coiled tubing-the future of underbalanced drilling

8/13/2019 Coiled Tubing-The Future of Underbalanced Drilling

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POOOS-97

ByD. B. Bennion, F. B. Thomas,R. F. Bietz, A.K.M. JamaluddinHycal Energy Research aboratoriesLtd.

Paper o be presented t the 5th nternational ConferenceonCoiled Tubing Technologies,Drilling-Completions-Field Operations-ServicesTo be held in Dallas, Texas January8-10, 1997

INTRODUCTIONABSTRACTWhat s UnderbalancedDrilling? Underbalanced rillintechnically classified as any situation in which the effeccirculating pressureof the downholedrilling fluid is less the pore pressureof the adjacent onnation (preferablyoveentire length of the exposedopen hole section of the paysomesituations, where he formation hashigh natural pres

an underbalanced ondition can be generated uite easily uconventional unweighted type drilling fluids (clear brineoils). This type of drilling is common in such placesasAustin Chalk in Texas and is often referred to as drilling . In situationswhere he reservoirpressures deplsome type of non-condensiblegas (either as a gasified fmist or pure gas injection operation) s generally requireengendera sufficiently low equivalent circulating densgenerate an underbalancedcondition. The underbaldrilling process s illustrated schematically s Figure I anbeen discussed n detail in the literature'.

Coiled tubing has been used with increasing requency nsome underbalanceddrilling (UBD) situations becauseofperceived superiority to conventional jointed pipe drillingtechnology.Much of the success f an underbalanced rillingoperationcentreson the ability to maintain an underbalancedcondition on a continuous basis, ransmit effective survey andpressure ataback o the surface n a unremitting fashionduringthe underbalanced rilling operation,clean he hole effectivelyand operate n a safe fashion with potentially high flowpressures t surface.Coiled tubing has distinct advantagesnthese areasover conventional ointed pipe due to the lack ofrequirementsor connections,use of an internal wireline andcapillaries for survey and geosteering purposes, and muchhigher operatingsurfacepressure imitations than conventionaljointed pipe.

Jointed pipe, however, is much less expensive to utilize andis readily available. Also, increasing experience andmodifications in technology (such as EMT surveying tools)have reduced or eliminated many of the problems initiallyassociated with its use for UBD applications. The commonapplication of jointed pipe technology for drilling reducesproblems associatedwith inexperienced crews implementing therelatively new technology of coiled tubing drilling. In addition,coiled tubing drilling technology is currently limited at 500-700m of horizontal outreach and relatively small hole sizes n orderto maintain sufficiently high annular velocity for adequateholecleaning purposes. High frictional losses, associated with thenecessity of injecting fluid through the entire length of thecoiled tubing string at all times, may also be problematic.

Why Drill Underbalanced? A number of reasoncommonly provided as motivations for underbalanced riThesewould include:. educed nvasive formation damage esulting n h

production rates. ncreased ate of penetration. reduction in drilling problems such as lost circulatiodifferential sticking. elimination of costly and exotic mud systemsubsequent isposal of such). ability to flow test while dtilling. flush production during the drilling operation. ess environmental mpact (generallyno mud pits)

This paper presents some of the advantages anddisadvantages f coiled tubing and jointed pipe methods orunderbalanced drilling, primarily from a reservoir andproductionoptimization perspective, ather than from a simplemechanical r operationalapproach.

Not all reservoirs, however, are ideal candidatunderbalanceddrilling and, if the technology is improapplied, more formation damage may result than if adesigned and executed overbalanced'operation badutilized2.J.


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COILED ruBING-THE FUruRE OF UNDERBALANCED DRILLING?0005-97pressure s still less than for conventional CT, which mayresult n CT being a potential consideration or situationswheresour fluids or high surface pressures are expected. Sourcirculating fluids in the drill string, when connections arerequired or jointed pipe operations,may also aggravate afetyconsiderations.CT drilling operations are also quieter thanconventional jointed pipe operatio.ns, hus reducing noiseconcernswith drilling personneland local residents.

prevent apid fluid fallback in the vertical annularsectionof wellbore. Bottomhole pressure fluctuations, however, situations where significant volumes of liquid are beproduced from the fonnation, can be minimized throuapplication of these echniquesbut not totally avoided.Alternate injection configurations, such a parasite tubstring or microannular njection techniques see Figures 3 a

4) can provide stableand unifonn bottomholepressure roffor jointed pipe operations similar to those seen during openstions. hesemodified UBD operations ncrease xpensthe well, however,both from an equipmentperspective ndaoperationallydue to the fact that more non-condensi~le arequired to maintain the same bottomhole pressure inconcentric or parasite string application in comparison ostandpipe njection mode in the samesituation.

