B* ....Society of Petroleum EngineerslADC/SPE 39321

A Robust Torque and Drag Analysis Approach for Well Planning and Drillstring DesignOpeyemi A. Adewuya, SPE, and Son V. Pham, SPE, Baker Hughes /NTEQIADC Members

Copyright 199S, lADC/SPE Drilling conference

This paper was prepared for pr=nbfion t the 1~ lADC/SPE Drilling Conference held inDallas, Texas S-S March 199S.

This pawr was seletid for presentation by an lA~SPE Prcgmm Cammitt- followingrww of information containsd in an abatract submitted by the author(s). Gntenta of thepaper, as presented, hava not hrr raviti by the International A=iation d Drillingcontractors or the Society of Patraleum Enginwra nd m subj6ct to mrrsction by theauthor(s), The material, as presented, d- not neceaaarily rc4fect any postion & the IADC orSPE, their officers, or mamhra. PaPra pr.saantd at the lADC/SPE meetings are subj~ topublication rwaw by Edkorial Committwa of the IADC and SPE, Electronic reproduction,distribution, or storage of any part of this Pawr for commercial PU- tihout the tienwnsent of the society of Petroleum Enginwra is prohib~, Parmiasion to reprdua in printis restdctd to an abstract of not mom than 300 words illustrations may not b mpiacf. Theabstract must contain conspicuous scknA8clgment & where and by tiom the paper waspr=ented. Write Librarian, SPE, P.O. Sox SS-, Richardson, TX 7~, U.S. A., fax01-972-952-9435.

AbstractThis paper presents a novel torque and drag analysisapproach and demonstrates its robustness when used with aversatile computer program. Torque and Drag amdysisremains an important evaluation prwss for assessing drillingfeasibility of directional wells, minimizing the occurrence ofcatastrophic drill string failures and avoiding prematuretermination of the drilling operation before reaching plannedtarget depth.

From a draft well plan, the drilling engineering analysis isinitiated with the development of a representative analyticalmodel using selwted entries in a Torque and Drag computerprogram. Several parameters and instances of evaluation arended to capture the physical behavior of modeled systemsand to produce technically sound results.

The availability of computational tools have not necessarilyimproved the drilling enginwring process or enhanee thequality of recommendations without a methodical approachand application of results.

To minimize the iterative steps required to reach aninterpretable result, the analytical process as presented in thispaper is accelerated with a directed search and a convergenceto the determinant drilling variables, The novel approachnarrows - the design search domain and tests sensitivities ofwell-plan characteristics, simulates drilling conditions andapplicable drillstring - to the dominant operating factors thatdetermine the boundaries of application.

A reeord extended reach well ~~ ratio of 2.9) with alateral displacement of approximately 6,000 ft. was drilled inthe GOM using this approach to select tubulars and theirposition in the well with respect to well trajecto~ objectives,bottom-hole assembly (BHA) performance and target reach.

IntroductionSuppose we define drilling mechanics analysis as consistingof a number of well-established activities, including well-path planning, torque and drag analysis, drillstring designand the selwtion of drilling systems. The subject of this paper- well-path design and, torque and drag analysis - maintainsa strong interest in the petroleum industry.

The process of well-path planning and drillstring design forgiven geological targets are subject to BHA directionalperformance, torque and frictional drag analysis, hydraulicsrequirements and mechanical strength of drillstringcomponents has seen progressive development, the currentsurge in Extended Reach Drilling (ERD) operations,Horizontal re-entries and other complex drilling programs isan excellent testimonial. Torque and drag analysis compriseswell-path description and drillstring load modeling processaimed at simulating the same mechanics and characteristicsof a real-life drilling operation. Torque and Drag analysis isnow considered a valuable tool used primarily for design,planning and application screening of drilling and completionsystems.

However, evolution of successful approaches has been doggedby heuristic concepts and rules of thumb which are noteffective when non-linear sititions exist or when dwisions-me sensitive to quantitative measures rather thanqualitative indicators. It is our belief that the evidentcomplexity of run-time problems does not permit solutionsbased only on the experience of the drilling group.

The design and troubleshooting ability of those whoundertake such analysis should not be limited to historicalexperiences and performance of the applied drilling system ifthe proper use of computational tools and methodicalapproaches enable&:lling program.

