Mehran University, Jamshoro, amjadspe@hotmail.com

OPTIMIZATION OF GAS WELL PRODUCTIVITY BYCONTROLLING WATER PRODUCTION

Amjad Hussain Shar

ABSTRACT

It is seen that gas wells if produced without plannedrates may produce water pre-maturely. Production ofwater basically loads the subject well by generatinghydrostatic head within the well tubing. Secondly thewater production alters the in-situ gas saturation in nearwell bore region resulting change in the relativepermeability. Both these reasons make the subject wellproblematic due to decrease in well productivity andincrease in the well operating cost. Also the handlingof produced water is uneconomical in many cases.

The work done for this study is general in nature, thoughthe essence of this work comes from data collectedfrom different gas wells having water productionproblem. This study gives an overview of all the methodsand techniques, which can be used for, expel-out suchproblems and optimize gas production. Main stress hasbeen given on Tubing Performance Curve study in orderto select best tubing size and calculation of the optimumrates against selected tubing size. General tubingperformance curve are also presented showing thebehavior of the well before and after water production.

On the basis of study of different techniques related tothe Gas well production optimization and waterproduction control we became, able to give a solutionof interchanging the well tubing strings withoutpurchasing any new string pertaining to large gas fieldswhere different tubing sizes are used in different wells.

INTRODUCTION

It is considered that wells producing dry gas have usuallylower flowing bottom-hole pressure [1]. Liquid loadinghappens when the gas does not have enough energy tocarry the water out of the wellbore. Water accumulatesat the bottom of the well, generating backpressure inthe wellbore and blocking gas inflow [2]. For theoptimization of the production of the gas it is necessaryto unload that water from the wellbore so that there

should occur a decrease in the backpressure of the staticwater column. The gas well loading phenomenon isone of the most serious problems that reduces, andeventually kills the production in gas wells [3]. Thevertical flow in tubing changes to the slug flow, andthe consequence of this slugging for a well is to havelarger pressure drop which reduces the well productivity.The total pressure drop I equal to the sum of pressuredrops due to Elevation (hydrostatic column pressure),friction and acceleration [1]. The importance of theterm elevation increases due to the accumulation of thewater in the well bore column. Friction and accelerationterms are well production rate dependant as for lowerrates these posses’ smaller values and for higher ratesthese become significant.

Fig. 1 shows the affects of gas flowing velocity onflow regimes in vertical tubing. Fig. 2 (a) shows themechanism of liquid loading wells. This figure clarifiesthat once the water enters into the wellbore, how itchanges the flow regime and Fig. 2 (b) shows the stagesof decreasing production.

Planning of proper tubing size can save from abovewater problems hence one can claim that the wellproduction can be optimize [2]. This study gives thetubing planning idea for larger gas fields which havemany wells with varying production-period and welltubing sizes. The idea based on this study is ofinterchanging the tubing sizes between the wells withoutinvesting money for new tubing strings. This paperdiscusses the critical flowing velocity and well tubingperformance. On the basis of these studies suggestionsand recommendations in general are made.

TECHNIQUES OF HANDLING WATERPRODUCTION

Production of water from Hydrocarbon producing wellsis always required to be avoided. There are many reasonswhy this is done; such as handling of produced wateris a big problem. The water contains contaminants

SPE/PAPG ANNUAL TECHNICAL CONFERENCE 2005November 28-29, 2005, Islamabad

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in abundance and chemically it cannot be drained incultivated land, near populated area or in any pond,river, watercourse, where people use water for drinkingpurposes. For making it utilizable, it needs muchexpensive treatment.

Besides all that, production of water makes economicloss by corroding production and facility installations.This is not the end, on the basis of study of large samplestaken from gas wells producing water; it was found thatnear about 20% of original reserves has been reduced,due to water production problems (National EnergyBoard for Canada, 1995).

Different well dewatering technologies are available inthe industry and some have been used successfully tocontrol water loading problem in gas wells.

1. Gas Compression Technology2. Liquid Diverters3. Gas Lifts4. Foaming5. Beam Pumping6. Flow Controllers7. Swabbing8. Coiled Tubing/Nitrogen9. Venting10. Plunger Lift11. Concentric Tubing String

Some of the above methods are discussed here, in orderto get comparative idea of dewatering the wellbore tooptimize production.

Gas Compression Technology

By introducing wellhead compression, well head flowingpressure can be decreased and the required gas deliverypressure can be achieved by compressing the gas to adesired pressure. This decrease in well head pressureincreases the gas velocity to an extent that it also carriesout with it the loaded water when the well is put onproduction. In this way the productivity of the well isincreased for the time being.

Well head compression considerations are given below:

v Will well head compression increase the rateeconomically and provide long-term effects?

v Match compression with the Tubing Performancecurve

v Compression should be applied on a group ofwells or on a large field.

v Type of compressor to be selected for throughput,

intake pressure, liquid tolerance, durability andbest economics.

Gas Lifts

Gas lift technology is applied for reducing the amountof water accumulated in the wellbore. This methodintroduces the additional gas in the tubing string inorder to decrease the flowing gradient so that increasedfluid velocity can be achieved. The method of injectingthe gas in the well bore is to inject gas through casingand allows it to flow in the tubing by means of gas liftvalve [1]. Fig. 3 shows dewatering of well by gasinjection technique.

