By D. Brant Bennion

CALGARY, ALBERTA. CANADA-Coiled tubing has been used with increas:ing rrequeocy in under-balanced drilling(UBD) situations because of perceivedsuperiority to conventional jointed pipedrilling technology.

Much of die success of an under-bal-anced drilling operation centers on theability to maintain an under-balanced con-dition on a continuous basis. transmit ef-fective survey and pressure data back tothe surface in an unremitting fashion 4ur-ing the under-balanced drilling operation,clean the hole effectively. and operate ina safe fashion with potentially high flowpressures at surface. Coiled tubing hasdistinct advantages in these areas overconventional jointed pipe because of dielack of requirements for connectiQns. theuse of an internal wireline and capillariesfor survey and geosteering purposes. andhigher operating surface pressure limita-tions than conventional jointed pipe.

Jointed pipe, however. is much lessexpensive to utilize. and it is readily avail-able. Also. increasing experience andmodifications in technology such as elec-tromagnetic (EMT) surveying tools havereduced or eliminated many of the prob-lems initially associated with its use forUBD applications. The common applica-tion of jointed pipe technology for drill-ing reduces problems associated with in-experienced crews implementing the rela-

D. BRANTI BENNION

D. Brant Bennion is president ofHycal Energy Research laboratoriesLId. He is a project engineer with 20years of technical expertise in the ar-eas of formation damage and fluidflow in porous media. Bennion servesas director of The Petroleum Society(if the CIM. and as general programchaimaanfor the Petroleum Society's50th ATM in 1998. He holds a B.S. in

chemical and petroleum engineeringfrom the University of Calgary.

JANUARY 1998 113

tively new technology of coiled tubingdrilling.

In addition. coiled tubing drilling tech-nology is limited at 2.(XX)-2,500 feet ofhorizontal outreach and relatively smallhole sizes in order to maintain sufficientlyhigh annular velocity for adequate holecleaning. High frictional losses. associ-ated with the necessity of injecting fluidthrough the entire length of the coiled tub-ing string at all times. may also be pr0b-lematic.

Under-8A-12-~ DriIHOIWhat il; under-balanced drilling? Tech-

nically speaking. UBD is classified as anysituation in which the effective circulat-ing pressure of the downhole drilling fluidis less than the pore pressure of the adja-cent formation (preferably over the en-tire length of the exposed open hole sec-tion of the pay).

The under-balanced drilling process isillustrated schernarically in Figure I. Thereare a number of motivations for under-balanced drilling. They include:. Reduced invasive formation damage,

which results in higher production rates~. Increased rates of penetration~ .. Reduced drilling problems. such aslost circulation and differential sticking;

. Eliminating costly and exotic mudsystems (and their subsequent disposal);. The ability to flow test while dri1ling~

. Flush production during the drillingoperation; and

. Less environmental impact (gener-ally no mud pits)

Not all reservoirs are ideal candidatesfor under-balanced drilling, and if thetechnology is improperly applied. morefonnation damage may result than if anover-balanced mode of operation had beenutilized.

CT Versus Jointed PIpeThe vast majority of wells drilled us-

ing under-balanced technology have uti-lized conventional jointed pipe technol-ogy and rotary rigs. In fact, more than 85percent of the wells drilled using the "ar-tificial" under-balanced type of technol-ogy in Canada have been drilled usingconventional jointed pipe. The percent-age is even greater if low head and flowdrill applications are considered on aworldwide basis. Major advantages anddisadvantages with respect to coiled tub-ing versus joint,ed pipe applications canbe summarized into categories relating to24 distinct issues:. Safety and surf~ pressure control;. Continuous bottom-hole pressuremaintenance;. Rate of penetration;. Hole cleaning;

.Total drilling time;

. Continuous cireulation;

. Mud spillage and environmental;

. MWD capabilities;

. Rig/site considerations, footprint;. Surface hole/casing considerations;. Hole size limitations;. Depth limitations;. Rotation;

. Torque, drag and weight-on-bit;

ing fluid (which has the capability to fOnDa stable, bri'dging, and-bopefuUy-readilyremovable filter cake).

