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Gas lift valve mechanics

The advent of the unbalanced, single-element, bellows-charged gas lift valve revolutionized gas lift application and installation design methods. The following topic describes the various types of gas lift valves and crossover seats currently used in the industry.

Contents

Unbalanced, single-element gas lift valves

The unbalanced, single-element gas lift valve is essentially an unbalanced pressure regulator. The analogy between these two devices is apparent in Fig. 1, where:

(a) Injection-pressure-operated gas lift valve and backpressure regulator responds to injection-gas pressure and upstream pressure, respectively

(b) Production-pressure-(fluid)-operated gas lift valve and downstream-pressure regulator respond to flowing production pressure and downstream pressure, respectively.

The closing force for a gas lift valve can be a gas pressure charge in the bellows exerted over the effective bellows area or a spring force, or a combination of both. The closing force for the regulator or gas lift valve can be adjusted to maintain a desired backpressure for injection-pressure operation. The regulator or valve remains closed until this set closing force is exceeded.

Generally, the major initial opening force for a gas lift valve is the pressure exerted over the effective bellows area minus the port area, and the lesser opening force is the pressure acting over the port area. In like manner, the major opening pressure for a pressure regulator is applied over an area equal to the diaphragm area minus the port area. The effect of the unbalanced opening force is far less for most unbalanced backpressure and pressure-reducing regulators than for gas lift valves. The reason is that the ratio of the port area to the total effective bellows area of a gas lift valve is much greater than the ratio of the port area to the total diaphragm area for most regulators. The operating principle remains identical for the gas lift valve and regulator, but the pressure applied over the port area has greater effect on the initial opening pressure of most gas lift valves.

Pilot-operated gas lift valves

There are numerous special application gas lift valves available. The operation of many of these unique valves can be analyzed using the static force-balance equations for the unbalanced, single-element, gas lift valve. The many different types of gas lift valves and the variation in calculations are not discussed in this section because of their limited application. However, one special-purpose valve of particular importance is the pilot-operated gas lift valve.

The pilot-operated gas lift valve in Fig. 2 has operating characteristics that are ideally suited for chamber installations and deep intermittent gas lift operations with low injection-gas operating pressure and small tubing in large casing. The pilot valve offers a very large main port with controlled spread and a predictable constant closing pressure. Spread is defined as the difference between the initial valve opening and closing pressures. This type of valve functions properly on time cycle or choke control of the injection gas. The pilot section operates in the same manner as a single-element gas lift valve with a small choke located downstream of the valve seat. The production pressure at valve depth is exerted over the ball/seat contact area of the pilot section as an initial opening force. When the pilot section begins to open, an increase in pressure occurs between the pilot valve seat and the main valve piston. This increase in pressure above the piston results in compression of the spring under the piston, and the main valve snaps open. An exceedingly high, instantaneous, injection-gas rate enters the tubing through the large main valve port. As the injection-gas pressure in the casing decreases from gas passage through the large main port, the pilot section begins to close. The pressure downstream of the pilot port remains approximately equal to the injection-gas pressure until the pilot port area open to injection-gas flow becomes less than the bleed-hole area in the main valve piston. When the pressure across the piston approaches equalization, the spring returns the main valve to its seat.

The closing pressure of a pilot valve is considered predictable because it is approximately equal to the theoretical closing pressure of an unbalanced, single-element gas lift valve. The pressure upstream and downstream of the pilot port is approximately equal at the instant the pilot section closes. Selecting the proper pilot port size controls the spread of a pilot valve. The high injection-gas throughput capacity of the large main valve port is unaffected by the pilot port size.

Valve specifications including full-open stem travel

Manufacturers publish gas lift valve specifications for their valves. Some manufacturers assume a sharp-edged seat for the ball/seat contact, and others arbitrarily add a small increase to the port ID to account for a slight bevel for the ball-seat contact. Because most manufacturers use the same sources for their supply of bellows, the effective bellows areas are considered the same. The generic gas lift valve specifications in Table 1 are representative of many actual unbalanced, single-element, gas lift valves. The theoretical fully open stem travel is not included in the valve specifications published by most manufacturers.