There are certain considerations for the installation ofthe gas lifting system for water removal in order tooptimize the gas production:

v Comparison of costs with wells should be madev It must be assured that high pressure gas injection

sources are available within approachv Entire field well-to-well cost comparison should

be made

Foaming

Foaming is introduced to decrease the hydrostatic headof the well column. This method is for temporarytreatment. Which optimizes production of gas from awell only for a short interval. The mixing of liquid/ gas/surfactant usually occurs downhole. This method worksfor water only. Foaming is not the best economic solutionin case if larger quantities of the surfactants are requiredto decrease the fluid pressure gradient. The foamproduces a less-dense mixture by increasing the surfacearea of the liquid with bubbles.

There are various methods of introducing surfactantsinto the wellbore and the simplest method is to batchor continuously inject chemical down the annulus ofthe wellbore. Also soap sticks can be dropped into thewellbore tubing for this objective [1]. Fig.-4 showsdewatering of well by foam injection technique.

Beam Pumping

Beam pumping systems are a common method ofdewatering gas wells. The application of this system isfor the conditions when there is high Gas Liquid Ratio.Gas may cause gas lock in the pump, which in turncan cease the production [1]. If the pump is correctlybuilt and spaced in order to obtain high compressionratio (CR) on the down stroke, as Fig. 5 shows.

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Swabbing

Swabbing is used for unloading the well. This isgenerally used during initial well operating conditionswhen the well is killed. Small swabbing cups are usedand well unloading is carried out, which causes decreasein hydrostatic pressure and the well flows. This job iscyclic by nature in a way that for a time-being well isunloaded and flows till next unloading time.

Coiled Tubing/Nitrogen

Nitrogen kick-off job is done for unloading wells bymeans of coil tubing. Nitrogen once enters into theliquid column reduces its density and so well startsflowing for some time.

Venting

Gas venting is made for liquid unloading as a cheapmethod. This usually does not need any extra mechanicalequipment at the surface for de-watering instead a gasvent line should be available. Well is opened by chokeand flow is diverted to atmosphere. Well can be flowedtill it becomes clear. This method can also be called asthe cyclic dewatering of the well.

Velocity String

The installation of velocity string represents a thirdpossible solution to the liquid loading problems in big-bore completion [2]. The velocity string will maintainhigh gas velocity, which will keep the well from dyingprematurely. Big bore completions have lower gasvelocities than those with smaller bore.

Installation of small tubing string in existing larger borecompletion will reduce diameter and hence there willbe increased velocity which keeps the well flowing atoptimum rates, preventing the well from loading.

CRITICAL GAS VELOCITYCALCULATION FOR WELL UNLOADING

It is very difficult to determine the flowing velocityvalues, so as to save the well from loading with waterand to produce it at optimum rates.

Turner et al established a method [1] of determiningthe minimum fluid velocity value for continuous wellunloading. This method is dependent on the model ofentrained liquid droplets in a high velocity gas stream.The basis of the calculation was a larger possible dropletwhich was needed to be flowed from well. The velocitycalculation on the basis of existing interfacial tensionwas calculated.

The equation suggested by Turner is given below [4]:

Vt = 1.915 [{s (rl – rg)} 0.25 / (rg0.5)] …..(1)

Where,V = Terminal Velocitys = Interfacial Tensionr = Density, subscripts t, l, g are terminal, liquid, andgas respectively.

It is observed that heavier phase always dictates thelimiting velocity, so incorporating the water propertiesin above equation we can determine the flow rate. Weknow that flow rate determines the combined effect ofvelocity and the area of the conduit.

Qg = 3056 (P Vt Ap ) / (Tabs Z) ……(2)

Where,Q = Rate of flowP = PressureV = VelocityA = AreaT = Temperature, andZ = Gas Deviation FactorThe Critical Gas Velocity and Flow rate Calculationsfor two wells, XX and XY are shown in table 1.

TUBING PERFORMANCE CURVES

The generalized form of Inflow performance relationship(IPR) and Tubing performance relationship (TPR) isshown in Fig. 7.

The performance curve shown in Fig. 8 is basically forthe same well to show the effect of the water production.After a year of water production, there has been a declinein the performance curve as shown in Fig. 8. The effectof altering Tubing String diameter on optimum flowrate is shown in Fig. 9.

EXAMPLE OF WATER PRODUCTION

After the collection of data regarding the waterproduction for a well, it was plotted and a typical plotwas found. This plot (Fig. 10) represents complexbehavior. First production of water increases and it getsa peak value of 1000gals, (note all values of water aregiven in unit of gallons), then it decreases and comesdown to about 170 gals as shown in Fig. 10. But thisis not the end, after this again water production increasesbecomes about 700 gals. The reason for decline in thewater production is that well started load up and ratedecreased. It was assumed that now the velocity of gasis not sufficient and hence it flows out at surface

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by channeling. This happened for a short period. Lastlywell was flowed in open atmosphere and it wasoffloaded. After its choke was decreased from 48/64 to24/64; the reduced choke caused the well to controlwater production. It was then found that the gaschanneling was because of larger tubing size; insteadif tubing size has been smaller than there must not havebeen water flow at the very starting period of the wellflow.

