, a benchmark
valve (Fig. 2) that
Figure 3Comparison of sharp-edged seat with beveled seat (at
different beveled angle).
Table 1GLV PORT/BALL AND STEM CHARACTERISTICS FOR GAS THROUGHPUT
TESTS
4 SPE-171342-MS
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has the body of an actual GLV with a controlledflowing area is
used for these sets of experiments.The banchmark valve is used to
test the valve per-formance at different stem positions. The only
phys-ical difference between a true GLV and the bench-mark valve is
the existence of the bellows assembly.The bechmark valve is not
equipped with a bellowsor dome section, but the stem position in
relation tothe valve seat is manually adjustable. This informa-tion
could be applied to predict the stem travel foractual GLVs when the
volumetric gas rate and up-stream and downstream pressures are
known.
For the modified design, different sizes of seatswere tested to
determine flow through values. A1-1/2-in. (IPO) Camco J-20 was used
in all exper-iments. The benchmark valves with identical seatswere
installed in an encapsulating tester, and then the stem travel was
adjusted at six different positionsrelative to the ball/seat
contact area. Therefore, the ball/seat distances were based on the
theoreticalminimum required to fully travel that the upsteam area
would be identical to the downstream area.Because a ball/stem was
involved as well, the benchmark valve was set at two other
positions greater thanthe theoretical fully open area.
The testing system used in this paper was based on API RP11V2
(2001). The testing procedure wasbased on a constant production
pressure test (CPPT), in which the downstream production pressure
usedwas atmospheric pressure. The testing was performed at
transient conditions; however, it could beperformed under steady
state conditions as well. The pressure-time data were recorded
using a very fastdata acquisition system (DAS) utilized by National
Instruments (NI) (2014). This system is capable ofrecording up to
12,000 samples per second per channel. In the experiments for this
paper, the sample ratewas set at 100 samples per second per
channel. Winkler (2010) shows that using 100 samples per
secondworks as well as using 12,000 samples per second. Data
analysis is also accelerated. Shahri (2011) andShahri and Winkler
(2011) published some discharge coefficient values as these tests
were underway.Their results show that as the dome-charged pressure
changes, the liner stem travel of the ball changes.
Figure 4Variable beveled seat arrangements for gas throughput
capacity measurements.
Figure 5Variable dimensionless flow area of variable
beveled-angleseat and different ball sizes for -in. port.
SPE-171342-MS 5
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This causes the value of the discharge coefficient tochange. Not
considering these changes can result inoverestimating the liquid
production by up to 30%.
The calculation is implemented at a critical con-dition and
orifice flow regime. To creat this situa-tion, the calculated ratio
of downstream pressure toupstream pressure (Pdown / Pup) should be
less orequal to 0.528 (the critical value for nitrogen).
Knowing the injection pressure, the pressuredrop at a depletion
time, and the capacity of work-ing gas allows the volumetric flow
rate to be calcu-lated. The effect of temperature was
includedthroughout the test. Because the testing time wasshort, the
temperature changes was insignificiantbut were incorporated into
the test.
The volumetric calculations are based on the realgas law. Some
values must be measured, such as the pressure as the gas is venting
from the system andthe corresponding temperature with time. Working
gas pressure (upstream) was recorded with an analog
Figure 6Measurement of flow throughput capacity of variable
beveled-angle seat for -in. port.
Figure 7Variable dimensionless flow area of variable
beveled-angleseat and different ball sizes for 5/16-in. port.
Figure 8Measurement of flow throughput capacity of variable
bev-eled-angle seat for 5/16-in. port.
Figure 9Variable dimensionless flow area of variable
beveled-angleseat and different ball sizes for 3/8-in port.
6 SPE-171342-MS
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dial gauge, as well as a high-speed digital DAS from NI. Gas
temperature (upstream) was read using anelectronic laser
thermometer. Because the length of each experiment was short, the
temperature variationswere negligible. Therefore, the temperature
measurements were not continuous and were measured at thebeginning
and at the end of each test. The temperature variations were to be
0.5 F. Atmosphericpressure was read using a barometer in mmHg and
then was recalculated to psia. The basis of standardpressure was
set to 760 mmHg or 14.696 psia. On the downstream side, the initial
temperature wasassumed to be equal to the atmospheric temperature;
however, because of the gas cooling effect, thattemperature varied
under experimental conditions. The gas compressibility factor was
calculated at eachpressure and temperature based on available
correlations. Some constant inputs were used in this analysisthat
were dominantly depenedent on the location and testing facility
such as the gas constant, ratio ofspecific heats of the active gas,
the capacity of the storage facility, and the specific gravity of
the workinggas.
ResultsThe experimental results confirmed the theoretical
predictions. Figs. 5, 7, and 9 compare the effective flowarea
obtained when using the sharp-edged seat with that obtained using
the new design for three differentport sizes: 1/4, 5/16 and 3/8 in.
Figs. 6, 8, and 10 demonstrate the gas throughput capacity of each
portsize respectively. At the same stem travel time, the new design
provides a larger flow area, and as the porttop diameter increases,
the flow area increases. This improvement should have a significant
impact on theGLV performance, which was measured to be between 5 to
30% more than the gas throughput capacityof GLVs using a
sharp-edged seat.
ConclusionsThe following conclusions are a result of this
work:
GLVs do not pop open as the injection pressure passes the
initial opening pressure; and thus, thestatic force balance
equations used to calculate opening and closing pressures are not
appropriatefor calculating dynamic flow performance.
A new GLV seat was successfully designed, manufactured and
tested. The port geometry and maximum stem travel effect on the
volumetric gas throughput of each GLVwere improved.
The experiment results showed that for the same stem travel,
GLVs using a beveled seat had alarger open area to flow compared to
those using a sharp-edged seat.
Figure 10Measurement of flow throughput capacity of variable
beveled-angle seat for 3/8-in. port.
SPE-171342-MS 7
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ReferencesISO 17078-2 Petroleum and Natural Gas Industries
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8 SPE-171342-MS
Improved Gas Lift Valve Performance using a Modified Design for
GLV SeatIntroductionStatement of the problemModified
DesignTestingResultsConclusions
References