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thorough and conciseThe implementation

evaluation of theof modeldenved

2 YEMI ADEWUYA, SON PHAM lADC/SPE 39321

analytical solutions shotid be brought about by providing atiework for system behavior dynamics to evaluate tiepossible system states created in the modeling process.

Therefore a robust modeling process is based on the logicalrepresentation of system states as functions of intervalobjectives at the modeling stage, providing solutions forextremely complex interplay of variables without necessarilysimplifying the system model, The approach we propose usesavailable theoretical foundations and analyses, combined withthe extensions to conservative criterion offered by practice toarrive quickly at feasible parameters for well path design,optimum tubular properties, and scope of drilling feasibility.

This paper is presented in two sections, in the first sectionbegifing with Well Planning Considerations we discuss theTorque and Drag implications of the Well-path Trajecto~Method used in survey calcdations for the well design.Completing this first ~tion is a discussion on the attributesthat makes this proposed modeling approach robust and thesteps demonstrating its value - minimized iteration time andease of implementation - using an example well. In ourconclusion we summarize, with emphasis, the most valuablecomponents of that process.

Well-planning Considerations

Well-path Trajectory Method:Many methods for calcdating well-path trajectory have

been formtiated to represent a suitable plan to reachgeological objectives. There are basically six differentmethods, which have been widely used in the directionaldrilling applications, are the Tangential, Average Angle,Balanced Tangential, Mereu~, Minimum Curvature andRadius of Curvature method. All except for the Tangentialmethod demonstrates relative accurate representation of thewell-bore trajectory [16], Readily available computationaltools naturally leads to the use of the more demandingMinimum Curvature Method in order to maximize on surveycalculation accuracy.

While the variation in smvey calculation methods plays aminor role in the overall torque and drag amdysis, it doescontribute to the overall accuracy and thoroughness of thewell-path design. Therefore Minimum Cmvature Method isthe formtiation of choice and is consistently utilized in thewell planning process.

Constraints Definition and Management:Most engineering systems are designed to operate within

specfled set of constraints which may be limitations onoperating load levels, modes or overall system response. Theconstraints define the lower and upper bounds of wleeteddesign variables and in terms of design performance becomesa yardstick for measuring compliance.

To allow for efficient processing of design steps, amechanism for defining constraint properties at each designstage is necessary [see Figure 1].

I. Structural @urface location and Target coordinates)Geophysicists and gwlogists work together to select take

points and reservoir intersection requirements. Candidatesurface Iwtions are chosen based on proximity, logisticrequirements, criteria to maximize slot recoveryopportunities and minimization of drilling costs for trajectoryand ancillary resources required to complete a well.

The choice of surface location relative to targetcoordinates define the design space for the trajectory of thewell. The geometric elements of the well are prescribed byother factors which include drag and allowable curvature fordrilling tools in applicable hole size.

2.Geometric specl~cations:The variables which shape the gametry of directional

well plans are Kick-off Point (KOP), Build-up Rate (BUR),hole inclination and casing program. Rehashing what iscommon knowledge today, having been the subject of muchresearch, it is clear that tie depth of kick-off contributessi@lcantly to the torque and drag characteristics andhorizontal reach of a well [5,6,7,8,9,10].

Build-up rates are a matter of comecting points along thewellbore to intersect target coordinates, but the choice of anoptimal BUR is determined by hole size, drilling toolcapability, anticipated drag effects and an over-all evaluationof the drilling objectives.

3.Casing Program:The casing design process requires tie selection of a

casing program to meet at the minimum design requirementssuch as imposed mechanical stress (hoop, radial and tri-axial)and loads (burst, collapse, tensile) among other prerequisiteswhich include estimated life-cycle of well, future re-entrywork formation isolation and casing wear tolerance.

Strategic casing placement to extend drilling assemblyperformance, although an opportunity cost issue, can bejustified by using the example well presented later in thispaper. For the work on which this paper is based, the casingprogram was spec~led for inclusion in the well-plan.

