spe-174092-pa

Some Design Aspects for Venturi GasLift Valves

A. R. Almeida, Petrobras Research and Development Center

Summary

The Venturi valve represents a significant advancement in thetechnology of gas lift in petroleum wells. Its use is expanding,and an understanding of the theoretical and practical aspects isfundamental to maximize the benefits of its application. Theapplication-related aspects are reasonably well-covered in the lit-erature; however, to achieve good performance and promote theexpected benefits, some valve-design aspects have to be takeninto account. This paper describes key points, including a recom-mended Venturi geometry, the influence of several Venturi designparameters on performance, and an analysis of the role of thecheck valve. The design guidelines provided in this work are sup-ported by experimental tests performed at Petrobras Gas LiftValves Test Unit and by sizable experience with Venturi valvesafter more than 600 valves run in wells or tested.

Introduction

The Venturi-nozzle (or simply Venturi) gas lift valve is beingused increasingly in continuous gas lift wells worldwide. Somearticles (Tokar et al. 1996; Faustinelli et al. 1999; Lyngholm et al.2007; Kartoatmodjo et al. 2008; Almeida 2010, 2011a, 2011b;Rilian et al. 2012) describe aspects of this technology. In Brazil,approximately 350 offshore wells are now equipped with thesevalves and more than 600 valves with Venturis were run in wellsor tested by Petrobras.

Roughly speaking, there are two types of gas lift valves. In thefirst type, there is some sort of mechanism to open or close the valveaccording to pressure (and, in most cases, temperature) conditionsin the well. A charged bellows is the most-used mechanism. Thesecond type involves valves that are always open (in the casing-to-tubing direction). They are composed of a choke (to restrict flowrate) and a check valve (to avoid reverse flow from tubing to cas-ing). The most-used choke is a cylindrical plate with a square-edgedorifice. Alternatively, a choke with Venturi geometry may be used.

Fig. 1 shows an experimental gas-flow curve of a Venturi valvecompared with that of a conventional square-edged-orifice valve.The most-important difference refers to critical flow (i.e., theregion of the curve where the gas-flow rate through the valve isconstant, irrespective of downstream pressure). This occurs whendownstream pressure is less than a certain fraction of the upstreampressure. This fraction, called critical ratio, is roughly 0.5 for anorifice valve and 0.9 for a Venturi valve. Thus, in view of the usualpressure differentials between casing (injection pressure) and tub-ing (production pressure), orifice valves operate under a subcriti-cal-flow regime and Venturi valves under a critical-flow regime.This difference in critical-pressure ratio is explained in detail inseveral references (e.g., Almeida 2011b) and is mainly caused bythe existence of a diffuser downstream of the main flow restrictionthat promotes a remarkable pressure recovery.

Observing Fig. 1, another difference that draws attention is theorifice lower critical-gas-flow rate when compared with a Venturithat has the same throat diameter and that is submitted to thesame upstream conditions and gas. The main reason is the pres-ence of a vena contracta downstream of the orifice. It is originatedby swirling eddies formed after the sudden gas expansion. The

main jet from the orifice is then contracted, forming the vena con-tracta, which is the smallest flow area. In a Venturi, the smallestflow area is the throat area itself.

Because most advantages of a Venturi valve come from thehigh critical ratio, which depends on the valve design, we define avalve as being a Venturi valve if the critical ratio is 0.9 or greater,considering the gas and pressure/temperature conditions of theactual application. Valves with lower critical ratios are not consid-ered as Venturi, but may be used depending on the case. However,the gas lift designer needs that information in hand to makethe decision.

In theory, Venturi valves may be used in all wells producingby (continuous) gas lift. In practice, some factors may prevail andindicate orifice valves instead. Thus, it is important to know theadvantages and limitations of Venturi valves when compared withorifice valves. Almeida (2011b) gives some advice on this,describing the most-attractive applications and the most-importantrestrictions. Calculation of gas-flow rate is treated in Almeida(2011a) and, in a simplified form, in Almeida (2010).