The stem travel required to fully open an unbalanced, single-element gas lift valve increases with port size, as illustrated in Fig. 3. The curves were calculated for gas lift valves with a square, sharp-edged seat and a ball on the stem that is 1/16-in. larger in OD than the bore diameter of the port. The calculated equivalent port area, before a valve is fully open, is based on the lateral surface area of the frustum of a right circular cone. The major area of the frustum is the ball/seat contact area, which remains constant. The minor area decreases with an increase in stem travel as the ball moves away from its seat.

Actual gas lift valve injection-gas rate throughput performance is seldom mentioned in published gas lift installation design literature. Gas lift valves with the larger ports may, or may not, fully open and have the predicted injection-gas capacity for gas lifting high-rate wells through large tubing or the casing annulus.

Gas lift valve port configurations

The port geometry and the maximum valve-stem travel affect the volumetric injection-gas throughput rate of a gas lift valve. Most gas lift valves have a polished carbide ball that is silver soldered to the valve stem. The valve seat can have a sharp-edged port or a taper. The chamfer may be very slight for breaking the seat line or may be of sufficient depth to assure that the ball remains in the taper for full-stem travel. A sharp-edged and a tapered seat with a 45° chamfer are illustrated in Fig. 4. Note that in

(a) The sharp-edged seat has an effective A p equal to the bore area through the seat

(b) The tapered seat has a 45° chamber measured from the horizontal (90° included angle)

The effective Ap in the Ap/Ab ratio is the ball/seat contact area and not the bore area through the seat. The example calculations in this section are based on the sharp-edged seat because the majority of gas lift valves in service have a sharp-edged seat or a very shallow chamfer for breaking the seat line. The calculations are basically the same for a sharp-edged seat and a seat with a shallow taper. The calculations for an equivalent area open to the injection-gas flow differ for a seat with a deep chamfer. There has been no standard angle adopted for the taper of a gas lift valve seat. Certain manufacturers use the same tapered seat for different stem-ball sizes, and the bore area through the seat may be the same. The area of the port used in the port-to-bellows area ratio must be redefined for a tapered seat when the ball/seat contact area is larger than the bore area through the seat, as shown in Fig. 4b. The Ap/Ab ratio is the ball/seat contact area, not necessarily the bore area through the seat, divided by the effective bellows area.

The specifications for the gas lift valve depend upon the ball size and the angle of the chamfer for valves with a port configuration similar to Fig. 4b. The selection of an angle for the taper, ball size, and the bore area through the seat can result in a ball/seat contact at the base of the taper. For this geometry, the bore area of the port would be used in the Ap/Ab term. The maximum stem travel in many gas lift valves with a deep taper is restricted to prevent the ball from pulling out of the taper, and the valve always remains in the throttling mode. A throttling mode implies that the generated area open to flow for the injection gas is less than the bore area through the valve seat. Certain types of gas lift valves with a deep tapered seat are designed to operate only in the throttling mode for continuous-flow application.

Crossover seats

Several types of gas lift valves have a crossover seat for a particular application. The crossover seat is designed to direct the downstream pressure into the valve body where the pressure is exerted over the effective bellows area less the ball/seat contact area. The upstream pressure is applied to the ball/seat contact area. The crossover seat in Fig. 5 is a schematic illustrating the principle of a crossover seat. A choke upstream of the port controls the maximum injection-gas rate and aids in keeping downstream pressure applied over the bellows area after the valve opens. An actual crossover seat has a group of bypass openings or a milled area around the main port. The total bypass area must significantly exceed the port area to ensure that a valve with a crossover seat will close.

An example of the need for a crossover seat is a production-pressure-operated gas lift valve installed in an injection-pressure-operated gas lift valve mandrel. Another application is a casing (annulus) -flow gas lift valve in a tubing-flow mandrel. In both examples, the gas lift valve is modified, rather than the mandrel. An example is a wireline-retrievable gas lift valve mandrel with pockets designed for injection-pressure-operated gas lift valves and tubing flow that has been installed in a well. The operator desires production-pressure operation. The solution is production-pressure-operated gas lift valves with a crossover seat.

