ThePennsylvaniaStateUniversity

TheGraduateSchool

DepartmentofEnergyandMineralEngineering

DEVELOPMENTOFANALGORITHMTOANALYZETHEINTERRELATIONSHIPAMONG

FIVEELEMENTSINVOLVEDINTHEPLANNING,DESIGNANDDRILLINGOFEXTENDED

REACHANDCOMPLEXWELLS

AThesisin

PetroleumandMineralEngineering

by

ArmstrongLeeAgbaji

2009ArmstrongLeeAgbaji

SubmittedinPartialFulfillment

oftheRequirements

fortheDegreeof

MasterofScience

May2009

ThethesisofArmstrongLeeAgbajiwasreviewedandapproved*bythefollowing:

RobertWilliamWatsonAssociateProfessorEmeritusPetroleumandNaturalGasEngineeringThesisAdviser

R.LarryGraysonProfessorofEnergyandMineralEngineering;GeorgeH.Jr.andAnneB.DeikeChairinMiningEngineering;GraduateProgramChairofEnergyandMineralEngineeringJoseA.VenturaProfessorofIndustrialandManufacturingEngineering

*SignaturesareonfileintheGraduateSchool.

ii

ABSTRACT

ExtendedReachDrilling(ERD)isanintegratedmethodologyfordrillinghighanglewellbores

with longhorizontaldisplacements.Typicalproblems thathavecometobeassociatedwith

this type of operation include: Directional Well Design, Torque and Drag Limitations,

HydraulicsandHoleCleaning,VibrationandWellboreStability,EquivalentCirculationDensity

Management,aswellasMudRheologyandSolidsControl.

Giventheseproblems,thedrillingoperationofERDwellsshouldbeaclosedloopprocessthat

startsat theprejobplanningphase, iscarriedon through thedesignandexecutionphase,

and results in the implementation of lessons learned. For high levels of performance

improvement to be achieved, it is essential to analyze the interrelationship among the

elements involved intheentiredrillingprocess.This isbecauseasystemthatconsidersthe

individual elements of the drilling system in isolation is inadequate to deliver desired

performanceimprovement.

Inthisstudy,analgorithmthatsetsforthadesignforadrillingprogramthatissuitabletodrill

anextendedreachwellwasdevelopedusingVisualBasic.Althoughthealgorithmtoucheson

several factors affecting extended reach drilling, themajor focus is on the five elements,

which are considered the critical factors. These are:Well Planning and TrajectoryDesign,

BottomHoleAssembly(BHA)Design,DrillStringDesign,TorqueandDragAnalysis,aswellas

HydraulicsandHoleCleaning.

iii

The proposed algorithm evaluates the interrelationship among these critical factors and

providesdirectionontheprocessesandtasksrequiredatallstagesinthedesignanddrilling

of an ERDwell to achievebetterdrillingperformance. It also affordsbothoffice and field

personnelthecapabilityto identifydrillingperformanceproblemsandbyevaluatingresults,

identifyremedialactionsquicklyandaccurately.

iv

TableofContents

ListofFigures....viii

ListofTables...x

Acknowledgements..xi

Chapter1.INTRODUCTION........................................................1

1.1.DefinitionofExtendedReachDrilling....3

1.2.ExtendedReachDrilling:AHistoricalPerspective.7

1.2.1.HorizontalandERDWells.....9

1.2.2.DualHorizontal/DualLateralWells....10

1.3.StatementoftheProblem..14

1.4.ObjectiveofStudy....15

1.5.ScopeofStudy.15

1.6.SignificanceofStudyandDeliverables..16

Chapter2.LITERATUREREVIEW..............................................17

2.1.WellpathPlanning....19

2.2.TorqueandDrag.20

2.3.HoleCleaning......22

2.4.CuttingsTransport...24

2.5.RotarySteerableDrillingSystems..28

2.6.ConcludingRemarksfromLiterature..31

v

Chapter3.CRITICALTECHNOLOGIESFORSUCCESSOFEXTENDEDREACHWELLS.33

3.1.WhatisDifferentaboutExtendedReachDrilling?.............................................................34

3.1.1.Torque,DragandBuckling......34

3.1.2.HoleCleaning......35

3.1.3.EquivalentCirculationDensities.....35

3.1.4.RigCapabilityandPowerRequirements.36

3.1.5.WellboreInstability,DifferentialStickingandStuckPipe.....37

3.1.6.WellControl......38

3.2.WellPlanningandTrajectoryDesign...38

3.2.1.TrajectoryDesign........39

3.2.2.CatenaryWellDesign....41

3.3.BuildRate...43

3.3.1.EffectofBuildRate.....43

3.4.HoleSizingandSelection....44

3.5.TorqueandDrag....45

3.5.1.Torque.....45

3.5.2.Drag......48

3.5.3.FrictionalForces.........50

3.5.4.InfluenceofFrictionFactor........52

3.5.5.TorqueandDragReductionMethods......53

3.6.EquivalentCirculationDensityManagement......55

3.6.1.EffectsofECD..........59

3.6.2.ECDManagementandControl..........60

3.7.DrillingHydraulicsandHoleCleaning.......61

3.7.1.FundamentalsofHoleCleaning.......62

3.7.2.FactorsAffectingHoleCleaning.......65

3.7.3.ConsequencesofPoorHoleCleaning..66

vi

3.7.4.TheCleanHoleConcept:WhatisaCleanHole?..67

3.7.5.HoleCleaningMechanism...68

3.8.WellboreStability.....72

3.8.1.Vibrations..72

3.8.2.WellboreStability....73

3.8.2.PreliminaryWellboreStabilityAnalysis....74

Chapter4.EXTENDEDREACHDRILLINGALGORITHM....................76

4.1.AlgorithmDefined....76

4.2.VisualBasicAlgorithmforERD....77

4.2.1.AttributesoftheModel.......79

4.3.AnalysisoftheIndividualModules...82

4.3.1.WellPlanningModule........83

4.3.2.BottomHoleAssembly(BHA)DesignModule....89

4.3.3.DrillStringDesignModule.....92

4.3.4.TorqueandDragAnalysisModule....98

4.3.5.HydraulicsandHoleCleaningModule.....101

4.3.6.EquivalentCirculationDensity(ECD)Module....104

4.4.SensitivityAnalysisExampleProblem...106

4.5.LimitationsoftheModel....116

Chapter5.SUMMARY,CONCLUSIONSANDRECOMMENDATIONS117

5.1GeneralObservations......117

5.2ConclusionsDrawnfromUsingtheModel....118

5.3Recommendations.....119

5.4ConcludingRemarks.....119

5.5TheFutureofExtendedReachDrilling....120

5.6FurtherStudies..121

AppendixA:ProgramCode...122

AppendixB:SampleReport..132

Bibliography.134

vii

ListofFigures

Figure1.1:IndustryExtendedReachDrillingExperience...4

Figure1.2:ERDLimit.6

Figure1.3:LimitofRotarySteerableERD.7

Figure1.4:EvolutionofExtendedReachDrilling.12

Figure3.1:WellTrajectoryProfiles....41Figure3.2:DiagrammaticRepresentationofTorqueGeneratingForces46Figure3.3:FrictionalandSurfaceactingForces47Figure3.4:DrillStringOpposingForces..49Figure3.5:DragandPickUpForces..50Figure3.6:CasingShoeECDDetermination......58Figure3.7:AngularDepictionofExtendedReachWellTrajectory......62Figure3.8:CuttingsTransportatDifferentInclinations.......63

Figure4.1:TheContinuousImprovementCycle.....78Figure4.2:DrillingDesignFlowChartforERD...80Figure4.3:WellPlanningandTrajectoryDesignModule84Figure4.4:InterrelationshipamongERDCriticalElements..88Figure4.5:FlowChartfortheDesignofBottomHoleAssembly90Figure4.6:BottomHoleAssembly(BHA)DesignModule.91

Figure4.7:FlowChartforDrillStringDesign..96

Figure4.8:DrillStringDesignModule.97

viii

Figure4.9:TorqueandDragAnalysisModule.100

Figure4.10:HydraulicsandHoleCleaningModule..103

Figure4.11:EquivalentCirculationDensity(ECD)Module.105

ix

ListofTables

Table3.1:WellTrajectoryOptions,AdvantagesandDisadvantages.....................40

Table3.2:RecommendedMinimumandMaximumFlowRatesforDifferentHoleSizes....71

Table3.3:RecommendedDrillStringRPMforVariousHoleSizes71

Table3.4:TypesofVibration..73

x

Acknowledgements

Iwould like toexpressmy sinceregratitude and appreciation tomy academic adviser,Dr.

RobertWilliamWatsonforhisconsistentadvice,fortitude,andreceptivenesstoopinionsand

ideaswhileIworkedonthisproject.Myappreciationalsogoestotheothermembersofmy

thesis committee,Dr. LarryGrayson andDr. JoseVentura for the unflinching support and

encouragementIgotfromtheminthecourseofdoingthiswork.

A lotofcreditgoestoJimOberkircher,PresidentandCEOofGFEagleCorporationwhofirst

gaveme the ideaandsuggestion toconduct researchonExtendedReachDrilling. Iwould

also like to acknowledge the help of individuals and companies in the oil industry that

contributed immenselytothesuccessofthiswork.Toponthe list isBakerHughesINTEQ. I

wanttothankPeteClarkwhofirsthiredmeasaninternandthusstartedmyrelationshipwith

the company. Special thanks also go to Charles van Lammerenwho is the brainchild and

initiatorofthe involvementandpartnershipofBakerHughesINTEQ inthisresearch. Imust

not failtothank JohnFabianandCarlCorson forthe immenseroletheyplayed inensuring

thatIgotallthematerialsIneededtocarryoutthiswork.Themanymeetingsanddiscussions

IhadwiththeBakerHughesTeamgreatlyensuredthat Istayedoncourseandonmessage

throughoutthedurationofthisstudy.

Of course Iwould like to thankmywife Elma, for staying strong and focusedwhile Iwas

severalmilesaway,workingonthisproject.It is impossibleformeto listallthepeoplethat

contributed inonewayor theother to the successof thiswork.Toallwhosenameshave

xi

xii

beenmentionedandthosenotmentioned,Isay,THANKSisasmallword,but itgoesa long

way to show howmuch I appreciate your love care and concernwhile Iworked on this

project.

ChapterOne

Introduction

Extended Reach Drilling (ERD) is an integratedmethodology for drilling highanglewell

bores with long horizontal displacements. ERD wells are typically kicked off from the

verticalnearthesurfaceandbuilttoanangleofinclinationthatallowssufficienthorizontal

displacementfromthesurfacetothedesiredtarget.Thisinclinationisheldconstantuntil

thewellbore reaches thezoneof interestand is thenkickedoffnear thehorizontaland

extended into the reservoir. This technology enables optimization of field development

through the reduction of drilling sites and structures, and allows the operator to reach

portions of the reservoir at amuch greater distance than is possiblewith conventional

directionaldrilling technology.Theseefficiencies increaseprofitmarginsonprojectsand

canmake the difference whether or not a project is financially viable (AlSuwaidi, El

Nashar,AllenandBrandao,2001).