If high surface njection pressuresare required, CT mayhave a nominal safety consideration,due to the fact that theentire length of the high pressureCT spool is at surface n alocation exposed o the rig crew and ongoing operations. Thepossibility of a leak or safety concerns is present, butappropriatemonitoring of the fatigue and cycle life of the CTstring should addressmany of theseconsiderations.Rate of Penetration (ROP) Issuesypically, jointed pipe underbalanceddrilling operationshave had an exceptional safety record when combined withproperly designedsurface control equipment and experienceddrilling personnel. Hence, all factors being considered. or

sweet, ow surfacepressureapplications,CT does not exhibitany appreciable dvantagewith respect o safetyconsiderationsover conventional ointed pipe.

Generally, ROP is significantly increased by UBD operatioAll factors being equal, ROP increases in CT vs jointed poperations are comparable, but this assumes hat equivalent hsize, weight on bit. torque and drag considerations, etc. present which is often not the case as will be discussed shorROP may be compromised in some CT applications, particulain extended reach applications, due to the fact that limiweight can be applied to the bit by the action of the CT injecat surface, and much of this force may be expended in deffects as the CT string is forced around the horizontal band down the helical well path that is typically generated wattempting to drill horizontally with CT.

Continuous Maintenance oran Underbalanced BottomholePressure ConditionOne of the main motivations for underbalanced rilling, inmany situations, s the elimination or minimization of invasiveformation damage caused by fluid and solids losses o thefonnation underconventionaloverbalanced onditions.Much ofthis benefit is negated f the underbalance ressure ondition is-periodically compromised2.J.his phenomenas schematically

illusb atedas Figure 2. Coiled tubing bas a major advantageover ointed pipe in this case,due to the fact that there is nonecessity o break for coMcctions. If standpipe njection of thenon-condensiblegas being used to generate an artificiallygenerated underbalancepressurecondition is occurring, thismay result in bottomhole pressure luctuations which may, ifimproperly handled, esult in conditionsof periodic hydrostaticoverbalancepressurebeing encountereddownhole. This maynegatemuch of the benefit of the UBD operation with respectto fonnation damageminimization). Damagemay actually beworse in some cases n comparison o the samesituation thanif a well-designed overbalanced rilling fluid {which has thecapability to fonn a stableand bridging and. hopefully, readilyremovable ilter cake)bad beenutilized.

Hole Cleaning IssuesHole cleaning presents a major problem in many Uoperations.Hole cleaning s affectedby fluid rheology.cuttsize and concentration which is in turn a function of bit tand ROP) and annular fluid velocity. The majority of operations are conducted using 2 or smaller CT striwhereashole size can be 6.25 or larger. This may resuhigh pump rates and frictional losses n the CT string brequired in order to maintain sufficient aMular velocitymaintaineffectivehole cleaning. nability to rotatecoiled umay further exacerbate he problems with hole cleaning.use of larger CT strings (to reduce rictional drop in the sand ncrease orrespondingannular velocity due o a reducin annularsize) may be useful. Increased se of aggressiv

(such as POC's) to reducecuttings size may also be usefulmay be offset by problems with torque generationand amplitude orque variation.ottomhole pressure luctuations using jointed pipe can beminimized by making fewer connections drilling with doubleor triple pipe stands- riples are becoming more common with

the increasing use of top drive rigs for UBD jointed pipeapplications), faster connections, circulating to pure gas toattempt o unload asmuch fluid from the horizontal and annularsection as possible prior to making a connection,and leavingthe annulus open during connections to allow bleedoff to

The ability to continually or periodically rotateconventjointed pipe may result in superior hole cleaningability dukeeping a competentcuttings bed from forming. WorkingCT string may also assist in hole cleaning ability in cesituations.Many CT applications or UBD aredrilled slim which reduces he problems associatedwith low annular


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4 D.B. BENNION. F.B. THOMAS, R.F. BIETZ. A.K.M. JAMALUDDIN P0005velocities n the horizontal section,althoughproblemsmay stillbe apparent n larger diameter uphole sections. injection is required in this application and thereconventionalMWD technologycan be usedwith no probleTotal Drilling Time The advent of CT technology minimized some of

problems associatedwith downhole MWD technology since, to the continuous nature of the CT string, a continuous troubfree internal wireline approach could be used to directransmit the data from the bottomhole MWD assembly bacsurface allowing for continuous telemetry measUrementswhmaintaining a continuously underbalanced condition.

