Towards Zero Flaring

An important issue in protecting theenvironment of the Gulf, which is no lessfragile than elsewhere in the world, is theeffect of hydrocarbon flaring from oilproduction operations. This causes manyforms of pollution – noise, toxic gases, soot,acid rain and the production of carbondioxide, the latter is one of the primarycauses of global warming.

In this article, Alp Tengirsek and NashatMohamed explain the progress of a projectin Abu Dhabi that has already eliminated oil flaring during testing and production,with the ultimate aim of eliminating allhydrocarbon flaring within a year.

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The first stage of the plan was toreduce oil burning by reinjecting oilrecovered from water back into theproduction stream. This wasintroduced successfully to threeADMA/ZADCO offshore rigs in 1998.To recover the oil, cleanup fluids wereused to reduce water acidity topH 5.5–6, thus avoiding damage toservice providers’ equipment, subseapipelines and onshore processingfacilities. The separated oil was thenreturned to the production pipeline.This first stage of the zero-flaringinitiative, however, did not addressgas flaring, residual fluids andseparated water that still containedspent acid and oil and continued to bedischarged into the sea. In this stageof the initiative, measurement of flowrates continued to be made byseparators and gauge tanks. This wasan incomplete solution requiringconventional cleanup.

The second stage virtuallyeliminated oil burning (Figure 1.2).This was introduced to the same rigsbetween February and March 2000.Acid in the effluent discharged fromthe well during cleanup wasimmediately neutralized using sodiumcarbonate, which allowed oil to beinjected safely into the production

Oil to production lineGas to burner

Separator Oil

Oil to flare

Transferpump

Well effluents

From wellhead

Chokemanifold

Surge tank

Contaminated waterto sea

Oily water

T he modern states of the Gulfregion are fortunate in having

inherited an untainted environmentallegacy. The recent, bold developmentof their industrial base has taken intoaccount the lessons learnt by otherregions that have undergoneindustrial revolutions. Pristineexpanses of land and sea are visibletestimony to their success to date.

However, the environment of theGulf is no less fragile than elsewherein the world. In order to preservetheir natural resources, environmentministers of the 21 Arab countriesmet in Abu Dhabi, February 2001,at the Gulf Cooperation Council’s(GCC) 19th Summit to define anenvironmental strategy for the region.The result was the adoption of theAbu Dhabi Declaration, which laysdown a strategy for the sustainableuse of the region’s environmentalresources in the 21st century.

An important issue the Abu DhabiDeclaration addresses is hydrocarbonburning or flaring from oil productionoperations. This causes pollution –noise, toxic gases, soot, acid rain andcarbon dioxide, the latter being one ofthe primary causes of global warming.

Rising sea levels and potentialflooding due to global warming areparticular concerns for the countriesof the Middle East. The low-lyingcoastal areas of the United ArabEmirates, and areas such as theheavily populated Nile Delta in Egyptand the lower reaches of the Tigrisand Euphrates river systems in Iraqare under threat from rising sea levels.

Recognizing and anticipatingmounting international concerns overenvironmental issues, the Abu DhabiMarine Operating Company (ADMA-OPCO) and the Zakum DevelopmentCompany (ZADCO) have achieveddramatic reductions in hydrocarbonflaring from on- and offshore oil fieldssince 1997.

The companies formed a joint teamand, with the help of Schlumberger,developed a multistage plan forreducing flaring emissions during wellcleanup and well-testing operations.The ultimate aim is to achieve zeroflaring within a short but realistic timeframe. Zero flaring will also enablesignificant cost reduction for cleanupoperations by allowing immediate flowof backflowed effluents through thetest separator.

Countdown to zeroDuring conventional cleanup and well-testing operations (Figure 1.1), bothoil and gas are burned away into theatmosphere. Water, collected by thetest separator and then dischargedinto the sea, may contain acid used to stimulate production fromcarbonate reservoirs that could causecorrosion problems in the pipeline.The discharged water also containsunacceptable oil levels. Each of these –oil, contaminated water and gas –represents an environmental hazardduring well testing and cleanup.

Figure 1.1:Conventional cleanupand well-testingoperations aredetrimental to theenvironment.Oil and gas areburned and releasedinto the atmosphere,while contaminatedwater is dischargedinto the sea

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Oil to production line

Oil and gas to production line

Oil and gasPhaseTester X

FRAMOmultiphase pump

From wellhead

Chokemanifold

Well effluents

From wellhead

Chokemanifold

Gas to burner

Transferpump

Transferpump

Surge tank

Separator Oil

Well effluents

From wellhead

Chokemanifold

Contaminated waterto sea

Oily water

Clean, treated waterto sea

Clean, treated waterto sea

Skimmer to treatseparated water

Injecting neutralizer

Injecting TFA 400 chemicals

Oily water

Injecting TFA 400 chemicals

Injecting neutralizer

Figure 1.3: Futurecleanup and well-testing operationswill eradicatepollutants, with oiland gas beingreturned to theproduction line andclean waterdischarged safelyinto the sea