However, to fulfill its expectations, the Venturi gas lift valvehas to be designed, manufactured, and handled with care to guar-antee the desired high critical-pressure ratio. The Venturi elementinvolves a well-known geometry, but details on the specific pro-file make a difference. For example, Fig. 2 compares experimen-tal performance curves of two commercial Venturi valves. Ingeneral, critical ratios improve for smaller throats, but in thiscase, the opposite happened for one of the commercial valvesbecause of a design mistake in the Venturi profile.

Next, design guidelines to achieve a high critical ratio in a Ven-turi valve are presented on the basis of theoretical analyses, experi-mental results, and practical applications. Not only must theVenturi itself follow an appropriate profile and finishing, but theentry ports and the check valve must introduce pressure losses thatare as small as possible. It is advisable that gas lift engineers payattention to some points to avoid surprise with a poor performance.

Venturi Profile

The Venturi geometry is well-known. However, small geometricvariations can exist between designs that result in different levelsof efficiency. Venturis are standardized in cases in which suchelements are used for flow measurement. A classical reference insuch cases is American Society of Mechanical Engineers (ASME)standard MFC-7M-1987: Measurement of Gas Flow by Means ofCritical Flow Venturi Nozzles (1987). According to this ASMEstandard, two types of Venturi profiles may be usedtoroidalthroat and cylindrical throat.

Fig. 3 shows a generic profile for a Venturi with a toroidalthroat. In this case, the nozzle profile is a circular arc that extendsto the point where the conical diffuser is tangent to the toroidalportion. The throat at x xb is in the toroidal portion, and the tan-gency point is at x xc.

Defining k rb/rg, the ASME standard recommends 3.6k 4.4 and 2.5 a 6 . Considering the available space for aVenturi inside a gas lift valve, it is recommended to adopt k 4and a 6 , which have proved to be adequate in practice.Expressing that profile in terms of equations gives the following.

For the nozzle (0 x xb),

y r2b xb x2 yb; 1. . . . . . . . . . . . . . . . . . . .

Copyright VC 2015 Society of Petroleum Engineers

Original SPE manuscript received for review 5 May 2014. Revised manuscript received forreview 12 November 2014. Paper (SPE 174092) peer approved 3 February 2015.

PO174092 DOI: 10.2118/174092-PA Date: 7-April-15 Stage: Page: 1 Total Pages: 8

ID: jaganm Time: 14:52 I Path: S:/3B2/PO##/Vol00000/150014/APPFile/SA-PO##150014

2015 SPE Production & Operations 1

where (xb, yb) are the coordinates of the center of the circumfer-ence with radius rb that defines convergent nozzle curvature.

The circumference arc has to satisfy some conditions. At x xb, y R rg, which implies that

yb R rg rb: 2

At x 0, y 0, which implies that

xb r2b y2b; or xb r

2b R rg rb

2 :

3

Combining Eqs. 1 through 3 results in

y r2b r2b R rg rb

2 x 2 R rg rb :

4

For the diffuser (xb x xt), the diffuser is usually conical.The nozzle arc is extended to promote smooth transition betweenthe curved nozzle and the conical diffuser. That is, the arc of cir-cumference is extended from x xb (the throat) to x xc, wherethe derivative is equal to [tan(a)].

Differentiating Eq. 1 results in

dy

dx

xb x

r2b xb x2: 5

Recalling thatdy

dx x xctana, one obtains

xc xbrbtana

1 tan2a: 6

Then, in the segment xb x xc, Eqs. 1 and 4 are valid. In theremaining part (that is, xc x xt), the following holds:

y yc x xc tana; 7

where yc is given by

yc r2b xb xc2 yb: 8

Then, the overall length of a full Venturi is given by

xt xcyc

tana: 9

Sometimes, there is no room for a full-length Venturi. In thiscase, the overall Venturi length xt is defined by the available space,and the Venturi has an outlet diameter smaller than its inlet diameter.