Gas lift valves with a crossover seat are not recommended if the proper mandrels can be installed to eliminate the need for a crossover seat. The maximum port size is limited for valves with a crossover seat. This limitation can be very serious in wells requiring high injection-gas rates. Another problem with a crossover seat is the possibility of partial plugging of the crossover bypass area. The physical bypass area should be at least 100% greater than the valve port area because the bypass openings usually are smaller and more likely to plug than a valve port that can be opened and closed. The production-pressure-operated gas lift valve does not close at the design closing pressure when the crossover area results in a significant pressure loss. The pressure exerted over the bellows area is between the flowing-production and injection-gas pressures rather than at the lower flowing-production pressure.

Many production-pressure-operated gas lift valves with crossover seats can be choked upstream of the ball/seat contact area. The same port size may be used in all valves, and the volumetric injection-gas throughput, for the upper unloading gas lift valves, is limited by a choke size that is smaller than the port area. The small inlet chokes tend to reduce the valve closing-pressure problem associated with production-pressure operation.

Bellows protection

All reputable manufacturers of gas lift valves have provided bellows protection in the design of their valves. A bellows should be protected from a high pressure differential between the bellows-charge and the wellbore pressures and from the possibility of a resonance condition that can result in high-frequency valve stem chatter. The bellows-charge pressure is atmospheric pressure for most spring-loaded valves. The maximum pressure differential across the valve bellows occurs in most installations during initial unloading operations when the lower gas lift valves are subjected to exceedingly high hydrostatic-load-fluid pressures in deep wells.

Gas lift valve bellows are protected from high hydrostatic pressures by several methods:

  • Hydraulically preformed bellows by a high pressure differential, with or without, support rings within the bellows convolutions
  • A confined liquid seal in the bellows with full stem travel
  • Isolation of bellows from outside pressure with full stem travel

The primary purpose of these methods for protecting the bellows is to prevent a permanent change in the radii of the convolutions after installation in a well, which in turn, can change the operating pressure of a gas lift valve.

The possibility of a valve-stem chatter condition is not predictable. The evidence of valve-stem chatter is a bellows failure and a dished-out seat if the valve seat is not manufactured from an extremely hard material. Many gas lift valves have some form of dampening mechanism, and the majority of these devices operate hydraulically. The bellows are partially filled with a liquid, generally a high-viscosity silicone fluid. A restricted liquid-flow rate within the bellows or a fluid-shear dampening mechanism prevents valve-stem chatter.

Stabilization of test-rack opening pressures

One of the most important procedures for preparing gas lift valves for installation in a gas lift well is the stabilization of the operating pressure. The test-rack set opening or closing pressure of an unbalanced, single-element, nitrogen-charged or spring-loaded bellows gas lift valve should be stabilized before installation in a well. Many operators and manufacturers call the process “aging.” The purpose of this procedure is to prevent the set operating pressure of a valve from changing after being run in a well. Another term for valve operating pressure changes is valve set pressure “scrambling,” which may prevent a gas lift well from unloading, cause inefficient multipoint gas injection, or may cause an unpredictable variation in operating valve depth.

After setting the test-rack opening or closing pressure, the gas lift valve is placed in a high-pressure vessel filled with water. The valve is fully stroked several times by alternately increasing and decreasing the pressure of the water confined in the vessel. The exact procedure varies among manufacturers. Typically, the maximum pressure ranges between 3,000 and 5,000 psig, and the minimum number of cycles is between 5 and 10. The valve is removed from the high-pressure vessel, and the test-rack opening pressure is rechecked at the base setting temperature. If the opening pressure varies more than a specified few psi, the valve must be reset and the procedure repeated until the test-rack opening or closing pressure stabilizes. Also, the process identifies valve bellows and bellows weld failures.

Bellows-assembly load rate

Bellows-assembly load rate is defined as: the psi increase exerted over the bellows area per linear unit travel of the valve stem. The controlled pressure is applied over the entire effective bellows area, and the valve-stem travel is measured by means of a depth micrometer. A typical gas lift valve probe tester is shown in Fig. 6. [1] The bellows-assembly load rate is the slope of the pressure vs. stem travel best-fit straight line in the linear portion of the curve in Fig. 7. [1]

The best-fit straight line represents an average between the stem travel measured for increasing and decreasing probe-tester pressures. The increase in nitrogen-charged dome pressure with stem travel is negligible, as compared with the load rate of a bellows-assembly in most bellows-charged gas lift valves. The load rate of a bellows assembly, which is analogous to the load rate of a helical spring, is far greater than the effect of the increase in dome pressure resulting from the decrease in dome capacity for the stem travel required to open a typical gas lift valve.