Figure 1.2: Second-stage cleanup andwell testing are much better thanconventional methods.Only gas is burned thistime. Oil is recoveredfrom produced waterand returned into theproduction line,leaving clean treatedwater to bedischarged

line as soon as the first barrel wasproduced. The oil-in-water contentwas reduced to less than 80 ppm byimproving separation efficiency, usingcarefully selected chemicals toaccelerate separation of the emulsion,and a skimmer. Using an emulsionbreaker in this way improves theefficiency of the skimmer whereemulsion is present. Gas wouldcontinue to be flared during this stageand water, no longer contaminated,was safely discharged into the sea.

The PhaseTester* portablemultiphase periodic well testingequipment has been introduced intothe system. It has enhancedoperational flexibility and providedaccurate phase flow rates. During thepost cleanup operations, thePhaseTester led to the elimination ofgas flaring. The minimal pressuredrop across the meter and its highpressure rating make it possible toflow the well effluents naturallythrough the sea line without

separating the well effluent phases.The deployment of this multiphaseflow meter in the system has resultedin a reduction in the gas flaring ofabout 60% during duration of the job.

The final stage of the zero-flaringplan (Figure 1.3), expected in late2002, is to fully eliminate gas flaringby returning gas, as well as oil, to theproduction line.

The use of gas compressors toovercome the pipeline pressure wasstudied, but was discounted for a

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number of reasons. These includedthe relatively large size of theequipment and its inflexibility interms of flow characteristics, whichwould present difficulties during thecleanup phase. However, bymeasuring flow rates without theneed for separation, the neutralizedoil and gas mixture could bereinjected into the production lineusing multiphase pumps, which allowthe flow of gas and oil together. Thishas been made possible by the recentintroduction of Vx* multiphase well-testing technology, embodied inPhaseTester X.

Coping with the pressureEliminating flaring by returning wellfluids to the production line is not atrivial task. The first challenge is toovercome the pressures needed toreinject oil from the low-pressureseparator or surge tank into theproduction line. This pressure isgreatest at the start of the cleanupoperation, where the presence ofcompletion fluid and pumped liquidcontribute to a much greaterhydrostatic pressure than that of a borehole filled purely withhydrocarbons. This is achieved usingsingle-phase pumps, typically thoseused to pump oil to a burner orinjection line, that have been modifiedspecifically to cover the entireoperating envelope of a wide range of

flow regimes and sea line pressures.The pumps must also enablerepresentative data to be acquiredduring production well testing. Arelatively low separator pressure isrequired to respect chokeperformance and for critical flowconditions, where downstreampressure should be 50% less thanupstream pressure.

Around 10 to 15 gallons per foot of15% hydrochloric acid (HCl) are usedfor stimulation, with even highervolumes in deviated oil and gas wells.In horizontal wells, now commonplacein the carbonate formations of theMiddle East, the long horizontalsections result in very large volumesof spent acid after stimulation. Evenafter the acid has been allowed tosoak into the formation for four hoursto allow proper contact with theformation, the spent acid at thesurface during cleanup is pH 2–3, andit is clearly desirable to avoiddischarging this into the sea. Toovercome this, and to allow effluentsto be passed immediately through thetest separator, neutralizing agents(sodium hydroxide or sodiumcarbonate) must be injecteddownstream of the choke manifold toraise effluent pH to above 5.5, which isthe acceptable limit for diverting theeffluents through the surfaceequipment and the production line.

Emulsification of well fluids causesfurther problems when reinjecting oilinto the production line. The

intermixing of oil- and water-basedfluids often forms high-viscosityemulsions. Water-in-oil emulsion,formed by invasion of drilling andcompletion filtrate or treatment fluids,has small droplets of water dispersedin a continuous phase oil. Oil-in-wateremulsion and multistage water-in-oil-in-water emulsion, formed by agitationof the simple emulsion, may bepresent in produced well fluids.

Emulsions can be stabilized bysurfactants and mutual solvents.Initially, four different chemicals wereidentified that, when added to theback-flow fluids, were effective inbreaking down the emulsion at 5%concentration. However, this wasunacceptable because the resultingwater would be discharged into the sea.A multifunction demulsifying agent(such as TFA 400) that is a blend ofsurfactants was preferred. This isinjected at the eruption manifold,which is connected to the flowlineupstream of the choke manifold.

A series of laboratory tests –compatibility with the crude oil, wellfluid and acid design – is used todetermine the suitability of thetreatment fluid formulation. Anemulsion test and an anti-sludge testare also carried out.