Eq. 2 shows that for rg R / (k 1), the center (xb, yb) of thearc that defines the nozzle is positioned at the x-axis of Fig. 3. Ifrg is reduced below that limit, this center moves into the interiorof the piece. In this case, the nozzle has the shape shown inFig. 4. The curved section does not start at y 0, and there is aflat portion at the inlet plane of the Venturi. The apparent incon-sistency regarding the imposed condition y 0 for x 0 in Eq. 3is explained by the dotted line in Fig. 4. The nozzle becomesmore and more like a simple orifice with rounded edges, whichcan degrade performance. In this case, it is recommended to setthe approach angle b at the nozzle inlet plane (see Fig. 3) and letrb fluctuate accordingly, instead of maintaining rb as a fixed pro-portion of rg. In this new configuration, Eqs. 1 through 9 remainvalid. Because the curvature radius rb is calculated by the newconcept of constant approach angle, there is a new condition:

At x 0;dy

dxtanb, which implies (see Eq. 5) that

rb1

tan2 b1 xb; 10

or, combining Eqs. 3 and 10,

rb R rg1

tan2b1

1

tan2b1

1

tanb:

11

. . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

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1.20

1.00

0.80

0.60

Mas

s-F

low

Rat

e of

Gas

(kg

/s)

0.40

0.20

0.0050 60 70 80 90 100

Venturi 1/4-in. throatOrifice 1/4-in.

110

Downstream Gauge Pressure (bar)

120 130 140 150

Fig. 1Dynamic performance comparison between two com-mercial 1.5-in.-outside-diameter Venturi and orifice gas liftvalves. All performance curves in this work were obtained atPetrobras Gas Lift Valves Test Unit in Aracaju, Brazil, with nat-ural gas and 140-bar upstream gauge pressure, except wherestated differently. If gas-flow rates are given in m3/d, they are atstandard conditions (atmospheric pressure and 20 C).

0.70

0.60

0.50

0.40

Mas

s-F

low

Rat

e of

Gas

(kg

/s)

0.30

0.20

0.10

0.0070 80 90 100 110 120


Manufacturer 1Manufacturer 2

130 140 150

Fig. 2Venturi design details may lead to different performan-ces. In this example, performances of two commercial 1-in.-out-side-diameter Venturi valves with a 3/16-in. throat are compared.The valve of Manufacturer 1 shows a much better perform-ance in terms of critical ratio. Note: for Manufacturer 1, theactual throat diameter is slightly greater than the nominalthroat diameter (3/16 in.), and to match critical flow rate for bothvalves, the flow rates of Manufacturer 2 were multiplied by 1.10,approximately.

y

R

rbxb xc xt x

R rg

Fig. 3General Venturi profile with toroidal throat.



2 2015 SPE Production & Operations

A particular case of interest is b 90 . In this case, yb 0,rb xb (R rg ), and

y x 2rb x : 12

The constant-approach-angle criterion not only solved the prob-lem when rg

geometric elements that are different from one valve to another.Thus, an Orifice Plate F (Table 1) was machined. The length ofthe orifice is equal to the length of the nozzle of the referenceVenturi A (i.e., from inlet plane to throat). The material, surfacefinishing, and tolerances are also the same, and both pieces wereadapted in the same housing of a commercial Venturi valve withexternal check valve. The tests were carried out without the dartand spring of the check valve to eliminate any influence of theseelements.

Fig. 6 compares these inserts at an upstream pressure of 140bar. The behavior is as expected. The Venturi presented a criticalratio of 0.93 and a discharge coefficient of 0.95. For the orifice, thevalues were 0.56 and 0.85, respectively. These values are compa-rable with the values obtained for the commercial valves in Fig. 1.

The Venturi nozzle is basically composed of three elements:nozzle, throat, and diffuser. Fig. 6 also compares the referenceVenturi A with the E and F pieces. The latter is the sharp-edgeorifice plate, and the former isolates the nozzle of Venturi Abecause it was cut exactly at the throat. The behavior of the iso-lated nozzle is intermediate between the orifice and the Venturi.However, the discharge coefficient of both the nozzle and theVenturi are essentially the same (in Fig. 6, the difference in criti-cal-flow rate becomes much smaller when the difference inupstream gas temperature between the tests is accounted for).This indicates that the nozzle geometry leads the gas to the mainrestriction in a way that avoids (or dramatically reduces) forma-tion of the vena contracta. Then, in a Venturi, the nozzle is largely