ExtendedReachDrillinginvolvespushingdrillingcapabilitiesneartheirlimits,whetheritis

toreachreservesfarfromexistingfacilitiesortoexposereservoirsectionsforproduction

andreservoirmanagementadvantages.Asamatterof fact,ERDneedstobeviewednot

onlyasExtendedDrilling,butalsoasExtendedReservoirDrilling,orbothExtendedReach

andReservoirDrilling(Payne,Wilton,andRamos,1995).Inthisperspective,ERDassumesa

criticalroleincurrentindustryoperations.

1

Thepurposeofdrillingextendedreach,horizontalandcomplexdesignwellsistoproduce

oilandgas inafield inthemostcosteffectiveway.Theuseofthesenewtechniquescan

make old nonprofitable fields profitable, prolong an existing fields economic life and

makenewanduncertainfielddiscoveriestechnicallypossible.

ExploitationofERDprocedurescouldalsobemadeto:

1. Developoffshorereservoirsnowconsidereduneconomical;

2. Drillundershippinglanesorunderenvironmentallysensitiveareas;

3. Accelerate production by drilling long sections of nearly horizontal holes in

producingformations;

4. Provideanalternativeforsomesubseacompletionsand

5. Reducethenumberofplatformsnecessarytodevelopalargereservoir.

SomeofthemanyadvantagesofusingERDtoaccessreservesarebasedontheelimination

of high capital cost items. For example, in Alaska, ERD can reduce the need for costly

drillingandproduction islandsandthe inherent logisticalproblemsassociatedwiththem

(Nelson,1997).Foroffshoredrilling,suchasintheNorthSeaandtheGulfofMexico,ERD

results inasubstantialreduction insubseaequipment, includingfewerpipelines.Thisnot

onlyhasanimpactoneconomicsbutalsoonenvironmentalconcernsandevenpermitting

(MasonandJudzis,1998).

2

1.1 DefinitionofExtendedReachDrilling

ERDwells are generally associatedwith accessing reservoirs at locations remote from a

drillsite.Generally,awell isdefinedasextendedreach if ithasaStepOutRatioof2or

more. StepoutRatio isdefinedas thehorizontaldisplacement (HD)dividedby the true

verticaldepth(TVD)attotaldepth.

Thedefinitionofwhatexactlyisanextendedreachwellremainsadebatableissue,butas

was mentioned earlier, current consensus says it is a well in which the horizontal

displacementisatleasttwicethetrueverticaldepthofthewell(Economides,Watters,and

DunNorman,1998).Thisdefinitionalso implies thatas theratioofdisplacementtoTVD

increases,thewelldifficultyincreases.WhilethereisnosuchthingasaneasyERDwell,

it is factual tosay thatwith increaseddisplacementcomes increaseddifficulty (Williams,

2008).However,thedepthofthereservestobeaccessedisalsocriticaltheshallowerthe

reserves themoredifficult it is toaccess thewellusingERD.Therefore,displacement to

TVDratioprovidesaneffectivemeasureoftotaldifficulty(Williams,2008).Extendedreach

drillingcanthusbeviewedasanintegratedmethodologyfordrillinghighanglewellbores

withlonghorizontaldisplacements(TolleandDellinger,2002).

There has been some debate on what exactly determines the difficulty in drilling an

extended reachwell.SomeauthorshaveusedERD ratio (HD/TVD)oreven (MD/TVD)as

measuresofdifficulty,whereMDisthemeasureddepth.Relativedifficultyisafunctionof

both HD/TVD ratio and absolute TVD and it is commonly referred to as Aspect Ratio.

3

Williams(2008)clearlysupportsthenotionthatthehighertheratiothemoredifficultthe

wellistodrill.Butinanearlierstudy,MasonandJudzis(1998)propoundedthatthisisnot

true,asdepictedinFigure1.1.

Source:BakerHughesINTEQ

Figure1:IndustryExtendedReachDrillingExperienceFigure1.1:IndustryExtendedReachDrillingExperience

Asseeninthefigure,thelargeststepoutwellshavebeendrilledatthehighestERDratio.

But this issue is farmorecomplicated than this.Forexample indeepwater fieldswhere

directionalworkisoftennotpossibleuntildrillingdepthisbelowtheseabed,thetotalERD

ratiowill normally be less than 1. This is the case even though the element of vertical

depth overwhich anydirectionalwork is carried outwould have a very high ERD ratio

(MasonandJudzis,1998).

4

Inspiteofthesecontroversies,thereisstillnostandardandgenerallyaccepteddefinition

foranERDwell.Thus,thereistheneedfortheindustrytoaddressthisissueandcomeup

withagenerallyacceptablemethodforcharacterizingallwelldesigns.

Itshouldbenotedhowever,thatthegenerallyaccepteddefinitionsofanERDwell

(LongwellandSeng,1996)include:

1. Wellshavinghorizontaldisplacementsgreaterthantwicethewellstruevertical

depth,yieldinginclinationanglesinexcessof63.4degrees;

2. Wellswhichapproachthelimitsofwhathasbeenachievedbytheindustryinterms

ofhorizontaldisplacement;

3. Highangledirectionalwellsthatapproachthecapabilitiesofthecontractedrig.

Typically, theExtendedReachDrilling limit is reachedwhenoneof the followingoccurs

(Demong,RuverbankandMason,2004):

1. The hole becomes unstable, due either to time exposure, geomechanical

interaction, adverse pressure differential, or drilling fluids interaction (or

incompatibility).Theonsetof these conditions isusuallyobserved in the sudden

increaseoftorqueanddraginthedrillstringnotrelatedtodoglegseverity(DLS)of

theholeorthelengthofthedrilledsection.

5

2. Thedrillstringwillnolongertraveltothebottomoftheholeduetoexcessivedrag.

Thissituationisdifferentiatedfromthecaseabovebecausethiseffectisnotrelated

to the friction factor which remains unchanged. Instead, it is related to the

cumulativelengthdrilledalongwiththeDLSoftheholeasdrilled,Figure1.2.

Source:Demong,K.,Riverbank,M.,andMason,D.(2004)

Figure1.2:ERDLimitReachedwhenfrictionexceedstheforceavailabletopushthedrillstringdownthehole

3. Whenrotationisusedtoovercomefrictionandadvancethedrillstring,suchasin

rotary steerable application. The limit is reached when you reach the torque

capacityofthetubularsasshowninFigure1.3.

6

Source:Demong,K.,Riverbank,M.,andMason,D.(2004)

Figure1.3:LimitofRotarySteerableERDReachedwhenfrictionexceedsthetorqueavailabletoturnthedrillstring

1.2ExtendedReachDrilling:AHistoricalPerspective

Horizontalwelltechnologyandextendedreachdrillingtechnologyhavebeenimplemented

intheindustrysince1987.Earlyeffortsofmoderndayhorizontalwelltechnologywereled

byAtlanticRichfieldinthelate1970s,ElfAquitaineintheearly1980s,andBPAlaskainthe

mid 1980s. Estimates by Philip C. Crouse and Associates Incorporated (1995) placed

worldwide implementationofhorizontalwell technologyatover11,000horizontalwells

duringtheperiodof1986through1996.

7

Followingtheendoftheeraofeasyoilandwithmajoroilfieldsbeingsituated indeep

watersanddifficultterrains,welltrajectoriesbeingplannedhadlonghorizontaldepartures

and increasinglybecamemorecomplex.ExtendedReachDrillingthusbecamethemeans

ofexpandingproductionoftheseagingoilfieldsandmadeitpossibletodrillandproduce

hithertoimpossiblereserves.

The first extendedreachwellwith a horizontal reach ofmore than 5,000mwas drilled

from Statfjord Well C10 in 19891990 (Rasmussen, Sorheim, Seiffert, Angeltvadt and

Gjedrem,1991). StatfjordWellC3,whichwasdrilledandcompleted in1999,holds the

recordofbeing the firstwell in the industry toexceed6,000m (Njaerheim andTjoetta,

1992). Ithadahorizontaldepartureof6.1km. In19921993,StatoilsWellC2 setnew

worldrecordsandbrokethe7kmdeparturebarrierforthefirsttime.WellC2achieved

8.7kmMDandadepartureof7.3kmat the Statfjord reservoirTVDof2,700m. In1994,

Norsk HydrosWell C26 set anotherworld record forwell departure. TheirWell C26

achieved9.3kmMDwithadepartureof7.85kmat2,270mTVD.

Since the Statfjord development projects, the industry has recorded breakthroughs in

extended reach drilling. Somemajor industry achievements that followed the Statfjord

successesarechronicledhereunder:

8

1.2.1HorizontalandERDWells

InSeptember1991,CliffOil&Gasdrilledthedeepesthorizontalwell intheNorthBayou

Jack field in Louisiana. The well reached TD at 4,675m TVD (Drilling and Production

Yearbook,1992).

InNovember1991,MaerskOil&GascompletedtheworldslongesthorizontalwellatTyra

WestBravofield,TWB11aintheDanishsectoroftheNorthSea.Thehorizontalextension

recordwas2,500matameasureddepthof4,770mandatotalverticaldepthof2,040m

(Drilling and Production Yearbook, 1992). At about the same period, Unocal had the

greatest horizontal displacement to true vertical depth ratio,HD/TVD of 3.95. Thiswas

achieved inWellC29 in theDosCuadreaFieldoffshoreSantaBarbara inCalifornia.The

well had a measured depth of 1,335m (Drilling and Production Yearbook, 1992). The

companyalsodrilledthelongestERDwellintheUSA.ThewellA21wasdrilledinJuly1991

fromtheIreneplatformlocatedinoffshoreCaliforniaandhasalateralreachof4,472m.

About thesameperiod,WoodsideOffshorePetroleumdrilleda longextendedreachgas

wellfromtheNorthRankinAplatforminAustralia.Thewellhadareachof5,009manda

MDof6,180m.

9

1.2.2DualHorizontalDualLateralWells

In1992,WellA36achieved5kmdeparture intheshallowerGulIfaksreservoirat2,160m

TVD,resultinginaMD/TVDratioof2.79.Itwasamongthehighestatthetime.

InMarch1992, Torch EnergyAdvisors completed the firstDualHorizontalWell,Basden

1H, inFayetteCounty,Texas(DrillingandProductionYearbook,1993).Afterdrillingboth

horizontallaterals,thetwoslottedlinerswererunintoopposinghorizontalwellbores.The

secondlinerwasrunthroughawindowinthefirst.Eachhorizontallateralextended800m.

In 1993, PetroHunt Corporation used a unique horizontal well design to optimize

developmentofan irregularly shaped lease in theMcDennandWellNo.1 in theAustin

Chalk formation in Texas (Cooney, Stacy and Stephens, 1993). Two mediumradius

horizontal boreswere drilled in opposite directions from one vertical hole tomaximize

horizontaldisplacement inthe lease.Thetwoboreshadhorizontalsectionsof825mand

625m, respectively. Underbalanced drilling techniqueswere used to prevent formation

damage.Thehighmechanical integrityoftheAustinChalk formationsuitedanopenhole

completionmethod.

In AugustSeptember 1991, Gemini Exploration Company drilled the longest total

displacement of two lateral wells in the Pearsall field of south Texas (Drilling and

Production Yearbook, 1992). The total combined displacement of the two lateralswas

2,687m.