Drilling times may be reducedusing CT due to the fact thattime for connectionsmay account or up to 25% of the totaldrilling time (in soft fonnation applications)when drilling withconventional ointed pipe. Since cOMCctions rc not requiredfor a CT application, his represents irect reduceddrilling timeand cost savings for the drilling operation. CT can also betripped much faster han conventional ointed pipe, which mayalso reduce on-site time if multiple trips are required in aparticular ob.

The use of newer EMT technology. has allowed successfulapplication of MWD technology to jointed papplicationswith many of the benefitsof a continuouswireused n a CT system.With EM technology,an electromagpulse s used o transmit the survey and pressuredata direthrough he ground back to a surface eceiver.The technohas been used extensively and successfully in many Uapplications.Depth and temperature imitations are the mproblemswith this technique,aswell as he inability of certypes of fonnations to effectively conduct he EM signal.

Continuous Circulation IssuesDue to the nature of coiled tubing, continuouscirculation spossibleboth while drilling ahead. nd also while hipping. Theadvantages f continuouscirculation in maintaining a uniformBHP while drilling havebeendiscussed reviously. The abilityto continuously circulate with CT while tripping, in general,reduces eaming requirements,allows for backreaming,whiletripping out (if required)and allows the hole to be cleaned ndmaintained n a better condition.

Rig/Site Considerations -FootprintCT drilling hassomeadvantages ver a conventional oinpipe rig with respect o site considerationsn remote ocator sites with limited space. A CT unit is a smaller, mportableassemblyhan a conventionaldrilling rig, and ypicrequires only about 50% of the footprint space on siteoperation n comparison o an equivalentsystemusing oinpipe technology. This lowers environmental impact on (which may be critical in certain sensitive areas)as wereducing site construction,use and abandonment osts).

Mud Spillage IssuesGenerally,UBD operationshave essenvironmental mpactthan conventionaldrilling, due o an absence f mud pits whena closed oop surfaceconb'ol ype system s utilized. CT use nUBD further minimizes environmental mpact as, once again,due o a lack of necessity or connections,mud spillageon thesite during connectionswhen ointed pipe s being usedand he

potential for contamination rising from such spillage isminimized. Surface Hole ConsiderationsIf CT is in use n a new drilling application. some advconditionsmay be mposedby the act hat a small conventrig will generally be required n order to spud he well. runsurfaceand possibly intermediatecasing or. in some casepull the production equipment rom an existing well. Thiscourse.may obviatesomeof the potential economicadvanof CT drilling since two completesets of mechanicaldrilequipmentmust be moved onto and ofT site with the econand ogistical hassleswhich this entails.

Measurement While Drilling (MWD) IssuesThe ability to MWD during an underbalanceddrillingoperation s essential.both from a geosteeringand trajectorycontrol perspective or a horizontal well. and also due to thefact that real time measurement f the BlIP is essential oensure he success f any underbalanced rilling operation.In the early daysof artificial UBD drilling. conventionalmudpulsed telemetry was used. Since the presence of an

incompressibleluid in the drill string is a pre-requisite or mudpulsed telemetty, this necessitates he cessation of non.condensible as injection in order to allow the propagationofa successful uite of data o surface.This, of course, esulted nthe appl cation of full hydrostaticpressureo the formation andnegated he underbalance ressure ondition and, hence,most,if not all, of the benefitsofunderbalanceddrilling with respectto formation damage mitigation were compromised. If aconventionalmud system s used or a flow drilling application,this, of course, s not a problem as no non-condensible as

Hole Size LimitationsAlthough there are some reportedcasesof larger hole sbeing drilled with coiled tubing, the current conventionalulimit for CT drilling is presentlyabout 6.25 . Largerhole sif required, typically tend towards using conventional oindrillstring. CT drilling in the pasthas ended o be restrictmore slim hole applications and reentries, rather than ldiameternew drill applications.As CT drilling is extended o using larger diamete


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coiled tubing-the future of underbalanced drilling

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