responsible for the high discharge coefficient. There is also agreater pressure recovery with the nozzle in relation to the orifice(10% of the upstream pressure vs. 2% for the orifice). Probably,nozzle smooth geometry introduces a much lower local head lossin comparison with the orifice. Another factor that may be influ-encing the recovery is the absence of the vena contracta. Thus,this result seems to indicate that, in a Venturi, the nozzle has itsshare of influence on the overall pressure recovery. The main con-clusion here is that a bad nozzle design may impair the dischargecoefficient strongly and may also affect the critical ratio.

Fig. 7 compares two possible nozzles in a Venturi. One isused in Venturi A (approach angle 77 , k 3.65), and the other,Piece B, was designed with an approach angle of 45 (k 15)and is therefore much longer. Fig. 7 shows that there is a visibledecrease (from 0.93 to 0.91) in critical ratio from Venturi A to B.Thus, smaller approach angles, which intuitively would be bestto conduct the gas more smoothly to the throat, impact nega-tively on the pressure recovery after throat. An approach angle of90 seems to work well, but more-comprehensive studies areneeded to define if there is an optimal angle between 45 and 90 .Experience indicates that this optimum angle may depend onthroat radius and is between 60 and 80 8. Then, it is recom-mended to maintain the nozzle-approach angle at approximately60 to 80 .

The isolated effect of a diffuser is shown in Fig. 8. The G pi-ece is composed of an orifice plate (same geometry as Piece F)followed by a conical diffuser with a half-angle of 6 . The curve

80 000

60 000

40 000

Gas

-Flo

w R

ate

(m3 /

d)

20 000

040 60 80 100

VenturiNozzleOrifice

Downstream Pressure (bar)

120 140 160

Fig. 6Comparison among Venturi A (blue line), Nozzle E (green line), and Orifice F (black line).

80 000

60 000

40 000

20 000

080 90 100 110

Reference Venturi

Venturi 45 approach angle

120


Gas

-Flo

w R

ate

(m3 /

d)

130 140 150

Fig. 7Comparison between Venturi A (blue line) and Venturi B (red line).




obtained is compared with the single Orifice Plate F and with thereference Venturi A. It can be seen that even without the nozzlewith a smooth geometry, there is a noticeable pressure recoveryacross the diffuser. Critical ratio of the piece G is 0.85 vs. 0.93 forthe reference Venturi and 0.64 for the square-edge orifice. Then,comparing an orifice plate with a Venturi, roughly two-thirds ofthe additional pressure recovery achieved by the Venturi resultsfrom the diffuser itself. It is clear that a bad diffuser design willimpair the critical ratio strongly.

Fig. 9 shows the influence of the diffusers length and angle.The reference Venturi A is compared with two options of trun-cated diffuser. In the piece D, the diffuser is simply cut to one-halfof its length (relative to the reference Venturi), keeping the half-angle of the diffuser at 6 . In the piece C, the diffuser has thesame length of Piece D (one-half of the reference Venturi dif-fuser), but the angle is changed to 12 to have practically the sameoutput area as the reference Venturi. The importance of the dif-fuser angle on the overall Venturi performance is clear. If, becauseof space limitations, it is necessary to shorten the Venturi from theideal length, it must be truncated without changing the diffuserideal angle. There is a loss of performance (in the case of Fig. 9,the critical ratio falls from 0.93 to 0.92), but much smaller than theloss that will happen if the angle is changed (from 0.93 to 0.78 inFig. 9). In terms of discharge coefficient, as was expected, therewas virtually no change because the nozzle sections are identical.