10

As the yearswent by (especially in the late 1990s), drilling techniques continued to be

developedtopushthelimitbeyondthethenmaximum60degreeinclinedwells.Thiswas

asa resultofacombinationofengineering research,applicationofnew technologyand

fieldexperience.Duringthisperiod,BPalsodrilledwellsthatstretched5km(3miles)then

8km(5miles)andthen10km(6miles)offshorefromtherigsitelocatedonshoreattheir

Wytch Farm development (Williams, 2008). The Wytch Farm project thus broke the

existingrecordandsetanewone.Thetechnicalsuccessofthisprojectopenedtheflood

gatesandextendedreachdrillingnotonlyincreasinglybecamethestandardpracticefor

expanding production of aging oil fields, it alsomade it possible to drill and produce

hithertoimpossiblereserves.

Figure 1.4 is an evolutionary plot,which shows that themost aggressive ERD activities

indeedtookplaceduringtheearly1990s.TheinnerenvelopedescribesERDwellsdrilledas

of1992while thesubsequentones to therightdescribe the industryachievementsover

theyears,basedonrecentindustryliterature.

11

Pre1992 Late90,s

CurrentLimit

Source:OilandGasGraphics,August(2008)

Figure1.4:EvolutionofExtendedReachDrilling

Asaresultofimprovementsinpersonnelcompetencyandtechnologicaladvancement,not

onlyweremoreERDwellsdrilled,thesewellscontinuouslypushedtheenvelope interms

of lateralreachandgeometriccomplexity inorder toaccessmorechallenginggeological

targets. The complexity of ERD operations has progressed from 2D to complex 3D

trajectorieswhere, forexample,anabandoned2DERDwellhasbeensidetracked to tap

shallowersatellitereserves,requiringahighlycomplexuphillwellprofile(Ruszka,2008).

12

In April 2007, ExxonMobil Corporation announced that its subsidiary, Exxon Neftegas

Limited (ENL), completed thedrillingof theZ11well in theSakhalinproject. Itwas the

longestmeasured depth ERDwell in theworld at the time. Located on Sakhalin Island

offshore Eastern Russia, the recordsetting Z11 achieved a total measured depth of

11,282moroversevenmiles.TheZ11wasthe17thERDproducingwelltobecompleted

aspartof the Sakhalin1Project. Itwasdrilled in61days,more than15daysaheadof

schedule andbelow expected costwithno safetyorenvironmental incidents. Since the

firstSakhalin1wellwasdrilled in2003,thetimerequiredtodrilltheseworldclasswells

hasbeen reducedbymore than fiftypercent.When compared to industrybenchmarks,

Sakhalin1wellsareamongtheworld'sfastestdrilledERDwells.

However,thecurrentworldrecord forthe longestERDwelldrilled isheldbyMaerskOil

Qatar.InJune2008,thecompanywasreportedtohavedrilledthewellintheAlShaheen

FieldoffshoreQatar.Thiswellbrokethepreviousrecordlengthbyover610m;reachinga

total depth of 12,289m, with a total stepout distance from the surface location of

10,902m.Inall,thewellset10recordsincludingthelongestwelleverdrilled,thelongest

alonghole departure (11, 569m), the longest 8in section (10,805m), the highest ERD

ratio(AHD/TVD=10.485),thehighestdirectionaldifficultyindex(DDI=8.279),thedeepest

directional control, the deepest downlinkMWD transmission and LWD geosteering (12,

290m), thedeepestbatterylessoperation, the longest reservoir contact (10,805m.)and

finallythelongestopenhole(DrillingContractorMagazine,July/August,2008).

13

Recordbreakingachievementscontinuetobemadeandthesetendtodemonstratethatit

isindeedpossibletoexceedthecurrentindustrylimitofERD.Whatislesswellunderstood,

however,aretherisksofdrillingsuchwells.Experiencehasshownthattheprobabilityof

encounteringsignificantdrillingproblems inERDwells isgenerallymuchhigherthanthat

experiencedinconventionalwells.Drivenbyaneedtominimizeenvironmentalimpactand

improverecoverablereserves, it isanticipatedthatERDactivitywillcontinueto increase.

Simultaneously, advances in drilling technology developmentwill continue to lower the

operational risk.As a result, ERD limitswill continuously be tested and pushed to new

levelsasthegrowingbenefitsoutweighthediminishingoperationalrisks(Ruszka,2008).

1.3StatementoftheProblem

Wellprofileshavebecomemorechallengingasaresultoftheneedtoreachnewtargets,

depths and departures that in the past seemed improbable. Regardless of our current

levelsofsuccess,thereistheurgencytodevelopmethodsandprocessesthatwillfurther

improvedrillingefficiency.Welltrajectoriesbeingplannedtodayhavesuchdeparturesand

complexity thatviability, riskandeconomicsareallvitalpartsof theevaluationprocess

priorto investing intheproject.These in largepartaredeterminedbytherigdesignand

capacitytodrilltheseextremereachwelldesigns.

To reduceoperational costs, the entire drilling processmust be improved substantially.

Researchers focusing on these challenges have identified the need for systembased

solutions.Suchanapproachevaluatesalltheelementsthat influencethedrillingprocess.

14

Assuch,itisimperativethatasystematic,logicalandquantifiableapproachfordeveloping

and implementingtechnologies inthesetypesofapplicationsbedefined.Asthe industry

continues to move toward the more costeffective technologies of computerbased

instrumentation,powerhandlingtools,andautomateddrilling,thereistheneedtohavea

definedandstructuredapproachfordrillingextendedreachwells.

1.4ObjectivesofStudy

Theobjectiveofthisstudy isto integratetheparameters involved intheplanning,design

anddrillingofextendedreachandcomplexwellsintoasinglecomputermodel.Thegoalis

todevelopanalgorithmthatwillevaluatethe interrelationshipamongtheseparameters

duringtheplanninganddesignphasesofthedrillingoperationinatechnicallysoundand

feasibleway.

1.5ScopeofStudy

As was mentioned earlier, the industry has made significant progress in developing

improved technologiesespeciallyas itaffects thedrillingofcomplexanddesignerwells.

Thisstudywillconcentrateonthefollowingareas:

WellPlanningofExtendedReachandComplexWells,and

DrillingParameterEvaluationinExtendedReachandComplexWells.

15

Underthesetwobroadheadings,thefollowingtechnologies,whichhavebeenfoundtobe

criticaltothesuccessofERD,wouldbeanalyticallyexamined:

o WellPlanningandTrajectoryDesign

o BottomHoleAssemblyDesign

o DrillStringDesign

o TorqueandDrag

o HydraulicsandHoleCleaning

o EquivalentCirculationDensityManagement

1.6SignificanceoftheStudyandDeliverables

Attheendofthisstudy,aVisualBasicalgorithmwillbedeveloped.Themodelwillhave

thecapabilitytodesignthedrillingprogrammostsuitabletodrillanextendedreachwell.It

willbeversatileandflexiblesuchthatthecomponentsofextendedreachdrillingcaneither

bechosenbytheuserbasedonavailability,orbytheprogramfrom its library/databank.

Evaluationofthesecomponentswillresultinanoutputthatwillproduceaviablewellplan

anddesignforthegivenERDwell.

16

ChapterTwo

LiteratureReview

Severalstudieshavebeencarriedoutonthetheoryandpracticeofdrillingextendedreach

andcomplexwells.Areviewoftheliteraturegivesusaninsightintosomeofthesestudies,

thetechnologiesusedandhowtheyhaveimpactedthedrillingindustry.

Foranygivendrillingoperation,numerousdrilling technologiesare typicallyavailable to

ensuremaximum recovery of the hydrocarbon from the reservoir.Over the years, the

industryhasmadesignificantprogress indeveloping improvedtechnologiestotacklethe

complexitiesencountered indrillingextended reachwells.Someof these improvements

arerelatedtorealtime formationevaluation,directionalcontrolwhiledrilling, improved

bottomholeassembly(BHA)components,anddrillingfluids.

Based on the many lessons learned in recent projects, technologies that have been

identifiedtobevitaltothesuccessofERD(Payne,CockingandHatch,1994)include:

TorqueandDrag DrillstringandBottomHoleAssemblyDesign WellboreStability HydraulicsandHoleCleaning DrillingDynamics RigSizing

17

Fromthedrillingfluidperspective,somecrucialissuesthatcanposesignificantchallenges

fortheoperator(Cameron,2001)include:

NarrowMudWeight/FractureGradientWindow

EquivalentCirculationDensity(ECD)Management

LostCirculation

Lubricity

Also related to these are surveying andwell profile design.Other technologies of vital

importance are the use of Rotary Steerable Systems (RSS) togetherwithMeasurement

WhileDrilling(MWD)andLoggingWhileDrilling(LWD)toolstogeosteerthewellintothe

geologicaltarget(Tribe,Burns,HowellandCickson,2001).

It iswellknown thatERD introduces factors thatcancompromisewelldelivery,and the

firstchallengepriortodrillinganERwellistoidentifyandminimizerisk(Karmaruddin,Lah,

Sering, Good and Khun, 2000). The literature is repletewith several experimental and

theoretical studies that have been conducted on extended reach exploration and

developmentdrilling.Researchershavemainlyconcentratedonthecriticalfactorsoutlined

above.This literaturesurveywillbecarriedoutby reviewing thestudies thathavebeen

made,basedonthesecriticaltechnologies.

18

2.1WellpathPlanning

Extended Reach Drilling is playing an important role in the economic development of

variousoilreserves.ForsomeERwells,carefulwellplanningandutilizingexistingdrilling

practicesaresufficienttoavoidproblemssuchaswellboreinstability,lostcirculation,and

stuck pipe.However, results from several studies have shown thatwhenwell stepout

ratios increase, some operational practices developed while drilling conventional wells

become inadequate tosuccessfullyandcosteffectivelydeliver theERwells. Itshouldbe

noted,however,thatcarefulplanningalonecannotalwayspreventdrillingproblems.

ERDwell profile design is not just a simple geometric curve design. It is an integrated

processthatrequiresanoptimumwellpathprofilewithrespecttotorqueanddrag.There

are twomain principles that should be obeyedwhen planning an extended reachwell

(Shanzhou,Genlu,Jianguo,andZhiyong,1998).Theseare:

MinimizingTorqueandDrag MinimizingWellLength

Severalpapershavebeenwrittenonthissubject.McClendonandAnders(1985)described

the use of the catenarywell planmethod for directional drilling. CatenaryWellplan is

explained indetail inChapter3.Amodifiedversionof thismethodwasusedwithgreat

successwhensomeoftheERDandhorizontalwellsontheStatfjordfieldweredrilled.The

methodwasmainly used to reduce torque and drag by reducing thewall forces in the

19

buildupsection.Thiswasdoneinthetoppartoftheholewherethedrillpipetensionand

sideloadswerehighest.

