Analysis of Gas Lift Installation Problems

Copyright 2000, Society of Petroleum Engineers, Inc.

This paper has been selected for presentation at the 9th Abu Dhabi International PetroleumExhibition and Conference held in Abu Dhabi, U. A. E., 15-18 October 2000.

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AbstractGAS Lift is an important mean of artificial lift method beingused for most of the producing wells of two major QGPCoffshore oil fields. This was best-suited method to keep andextend the plateau production rate as much as possible. Bymeans of a variety of field examples, this paper describes howquickly the trouble shooting action is required before gas-lifting problems arise.The proper analysis of a gas lift installation can becomeextremely important in determining whether or not it isperforming satisfactorily. The general opinion and acceptedpractice in most cases have been to wait until some type oftrouble develops before analyzing a gas lift installation.Preferably, the gas lift installation of each well should beanalyzed while it is performing satisfactorily. Not only willthis show whether or not the installation has been properlydesigned, but provide valuable information for futurereference in the event of trouble. Where improvement appearspossible, design changes may then be made with completeconfidence. If this information is not obtained, and the tubingand gas lift valves are pulled because of some trouble, achange of design, including such things as valve spacing,valve pressure, and type of gas lift valve would still involvesome guess work. There are many ways of analyzing gas liftinstallation problems. These can be divided into surface andsubsurface methods. Surface observation methods includebackpressure, total injected and produced gas volume as wellas operating gas injection pressure and its behavior analysis.

Subsurface methods include flowing pressure/temperaturesurvey analysis. Nodel analysis can also used as an effectivetool for evaluating the gas lift valves performance.

IntroductionGas Lift in general and as a system can be the source of manyproblems, but gas lifting installation problems have become avery important issue in QGPC and in the oil industry. A gaslift installation is designed to unload and work down to theoperating valve for continuos production in order to achievethe maximum production rate as shown in the typicalperformance curve for a gas lift well (Figure-1). In order tomaintain this maximum rate and get maximum revenue, wemust keep a gas lift installation operating correctly andefficiently. Continuos observation of surface indications ofthe well performance is necessary. Such observations andmonitoring include the continuos analyzing of well tests, gasinput volumes, and casing/tubing pressure charts as well asflowing and static bottom hole pressures and flowingcharacteristics.In order to properly evaluate the efficiency of the continuosflow of the well by gas lifting, it is necessary to analyze theinstallation performance on a regular basis. When theinstallation problems arise and cause decline in production,the problem should be properly analyzed and solved.An improvement of oil production could be made by bringingthe well back to maximum production rate. This paperdescribes and analyzes the problems associated with the gaslift well installations which as such are not performing asrequired, to get the maximum production rate. The usefulmain tools utilized to determine the trouble spots in theinstallation of gas lift well are:

1- Well head pressure recorder.

2- Subsurface pressure and temperature surveys.

Mazin Zain Al Abdin – QGPC, Doha, QATAR

SPE 87278

Analysis of Gas Lift Installation Problems

2 MAZIN ZAIN AL ABDIN SPE 87278

Gas Lift SystemThe flow path of a typical modern gas-lift system consists ofthe reservoir, perforations, tubing string, Side PocketMandrels which contain gas lift valves, wellhead regulator,surface piping, fluid separation facilities, compressor and gasinjection pipelines in addition to injection and productioncontrol. Lift gas is injected into the casing annulus where itpasses through downhole gas lift valves. Produced fluid andgas, along with injected gas, then flows into the separator.Produced oil is pumped to storage while injected andproduced gas is returned to the suction side of thecompressor. After the gas is recompressed, the rotative cycleis completed. Make up gas from another gas producing wellis normally being used for compressor start up. The typicalgeneral gas lift system is shown in (Figure-2).The main gas lift parts of the system considered in this paperare surface and subsurface installations. The surfaceinstallation include the wellhead casing control to regulatethe gas injection and wellhead tubing control to monitor theproduction and gas lift valve performance. The subsurfaceinstallations used for unloading the well for continuos gaslifting are the gas lift valves, located in the tubing string,sized and spaced according to the overall design. The typicalgas lifting well completion is shown in (Figure-3). Themethod of operation and type of installation depend largelyon the type of valves used. The valves are distinguished bytheir sensitivity to the casing and/or tubing pressure needed toopen and close them. Knowledge of mechanical operation ofgas lift valves is required in order to understand why thevalve operates badly under certain conditions in a well. Thereare many diagnostic tools available to assist evaluating gaslift installation performance. These tools can be usedindividually or collectively to give a picture of what ishappening downhole. The techniques to be considered aretwo-pen recorder charts and flowing pressure and/ortemperature surveys.