Generally, rate of penetration is sig-nificantly increased by UBD operations.All f~tooI being equal, ROP increases incr versus. jointed pipe operations arecoinparabl~ but this assumes that equiva-lent hole size, weight-on-bit, torque anddrag considerations. etc. are present That

I is often not the case. ROP may be com-promised in some cr applications, par-ticularly in extended-reach operations. be-cause of tbe limited weight that can beapplied to the bit by the action of the crinjector at surface, Much of this force may~ expended in drag effects as the crstring is forced arooOO the tKJrizootal beIkIand down the helical well path that is typi-cally generated when attempting to drillhorizontally with cr.

plications, cr does not exhibit any ap-preciable advantage with respect to safetyconsiderations over conventional jointedpipe.

Major AdvantageIn many situations, one of the main

motivations for UBD is eliminating orminimizing invasive formation damagecaused by fluid and solids losses to thefonnation under conventional over-bal-anced conditions. Much of this benefit isnegated if the under-balance pressure con-dition is periodically compromised. Thisphenomena is schematically illustrated inFIgure 2. Coiled tubing has a major ad-vantage over jointed pipe in this case,because there is no necessity to break. forconnections.

If standpipe injection of the non-con-densible gas being used to generate anartificially generated under-balance pres-sure condition, it may result in bottom-hole pressure fluctuations during connec-tions. This may, if improperly handled,result in conditions of periodic hydrostaticover-balance pressure being encounteredoownhole. which can negate much of thebenefit of the UBD operation (with ~-spect to minimizing formation damage).Damage may actually be worse in somecases in comparison to the same siwationusing a weU-designed over-balanced drill-- ~

Hole OeaDiDIHole cleaning presents a major pr0b-

lem in many UBD operations. Hole clean-ing is affected by fluid rheology, cuttingssize and concentration (which is, in turn,a function of bit type and ROP) and an-nular fluid velocity. The majority of croperations are conducted using 2.(}'inchor smaller cr strings, whereas hole sizecan be 6.25 inches or larger. This may re-sult in high pump rates and frictional

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. Downhole motor;. Tubing life limitations;. Torque limitations;. Flow hydraulics limitations;

. Orientation and steering difficulties;

. Bottom-hole assembly (BUA) con-siderations;. Availability; . .

.Experience;

. Economics; and

. Hybrid rig apPlications.

Safety I8IesSafety and well co~trol concerns are

always a major issue in any under-bal-anced drilling operation. particularly insituations where high reservoir pressuresor sour fluids exist The smooth and con-tinuous surface of a cr string and thebuilt-in stripping ~banism in cr injec-tion units provides perceived superiorsafety when operating at high annularsurface return pressures in an under-bal-anced drilling operation.

Considerable development work hasoccun'ed in the last few years in a varietyof different types of rotating control headsfor similar control flexibility for opera-tions using jointed pipe. The new genera-tion of control heads allows rotating op-erations at pressures up to 2,500 psi witha 5,<xx> psi static pressure rating. The r0-tating working pressure is still less thanfor conventional cr, which potentiallymay result in cr being considered forsituations where sour fluids or high sur-face pressures are expected. Sour circu-lating fluids in the drill string, when con-nections are required for jointed pipe op-erations, may also aggravate safety con-siderations. cr drilling operations arealso quieter d1an conventional jointed pipeoperations, reducing noise concerns withdrilling personnel and local residents.

If high surface injection pressures arerequired. CT may have a nominal safetyconsideration, owing to the fact that theentire length of the high-pressure CTspool is at surface in a location exposedto the rig crew and ongoing operations.The possibility of a leak or safety con-cerns is present. but appropriate monitor-ing of the fatigue and cycle life of the CTstring should address many of these con-siderations.

1'ypica1ly, jointed pipe under-balanceddrilling operations have had an excep-tional safety record when combined withproperly designed surf~e control equip-ment and experienced drilling personnel.Therefore, when all factors are consid-ered. for sweet. low surface. eressure ap-

The ability to ~ while drillingduring an under-balanced drilling opera-tion is essential. both from a geostceringand trajectory conb'Ol perspective for ahorizontal well. and also because reaI-time meas~nt of bottom-bole pres-sure is essential to ensure dte success ofany UBD operation.