2.2TorqueandDrag

The ability to confidently predict that you can reach a reservoir target is probably the

biggestproblemwhendrillinganextendedreachwell.Thismakestheaccurateprediction

of torque and drag very essential. Drilling torque losses calculated on various plan

trajectoriesare a good indicationofdrillabilityof awellplan.A reviewof these torque

valuesandadjustmentstotheplannedtrajectoryisawaytoimprovetheplanorfindthe

easiestpath todrill. Selecting the appropriatewellprofile followedbymodifying casing

and tubulardesignsshouldbe thekey initialsteps in torque reductionmeasures (Aston,

Hearn andMcGhee, 1998). Torque levels in ERwells are generallymore dependent on

wellbore length and tortuousity than tangent angle. Yet, higher anglewells do tend to

reduceoveralltorque levelswhiledrillingsincemoreofthedrillstring is incompression,

andthetensionprofileinthebuildsectionisreduced.However,highertorquevaluesand

associatedproblemssuchasacceleratedcasingwearandkeyseatsareseenduringback

reamingoperations(Modi,Mason,ToomsandConran,1997).

One of themajor factors affecting torque and drag in an extended reach well is the

tortuousityofthehole(Banks,HoggandThorogood,1992).Hightortuousityresultsfroma

lackofcontroloftoolfaceanddeviationrate.Thus ifasteerablemotor ishardtoorient,

this will result in higher torque and drag, which compounds the original orientation

20

problem.Lowbuilduprates,deepkickoffpoints,eliminationofexcessivetortuousityand

optimumhole cleaningare themain factors thatwillaffect the torqueanddragvalues.

Banksetal.(1992)alsodiscussedtheeffectofexcessivetortuousity.Theystipulatedthat

excessive tortuousity can severely limit the drillable depth and the elimination of this

effectisacriticalfactorinsuccessfulERDoperations.Theyrecommendthatthisshouldbe

usedasanobjectivemeasureofthedirectionaldrillingcontractorsperformance.

Highdrilling torquecanposesignificantproblems inERDwells.Meader,AllenandRiley,

(2000) investigatedmethodsof torque reductionat theWytch farmproject.Theproject

hadusedBarafibre(crushedalmondshell)astheprimemethodoftorquerelief.Butduring

thedrillingofWellM16,atrialofanalternativemudadditivewasconducted.Theproduct

was Lubraglide (small plastic beads). The trial was conducted by stripping out all the

Barafibre from thesystemand replacing itwithLubraglide.The initial resultswerequite

impressive.The results showed thatas theBarafibrewas stripped from the system, the

torque level rose from 40,000 ftlbs to the limit of 45,000 ftlbswhere the top drive

stalledout.AstheLubraglideenteredtheopenholethetorqueleveldroppedto25,000ft

lbs.Thisisadramaticresultbutunfortunatelythetorquereductionwasnotsustainedfor

thedurationofthe8holesection.Thetorquewasmanagedfortherestofthewellwith

a combinationof LubraglideandBarafibre.The reasonswhy Lubraglidealone couldnot

consistentlyreducethetorquearenotwellunderstood,andthisunderlinestheneedfor

extensivedatagatheringsothatthoroughpostwellanalysiscanbeconducted.

21

Manyoperatorshave reported theuseofmechanical and chemical friction reducers to

minimize torque and drag. Rubber element and plastic casing protectors have been

reported to be successful in reducing torque. Some operators, (Ikeda, Takeuchi, and

Crouse, 1996) reported torque reductions of over 25% by usingNonRotatingDrill Pipe

Protectors (NRDPP). Bidirectional rollers for both drillpipe and casing have shown the

most encouraging results especially inside casing while running liners and drill pipe

conveyed logging tools.Other torque reduction strategies thatare known tohavebeen

implemented by operators include torque reduction subs, beads, and downhole tractor

systems.ExperienceonpreviousERwellshavealso indicatedthattheuseoffibrousLost

CirculationMaterials (LCM)have a verybeneficial effect in improvingwellbore cleaning

and reducing both torque and drag (Cocking, Bezant and Tooms, 1997).When torque

valuesbegin to increase in the8section, lostcirculationmaterialsareknown tohave

beenusedtoreducetorquebyimprovingholecleaningeffectiveness.

2.3HoleCleaning

ERD wells typically require higher density fluids to maintain stability as compared to

verticalwells.High inclinationsalsoexposemoresurfaceareaofaparticularzonetothe

drilling fluid foragreaterperiod, thus increasing thepotential fordifferentialstickingor

lost circulation. It has been reported that hole cleaning is critical to the success of

extended reachdrilling,especiallywhere thehole inclination ishigh (Payneet al ,1994;

Ryan,ReynoldsandRaitt,1995;EckOlsen,Sietten,ReynoldsandSamuel,1994;Aarrestad,

andBlikra,1994;andIkedaetal.1996).

22

Poorholecleaninghasresultedinproblemsof:

1. Inabilitytotransferweighttothebit

2. Unexplainedandunplannedchangesindirection

3. Lostmud in the pay zone, which limits ultimate productivity due to formation

damage

4. Stuckpipeproblems

Severalpapershaveshownthe importanceofdrillpiperotationasameansof improving

holecleaning (Brett,BeckettandHolt,1989;Guildand Jeffrey,1994;Sanchez,Azar,and

Martins,1997).As thedrillpipe is rotated,cuttingsareagitated into the flowstreamand

circulatedoutofthewellbore.Toenhanceholecleaning,drillstringrotation isusedwhile

drillingandbackreaming,andwiththepipeoffthebottom.Inanotherstudy,Sanchezet

al.(1997)reportedthatthereductionincuttings'weightintheannuluscouldbeashighas

80% due to pipe rotation. There are obviouslymany factors involved inmodeling hole

cleaning, such as annular velocity andmud rheology. However, having continuous drill

stringrotationispossiblyoneofthemostsignificant.Deviatingtheholebyslidingamotor

willalsohinderholecleaning(Warren,1997;Ikedaetal.1996).

InreportingStatoilsexperiencesindrillinghorizontal,extendedreach,andcomplexwells,

Blikra,DrevdalandAarrestad(1994)propoundedthatdeepkickoffpointandlowbuildup

rateswillreduce the inclination inthe largerdiameterholesections.Thisgenerallygives

betterholestabilityand improvesholecleaning.Accordingtothereport,highrotationof

23

thedrillstring in theregionof150180rpmhavebeenknown to improveholecleaning,

especiallywhiledrilling the12and8hole sectionswithoilesteroretherbased

mudsystems.Theyalsoreportedthatdisturbingthecuttingsonthe lowsideofthehole

andcreatingaturbulentflowpatternaroundthedrillpipecan Ieadto improvedcuttings

removal. Blikra et al. (1994) also showed that backreaming in open hole and inside

casing/linersathighrotationspeedhasbecomestandardprocedureforERDwellsandhas

also resulted in improved cuttings removal. Other factors that have been known to

enhanceholecleaning includehigh flowrates, tightcontrolofmudproperties (especially

mudrheology)andcontrolleddrillingrates.

Additional research isbeingconducted toaddress theproblemof lostcirculation inERD

wells.Thehighwellbore inclinationpresentssignificantchallengesconcerningplacement

techniques,sufficientcoverageofthelosszone,andselectionofnondamagingmaterials.

Additionof lost circulationmaterials (LCM)hasbeen found togreatly reduceopenhole

torques through the formation of a lowfriction bed between the drill string and the

formation. Thus current investigations focus on a better understanding of loss and

propagationmechanismsand theapplicationof crosslinkpolymersandother fluids that

providelowcompressivestrength.

2.4CuttingsTransport

Efficient cuttings transport isone of theprimarydesign considerations for an extended

reachdrillingfluid,especiallyforwellswithhorizontalandhighlyinclinedsectionsofmore

24

than 6,000m (Guild,Wallace, andWassenborg, 1995; Gao, and Young, 1995; Schamp,

Estes,andKeller,2006).Guildetal.(1995)reportedthat,whilebeingtransportedfromthe

hole,cuttingscanbegroundtofinersandparticles,especiallywhenrotarydrillingisbeing

used.Under these conditions, drillingmay not be able to proceed if cuttings transport

remainsaproblem in thehole.Asaresultofexcessive torqueanddragcausedbysmall

cuttings settled at the lower side of the horizontal or inclined section, itmay not be

possibletoruncasinginplace,evenifdrillingtothetargetdepthisachievable.Horizontal

and highly inclinedwells drilled through unconsolidated sand reservoirs have also been

knowntohavethesesameproblems.

There is the argument that inefficient transport of small cuttings is themain factor for

excessive torque anddragduring extended reachdrilling.However, very little is known

about the transportbehaviorof smallcuttings.Experimentalobservations (Parker,1987;

Ahmed, 2001) show that small cuttings are more difficult to transport under certain

conditions.Ahmed(2001)alsosuggestedthatsmallerparticlestendtomoreeasilystickto

drillpipedue to theircohesiveeffects.Aboveall, it isevenmoredifficult to release the

pipeonce itgets stuckby small sandsized cuttings.Bymeasuring totalannular cuttings

concentration, Parker (1987) observed that smaller cuttings are easier to transport in

verticalwells,butslightlyhardertotransportinhighlyinclinedwells.Thisisconsistentwith

Larsens(1990)observationthatathighholeinclinationangles,smallercuttingsareharder

to clean out. These cuttings need a higher fluid velocity to keep continuous forward

movement.

25

Duan,Miska, Yu, Takach and Ahmed (2006) studied the transport of small cuttings in

extended reach drilling. Their study involved extensive experimentswith three sizes of

cuttingsinabidtoidentifythemainfactorsaffectingsmallcuttingstransport.Theeffects

ofcuttingssize,drillpiperotation,fluidrheology,flowrateandholeinclinationwerealso

investigated as part of the study. The results show significant differences in cuttings

transportbasedoncuttings size.Their studyproduced somevery importantconclusions

amongwhichare:

1. Intermsofcuttingsconcentration,smallercuttingsaremoredifficulttotransport

than larger cuttings in thehorizontalannuluswhen testedwithwater.However,

smallercuttingsareeasiertotransportthan largercuttingswhen0.25 Ibm/bblof

PolyanionicCellulose(PAC)solutionswereused.

2. Piperotationand fluidrheologyarekey factorsaffectingsmallcuttingstransport.

Improvementbypiperotation inthetransportefficiencyofsmallcuttings isupto

twice as large as the improvement in large cuttings transport. Compared with

water, PAC solutions significantly improve smaller cuttings transport, while the

transportoflargercuttingsisonlyslightlyenhanced.

3. Holeanglehasonlyminoreffectsoncuttingsconcentrationandbedheightwithin

therangeof7090degreesfromvertical.

26

Duanetal.(2006)concludedthisstudybyrecommendingthatdrillpiperotationcombined

withpolymericdrilling fluidsshouldbeusedtoefficientlytransportsmallcuttingsduring

extendedreachandhorizontaldrilling.

The factors thataffectcuttings transporthavebeenwelldescribed in the literatureand

theyinclude:

Drillingfluiddensity Lowshearraterheology Flowrate Cuttingsizeandconcentrationintheannulus Drillpipesize Rotaryspeed Shapeofparticles Drillstringeccentricityinthewellbore

Studies have also been conducted to determine the critical transport fluid velocity

necessarytopreventcuttingsbedformation(Larson,PilehvariandAzar,1993).However,

cuttingssizeisnottheonlyfactorthataffectseffectiveholecleaning.SiffermanandBecker

(1990) noted that cuttings size in itself only hasminor effects on hole cleaning,but its

influenceontheeffectsofotherparametersisnoteworthy.Inalaterstudy,Bassal(1995)

foundthattheeffectsofcuttingssizeoncuttingsconcentrationinahorizontalannulusare

quitedependentonotherparameters.Withlowviscositymud,smallercuttingsareharder

to transport than larger ones at all pipe rotary speeds and flow rates. However,with

highviscositymud,thetendencymayreversedependingondifferentflowrates.