Gas Lift Problem AreasGas lift problems are usually associated with three areas:inlet, outlet and downhole problems as shown in (Figure-4).

The inlet problems are related to gas injection facilities suchas pipeline size restrictions, small/large choke size andhigh/low gas injection pressure and rate. The pipeline sizeshould be enough to handle the quantity of gas required forinjection. The choke size should be suitable to pass thequantity of gas required. The gas injection pressure and rateshould be according to downhole gas lift valves design andproduction optimization.

The outlet problems are related to fluid production. Thebackpressure should be kept to a minimum since thetransmitted pressure to the bottom of the hole reduces thedifferential into the wellbore thereby reducing production.The valves at the tree and header should be fully open without

obstructions in the pipeline from the wellhead to theseparator. This condition can sometimes prevent the wellfrom fully unloading, or cause heading. The separatorpressure should be maintained as low as possible.

The downhole problems are related to gas lift valvesperformance/efficiency. A passing gas lift valve is a typicalexample. Such valve does not close at the stipulated drop inthe casing injection pressure and as such a problem are lossof the dome pressure, valve stem stuck in open position due tocorrosion/mechanical failure and erosional wash out acrossthe stem /seat. Another example associated with the gas liftvalve problem is the failure of a gas lift valve to open at astipulated pressure. This problem is basically caused due toan excessive pressure in the dome, a situation which canresult due to an ingress of external pressure into the dome atthe time of tubing test.

Gas Lift ValvesThe schematic view of a typical subsurface gas lift valve usedfor gas lift is shown in (Figure-5).The dome is charged with gas prior to installing the valve tothe tubing string. The dome pressure is chosen in such amanner so as to cause the valve to open at specificcombination of casing pressure (Pc) & tubing pressure (Pt).When the gas lift valve is installed in a tubing string andplaced in a well, the bellows are subjected to forces caused bythe dome (F1), casing (F2), and tubing (F3) pressures. Whenthe forces acting to open the valve exceed those acting toclose it, the valve opens, allowing gas to flow through anorifice in the valve and into the tubing. (Refer to Figure 5).

Closing Force F1 = Pd. A1

Opening Force F2 = PC. (A1-A2) (Casing)

Opening Force F3 = Pt. A3 (Tubing)

Valve Closes when F1 larger than (F2+F3) and Openswhen (F2+F3) larger than F1.

The standard gas lift valves used by QGPC are the gasinjection and fluid – operated valves. The gas injectionpressure operated valve is a pressure regulator controlled bythe injection (casing) gas pressure applied to the outside ofthe bellows. A schematic of a typical pressure operated gaslift valve installed in the tubing string is shown in (Figure.6).Nitrogen is normally injected inside the dome and maintainsthe valve in the closed position until casing pressure iselevated to the valve opening pressure. The valve remains inthe open position allowing gas to pass from the casing to thetubing, until casing pressure is reduced to the valve closingpressure. The other type of gas lifted valves is fluid-operated

SPE 87278 ANALYSIS OF GAS LIFT INSTALLATION PROBLEMS 3

valve. A schematic of a typical fluid operated gas lift valveinstalled in a tubing string is shown in (Figure.7) The port isexposed to the casing pressure and the bellows is exposed tothe tubing pressure. The tubing pressure is acting to open thevalve and the casing pressure is acting on the seat area to helpopen the valve. The fluid operated gas lift valve design isespecially suited for dual completion.