The toroidal profile (Fig. 3) has been preferred for Venturivalves. In this case, the Venturi throat has an infinitesimal thick-ness (i.e., it is reduced to a single cross section). However, anotherpossibility is the cylindrical profile (Fig. 5), in which the throathas a finite length. On the one hand, the existence of a well-defined throat brings some advantages. The manufacturing isfacilitated, and it becomes easier to ensure the diameter and asso-ciated tolerances of the main flow restriction. On the other hand,what would be the impact of a finite throat in performance?Fig. 10 answers this question by comparing the reference VenturiA with two other valves with the same nozzle and a tapered dif-fuser with a half-angle of 6 8, but with throats of 1.0 and 9.5 mm(same thickness as Orifice F) in length. Fig. 10 shows that a throatthat is excessively long affects both critical ratio and dischargecoefficient. Small throats, however, do not introduce any signifi-cant effect and can be used without problems.

The most-used material to manufacture Venturis for gas liftvalves is tungsten carbide. Comparison among the reference Ven-turi A (titanium) and others made of tungsten carbide and stain-less steel, with the same prescribed surface finishing, indicatesthat there was no significant influence of the material itself. It isworth remembering, however, that certain materials can providesuperior surface finish, and moreover, the dynamic performance isnot the only factor that counts in the choice of material. Duringthe study of constructive alternatives, some Venturis were made

40 000

35 000

30 000

25 000

20 000

15 000

10 000

5 000

010 20 30 40

Reference Venturi A

Orifice with diffuser G

Orifice F

50


Gas

-Flo

w R

ate

(m3 /

d)

60 70 80

Fig. 8Comparison between the reference Venturi A (blue line), the single orifice plate F (red line), and the orifice followed by a dif-fuser with a half-angle of 6 G (black line). Curves with 69-bar (1,000-psi) upstream gauge pressure.

80 000

60 000

Venturi A (reference)

Venturi D

Venturi C

40 000

20 000

080 90 100 110 120

Downstream Pressure (bar)

Gas

-Flo

w R

ate

(m3 /

d)

130 140 150

Fig. 9Performance comparison among Venturi A (blue line), Venturi D (green line), and Venturi C (red line).




with a stainless-steel housing and a tungsten carbide insert cover-ing much of the region of interest, from the inlet nozzle plane tothe beginning of the diffuser, a small distance past the throat. Theresults were very good, indicating that composite Venturis may beused. Surface treating (e.g., hardening of stainless steel) was alsotested, with excellent results.

The surface finishing of the Venturi is important and greatlyinfluences the degree of pressure recovery in the diffuser. This isone of the factors that should be considered when selecting thematerial and the manufacturing process. MFC-7M-1987 providessome values for surface finish and dimensional tolerances. It rec-ommends a maximum roughness of 0.5 mm, with a surface rough-ness of 0.1 mm or less being the best, giving an extra performancegain. MFC-7M-1987 also warns that the Venturi should beinspected visually and there should be no defects around the tran-sitions between the nozzle and the throat or between the throatand the diffuser.

The dimensional tolerance in the throat diameter is particularlyimportant for the Venturi valve, because it is designed to alwaysoperate in critical flow. For a desired tolerance of d% in the criti-cal flow rate, for example, the dimensional tolerance of the throatarea has to be d%; that is, (d/2)% in throat diameter. For a throatwith a 6.35-mm (0.25-in.) diameter, and a tolerance of 61% offlow rate, this means that the actual diameter of the throat must bebetween 6.32 and 6.38 mm, a variation of only 60.03. A dimen-sional tolerance of 0.05 mm or lower is recommended. This leadsto variations in flow rate within6 3% for the smaller throat diam-eters and 61% for the larger diameters. For a maximum variationof 61% of flow rate over the entire usual range of throat diame-ters, this tolerance should be 60.015 mm.

As the orifices in conventional orifice valves, it is natural thatVenturis are provided at discrete throat diameters at intervals ofapproximately 0.4 mm (1/64 in.) or approximately 0.8 mm (1/32in.). For orifice valves, such a diameter sequence works well.They operate in the subcritical regime and respond strongly tochanges in injection pressure, making it easy to obtain the desiredgas-injection-flow rate. Venturi valves are different because theyoperate in critical flow, and gas flow is much less sensitive to an-nular pressure adjustments. When the gas lift designer defines therequired gas-flow rate and, consequently, the throat diameter, he/she must choose the Venturi that is closest to the calculated one,considering the available-throat-diameter sequence from the man-ufacturer. This may mean a decision between injecting more orless gas than required within a relatively large range, which is par-ticularly problematic for small and intermediate throat diameters.A possibility is to manufacture tailor-made Venturis, but this maynot be practical commercially. Thus, a sequence that minimizesthe differences in gas flow between one diameter and the next