27

Allthestudiesthathavebeenconductedoncuttingstransporthaveuseddifferentcuttings

sizes.Thus, the conclusions about theeffectsof cuttings sizeson cuttings transportare

quitevariedand,insomecases,contradictory.Theexperimentsuponwhichmostofthese

conclusionsarebasedwereconductedunderseparateandunequalconditions;therefore,

it cannotbeunequivocally stated that smaller cuttingsareharderoreasier to transport

than largerones.Anyconclusion should thus representacombinationofvariousdrilling

parametersbeforeitcanberegardedasdefinitive.

2.5RotarySteerableDrillingSystems

Before theadventofRotarySteerableDrillingSystems, therewere twomajormeansof

directional drilling. The first method was the use of conventional rotary assemblies

designed to give predictable deviation in a certain direction. Typical examples of such

rotaryassembliesarependulum,packedholeandbellyassemblies.Thesecond istheuse

ofadownholesteerablemotor.Thisrequiresthedrillstringtoslidealongthehole,without

drillstringrotation,tocontroldirection.

Variablegauge stabilizershavebeenused to fine tunecontrolofhole inclination (Odell,

Payne and Cocking, 1995; Bruce, Bezant and Pinnock, 1996), with both of the above

methods,buttheyhavenotbeenknowntocontrolazimuth.Steerablerotarydrillinggives

control of both inclination and azimuth and has a number of advantages over these

methods (Barr, Clegg and Russell, 1996). Rotary Steerable Systems enhance the

28

penetrationrateandincreasethereachofERDwells.Thisbooststheefficiencyandlowers

the overall cost of ERD operations. Using these systems, operators have been able to

optimize hole quality andwellbore placement, achieve faster rates of penetration, and

enhanced reservoirdeliverability.Rotary steerabledrilling systemswereusedon several

extendedreachwellsattheWytchFarm(Colebrook,Peach,Allen,andConran,1998).

Oneofthebiggestadvantagesofrotarydrillingisinapplyingweighttothebitinextended

reachwells.Asdepartureincreaseswithrespecttoverticaldepth,itbecomesincreasingly

difficult toapplyweight to thebitand subsequently tocontrol thatweight,due toaxial

friction(Cockingetal.1997;Warren,1997;Payneetal.1994).Drillstringrotationhasbeen

describedasavirtualcureall for thegeneral reductionofdrag (PayneandAbbassian,

1996)andtheapplicationofweighttothebit(Modietal,1997;Ryanetal,1995).Useof

rotary steerable systems allows the drilling parameters to be optimized for the bit and

formation. The reactive torque from the bit does not disturb the toolface and so both

weightonbitandrotaryspeedcanbeadjustedwithinwidelimits.

It isastatementoffactthatExtendedReachDrilling,usingrotarysteerablesystems,has

made itpossible todrill andproducehitherto impossiblewells. Several verydeepwells

wouldnothavebeenpossible todrillwithout rotarysteerablesystemsbecausesteering

beyond 8,500m was not possible as axial drags were too high to allow the oriented

steerablemotorandbittoslide(Colebrooketal.1998;Meaderetal.2000).Inplaceslike

BrazilandNigeriawheremajoroilfieldsarelocatedindeepwaters,ERDwellsmightbe,in

29

somecases,theonlyeconomicallyviabledrillingsolution(Cunha,MartinsandFernandez,

2000).

Ikeda et al. (1996) initiated a study that investigated developments, activities, and

philosophies of the larger oil and gas industry in implementing horizontal well and

extended reach technologies. North American and European operators, contractors,

service companies, government agencies and educational institutions were directly

interviewed.Alongwiththedevelopmentofdataresources,referencesand industrywell

information,significantfindingsandrevelationswerereported.Amongthemanyfindings

from the investigation is the fact thatmanyoperatorsarenow steering theirwellswith

MWDand LWD tools.According to the report,operatorsnoted that steeringawellhas

been thebestway tomaximizepay zonepenetration.The report also showed that the

most common failures seen bymost operators have been inmotor andMWD failures,

whichcausesdowntime.Rotorandstatorproblemswerecommonlydiscussed formotor

failurealongwithothercomponents,and itwasgenerallyagreedthatthefailureratefor

MWDincreasedwith:

o Increaseintemperature

o Increaseinpressure

o Rapidtemperatureincreaseorseverepressurechanges

o Smallerholesize

o Longerholelength

30

In underbalanced drilling situations,motor andMWD problems appeared to be higher.

Operatorsalsopointedout that thebiggest improvementsover the last coupleofyears

havebeenmadeinMWDandmotors,eventhoughtheymaystillcontributetodowntime.

Operatorscautioned that the limitingboundary forMWDpulsingsignals isnotcurrently

well defined by the vendor companies, and they also noted the fact that BHA steering

capabilitiesarereducedastrajectoryincreases.

AllofthefactorsindicatedaboveleadtotheconclusionthatamuchimprovedoverallROP

will be achieved while drilling with a rotary steerable drilling assembly than with a

steerablemotor. Indeed in certainextended reachoroverbalanceddrilling applications,

rotarydrillingisessential.

2.6ConcludingRemarksfromLiteratureReview

Because longterm profitability is a primary concern of the oil and gas industry,major

efforts are under way to optimize drilling procedures to make operations more cost

effective. All phases of drilling are being subjected to economic analysis and, now

emerging, is the possibility of obtaining substantial savings by upgrading conventional

directionaldrillingtechniques intoadvancedproceduresfordrillinghighangle,extended

reach and designer wells. Such a capability offers a variety of applications with the

potentialforsignificantreductionsincost.

31

Advances in drilling technology now enable the exploration for and development of

offshoreoil and gas resourceswithout the installationof facilities andwells inoffshore

marineenvironments.Nowadays,moreoperatorsare identifyingapplicationsofERD for

reservoirdevelopmentthatwillprovidesubstantial financialbenefit. InAlaska,anumber

ofNorthSlope fieldsextendoffshoreand/oraway fromexistingdrill sitesand reserves.

ThesewillwarrantERDoperations.ExxonMobilandtheCaliforniaStateLandsCommission

both endorse ERD development of the South Ellwood Offshore Field using ERD from

onshore sites (Starzer,Mount and Voskanian, 1994). BP is also considering ERD for its

Liberty Project in Alaska. In the Gulf ofMexico, China, South America, the North Sea,

Indonesia, the Middle East and elsewhere, many other ERD opportunities are being

evaluated to optimize development drilling of newmajor projects (Payne,Wilton and

Ramos,1995).

Thisstudywilltakeextendedreachdrillingimprovementonestepfurtherbyintegratingall

the critical technologies for success of extended reach drilling into a singlemodel. The

modelwilldesigntheprogrammostsuitabletodrillanextendedreachwell.

32

ChapterThree

CriticalTechnologiesforSuccessofExtendedReachWells

Theliteraturehasshownthatextendedreachexplorationanddevelopmentdrillingusually

involves some significant issues that canpose significantchallenges for theoperator.As

hasbeenmentionedseverally,technologiesthathavebeenfoundtobevitaltothesuccess

ofERDinclude:

WellTrajectoryDesign TorqueandDrag DrillstringDesign WellboreStability HydraulicsandHoleCleaning Casingdesign DrillingDynamics RigSizingandSelection NarrowMudWeight/FractureGradientWindow EquivalentCirculationDensity(ECD)Management LostCirculation Lubricity

Thefollowingsectionswillindividuallyandcollectivelyanalyzethesesuccessfactorswitha

view tounderstanding the scienceand technologybehind them.However,before this is

done, it is vital to firstdiscuss thedifferencebetween theeffects these issueshaveon

extendedreachwells,ascomparedtoconventionalwells.

33

3.1WhatisDifferentaboutExtendedReachDrilling?

There are numerous issues that are different, or more critical for ERD wells when

comparedtoconventionaldirectionalwells.Insomecases,thechallengesonERDwellsare

thesameasthoseofconventionaldirectionalwells,onlythatinextendedreachwells,they

aremagnified. Inothercases,the issuesarespecifictothetypeofERDwellthat isbeing

drilled (K&M Technologies, 2003). There are both general and specific differences in

challenges between ERD and conventional directional wells. Some of the general

differencesareoutlinedhereunder.

3.1.1Torque,DragandBucking

These factorsareregularlyencountered inERDwellsduringbothdrillingandcompletion

phases.Torque isgenerallyonlyasignificant limitingfactoron longERDwellsoronslim

holeERDwellswheresmalldiameterdrillpipeisused.Itiscommontorotatecasingliners

stringsonERDwellstoensurethatagoodcementjobisobtained.Thisisoftenthecritical

torquelimitation(K&MTechnologies,2003).InERwells,torquelimitscanbereachedina

numberofwaysincluding:

Topdriveorrotarytableoutput Drillpipetooljoints Casingconnections Combinedpowerusage

34

Axialdragoccursduetotheinteractionofthedrillpipeorcasingwiththewellboreasitis

runin(slackoff)orpickedup(pickup)outofhole(K&MTechnologies,2003).InERwells,

slackoff and pickup become problems as the well gets deeper and as inclination

increases.

Bucklingofdrillpipeandcasingresultsfromexcessivecompression loadsthatbuildup in

the string due to axial friction.Drill string and completion string buckling are common

problems in ERDwells. This is because, as the length of high angle sections becomes

longer,unconstrainedpipetendstobendinthewellbore.Asthissituationbecomesmore

severe,ahelixwilldevelopandeventuallypreventsthepipefrommovingwithinthehole.

3.1.2HoleCleaning

Field experiencehasdemonstrated thathole cleaningpracticesused for verticalor low

anglewellsaregenerallynotsuccessfulinhighangleERDwells.Athoroughunderstanding

ofthedynamicsofholecleaningisthereforecriticaltothesuccessofERDwells.

3.1.3EquivalentCirculationDensities

Extendedreachwellsgenerallyhavehigherequivalentcirculationdensityfluctuationsthan

conventionalwells.WiththeadventofMeasurementWhileDrilling(MWD)basedPressure

While Drilling (PWD) technology, the industrys understanding of ECDs has been

challenged (K&MTechnologies,2003).Studieshave shown that themagnitudeofECD

35

fluctuations inERDwells is fargreater thanpreviously thought.According to findingsby

K&MTechnologies,ECDsaremoreseriousinERDwellsbecause:

i. Themagnitudesofthefluctuationsareworseduetolongermeasureddepth(MD)

relativetotrueverticaldepth(TVD).

ii. Wellborestability, lostcirculationandotherkeyeffectsaregenerallymoresevere

andlesstolerableinthesewells.

iii. Temperatureandpressurevariations(andtheireffectsonmudproperties)arealso

moreextremeinthesewells.