The measured bellows-assembly load rate is not identical for all gas lift valves with the same size bellows. The typical three-ply, seamless Monel bellows that is used in many 1.5-in.-outer diameter (OD) gas lift valves has a reported effective bellows area of 0.77 in.2. The typical bellows-assembly load rate for a valve with a nitrogen-charged bellows ranges from 400 to 600 psi/in. in the linear portion of the curve for a valve with a test-rack opening pressure between 600 and 1,200 psig. The three-ply, seamless Monel bellows in the 1-in.-OD valve has a reported effective area of 0.31 in.2 and a bellows-assembly load-rate range of 1,200 to 2,200 psi/in. for a valve with a nitrogen-charged bellows and a test-rack opening pressure between 600 and 1,200 psig. The bellows-assembly load rate for a spring-loaded 1-in.-OD valve can range from near 2,000 to more than 3,500 psi/in. It is similar to the load rate of a spring. The load rate of a spring depends on the wire size, material, and number of free coils. The purpose in noting the magnitude of the bellows-assembly load rate for typical gas lift valves is to emphasize the fact that an unbalanced, single-element, gas lift valve will not "snap" open. An increase in injection-gas pressure, or in flowing-production pressure, or a combination of an increase in both pressures, is necessary to stroke the valve stem. The larger-OD gas lift valves should be selected for installations requiring high injection-gas rates because the smaller valves do not have the same gas throughput rate performance as the larger-OD valve with the same port size. Valves with the smaller bellows assembly are not recommended for low-pressure injection-gas systems that may be used to gas lift shallow wells. The low closing force and bellows stiffness can result in leaking valve seats because of poor ball/seat seating characteristics at low injection-gas valve opening pressures.

Static force-balance equations for unbalanced, single-element, bellows-charged gas lift valves

Most gas lift equipment manufacturers test-rack set valve opening pressures are based on 60°F for nitrogen-charged gas lift valves. The valve is submerged in a 60°F water bath to ensure a constant nitrogen temperature in the bellows of each valve during the test-rack setting procedure. The initial test-rack opening pressure is measured with the tester pressure applied over the effective bellows area less the ball/seat contact area while atmospheric pressure (0 psig) is exerted over the ball/seat contact area. The valve actually is closed and begins to open from an opening force that is slightly greater than the closing force. The tester gas rate through the valve seat is very low. Although most gas lift valves are set with an initial opening pressure, certain types of valves with high production-pressure factors and valves with unique construction may be set at test-rack closing pressures.

The test-rack closing pressure is obtained by bleeding the tester gas from the downstream side of a gas lift valve. This theoretical closing pressure is noted when the downstream pressure continues to decrease and the upstream pressures remain constant. The upstream and downstream pressures are equal momentarily at the instant a gas lift valve closes. An accurate closing pressure is more difficult to observe than an initial opening pressure and can be affected by the rate of decrease in the tester pressure during bleedoff of the tester gas. An encapsulating tester with gas capacity rather than a ring-type tester is recommended so that any small leaks in the tester piping will not prevent observation of the true gas lift valve closing pressure. The pressure should be bled off of the downstream side of the valve through a very small orifice.

The equations for initial valve opening pressure in a tester and in a well, and a tester closing pressure, are based on static force-balance equations. These equations also apply to spring-loaded gas lift valves. The spring-load effect replaces the bellows-charge pressure of the valve as the closing force. Several manufacturers with spring-loaded gas lift valves report a test-rack closing pressure. The spring is adjusted until the force exerted by the spring is equal to the desired test-rack closing pressure. A base-temperature correction does not apply to the opening- or closing-pressure calculations of spring-loaded gas lift valves. If the total closing force for a gas lift valve is a combination of a bellows-charge pressure and a spring load, the spring-load effect must be subtracted from the total closing force to obtain the bellows-charged-pressure portion of this closing force before calculating the bellows charge pressure from well to tester base temperature.