would be very useful. Instead of a sequence in terms of incre-ments in diameter, it is suggested a sequence that provides a vari-ation of approximately 15% in throat area (and, consequently, inflow rate) between one diameter and the following, which is rea-sonable in terms of minimizing the operating difference betweenthe calculated and selected throat diameter. It is also advisable tocheck the real throat diameter that corresponds to the nominal onewith the manufacturers because some differences may occur onthe basis of fabrication issues. In general, such differences arevery small, but for Venturi valves, it may represent a significantdifference in flow rates.

The Venturi element has a length greater than that occupied bythe traditional orifice plate. Thus, its adaptation in existing valveshas the requirement that there is space enough for a diffuser withsuitable length. If such space does not exist, the body has to beredesigned. Note that, as shown in Fig. 9 and the associated text,it is better to truncate the diffuser than to increase its angle.

A good Venturi design is not sufficient. The overall valve criti-cal ratio may be severely affected by local pressure losses at entryports and at check valve/exit ports. More and larger entry portsare required in comparison with conventional orifice valves. Theentry section of the valve body has to present an external diametersmaller than the rest of the valve body, allowing a sufficientlylarge annular area between the valve housing in the mandrel andthe valve itself. The check valve must have a gas-passage areathat is as large as possible.

The requirement to reduce flow restrictions in Venturi valvesnaturally led to the choice of a check-valve model in which thedart is positioned at the nose of the gas lift valve (externalcheck valve), which presents a much-lower pressure loss than theusual internal check valve of traditional orifice valves. Moreover,in the latter case, the dart rotates and vibrates and, frequently, itslegs break and the dart turns upside down, blocking the gas pas-sage (working as a check valve in the wrong direction). For wellswith high gas-injection rate it may happen in a relatively short pe-riod of time, causing a premature intervention that is very expen-sive in subsea wells (accompanied by significant production lossin the eventuality of a shortage of rigs).

The importance of a good check-valve design was demon-strated in Mendes and Almeida (2008) by use of the computa-tional-fluid-dynamics technique. Even for a well-designedexternal check valve in a 1.5-in.-outside-diameter (OD) Venturigas lift valve and usual operating conditions at Petrobras, the headlosses at this check valve become unacceptable for gas-flow ratesgreater than 300 000 m3/d (standard conditions). As expected, theeffect of the check-valve pressure losses is more important forlarger Venturi throats than for the smaller ones. Fig. 11 illustratesthe practical effect of the check valve in terms of flow

40 000

35 000

30 000

25 000

20 000

15 000

10 000

5 000

010 20 30 40 50


Gas

-Flo

w R

ate

(m3 /

d)

Reference Venturi A

Venturi with 1-mm throat

Venturi with 9.5-mm throat

60 70 80

Fig. 10Comparison between the reference Venturi A (blue line) and Venturis with cylindrical throat of 1.0 mm H (black line) and9.5 mm I (red line). Curves with 69-bar (1,000-psi) upstream gauge pressure.




performance for a Venturi valve. Although the global critical ratiowith the dart and spring is acceptable, it is clear that an improve-ment in the check-valve passage areas would result in a much-bet-ter performance.

Internal channels of the Venturi valve as a whole should alsobe expanded. It is also a good idea to review the mandrel designbecause larger entry orifices and gas-passage regions would prob-ably be beneficial, particularly for high gas-flow rates.

Some Venturi gas lift valves in the market have springs andother devices to help in closing the check valve and improve seal-ing. If the spring stiffness is excessive, there may be degradationof critical ratio. However, the effect of the spring stiffness on per-formance depends on the upstream pressure. This suggests thatstiffer springs can be used without problems at higher pressures,but a relationship of maximum stiffness as a function of operatingpressure is not yet established.