3.1.4RigCapabilityandPowerRequirements

Extendedreachwellschallengethecapabilitiesofthedrillingrigmorethanaconventional

directional well of the same measured depth except for pickup loads (K & M

Technologies,2003).Theneedtousecontinuoushigherflowratesathigherpressures,use

ofhigherpipeRPMandhigher torqueanddrag forceswillalsocontinuously taska rigs

outputcapability.Also,powermaybelimited,especiallyinabackreamingscenariowhere

pickup, torqueandpumpsarealloperatingat/ornear their limit.Thecombinedpower

usagewhendeeponalongERDwellmaythusbecomeanissuebecauseitisoftenatthis

pointthatmaximumoutput levelsarerequired fromthemudpump,drawworksandthe

rotary system. Many of the industrys rigs that are being utilized for conventional

directional drilling do not have the capability to meet these combined output

requirements.

36

3.1.5WellboreInstability,DifferentialStickingandStuckPipe

WellboreinstabilityisusuallymorecriticalinERDwellsdueto:

o Theincreasedwellboreangle

o Increasedholeexposuretime

o IncreasedECDfluctuationsandeffects

DifferentialStickingisalsoasignificantprobleminERDwellsbecause:

o MudweightisoftenhigherinERDwells(forwellborestabilityathighangles)

o Theexposedreservoirintervalsarelongerinlength

o Theexposedreservoirintervalsareopenforalongerperiodoftime

o Thedrill stringandBHAwillbeon the low sideof theholeandat leastpartially

buriedincuttingsthroughoutthereservoirsection

Differential sticking is evenmore important for wells where torque, drag, or buckling

problemsexist,sinceevenminordifferentialsticking increases friction.Furthermore, the

abilitytojarorworkthepipefreeisreducedonERDwells.Thisisasaresultofthereduced

abilitytogetweightortorsiondowntheBHA.

37

3.1.6WellControl

This isgenerallymorechallenging inERDwellsbecausekicksareoftenmoredifficult to

detectandmeasureasaresultofthegeometriesofthewells.Notonly iskickdetection

moredifficult inERDwells, theability tomanageandkillthewellcontrolproblemare

evenmoredifficultasaresultoftheflowmechanics.

Other issues thathavebeenreported tobe,notonlydifferent,butmorecrucial forERD

wells than for conventional directional wells include: Survey Accuracy and Target

Definition,Logistics,TimeandCost,HSEIssues,andWellPlanning.Havingestablishedthe

differences in the challenges faced in conventional directional wells as compared to

extended reachwells, the following sectionswillexamine theparameters thathave the

greatestsignificanceinthesuccessorfailureofanERDproject.

3.2WellPlanningandTrajectoryDesign

As is the casewith all drilling operations, detailed engineering preplanning is of critical

importanceinsuccessfullydrillingERDwells.TosuccessfullydrillanERwell,itiscrucialto

beabletoaccuratelypredictthefollowingparametersunderactualdownholeconditions:

1. StaticandDynamicTemperatureProfileintheWell

2. HydraulicPressures

3. AnnularPressureLoss

4. EquivalentCirculationDensity

5. MudRheology

38

Theoperational requirements todrillextreme reachwells startwithextensiveplanning.

Well planning is usually an iterative process to determine the optimal balance among

wellpath,fluidandhydraulicrequirements,drillstringdesign,torque&draganalysis,casing

settingdepthetc.The iterativeprocessnotonlycoversthewellpathdesign,butalsothe

operations planning aswell. Thewell design has a large impact on the operation and

planningofERwellsandsincetherearemanyvariablesindesigningawell,suchasKickoff

Point(KOP),DoglegSeverity(DLS),Departure,etc., itbecomesan iterativeprocessaimed

atyieldinganoptimaldesignwhichshouldresultinthesimplestpathwhilestillachieving

allgeologicaltargets.

Inothertoachieveanoptimumwellpath,severalfactorshavetobeconsideredandput

into perspective. Based on reported industry experience gained from earlierwells, and

confirmedduringthedrillingofmorerecentwells,thefollowingaspectsareconsideredto

bekeyfactorsinwellplanning:

WellTrajectory BuildRate SurveyingandTargetSizing

3.2.1WellTrajectoryDesignTodeterminetheoptimumwelltrajectorythatwillachievedirectionalobjectives,themost

critical operations orwellbore characteristics,which are the limiting factors have to be

identified.Thereareseveralapproachestotrajectorydesigntoachievelongreacheswith

the fewest possible limitations on other downhole operations. Table 3.1 is a general

39

comparison of the major options while Figure 3.1 is a representation of the various

trajectoryprofiles.

Table3.1:WellTrajectoryOptions,AdvantagesandDisadvantages

Option Advantages Disadvantages

Multiple Build Profile: Rate ofbuild increases with depth inseveral discrete steps totangent angle, hold constanttangentangle

Very long reach, lowtorque/drag values, lowcasingwear

Hightangentangle

BuildandHold:Constantbuildrate to tangent angle, holdconstanttangentangle

Simple, long reachesachievable, low tangentangle

Potentiallyhighcontactforceinbuild(torque,casingwear)

Double Build: Buildholdbuildhold trajectory, can use twodifferent BURs in the buildsections

Very long reaches possiblewith low contact forces inupperbuild

May require deep steering,Highsecondtangentangle

UnderSection: Build and holdwithdeepKOP

Reducing hanging weightbelow build section, reducescontactforceinbuild

High tangent angle, shorterreach

Inverted: Tangent angle abovehorizontal so the wellboreenters the reservoir fromunderneath

Flexibility for multipletargets,avoidsgascap

Higher axial (buckling) loadsto push string uphill, deepsteeringrequired

3D: Any of the above withsignificantazimuthchanges

Flexibility to handle anticollision andmultiple targetrequirements

Morecurvaturemeansmoretorque and drag, deepsteering may be required,shorterreach

Source:BPExtendedReachDrillingGuidelines,1996

40

Figure3.1:WellTrajectoryProfiles

3.2.2CatenaryWellDesign

Anotherwellboreprofile,which is frequentlydiscussed, is the catenarywelldesign that

was firstproposedandpatentedbyDaileyPetroleumServices. Inacatenaryprofile, the

rateofinclinationbuildcontinuouslyincreaseswithdepthtomimictheshapeofahanging

cable.Thisdesignstartswithalowbuilduprate(say0.5o1.0o/100ft),andacceleratesto

higherbuildratesastheangleincreases(sayupto4o5o/100ft).Theoretically,acatenary

producesverylowtorqueanddragasaresultoflowcontactforcesbetweenthestringand

thewallof thehole.However, catenarieshavenotbeenwidelyused since creating the

41

catenary shape is impractical and costprohibitive even with the most modern BHA

configurations. One of the downsides of the catenary design is that it considerably

increasesboth the tangentangleand theoverall totaldepth. This increasedanglemay

also make wellbore stability more difficult to manage, which in turn can create hole

cleaningproblems.

Inchoosingamongtheseoptions,ausefulconcepttokeepinmindistheCriticalTangent

Angle.Thisanglerepresentsthelimitbeyondwhichatoolwillnotslidedownholeunderits

ownweight,meaningthatitwillhavetobepushedfromabove.TheCriticalTangentAngle

ismathematicallyrepresentedby:

qCos=qSin

Tan= 1(3.1)

Where:

q=Pipebuoyantweight,

=Frictionfactor

=Criticaltangentinclinationangle

Oneapproachtooptimizingthetrajectory istotrytopositiontheKickoffPoint(KOP)so

thatthetangentinclinationequalsthecriticalangle.Ifpossiblegivenotherconstraints,this

willallowlongreacheswithreducedslidingproblems.

42

3.3BuildRate

Anextendedreachwellneedsoptimumwelldesignwithrespecttotorqueanddragand

wellbore stability.Utilizing a lowbuild rate is considered tobe very importantboth for

torqueanddragatTD,andforwellborestability intheupperpartofthewell.Dueto its

impact on casing wear and torque and drag, build rate can be viewed as the most

importantconsiderationwhendesigninganextendedreachwellprofile.Whencompared

to a path having a single build rate, a combination of build rates usually offers a

compromise.Areviewofrecordextendedreachwellsshowsthatmostofthewellsbegan

withalowbuildrate,thengraduallysteppinguptoamaximumof1.5/100ft.However,in

situationswheretheTVDofthetargetisshallow,asinglemoreaggressivebuildratemay

be the better option. For extended reach wellpath, the industry is graduallymoving

towardacatenarydesignwhereverylowbuildratesareusedandsailanglesareathigher

anglesasageneraltrend.

3.3.1EffectofBuildRate

Buildratemayhaveamarginaleffectontorqueanddrag levelsforveryhighratiowells.

Thisisduetotheincreasingpercentageofstringweightsupportedonthelowsideofthe

holeresultinginlowertensileforcesatsurface.Howevercontactforcesmaybesufficient

to promote unacceptable casing wear at the higher build rates, especially when well

operations such as extended backreaming are anticipated due to poor primary hole

cleaning.Asaguideline,buildratesinexcessof2.5o/30mmaycauseconcernwithrespect

tohighcontactforces.Ifhigherbuildratesthanthisareplanned,thedifficultyofachieving

43

asmoothbuildalsohastobeconsideredwhereanincreasingpercentageofthebuildwill

beperformedwhileslidingandnotrotatingtheassembly.Someotherimportantthingsto

noteaboutbuildrateare:

a) HighReach/TVDratiowellsmaytoleratehighBURbecausethestringtensioninthe

curveislowandmayevenbeinmechanicalcompression.

b) LowReach/TVDratiowellsdonottoleratehighBURsincedrillstringtensioninthe

curveishigher.

c) Highbuildratescancausecasingwearproblems,especiallyinhighReach/TVDratio

wellswheretheremaybehightensile loadsthroughthebuildsectionduringtrips

outoftheholeandbackreaming.

d) LowBURsresultinlowercontactforces.Thistypicallymeanslowercasingwear.

e) Low tortuosity is also achievablewith lowBURs. It tends tobemoredifficult to

maintainlowtortuositywithahighBUR.

f) Generally,withlowerbuildrate,morecanbeachievedwhilerotatingtheassembly

andthusthechancesofachievingthedesiredsmoothbuildwillbegreatest.

3.4HoleSizingandSelection

ThemajorityofERDwellsdrilledaroundtheworlduseacombinationof17,12,and

8holesizes(K&MTechnologies,2003).AccordingtoK&MTechnologies,thereasons

44

for this include theavailabilityof toolsandequipment,ability todrill smallerhole sizes,

andsimplythedepthofexperienceinthesesizes.

However,therearesomebenefitsassociatedwithtwostringwelldesignsand97/8holein

particular. In ERD applications where two casings strings can be reliably used to TD,

considerationmaybegiven tousing13and97/8hole sizes (asanalternative to the

traditional17 x 12 x 8design).Thesmallerholesizesrequire lessflowrateto

keepthemclean,ortheycanbecleanedfasterwiththesameflowrate,therebyallowing

for faster penetration rates (97/8 hole has 50% less volume than 12 hole).Where

stabilityisaprimaryconsideration,thesmallerholesizesarealsoinherentlymorestable.

Thedownsideofthisstrategyhowever,isitsinabilitytoaffectpipemovementduringthe

cement job and therefore, limiting the probability for successful zonal isolation. It has

thereforebeensuggestedthatwhendesigninganERDwell,standardholesizesthathave

beenusedinthepastshouldnotjustbeautomaticallyselected.Thisisbecause,theremay

beadvantagesthatmightbegainedbyevaluatingdifferentholesizesandcombinations.