CONCLUSION

From the study of different gas well unloadingtechniques, we came to know that there are cost effectivemethods and economically feasible in most of theconditions as gas venting in atmosphere (in case if itcontains contaminants or diluents then venting shouldbe through flare), well shut-off etc, all these are for alimited time interval.

Among the discussed methods, the Interchanging ofthe tubing string between two or more wells will be themore suitable decision for optimization of gas wellproductivity for controlling the water production. Thewell, which is producing water, should be equippedwith smaller ID tubing string. The gas velocity willincrease in this tubing string, and the liquid liftingpotential will increase which in turn will prevent thewell from load-up. When the well is not loaded we willget increased production.

RECOMMENDATIONS

1. On the basis of above study made for productionoptimization of wells, which produce water, it isnecessary to determine the exact water productionvolumes and rates should also be known.

2. Water production monitoring can be made bymeans of Flocometers, which note theapproximate water volume passed through it.

3. Simplest method should be utilized to minimizethe production of the water.

4. Among the analysis of certain methods (as shownin Fig-6) there are cost effective methods andeconomically feasible in most of the conditionsas gas venting in atmosphere (in case if it containscontaminants or diluents then venting should bethrough flare), well shut-off etc, all these are fora smaller time period.

5. The best and cheap method keeping wellfluids to be produced at sufficient rate is theinterchanging of the tubing string between twoor more wells.

6. The well, which is producing water, should beequipped with smaller ID tubing string. The gas

velocity will increase in this tubing string, andthe liquid lifting potential will increase which inturn will prevent the well from load-up. Whenthe well is not loaded we will get increasedproduction.

ACKNOWLEDGEMENT

The author is highly grateful to Dr.Hafeez-ur-RehmanMemon, Professor, Institute of Petroleum and NaturalGas Engineering Mehran University and Assistant Prof.Mr. Shahzad Ali Baladi for providing their guidanceand technical support.

My profound thanks to Prof. Sikandar Ali Arbani(Director Instt: of Petroleum and Natural Gas Engg:Mehran University Jamshoro) and to all the teachersfor their support and encouragement, which has doneso much good to me.

REFERENCES

[1] Robert P. Sutton, Stuart A. Cox, E. Glynn Williams:“Gas Well Performance at Sub critical Rates”. SPE80887.[2] John Lee, Robert A. Wattenbarger, “Gas ReservoirEngineering”.[3] James F. lea, Henry. V Nickens,: “Solving Gas –Well Liquid – Loading Problems”, SPE Distinguishedauthor Series.[4] Fitrah Arachman, Kalwant Singh, James K. Forrest,Monas O. Purba, “Liquid Unloading in a Big BoreCompletion: A comparison among Gas Lift, IntermittentProduction, and installation of Velocity String” SPE88523.[5] Benesch, J.M., Nor, Nazri, and Ngatijian:“Optimization of Big-bore HPHT Wells to exploit aLow Pressure Reservoir in Indonesial,” paper SPE/IADC87171.[6] M.A. Nosseir, T.A Darwich, M.H Sayyouh, M. El,:“A new Approach for Accurate Prediction of Loadingin Gas Wells Under Different Flowing Conditions”,SPE Prod. & Facilities 15 (4), November 2000.[7] Turner, R.G., Hubbard, M.G., and Dukler, A.E:“Analysis and Prediction of Minimum Flow rate forthe continuous Removal of Liquids from the Gas wells,SPE 30197.[8] H. Dale Beggs, "Gas Production Operations", OGCIPublications.

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Fig. 1 - Flow type variation due to gas production.

Fig. 2(a) - Phenomenon of Well Load – up by Water.

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Fig. 2 (b) - Phenomenon of Formation of Heavier Slugs till Well Load up.

Fig. 3 - Dewatering of Well by Gas InjectionTechnique.

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Fig. 5 - Plunger Lift Method.

Fig. 4 - Dewatering of Well by Foam Injection Technique.

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Fig. 6 - Comparative analysis of some methods for dewatering a Well.

Table - 1: Critical Gas Velocity and Flow Rate Calculation.

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Fig. 8 - Well Inflow Performance Curve Variation after well-started water production.

Fig. 7 - A generalized Form of IPR and TPC.

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Fig. 10 - Month wise water production of well XX.

Fig. 9 - Effect of varying Tubing Sizes on Well Deliverability.

Month wise water production of well XX

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ABOUT THE AUTHORS

Amjad Hussain

Amjad Hussain is thestudent of Petroleum andNatural Gas Engineering,currently studying in Thirdyear in Mehran University ofEngineering and TechnologyJamshoro, Pakistan. In 2004(June-July 2004) worked withMari Gas Company Limitedfor 6 weeks as an Internee and with OGDCL for 2 weeksas an Internee in December 2004. He is also workingas President 2005-06, for SPE Mehran Student Chapterat Mehran University of Engineering and TechnologyJamshoro, Pakistan.