In the early days of artificial UBDdrilling. conventional mud-pulsed telem-etry was used. Since dte presence of anincompressible fluid in d1e drill string isa prerequisite for mud-pulsed telemetry.d1is necessitated the cessation of non-<:on-densible gas injection in order to allowdte propagation of a successful suite ofdata to surface. This resulted in die appli-cation of full hydrostatic pressure to dieformation and negated dte under-balancepressure condition-as well ascompom is-ing most. if not all. of the benefits of UBDwith respect to mitigating formation dam-age. Of course. this is not a problem if aconventional mud system is used for aflow drilling applications because no non-condensible gas injection is required. al-lowing conventional MWD technology tobe used.

The ~\'ent of cr ttA:hoology ~some of the problems associated widtdownhole MWD technology since a con-tinuous. more reliable internal wirelineapproach could be used to directly trans-mit data from the bottom-hole MWD as-sembly back to surface. allowing for con-tinuous telemetry measurements whilemaintaining a continuously under-bal-anced condition.

The use of newer EMT technology hasallowed the successful application ofMWD technology to jointed pipe appli-cations with many of the benefits of a con-tinuous wireline used in a cr system.With EMT. an electromagnetic pulse isused to transmit d1e survey and pressuredata ~y throogh d1e ground back toa surface receiver. The technology hasbeen used extensively and successfully inmany UBD applications. Depth and tem-perature limitations are the major prob-lems with this technique, as well as theinability of certain types of formations toeffectively conduct the elecb'Omagneticsignal.

unit is a smaller and more portable as-sembly than a conventional drilling rig,and typically requires only about half thefootprint space for operation as an equiva-lent jointed pipe system. This lowers en-vironmental impact 00 site (which maybe critical in certain sensitive areas), aswell as reduced site consbUction, use andabandonment costs).

In regard to surface hole considera-tions, if cr is used in a new drilling ap-plication. some adverse conditions maybe imposed by the fact that a small con-ventional rig will generally be requiredin order to spud the well. run the surfaceand possibly intermediate casing, or insome cases. pull the production equi~tfrom an existing well. This. of ~ mayobviate some of the potential economicadvantages of cr drilling. since two com-plete sets of mechanical drilling equip-ment must be moved on and off site withthe economic and logistical hassles thatentails. Hybrid rigs that incorporate bothcr and conventional jointed pipe tech-nology are in the evolution process. andare being seen more in the industry.

In terms of hole size limitati'ons, aI-dtough there; are some reported cases oflarger hole sizes being drilled with coiledtubing, the current conventional upperlimit f(X" cr drilling is 6.25 inches. Largerhole sizes, if required typically tend touse conventional jointed drill string. In thepast. cr drilling has tended to be re-stricted to more slimhole applications andre-entries. rather than large-diarneter newdrill applications. As cr drilling is ex-tended to llSing larger-diarnetercr strings,the possibility of larger hole diameters inthe future becomes more feasible.

Both depth and horizontal outreachalso presently limit coiled tubing. becauseof the inherent helical buckling charac-teristics wId limited power of the surfaceinjector Ix:ads. Maximum horizontal out-reach with today's technology is in therange of 2,500 feet (although some ap-plications at greater lengdIS have been re-corded), which severely limits the use ofcr applications in many horizontal situ-ations where extended-reach wells arerequired because of reservoir or surfacelocation considerations and cons~.

On-going research in using cr tractorunits, which can be used to apply weightto the bit in cr drilling applications. andusing larger cr strings may extend thereach capability of cr in the future. butthis remains one area in which conven-tional drill pipe exhibits considerable ad-vantages over cr.

losses in the cr string in cxder to main-tain sufficient annular velocity to main-tain effective hole cleaning.