3.5TorqueandDrag

3.5.1Torque

Torqueisarotationalforceanditcanbedescribedastheabilitytoovercomeresistanceto

rotation. Itsmagnitude ismeasuredbymultiplying theperpendicular componentof the

forceappliedbythedistancebetweentheaxisofrotationandthepointwheretheforceis

45

applied. Indrillingapplications thisdistancewouldof coursebe thedrillpipe radius.As

depictedbyFigure3.2below,torqueismathematicallyrepresentedas:

Torque=ForcexDistance

Where:d=DrillpipeOD

Figure3.2:DiagrammaticRepresentationofTorqueGeneratingForces

Torquecanalsoberegardedasaforcethatproducestorsionorrotation.Itisgeneratedby

powerequipment from surface,aswellasby frictionworkingagainst thepipe rotation.

Drillstring torque is the force required to rotate the pipe. (Unit: Nm (metric), ftlbs

(imperial)1ftlbs=1.3558Nm).Figure3.2isarepresentationoftheforcesresponsiblefor

creatingtorque.

46

OpposingForceDuetoFriction=M

Figure3.3:FrictionalandSurfaceactingforces

RotationalForce=M

Indrillingoperations,thetorqueatthesurfaceisgivenby:

TQ@Surface=TQ@Bit+TQalongthewellbore+MechanicalTQ

Where:

TQ@Bitorbittorque=theproductivecomponentofthetorqueand itdependsonbit

aggressiveness,WOBandbitdiameter.

TQbit=WOB*BitDiameter*BitAggressiveness

TQalongthewellboreorfrictionalstringtorque=theresultoftheinteractionbetween

thedrillstringandtheboreholewall.Itincreaseswithincreasedtorsionalfrictionlossesas

thedrillingprogressesand this isa functionof friction factorandside forces (axial load,

wellprofile ...etc.).MechanicalTorque= isgeneratedby cuttingbeds, stabilizereffects,

linercentralizeretcandisdifficulttoquantify.

47

Dependingonwelldesignanddrillingoperation,thetorquewilldevelopindifferentways

alongthewellbore.Analysisandprojectionsoftorqueshouldrecognizethattotalsurface

torqueiscomprisedof:

TotalSurfaceTorque=Stringtorque+Bittorque+Mechanicaltorque+Dynamictorque

Clearly,separatingthesetorquecomponentsallowsmoreaccuratedefinitionoffrictionfor

torqueprojectionsandforprioritizingmeasuresfortorquemanagement.Withtechniques

available for predicting bit torque, the implications of using different bit types can be

assessed.

3.5.2DragDragisaresistanceforcetothemotionofanobjectanditactsintheoppositedirectionof

its axialmovement. It is as a force that resistsmotion along a straight path. In drilling

operations,dragresultsfromcontactbetweendrillstringcomponentsandboreholewall

orcasingas thestringmovesupordown. It isgeneratedby frictionofdrillpipeagainst

holewalloragainst insideofcasing.Dragwillalwaysoperate intheoppositedirectionto

thatinwhichthedrillstringisbeingmoved.Dragisexperiencedasanextraloadoverthe

rotatingstringweightwhentrippingoutofthehole.UnitN(metric),lbsf(imperial)1lbsf

=0.4448N.

Itaccumulatesmainlywhenpickingup,slackingofforduringorienteddrillingwithmotor.

It increaseswith increased hole inclination and curvature due to the gravity effect and

compressionpushingthedrillstringagainstthe lowsideoftheboreholeandduetodrill

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stringtensionpullingupthedrillstringtothehighsideofthehole.Dragistheincremental

force required tomove thepipeupordown in thehole.Figures3.3and3.4 shows the

forcesresponsibleforcausingdrag:

Forcestoliftdrillstring=F OpposingForceDue

toFriction=Drag

Figure3.4:DrillStringOpposingForces

49

PickUpForce=Drag+AxialForce

Figure3.5:DragandPickUpForces

Drag=SideForcexFrictionFactor

Dragwhiledrillingawellisaresultofacombinationofseveralfactorslikewellpathdesign,

drilling fluid properties (lubricity),wellbore quality, tortuosity, formation type and drill

stringbuckling.

3.5.3FrictionForces

ThekeyfeaturesofERDarethathorizontaldepartureislongandinclinationishigh.These

typicallygives rise toconsiderable torqueanddrag fordrillstringandcasingstring, thus

makingtorqueanddragamajorlimitationforhorizontalreachinERD.Thisalsomakesthe

effective transmission of drill stringweight and rotation to the Bottom Hole Assembly

(BHA),majorconsiderationsintheplanningofERwells.

50

Apuzzlethatstillremainsunansweredinextendedreachdrillingis:Whatmakeshighratio

ERDwellssodifficulttodrill?Theanswer,accordingtoWilliams(2008)isfrictionandhow

besttoovercomeit.

Friction isa functionofbothtorqueanddrag.Fromthedefinitionsabove itcanbeseen

thattorqueistherotationalelementwhiledragistheaxialelement.Torquelossesareof

prime importancewhiledrilling, anddrag losses aremore importantwhile tripping and

running casing strings. Inotherwords:AxialFriction (Drag)actsalong the lengthof the

pipebody(longitudinalaxis)intheoppositedirectiontothedirectionofpipetravel(POOH,

RIH) while Torsional Friction (Torque) acts along the circumference of the pipe body

oppositetothedirectionofrotation.

Whilethereareamyriadofchallenges inERD,minimizingtorqueanddragwhiledrilling,

runningcasing,andduringcompletionscontinuetobeoneofthegreatestchallenges.As

thelengthandpercentageofanywellathighangleincreases,frictionalsoincreasesdueto

the increasedcontactareabetweenany string (drillingorcasing)and thewellborewall.

This is further compounded by themass of this string at high angle. Themeaning and

implicationofthisisthat,whenrunningdrillstringinandoutofthehole,wegettoapoint

where enoughweight (push) from surface cannot be exerted tomove the string. The

conventionalapproachtosolvingthisproblemistorotatesoastoreducedrag.Whilethis

temporarilysolvestheproblem,asdrillingprogresses,wewillreachanotherpointwhere

enoughtorquecannotbegeneratedatthesurfacetoturnthestring.There istherefore,

51

theneed to comeupwith techniques to reduce torque anddrag ifwewant to reduce

downtimeandsuccessfullydrillERwells.

3.5.4InfluenceofFrictionFactorFrictionFactor(FF)alsoknownasfrictioncoefficient,expressestherelationshipbetween

theforcesneededtoovercomefrictionalforceandthenormalforceoftheelement. It is

defined by the interaction between twomaterials. Since torque and drag can be the

limiting design parameters, the optimum trajectory design depends heavily on our

representationofwellborefriction.Weusecasedholeandopenholefrictionfactorsinour

torqueanddragstudies,buttheyarenotnecessarilyreflectiveofthecoefficientsofsliding

frictiononemightmeasure ina lab.Friction factorsshould thusbecalculated from field

torqueanddragdatawhichdependuponanumberofconditionsincluding:

o Mudcompositiono Holecleaning(cuttingsbeds)andcuttingtypeo Operationalproceduresandtypeofoperation(e.g.slidingorrotating)o Wellboretortuosityo Formationtype

Thereareseveralissuesthataffectfrictionfactor.Theyinclude:

Drillstinginteractionwithcasingorwithopenhole. TypeofDrillingFluid(WBM,OBM,Foam,Air) Mud rheology and properties, whichmight change while drilling particularly in

HTHPapplications.

52

Bythesametoken,therearesomeissuesthatdonotaffectFrictionFactor.Theyinclude:

Drillingmode(WhetherRotatingorSliding) Theloadapplied

ItshouldbenotedthatTorqueandDragpredictionsareonlyasgoodasthefrictionfactors

applied.Thus,itisrecommendedthateachdrillingprojectattheverybeginningestablisha

databaseof local casingandopenhole friction factors for themud typeused.Standard

defaultfrictionfactorshavebeenderived(usingcommerciallyavailablesoftwarepackages)

fromanalysesofwelldatacoveringnormaldrillingoperationsinarangeofwells.

3.5.5TorqueandDragReductionMethods

Therearemanywaystoreducetorqueanddrag.Optimizationofthewellprofile isone.

We should selectwell trajectory thatmakes torque anddrag values as low aspossible.

Mud additives and drillstring tools can also be used to reduce torque and drag.

Constructionofhigherspecificationrigscapableofgeneratingenoughweightandtorque

torunstringsinandoutoftheholeandalsotorotatethepipeisanothertorquereduction

method.

BasedonfieldexperienceandendofwellreportsfromseveralERDprojects,thefollowing

hasbeenrecommendedforreducingtorqueanddrag:

53

ToreduceTorque:

UseofalighterBHA,andreducethenumberofHWDPandDC's.MakesurethereisenoughWOBavailable,

Useamudsystemwithlowerfrictionfactor(semioilbasedoroilbased), UseNonRotatingDrillPipeProtectors(NRDPP)forcasedholetorquereductionor

bearingsubforopenholetorquereduction,

Redesign thewellwhenpossibleandconsiderdesignerwellswithcatenarycurvebuildsections(gradualincreaseincurvature).

ToreduceDrag:

Optimizewelltrajectoryanddrillstringdesign InhorizontalwellsuseHWDP/DCneartheverticalsection. Userotarysteerablesystems Usehighpowermotorstoincreasestallingresistance Considercasingflotation.

Mechanical and chemical friction reducers are used by operators to reduce torque and

drag. Rubber element and plastic casing protectors have also been reported to be

successful inreducingtorque.Severaloperatorshavereportedtorquereductionsofover

25%byusingnonrotatingdrillpipeprotectors.Othertorquereductionstrategiesthathave

beenmentionedbyoperatorsincludetorquereductionsubs,beads,anddownholetractor

systems.

54

NonRotatingDrillPipeProtectorshavealsobeenused to reduce casingwear.Although

there isasignificant torque reductionwith these tools, there isalsoan increase indrag.

Whenthedrillpipeprotectorsareused,theremustbeacompromisebasedonexpected

requirementsforrotaryororienteddrilling.

Torque andDrag valueswill increasewith thewell length up to the pointwhere a rig

equipmentupgradewillberequired.New techniqueswillalsoberequired.Forexample,

runningthe95/8casinginflotationwillbenecessarybeyondagivendepartureandsailing

angle,whereallavailableweightisabsorbedbythedrag.

3.6EquivalentCirculationDensityManagement

For easy comparison to critical mud density limits, the annulus pressure is often

transformed intoa virtualdensity value calledEquivalentCirculatingDensity (ECD).ECD

canbedefined as theadditional mudweight seenby thehole,due to the circulating

pressurelossesofthefluidintheannulus.

ECD is thedensitya static fluidneeds tohave inorder tocreate the samepressure the

actualfluidcreateswhilecirculatingintheannulusatacertaindepth.Ittakesintoaccount

the influenceofpressure lossesofthefluidflowinguptheannulusaswellaschangesof

thefluidsaveragedensityduetothecuttingsloadintheannulus.