The following equations for the initial gas lift valve opening pressures in a tester and in a well are derived for a bellows-charged injection-pressure-operated gas lift valve because most gas lift installations are gas lifted with this type of valve In Fig. 8, shows

(a) The determination of the test-rack opening pressure, Pvo, by flowing supply gas at a low rate into a ring type tester with atmospheric pressure applied to the port area

(b) The test-rack closing pressure, Pvct, is obtained by opening the gas lift valve, closing the supply valve, and slowly bleeding off the encapsulating tester pressure downstream of the port.

(c) The initial valve opening pressure in a well, PoD, is based on the injection-gas and flowing-production pressures at valve depth. The injection-gas and flowing-production pressures are interchanged for production-pressure-operated (fluid-operated) gas lift valves.

Initial valve opening pressure in a tester at 60oF

(Fig. 8a) Closing force = opening force.

Vol4 page 0552 eq 001.png....................(1)

Vol4 page 0552 eq 002.png....................(2)

Vol4 page 0552 eq 003.png....................(3)


Valve closing pressure in a tester at 60oF

(Fig. 8b) Closing force = opening forces.

Vol4 page 0552 eq 004.png....................(4)

Vol4 page 0553 eq 001.png....................(5)

Vol4 page 0553 eq 002.png....................(6)


Initial opening pressure in a well

(Fig. 8c) Closing force = opening forces.

Vol4 page 0553 eq 003.png....................(7)

Vol4 page 0553 eq 004.png....................(8)


Solving for the injection-gas initial valve opening pressure in a well

Vol4 page 0553 eq 005.png....................(9)

Vol4 page 0553 eq 006.png....................(10)

Vol4 page 0553 eq 007.png....................(11)


Additional related valve mechanics equations

Vol4 page 0553 eq 008.png....................(12)

Vol4 page 0553 eq 009.png....................(13)

Vol4 page 0554 eq 001.png....................(14)

Vol4 page 0554 eq 002.png....................(15)

and

Vol4 page 0554 eq 003.png....................(16)

where

Ab = total effective bellows area, in.2,
Ap = valve port area (ball/seat line contact area for sharp-edged seat), in.2,
CT = temperature correction factor for nitrogen from PbvD at TvuD to Pb at 60°F, dimensionless,
Fp = production-pressure factor, dimensionless,
n = valve location designation (n = 1 for top valve),
Pb = nitrogen-charged bellows pressure at 60°F, psig,
PbvD = nitrogen-charged bellows pressure at valve temperature, psig,
Po = surface initial valve opening pressure, psig,
PoD = initial gas lift valve opening pressure at valve depth, psig,
Pot = tester pressure upstream of gas lift valve port, psig,
Ppe = production-pressure effect, psi,
PpfD = flowing-production pressure at valve depth, psig,
Ppft = tester pressure downstream of gas lift valve port, psig,
Pvct = test-rack valve closing pressure at 60°F if Ppft = Pot at instant valve closes, psig,
PvoD = initial gas lift valve opening pressure at valve depth if PpfD = 0, psig,
and
ΔPpe = variation in production-pressure effect, psi.

Initial opening and closing pressures of an unbalanced, single-element gas lift valve

An understanding of the relationship between the initial opening and closing pressures of an unbalanced, single-element gas lift valve is important for calculating gas lift installation designs and analyzing gas lift operations. An unbalanced, single-element, gas lift valve does not have a constant closing pressure as noted in many publications, and the valve does not "snap" fully open at the initial injection-gas opening pressure. This type of gas lift valve initially opens and closes at the same injection-gas pressure if the flowing-production pressure and valve temperature remain constant. In like manner, an unbalanced backpressure regulator opens and closes at the same upstream pressure if the downstream pressure remains constant.

Fig. 9 shows a plot of the initial injection-gas opening pressure vs. the flowing-production pressure curves for both a 1/4-in.- and 1/2-in.-ID sharp-edged port in a 1.5-in.-OD gas lift valve having an effective bellows area of 0.77 in.2. Most manufacturers use this bellows size in the 1.5-in.-OD gas lift valve.