The inability to rotate coiled tubingmay further exacerbate problems withhole cleaning. Using larger cr strings toreduce frictional drop in the string and in-crease corresponding annular velocity byreducing annular size may be useful, anda number of 3.5-inch cr drilling rigs arecoming into use in the market The in-creased use of aggressive bits such asPDCs to reduce cuttings size may also beuseful. 00t may be offset by problems widttorque generation and high amplitude~ variation.

The ability to continually or periodi-cally rotate conventional jointed pipe mayresult in superior hole cleaning ability bykeeping a competent cuttings bed fromforming. Some new cr rig variants al-low slow rotation of the cr string to com-bat the problem, but the technology re-mains somewhat ~ and UDpoven. W~-ing the cr string may also assist in holecleaning ability in catain sibJatioos. Manycr applications for UBD are drilled slimhole, which reduces the problems associ-ated with low annular fluid velocities inthe horizontal section, although problemsmay still be apparent in iarger-diametersections up hole.

Drilling times can be reduced using crbecause of the fact that time for connec-tions may account for 25-30 percent oftotal drilling time when drilling with con-ventional jointed pipe. Since connectionsare not required for a cr application, thisrepresents direct reduced drilling time andcost savings for the drilling operation. crcan also be tripped much faster than con-ventional jointed pipe, which may alsoredIK:e on-site time if multiple trips arerequired. .'

Because of its nature, continuous CIr-culation is possible both while drillingahead with coiled tubing, and while trip-ping. The ability to continuously circu-late with cr while tripping helps red1x:es

reaming requifC;m~nts, per:mits ~ack-reaming while tripping out (if 1'eIquired),and allows the hole to be cleaned andmaintained in a better condition.

Generally, UBD operations have. lessenvironmental impact than convennonaldrilling because of the absence of mudpits when a closed-loop surface controltype system is utilized Using cr ~ UBDfurther minimizes environmental impactbecause the potential for contaminationarising from mud spillage while connect-ins jointed pipe is eliminated.~

Rig Footprintcr drilling has some advantages over

a conventional jointed pipe rig with re-spect to site considerations in remote lo-cations or sites with limited space. A cr

The an)OUDt of weight on bit d1at canbe awlied in vertical wm is limited byd1e fatigue life of d1e tubing. The amountof fon:e at d1e bit for directional or hori-zontal work is limited by the bucklingpoint of d1e coiled tubing and associatedfrictional drag considerations. Jointedpipe obviously has advantages in thesesituations.

Downhole Motorcr drilling obviously requires d1e use

of a downhole motor. Because of d1e holediameter associated with cr drilling, thisnecessitates the use of small-diametermotors that have generally had a less re-liable perronDance history than do larg~-diameter n1otor assemblies. In some cases,expenses associated with downhole f1}()-tors. hipping and motor replacement havea(X:OUnted for up to 25 percent of a job'scost.

With res~t to tubing life issues, atypical cr string may be plastically de-fonned multiple times during a typicaldrilling operation. There is a limited cy-cle life for a typical cr shing, after whichsafe usagt: for a drilling application is nolonger fe.lSible. This may significantlyaffect the economics of cr drilling, par-ticularly for larger-diameter cr strings.New technology for the way cr is usedhas significantly reduced d1e number ofplastic defonnations required in a givenoperation. and will likely lead to a treIKiof significantly enhanced CT life andmore cost-effective use.

The torque limits imposed by d1e CTstring often limit the type and size ofmotor assembly that can be used in a givensituation. In addition. for a cr applica-tion. tonIue wind-up may have an adverseeffect on directional tool face orientation.

cr drilling may also have significantadverse flow hydraulics limitations withrespect to conventional jointed pipe op-erations. These limitations include:

. A cr string of sufficient length to~h ~ depth must be brought on sitefor use in the drilling operation. Injectingd1e drilling fluid for the entire drillingoperation must occur through die entirelength of the cr string. This is in com-parison to a jointed pipe ~tion, w~a variable length of pipe correspondingto the depth of the cunent drilling loca-tion is used. This results in considerablymore horsepower being required continu-ously for the entire length of the cr drill-ing operation.

ec:cxIOIni<:s of a cr drilling application inromparison to conventional drilling withjointed pipe. The large number of depre-ciated and readily available land rigs thatcan be used for UBD operations maymake cr an uneconomic alternative inmany cases. An increased number of crunits will foster a more competitive mar-ket with more depreciated equipment thatmay aid in reducing costs and improvingthe economics of cr dri lIing as comparedto jointed pipe.