Defining the Gas lift Installation problemsThe design objective of Gas Lift installation is to control thevertical fluid gradient so as to give the desired productionrate. The design is constrained by the limitations of gasinjection pressure, available gas volumes, the nature of theproduced fluids, the well’s inflow performance and the tubingsize. Installation design calculations are divided into twoparts. The first part is the determination of the gas lift valvedepths, and the second part is the calculation of the test-rackopening pressures of the gas lift valves. The test rack setopening pressures are calculated after the valve depthsbecause the operating pressure and temperature duringunloading is based on these valve depths. The installationproblems effect into the following: -

One) Bring a well into production and reach the deepestpossible injection depth in the most economical way.

Two) Tailor the amount of gas injected at the deepestpossible depth.

Three) Obtain the required optimum amount of oilproduction.

There are numerous ways of controlling the gas injection andfluid production. A choke is simply installed on the injectiongas line and sized to pass the desired volume of gas into thetubing string. When producing wells are completed or re-completed, the casing/tubing annulus is left full withcompletion or workover (kill) fluid. This fluid must beremoved, down to the depth of the desired operating gas liftvalve by a process called Unloading, before a well can beplaced on gas lifting. Also, whenever a gas-lift well is stoppedfor a period of time, the fluid level in the production stringwill rise to the level, which can be supported by the shut-inbottom-hole pressure. When a gas-lift well is placed back onproduction, the well must be re-started, by a process calledkick off, to resume gas-lift injection from the desiredoperating gas-lift valve. Care in unloading a gas-lift well isextremely important since more gas-lift valves are damagedat this time during the life of the well. Preventing excessivepressure differentials across gas-lift valve reduces the chancefor valve failure due to sand and liquid cutting.

The unloading process is as follows: (Figure- 8)

Static ConditionAs an example four-gas lift valves located in the string are

fully open. The well is full with kill fluid all the way to thesurface.

Start Unloading OperationsInjection gas is applied slowly through a choke to theannulus. Immediately the kill fluid will start coming out ofthe tubing string. As gas is continuously applied to theannulus a gradual increase in casing pressure is required tokeep fluid moving out the tubing string. Valve number 1 willeventually uncovered and gas is then injected into the tubingstring.

Lowering of Fluid ColumnGas will be continued into the casing annulus and the liquidin the casing annulus will continue to be lowered until valveNo .2 is uncovered and then gas will be injected through it.

Unloading Operations ContinuedGas is continuously injected until valve no.3 is reached andthe same event is repeated until valve no.4 is reached. Atsome point during the unloading process, the bottom holepressure will be lowered to the extent that fluids will startfeeding in from the formation.

Flow Condition StabilizedThe fluids in the tubing string will start being a combinationof U-tube fluid from the annulus and produced fluid from theformation. This will tend to slow the unloading process untilthe operating valve is reached. The casing pressure will bestabilized and the well will be placed on production.

Gas Lifting Installation problems AnalysisThere is a natural tendency to increase gas injection when awell is not making its required production and to leave theinstallation alone when the well flow appears reasonable. Inmany of these instances, considerable improvement can bemade both in the rate of production and in the injection gasusage by a brief analysis of the gas lift installation.

The following list summarizes the various measurements usedto analyze a gas-lift installation.

1- Recording surface pressure in tubing and casingRecording the surface tubing flowing and casing gas injectionpressures can be done with two pen recorders. Theserecorders will record on a chart any change in surface tubingflowing and casing gas injection pressures. A change in therecorded pressure will usually be significant and frequentlywill indicate some physical change in the system. Increasedflowing tubing pressure indicates increase restriction in theflow line or an increase well pressure from the well flowingor from gas injection. Decreased tubing pressure indicates adrop in supply gas pressure or volume. A change in thecasing gas injection pressure indicates gas lift valveperformance.