The closing force for an unbalanced, single-element gas lift valve is assumed to remain constant for this analysis. The gas lift valve is actually closed on the line that represents a balance between the opening and closing forces in Fig. 9. The valve begins to open above the line and is closed below the line. The valve can be opened by increasing the injection-gas pressure with a constant flowing-production pressure, increasing both the injection-gas and flowing-production pressures simultaneously, and increasing the flowing-production pressure with a constant injection-gas pressure.


Production-pressure factor and valve spread

The production-pressure factor, Fp, is a relationship based on the effective bellows and ball/seat-contact areas for an unbalanced gas lift valve. Unbalanced implies that the flowing-production pressure is exerted over the entire ball/seat contact area as a portion of the initial opening force for a valve. In terms of gas lift valve operation, the production-pressure factor is the ratio of the incremental difference in the initial injection-gas opening pressures to a difference in the corresponding flowing-production pressures. If the flowing-production pressure increases, the initial injection-gas opening pressure decreases, and vice versa. The production-pressure factor can be obtained from the slope of the force-balance lines in Fig. 10 or can be calculated from the specifications for the valve.

Valve spread is defined as the difference between the initial injection-gas opening and the injection-gas closing pressures of a gas lift valve. The valve spread is zero for a constant flowing-production pressure because a valve initially opens and closes at the same injection-gas pressure. The valve spread observed in intermittent gas lift operations results from a large port and the change in the flowing-production pressure at the depth of the operating gas lift valve during an injection-gas cycle. The production pressure at valve depth approaches the injection-gas pressure beneath a liquid slug during gas injection, thus decreasing the valve closing pressure, which results in a spread between the initial opening and closing pressures of the operating valve. This can be a very important consideration for a chamber-lift installation where the initial opening pressure of the operating gas lift valve is high because of low tubing pressure. The operating gas lift valve is located above the chamber, and the tubing pressure, at valve depth exerted over the ball/seat contact area when the valve initially opens, is very low. The tubing pressure may approach the injection-gas pressure at the time the valve closes, thus resulting in a low closing pressure.

Injection-Gas Volumetric Throughput Rates for a Fixed Choke Compared to Unbalanced, Single-Element Gas Lift Valves

The difference in the injection-gas rate throughput performance of unbalanced, single-element, injection-pressure-operated gas lift valves and a fixed-size choke is illustrated in Fig. 11. The flowing-production pressure is a constant 500 psig. The gas lift valves have an injection-gas initial opening pressure of 1,000 psig. As soon as the injection-gas pressure exceeds 500 psig, injection gas enters the production conduit through the 1/4-in.-inside diameter (ID) choke. The injection-pressure-operated gas lift valves are backpressure regulators and metering devices because of the bellows-assembly load rate. These valves prevent injection-gas entry into production conduit until the injection-gas pressure exceeds the 1,000-psig set pressure. The difference in the injection-gas throughput performance of these two gas lift valves with the same 1/4-in.-ID sharp-edged port is from the bellow-assembly load rates. A greater injection-gas pressure increase is required to stroke the valve stem of the valve with the higher load rate.

The effective area of the bellows, the bellows-assembly load rate, the stem/seat configuration, and the linear valve-stem travel control the injection-gas throughput performance of a gas lift valve. The 1-in.-outside diameter (OD) valve with a 0.31-in.2 bellows area has the higher load rate of 1,800 psi/in., and the 1.5-in.-OD valve with a 0.77-in.2 bellows area has the lower load rate of 400 psi/in.


References

  1. 1.0 1.1 1.2 1.3 API Spec. 11V1, Specification for Gas Lift Equipment, first edition. 1995. Washington, DC: API.

Noteworthy books

Brown, K. E. (1967): GAS LIFT THEORY AND PRACTICE. Petroleum Publishing Co., Tulsa, Oklahoma.

Takács G. (2005): GAS LIFT MANUAL. ISBN 0-87814-805-1 PennWell Books, Tulsa Oklahoma, 478p.

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See also

Gas lift

Gas lift system design

Gas lift equipment and facilities

PEH:Gas Lift