There has been interest in the devel-opment of hybrid drilling rigs that incor-porate the desirable features of bothjointed pipe and cr drilling applicationsinto a single unit. This is particularly ap-pealing in some offshore applicationswhere very limited footprint size is avail-able for a given drilling operation. Onceagain, die cost of deveJopmcnt and con-sb'Uction of hybrid rigs may make dleirinitial use prohibiti~ly expensi~ exceptin certain special applications.

AppUcatioos favoring crSelecting cr drilling over conven-

tional jointed pipe for an UBD operationis not a specific one. and depends stronglyon the specific set of reservoir, drilling,site and economic considerations of agiven situation. However, coiled tubingdrilling may be favored in applicationswhere:

. Small-diarncter holes (less than 6.25inches) are required;

. Shor1-~us re-entries are required;

. Limited horizontal outreach is nec-

essary (less than 2.500 feet);. Sour gas/oil exists;. High expected surface return pres-

sures are anticipated;. Difficulty in MWD is expected with

cunent EM technology; and. Maintaining a continuous under-bal-

anced axktition to mitigate f(X1n8tjon dam-age is essential.

cr drilling in the under-balanced mar-ket is expected to occupy an increasingniche technology position. Because of~ of die eAX)OOInjc and tcchnical1imi-taboos of cumnt cr technology, cr drill-ing is not expected to rcpl~, oot ratherimpact die number of wells drilled under-balanced with jointed pipe in the near fu-ture. Operating improvements in usingjointed pipe for UBD operations have re-moved many of the drawbacks associatedwith its use in the early application ofUBD technology.

. High press~ at the injection pointof the cr string are required continuouslyto maintain flow. and the entire volumeof the pressurized cr string is exposed atsurface at the beginning of the drillingoperation.

. Using smaII-diameta-cr strings lim-its tile flow rates to be used for a drillingoperation because of excessive frictionalpressure drops resulting in high surfacelX'eSSuI'es. which may negate safe and ec0-nomic cr drilling.

Orientation and steering issues comeinto play because cr drilling requires theuse of an orienting tool to steer the BRAfor directional or Imzontal work. This isnewly developed technology with ass0-ciated problems. A muleshoe sub is usu-ally ~uired to ascertain toolface orien-tation.

cr drilling also necessitates using ashear sub in the event that the BRA be-comes stuck during the drilling operation.A coiled tubing adapter suitable for drill-ing induced stresses to connect the crstting to the BHA is ~uired. This as-sembly is generally purpose built to al-low encapsulated insertion into an under-balanced well.

Constraints And EconomicsIn many cases. using cr drilling tech-

nology for an under-balanced drillingoperation is desirable. but operators en-counter a shortage of suitable cr unitsfor their particular applications. as com-pared to the availability of conventionaljointed pipe rigs. This results in manyapplications of jointed pipe technologywhere cr may have been superior sim-ply because of availability constraints. Asmore cr drilling units become available.this issue may become less problematic.

Although there are many experiencedcoiled tubing operators. there are fewhighly experienced cr drilling operators.especially for UBD applications. This re-sults in problems in many applicationswith under-trained crews attempting tooperate equipment in a highly non-stand-ard situation. and may mean less-than-optimal performance and results. Onceagain. as more cr drilling units come intothe market and the number of cr drillingoperations increases. the mean expertiselevel of cr drilling crews will increasecorrespondingl y.

A combination of high-demand. new(non-paid out) equipment and the limitedlife ofcr strings may adversely affect the

Today, cr occupies a 10-15 percentshare of me Canadian artificial UBD mar-ket. That market share is expected to in-crease significantly over the next fewyears, to perhaps 25-30 percent of the to-tal UBD market. However, jointed pipewill remain the dominant drilling tech-nique for the near future. (]