2- Measurement of gas volumeInjection gas volume to be measured to determine gas-liftefficiency.

3- Surface temperature readingsWellhead temperature measurement is required because of theeffect of temperature on the operating characteristics of mostgas lift valves.

4- Visual observation of surface installationSome factors that can be determined by visual observation ofthe surface installation as are long flowline, high separatorbackpressure, and restriction in wellhead and flowlines, gasfreezing and restriction in gas injection lines causing lowinjection pressure and volume.

5- Well tests for oil and water productionWell test to measure oil and water production providesimportant parameters for determining the efficiency of a gaslift operation.

6- Subsurface static and flowing pressure surveysThe best and most widely used method of properly analyzinggas –lift installation. Static bottom hole pressure willdetermine static fluid level, static gradient. A flowingpressure and temperature survey, on the other hand, willlocate the point of gas injection, leak in the tubing, valvefailures and multi-point injection. A flowing pressure surveywill also determine the flowing gradient above and below thepoint of injection.

7- Subsurface temperature surveysA flowing temperature survey can locate valve performancefor proper design of gas lift valves.

8- Fluid level determinationFluid level is required to locate the operating valve.

Methods of Problems AnalysisThere are many ways of analyzing continuos flowinstallations. These can be divided into detailed andobservation methods. The detailed methods include analysisof surface recording of casing and tubing pressures,subsurface flowing pressure and temperature surveys. Theobservation methods include surface backpressure, total fluidrecovery, Injection and total produced gas volumes,temperature of flow lines, operating injection pressure and itsbehavior.

The tools used for detail installation problems analysis are asfollows:

1-Two- Pen Pressure Recorder ChartsThe two most significant pressures acting on any gas liftvalve are the production pressure and the gas injectionpressure. From the recorded pressures at the surface(recorded on a two-pen chart), the respective downholepressures existing in the tubing and casing can be calculated

and compared with the operating characteristics of the type ofgas lift valve in service. From this information, it is possibleto estimate the point of operation. Two-pen recorder chartscan be used to optimize surface controls, locate surfaceproblems, as well as identify downhole problems. The two-pen recorder connected to the wellhead as shown in (Figure-9). Chart interpretation can be useful in identifying andremedying such installation problems.

(See Appendix-1 for more details and examples)

2-Flowing Pressure/Temperature SurveysIn this type of survey, pressure/temperature-recordinginstruments are run in the well under flowing condition whilethe well is being tested. The pressure/temperature recordinginstrument is stopped above and under/ or below each gas liftvalve for a period of time, and records the pressure/temperature at each valve. Making additional stops betweenvalve stations can be helpful in plotting and interpreting thesurvey data. From this information the exact point ofinjection can be determined as well as the actual bottom-holepressure and temperature. This type of survey is the mostaccurate way to determine a gas lift well’s performance.


Problems Analysis by PSMProduction System Models integrate reservoirs, wells,gathering systems and process constraints into a single modelproviding a practical tool for production optimization. Thetool allows analyzing fields as integrated systems enabling theeffects of each component of the systems to be evaluated as apart of the big picture.In addition to normal production optimization for the two offshore fields, the system was used successfully to help solvinggas lift installation problems. Prosper Quick look is a gas liftdiagnostic tool for checking performance of unloading valvesand identifying injection points. Quick look can quickly checkpossible causes and identify the correct remedial action.


Conclusions1-The proper analysis of gas lift installation can becomeextremely important in determining whether or not it isperforming satisfactorily.

2-The problems discovered and improvements made afteranalyzing the installation are:

A- Locating multi-point injection.B- Valve leaks.C- Respacing valve to lower operating point

3-One of the first checks on any installation that is notperforming satisfactorily is the injection gas pressure beingutilized in the well head. A low gas injection pressure will


explain low production rate or tubing or valves leaking.