Besides the already-cited gas-flow-rate limit caused by check-valve restriction, the small space inside the valve also imposes athroat-diameter limit beyond which the Venturi valve perform-ance degrades, especially in terms of critical ratio. It is recom-mended, in the absence of other geometric limitations of thespecific valve model, to limit Venturi throat diameters to 8.0 mm(5/16 in.) for 1-in.-OD valves and 9.5 mm (3/8 in.) for 1.5-in.-ODvalves. Larger throats may be used, but the gas lift engineer has totake into account that it will probably result in critical ratios lowerthan 0.90. Alternatively, two Venturi valves may be used, divid-ing the flow rate. The use of two orifice valves to inject thedesired gas-flow rate is not new, but practice has shown that ithardly works, particularly if the valves depths are close in thewell. The dynamic behaviors of the valves interfere with eachother, impairing control, and the resulting gas-flow rate is in gen-eral unstable and smaller than previously calculated. Two Venturivalves in critical flow will not be affected in this manner, and thechances to work properly are good. Another possibility is the useof two Venturis in the same valve, one injecting upward and theother downward. However, these solutions introduce an additionalpossible failure or leak point in the tubing, and that concern mustbe evaluated. A better solution is to use 1.75- or 2-in.-OD valves,which allows for larger throats while still keeping goodperformance.

Despite all the preceding considerations, practice has shownthat it is advisable to conduct performance tests on every gas liftvalve to be run on subsea wells and compare the curves with pre-vious ones (i.e., with a signature of the valve). An alternative isto perform some basic tests and an in-depth inspection of eachvalve. Such procedures have identified many unexpected prob-lems before sending the valve to a well.

Conclusions

The Venturi gas lift valve is now standard equipment and maturetechnology at Petrobras. It has been preferred to orifice valves,particularly for offshore wells. In this authors opinion, the mainrestrictions to general use of Venturi gas lift valves are those casesin which there is a need for relatively large adjustments in gas-flow rate over time within a limited injection-pressure range, andthe change of the valve is not possible or economical.

The simple insertion of a careless Venturi-shaped piece inany commercial gas lift valve body does not result in a real Ven-turi valve. To be considered so and provide the expected benefits,the overall critical pressure ratio has to be at least 0.90 and thatobjective depends strongly on valve design. Guidelines were pre-sented here that help in a correct design of the Venturi itself andof the valve as a whole to attain that objective. Experimentalresults were also presented and show the influence of some impor-tant Venturi parameters on overall performance, supporting theseguidelines. The main design conclusions are

The recommended Venturi profile is the toroidal throat withk 4 and a 6 (see Fig. 3). Depending on the specific valvebody and the range of throat diameters, rg may be less than R /(k 1) and a relatively large flat portion at the inlet plane mayappear, which may degrade performance. In this case, it is rec-ommended to increase k. It is also recommended to maintainthe approach angle b at approximately 60 to 80 . If manufac-turing concerns lead to the adoption of the cylindrical-throatprofile (Fig. 4), throat lengths must be small, preferablyapproximately1 mm.

Good surface finishing inside the Venturi is essential. A maxi-mum roughness of 0.5 mm is recommended.

Slight variations in throat diameter may represent relativelylarge variations in gas-flow rates. Considering the usual throat-diameter range, a dimensional tolerance of 60.01 mm in all rel-evant profile dimensions is recommended.

The adaptation of an existing gas lift valve body requires roomfor a full-length Venturi. Then, redesign of the valve body maybe required. If space limitations persist, truncating a Venturidiffuser is better than increasing the diffuser half-angle.

Besides a good Venturi element design, it is crucial to havelarge gas-passage areas along the valve, especially at the checkvalve, which is a key element in terms of overall performance.All tests reported here were performed at Petrobras Gas Lift

Valves Test Unit, which is part of a field-scale multiphase labora-tory at Aracaju, Brazil. Experience has shown that it is worth-while for operators such as Petrobras to have expertise in keyartificial-lift equipment and its own experimental facilities tostudy and evaluate this equipment.