55

EquivalentCirculationDensitycanbecalculatedasfollows:

ECD=MW(ppg)+)(*052.0

)(ftTVD

psiPA (3.2)

Where:ECD=EquivalentCirculatingDensity[ppg]

MW=MudWeight[ppg]

PA=MeasuredAnnulusPressure[Psi]

TVD=TrueVerticalDepth[ft]

The principal variable in this equation is annulus pressure loss. This is affected by the

followingfactors:

Drillpipeconfiguration Mudweight Mudrheology Flowrate StringRPM Holecleaningefficiency Tripspeed AnnularClearance

Duringdrilling,themudcolumnpressurehastobecontrolledandregulated.Wedothisby

comparingit(i.e.themudcolumnpressure)tothe:

PorePressureGradient(PPG) FormationBreakGradient(FBG) FracturePressureGradient(FPG)

56

Weneedtocontrolthemudcolumnpressurebecause:

1. Wedonotwantporefluidstoentertheborehole

2. Wedonotwanttofracturetheformation

TheECDisusedtoanalyzetherelationshipofthedownholepressuretotheporepressure

and the fracturepressureandhas tobe lookedat inconnectionwithporeand fracture

pressuregradients.Itneedstobeinsidethewindowcreatedbythedifferencebetween

pore and the fracture pressure gradient. If the ECD is lower than the pore pressure

gradient, the well is prone to well control problems. If it is higher than the fracture

pressuregradient,thewellispronetoholestabilityproblems.

A critical ECD issue to lookout forwhendrillingdeviatedwells is the casing shoe ECD,

whichisthepressureatthecasingshoe.Aspreviouslynoted,thisisbecausetheformation

pressure and the pore pressure are a function of the TVD and the pressure drop

contributingtotheECDisafunctionofthemeasureddepth.ThisisexplainedinFigure3.5.

57

Figure3.6:CasingShoeECDDetermination

FromFigure3.5,wecanseethatsolvingoneproblembycreatinglowpressureatthebit

willcreateanotherproblem.Wewillbeindangerofcreatingacriticallowpressureatthe

casingshoeasthepressuredropoverthislongdistance(L2)isbiggerthanthechangein

formationpressureoverthisverticaldepth(TVDL1).Fromtheabove,itcanbeseenthat

overarelativelysmallverticaldepthchange,i.e.asmallformationpressurechange,there

isabigECDchange.Thiscouldleadtoacollapsingholeorincomingporefluids.Theriskof

thishappening ismostrelevantatthecasingshoe,sincetheECD(mudpressure) isat its

lowesttherewithoutbeingprotectedbythecasing.

58

SomeotherimportantfactstonoteaboutECDare:

1. IfpipeODandwellboreIDdonotchangeandinclinationiszero,ECDisconstant.

2. TheECDincreasesintheopenhole,wheretheannularclearanceislessthaninthe

casing.Ifthewellisverticalandinfinitelylong,theECDwillreachaconstantvalue

again.

3. IfthewellboreIDdoesnotchange,butthedrillstringcontainsdrillcollarswithlarge

OD,theECDincreases.

4. If thewellboreopensup,or if thedrillstringchanges tosmaller IDs, theECDwill

decrease.

5. ECD increaseswith increasing inclination.This isdue to the fact that theannular

pressuredropisafunctionofmeasureddepthwhileECDisafunctionofTVD.

6. Inaverticalwell,ifPressureGradientisconstant,theECDwillbeconstantaswell.

7. IftheopenholehaslargerIDthantheCasingabove,thetotalannularpressuredrop

isless,comparedtoaconventionallydrilledwell.Therefore,theECDisalsoless.

3.6.1EffectsofECD

ECDsaregenerallyamoresignificantissueinERDwellsthanforconventionalwells.Thisis

asaresultoftheirlongmeasureddepthintervalsrelativetotheverticaldepths.Also,ERD

wellsaregenerallyshallowinnature.ThismakesthemparticularlypronetoECDproblems

as their formations are often so shallow as to have very little integrity.Other reasons

include the fact that extended reach wells generally use larger diameter drill pipe to

59

combathydraulicsorbucklingproblems; they requiremoreaggressiveparameters (flow

rateandrpm)forholecleaning;andtheyhavelongerexposuretimeswithlongintervals.

SomeoftheeffectsofECDsare:

i. HighECDsincreasetheriskoflostcirculation,especiallywhile(a)drillingthe8or

smallerdiameterhole size,or (b)while runningor circulating long casing strings.

Also,ifECDsarenotminimized,itcouldleadtoreservoirdamage.

ii. Theconstant flexingand relaxingof thewellborewhen thepumpsare turnedon

andoffcan leadtowellbore instability.This isparticularlytrue ifthe formation is

brittle.

iii. Casing collapse can be initiated by ECDswhile running buoyancy assisted casing

stringsonlongdeepERDwells.

3.6.2ECDManagementandControl

Reducing flow rate isgenerally the firstoption ifECDbecomesan issuewhiledrillingan

extended reachwell. It is important that any reduction in flow rate iswithin the hole

cleaninglimitationsofthedrillingsystem.Theminimumallowableflowrateisdependent

onmanyfactors includingmudrheology,RPM,slidefrequency,holesize,ROP,andother

practices.Aswasmentionedearlier,piperotationalsoaffectsECDsespecially inthe8

holeor smaller.Aswith flow rate above, reducingpipeRPM is also anoption to lower

ECDs.

60

3.7DrillingHydraulicsandHoleCleaning

Hydraulics calculations are generally carried out to estimate the required rig pumps

capacitytodrilltheobjectivewell.Thedrillinghydraulicsystemisafunctionofthedrilling

fluidcharacteristicsanditsabilitytodeliverefficientdrillingandensurewellboreintegrity

andstability.Thereforetherequiredpumppressuremustbecapableofprovidingtheflow

rateneeded to transport thecuttingsupandoutof thewellbore,aswellasovercoming

theaccumulatedpressure lossesassociatedwith the surfaceequipment, thedrill string,

thebitandtheannulus.

Thehydraulicsapplicationsandanalysesarebasedondrillingfluidbehavior.Thisbehavior

is governed by rheology and hydraulics studies,which interrelate between each other.

Rheology is the study of howmatter (substance) deforms and flows while Hydraulics

describeshowfluidflowcreatesandusespressures.

AsidefromECDManagement,othercriticaldrillingfluidsuccessfactorsintheplanningand

constructionofextendedreachwellsinclude:

BoreholeStabilization HoleCleaning Lubricity

61

TheselectionprocessforERDdrillingfluidsmustconsideranumberofcriticalfactors.The

fluidmust:

a) Provideastablewellborefordrillinglongopenholeintervalsathighangles.

b) Maximizelubricitytoreducetoqueanddrag

c) Developproperrheologyforeffectivecuttingstransport

d) Minimizethepotentialforproblemssuchasdifferentialstickingandlostcirculation

e) Minimizeformationdamageoftheproductionintervals

3.7.1FundamentalsofHoleCleaning

Hole angle determines the mechanisms for cutting removal. Figure 3.6 is an angular

depictionofatypicalextendedreachwelltrajectory.

Figure3.7:AngularDepictionofExtendedReachWellTrajectory

62

Indeviatedwells,cuttingstendtosettleonthelowsidewallandformcuttingsbeds.These

cuttingsareoftentransportedalongthelowsideoftheholeeitherasacontinuousmoving

bedorinseparatedbeds/dunes.

Holecleaningcanbedividedintothreecategoriesbasedonthewellboreinclination.Figure

3.7belowshowsthemovementofcuttingsinthevariousangularregionsofthewellbore.

From the figure, it is obvious that the cuttings transport, and by extension the hole

cleaningstrategy,willbedifferentforeachinclinationrange.

Figure3.8:CuttingsTransportatDifferentInclinations

63

Basedon the following references [AzarandOkrajni,SPE14178;ClarkandBickham,SPE

28306; Saasen and Loklingholm, SPE 74558], the cuttings characteristics of the various

inclinationanglescanbegivenas:

035deg:Cuttingsdonotform

3545deg:Cuttingsbedstarttoform

4565deg:Avalanchingstarts

y 6590deg:Stablebeds

NearverticalWells(0350):CuttingBedsdonotforminthisholesection.Thisisbecause

transportvelocity isgreater thanslipvelocity, thus thecuttingsareeffectivelycarried in

suspension. The annular fluid velocity acts to overcome the cuttings settling force and

there isanetupwardmovementofthecuttings. Holecleaning issimplyprovidedbythe

viscosityand flowrateof thedrilling fluid.When thepumpsare turnedoff, cuttingsare

suspendedbytheviscousdrillingfluid,althoughsomesettlingwilloccurwithtime.

IntermediateAngles(35650):Inthissection,thereisunstable,movingcuttingsbed.This

isbecause transport isvia liftingmechanism. In this inclination range,cuttingsbegin to

form dunes, as thedistance for them to fall to thebottom isnow veryminimal. The

cuttingsmoveuptheholemostlyonthelowside,butcanbeeasilystirredupintheflow

regime. Themost notable feature of this inclination range is thatwhen the pumps are

shutoff,theduneswillbegintoslide(oravalanche)downhole.Thissignificantlychanges

the hole cleaning strategy with respect to the vertical well scenario. Typically most

64

problems associated with hole cleaning in deviated wells occur in this section. This is

because;itistheregionwheregravityeffectscancausecuttingsbedstoslumpdownthe

hole.

HighAngles(greaterthan650):Stationarycuttingsbedforminstantaneouslyinthisregion.

This is because transport is via a rollingmechanism. This presents a different set of

operationalcircumstances.Here,cuttingswill fall to the lowsideof theholeand forma

long, continuous cuttingsbed.Allof thedrilling fluidwillmoveabove thedrillpipe,and

mechanical agitation is required to move the cuttings, regardless of the flowrate or

viscosityofthemud.

Toareasonableextent,holecleaningoperationcanberemediedbyagoodcombination

of:

1. AppropriateMudProperties

2. Optimizedwellprofile

3. Standarddrillingpracticesprocedures.

However, continuous ECD measurement remains the best method to monitor hole

cleaningandminimizelostcirculationproblemsorstuckpipe.

3.7.2FactorsAffectingHoleCleaning

Perhaps the singlemost importantaspectofERDwellplanning isensuring that the rigs

pumps, solids control equipment, drillstring components and selectedmud system are

65

adequate to keep the hole clean. Hole cleaning is one of the most crucial areas in

successfullydrillinganERDwell.Theparameterswhichaffectholecleaningmorethanany

other(Payneetal,1994);(EckOlsenetal,1994)are:

o FlowRateDeterminestransportandannularvelocityo HoleAngleDeterminesmechanismofremovalo Fluidrheologyandflowregimeo MudDensityo Rateofpenetrationo Drillpiperotationo Geometryo Eccentricityo CuttingsSizeo CuttingsShapeo CuttingsDensityo Formation

3.7.3ConsequencesofPoorHoleCleaning

Cuttings generatedwhile drilling needs to be removed from the hole and transported

throughtheannulustothesurface.Apoorlycleanedholewillleadtoabuildupofcutting

bedsandareduction intheannulusarea.Thismayresult inmanydrillingproblemssuch

as:

i. Formationofastationarycuttingsbed