Finally, the problem of dischargingwater with a high oil content must beaddressed. Retention times whenpassing oil and water at 5000 B/Dthrough a three-phase separator are

19980

100

1999 2000 2001

Zero

oil

and

gas

flarin

g

Two single-phase pumps added

Additional pump

Acid neutralizationintroduced

Projected:Add multiphase

pumps andPhaseTester X

Oil flaring reduced by 35%

Oil flaringreduced by

65%

Zero oil flaring

100% gas flaring

Projected

Oil a

nd g

as fl

arin

g %

Oil flaring Gas flaring

Figure 1.4:Remarkableadvancementshave beenachieved incleanup and well-testing operationsin a short periodof time

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Clean and cost effective

The two-stage cleanup employed todate has resulted in spectacularreductions in the amount of oil flaredduring well tests and cleanups. Byneutralizing spent acids beforeseparation, and adding equipment toreduce oil in water, the objective ofzero oil flaring is achieved while thequality of the discharged water meetsenvironmental standards. Untilrecently, water discharged from theskimmer contained 100–120 ppm ofoil and had a pH above 5.5, but a newenvironmental standard demands areduction of oil in water to 15 ppm orless. This will be met with theintroduction of the WDOU* Water De-Oiling Unit, which is a combinationof a degasser, coalescers, a feedpump, a hydrocyclone and an oil tank,all contained on one skid. The WDOUshould be in place in Abu Dhabi bythe end of 2001. These improvements,realised through the use of existingequipment such as single-phasepumps, have become ADMA-OPCOstandard procedures. Theintroduction of new equipment, suchas multiphase pumps and multiphasetesters, will bring about true zeroflaring with the further elimination ofgas flares. In addition to the obviousenvironmental benefits thesetechniques have brought, there is avery attractive additional bonus tosatisfy even the most scepticalanalysts. This environmentalimprovement comes with clear andsignificant cost savings.

around 2 min. The gravity-precipitation velocity of waterdroplets into the bottom of theseparator is not high enough toensure complete separation in thistime. As a result, the dischargedwater can contain more than2000 ppm of oil, and some water iscarried into the production facilitiesin the oil phase.

To improve water quality afterseparation, the retention, or settling,time is reduced using a skimmer unit,which decreases the oil-in-waterconcentration to 100–120 ppm beforethe water is discharged. This ispossible because the skimmer has ahigher volume than the separator andoperates at a lower pressure. Oil-contaminated water from the bottomof the separator is routed into theskimmer with its two compartments.Fluid enters the water compartment,where it is treated with demulsifier.The increased separation time allowsthe oil droplets to separate out andfloat to the surface until they overflowinto the other compartment, theoil–water interface being automaticallycontrolled. The separated oil is thenpumped into the production line.

Oil from the separator, mixed withdemulsifier, is routed to an 80-bblsurge tank, where further residues ofwater that may have carried over areallowed to separate. The oil, having anacceptable pH value, is then pumpedinto the production line. Water issampled frequently at various pointsin the separator inlet, the outlet andthe bottom of the surge tank to checkthe effluent pH. Finally, gas from theseparator, surge tank and skimmer is burned.

Action in Abu Dhabi In Abu Dhabi, ADMA and ZADCOwere burning oil during drilling-relatedcleanup and testing operations. It wasdecided to equip a number of rigs tocarry out the first two stages of theflaring emission reduction project,following completion and acidization.

Two pumping systems, selected toserve the companies’ full operatingenvelope for oil reinjection, wereintroduced during May 1998. The

criteria under which reinjection of oilwas allowed to proceed, in terms ofbasic solids and water (BSW) and pHlevels, were specified by eachcompany to avoid any compromise toproduction-line integrity. Under theseconditions, oil flaring was reduced by38% (almost 100,000 bbl) by the endof 1998. A third pumping system wasadded in December 1999. Thisincreased the reduction of oil flaringto 65% (195,000 bbl).

The next step was to addneutralization units to allow immediateflow of well fluids through theseparator. Sodium carbonate was usedas a neutralizing agent for severalreasons: as a constituent of drillingmud it is always available at the rig site,it produces soluble products in waterand, it is cost effective. The pH of thebackflowed effluents was raised toabove 5.5 – the acceptable limit fordiverting effluents through contractors’equipment and the sea line.

Adding surfactants to the well fluidto control emulsion, and installingskimmer units to reduce oil-in-watercontent by increasing retention time,completed the planned second stageof the project. In those installationswhere stage two has beenimplemented fully, liquid flaring hasbeen eliminated.

Turning off the gasThe next, and final step towards zeroflaring will be to eliminate gas flaring(Figure 1.4). The treated oil and gasmixture will flow directly through themultiphase PhaseTester X, whichdetermines flow rates without theneed for oil and gas separation.

Various methods for reinjecting gasand oil into the production line havebeen evaluated, including the use ofgas compressors. However, thesewere considered unsuitable becauseof their relatively large size and their inflexibility in terms of flowcharacteristics. The most suitablemethod will be to replace the single-phase injection pumps withmultiphase pumps, where gas and oilcan flow together through the pumpimpeller without damaging it.