80 000

70 000

60 000

50 000

40 000

30 000

20 000

10 000

070 80 90 100 110

Without dart and spring

With dart and spring

With dart at half way

120


Gas

-Flo

w R

ate

(m3 /

d)

130 140 150

Fig. 11The three performance curves in this figure were obtained with the reference Venturi A inside the valve. The blue linerefers to the situation without a dart and spring inside the valves nose (no check-valve interference on gas flow). The black line isfor the usual valve assembly (dart and spring in place), and the red line is for an artificial situation with the dart locked at one-halfits way.




Nomenclature

For more clarity, please refer to Fig. 3.rb nozzle curvature radius, L, mmrg throat radius, L, mmR radius of the inlet section of the Venturi, L, mmx abscissa along the Venturi profile, L, mm

xb abscissa at the throat, L, mmxc abscissa at the tangency point, L, mmxt overall length of a full Venturi, L, mmy ordinate along the Venturi profile, L, mm

yb ordinate at the throat, L, mmyc ordinate at the tangency point, L, mma diffuser half-angle, degreesb approach angle at the nozzle inlet plane, degreesk ratio rb=rg, dimensionless

Acknowledgments

The author acknowledges Carlos A. Dias da Silva for the manu-facturing drawings and valuable suggestions and the crew of theGas Lift Valves Test Unit of Petrobras for the many required per-formance tests, in particular Fabio S. de Lima and L. Marlon R.Valenca.

References

Almeida, A. R. 2010. Practical Equations Calculate Gas Flow Rates

through Venturi Valves. Oil Gas J 108 (5): 4145.Almeida, A. R. 2011a. A Model to Calculate the Theoretical Critical Flow

Rate Through Venturi Gas Lift Valves. SPE J. 16 (1): 134147. SPE-

126184-PA. http://dx.doi.org/10.2118/126184-PA.

Almeida, A. R. 2011b. Advantages and Limitations of Venturi Gas Lift

Valves. World Oil 232 (7): 7179.API RP 11V2: Gas Lift Valve Performance Testing. 2001. Washington,

D.C.: American Petroleum Institute (API).

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Conference, Caracas, Venezuela, 2123 April. SPE-53959-MS. http://

dx.doi.org/10.2118/53959-MS.

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Approach to Field Surveillance Improves Efficiency in Gas Lift Opti-

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Petroleum Technology Conference, Kuala Lumpur, 35 December.

IPTC-12225-MS. http://dx.doi.org/10.2523/12225-MS.

Lyngholm, A., Opsal, M., White, T. et al. 2007. Technology Extends Gas

Lift Reach. World Oil 228 (4): 9598.

Mendes, R. and Almeida, A. R. 2008. Optimizing Gas Lift Equipment

with CFD Techniques. Presented at the 31st ASME/ALRDC Gas-Lift

Workshop, Houston, 48 February.

MFC-7M-1987: Measurement of Gas Flow by Means of Critical FlowVenturi Nozzles. 1987. New York: American Society of Mechanical

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ference and Exhibition, San Antonio, Texas, USA, 810 October.

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Alcino R. Almeida is a senior petroleum engineer with the Pet-robras Research and Development Center (CENPES) in Rio deJaneiro. He specializes in gas lift optimization, including new-equipment engineering, flow modeling, and software devel-opment. Almeida joined Petrobras in 1984 and CENPES in1988. He holds BS and MS degrees in mechanical engineeringfrom Federal University of Rio de Janeiro. E-mail: [email protected].

SI Metric Conversion Factors

bar 1.0* E 05 Pa

ft 3.048* E 01 m

ft3/sec 2.831 685 E 02 m3/s

lbm/hr 1.259 979 E 04 kg/s

m3/d 1.157 407 E 05 m3/s

psi 6.894 757 E 03 Pa

*Conversion factor is exact.



http://dx.doi.org/10.2118/126184-PAhttp://dx.doi.org/10.2118/53959-MShttp://dx.doi.org/10.2118/53959-MShttp://dx.doi.org/10.2523/12225-MShttp://dx.doi.org/10.2118/159645-MShttp://dx.doi.org/10.2118/36597-MS

spe-174092-pa

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