4-Semi-closed installation is recommended for mostcontinuos flow installations. When a packer is used, thecasing is only unloaded initially and when the well is shut-in,the fluid level in the casing remains constant therefore bettercontrol injection.

5-The large tubing size used for gas lifting increases theproduction capacity specially when the flowing densitydecreases.

6-The small tubing used for gas lifting decreases theproduction capacity but reduces the gravitational effectassociated with lower producing rates.

7-Small port size selection for pressure operated valves is notrecommended due to the following reasons: -One) Can prevent unloading.Two) Loss of energy by requiring a higher operating

injection gas pressure than would be required.Three) More than one valve to pass adequate gas volume

requires lifting the desired producing rate, which resultsin inefficient multipoint gas injection.

Four) Port can plug easily, particularly if the casing isscaly or the well has a deposition problem.

Five) Improper surface control of injection gas.

8- Flowing gradient surveys results define what is occurringin the system at a given set of conditions. Such predictionscan be made for well performance under a different set ofconditions.

9- When choke installed in the injection gas line, some trialand error adjustments may have to be made to obtain thecorrect size of the choke.

10- The accurate prediction of the flowing production fluidtemperature at valve depth is important in the design andanalysis of many gas lift installations with nitrogen chargedgas lift valves.

11- Two – pen recorder charts can also be used to optimizesurface controls, locate surface problems as well asidentify downhole problems.

Recommendations1- The new installation of each well should be analyzed

while it is performing satisfactorily.2- It is essential that the flowing gradient survey must be

initiated under stable flowing conditions and the testperformance must accurately duplicate normal producingcondition

3- A check on total fluid production along with ameasurement of the input and output gas liquid ratios,and a recording of casing and tubing pressures should be

made simultaneously with the pressure survey.4- Pressure and temperature curves should be plotted on

suitable graph paper to make complete analysis.5- Pressure and Temperature surveys should be run before

trouble develops in order to have the informationnecessary for over-all installation improvement.

6- Fluid operated valves should utilize the smallest portpossible.

7- Prevent continuos passage of liquid through the valvesbelow the fluid level to avoid damage to the valve seats.

8- Flowing gradient stops should be made directly belowand above each gas lift valve.

9- Direct connection from compressor or distribution pointto each well should be recommended for having morecontrol in injection and monitoring.

10- The ascertain the accuracy of the two-pen recorder itshould be calibrated periodically by using high resolutionpressure gauges or dead weight tester.

11- The two-pen recorder pressure chart reading should beinterpreted for the underlying causes of fluctuations inthe production pressure and injection gas pressure ratherthan just attempting to compare chart patterns from otherwells.

AcknowledgementsThe author wishes to thank the Qatar General PetroleumCorporation and Mr Ahmed A. Al-Siddiqi the Manager,Reservoir and Field Development for permission to publishthis paper. He also thanks the individuals who contributed tothe practical and theoretical comments related to this paper,especially Mr. Talib Al-Salat Head of Subsurface Studies forhis continuous support and valuable comments andinstructions.

References1- Brown K.E: The Technology of Artificial lift

Petroleum Publishing Co. Tulsa2- Brown K.E: Field Operation Handbook”

OTIS Engineering Corporation3- Winkler, H. W and Smith: “Camco Gas Lift Manual”

CAMCO Inc., Houston.3-D.R. Skiner “ Introduction to Petroleum Product “

Volume 24- Exxon Product Research Company, Houston, Texas “Typical Two- Pen Pressure recorder Charts from gas lift wells”5- “Gas Lift Production Technology” CAMCO6- H. Dale Begged “Production Optimization Using

NODAL Analysis”7- Cravo Norte Block “Well Completion and Artificial Lift

Study”.8- John Martinez “ Gas Lift Analysis and Troubleshooting”



















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Analysis of Gas Lift Installation Problems

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