4.Geological obstacles:Crooked well-paths or 3-D trajectories are not well-

profiles of choice. Furtive views of local geology obtainedfrom seismic data provides information on enroute geologicalobstacles such as sensitive shales, unstable sandstonestringers, dips, faults and the prominent water or gas sandssubtended by the oil bearing reservoir.

S.Drilling ~stem operational compatibility:From an automated well design tool, BUR necessary to

connect geometric markers (End of Build, End of Hold, etc.)is obtained routinely, optimization of the well-design isachiev~ when consideration is given to the interval hole sizeand applicable performance drilling system.

Top hole sections necessarily are large holes requiring the

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use of large diameter tools. The mechanical constraints of thelarge diameter tools limits the degree of curvature appropriatein the top hole section.

In addition, the lower bending capability leads to highlateral loads and the attendant drag and torque effwt. The useof collar-based Measurement While Drilling (MWD) toolsintroduces even greater rigidity which places firtherlimitation on planned well-bore curvature.

Conventionally, most top hole sections are drilledvertically to a selected kick-off depth, to allow drilling largehole sections and setting conductor casing. However whentrajectory efficiency requires early dirwtional work, relativelysmaller hole size i.e. 12-1/4 can be drilled out of largecasing, enabling the use of higher BUR. When the intervalTD is reached, the hole is opened up to 17-1/2, as was donein the example well, to accommodate a 13-3/8 casing string.

In smaller hole sizes, BUR ranges cover a wider spectrumallowing flexibility in trajectory geometric properties. At thehigh end of this wide spectrum is a drilling assemblylimitation posed by push-through radius. That discussion isbeyond the scope of this work.

So far we have discussed constraints as defined in thepreamble to this subsection, suppose a constraint were to beused to advantage, for instance, designing a well path tomaximize drilling assembly rotation and exploiting the droptendency of the drilling assembly from gravitational effects totrack the well into the target location.

Well-plan Drillstring Optimization - SupplementaryIssues:

In Exlended Reach directional wells, what remains aprotracted optimization issue is not simply DLS minimizationbut the effect of the inter-play between inclination, azimuthrdchange, drag and buckling.

Micro-loading studies into sensitivities of drillstring tovarying well-profile pursue the quantitative indexing ofdominant load factors towards achieving optimization. Asreported by Payne and Abbassian [4], critical well-boreinclination i.e. angle at which pipe no longer falls at ownweight, is one of the several factors that shape ERD well-boredesign [see Figure 2].

Logically, lower inclination angles produces less drag, butlacks the well-bore support (cradling effect) needed tomanage the severity of buckling. An interesting observationalso presented by Payne and Abbassian though empirical,identiles the sensitivity of hole inclination to ~ ofoperation and briefly stated, a high KOP well profile isfavorable to a 12-1/4 hole by 9-5/8 casing/coiled tubingrun, while a low KOP well profile is preferred for an 8-1/2hole by 5-1/2 liner/pipe runs.

Steering in well-bores with azimuthal and inclinationchanges combined with long tangent sections present achallenge to the transmission of mechanical forces. Preciseorientation of tool-face in the presence of significant torque

couples (normal/contact force, circular frictional drag)aggravates the uncertainty of heading and achievinggeometric drilling objectives.

Drillstring Torque and Drag Modeling and DesignDriI1ing enginaring aIgorithm developers are constanffy

striving to produce sophisticated computational engines frommathematical representations of drill string dynamics whichoffer greater accuracy and more realistic results. While thecomputational engines improves, the results produced aremore intricate and refined. The impressive developments inareas such as trajectory simulation are to be immenselyappreciated but each step brings its own problems for the end-user.

Software Tools:Robust amdysis of modeled mdtivariate ~stems require

considerable computational processing before meanin~results are obtained. The Torque and Drag analysis tool usedin this work is one of the seven mochde suite of BakerHughes INTEQ proprietary drilling engineering softwaretools.

In the Torque and Drag calculation mode the softsvarecomputes tie sutiace-to-bit load, stress and lateral forceinformation for rotary and oriented drilling operations atuser-spectiled evaluation depths. Operating load casesincluding magnitude, location and mode of occurrence (e.g.drilling, rotating-on-bottom, tripping, etc.)

The computational engine allows fast and rigorousenginwring mechanics analysis of tie modeIed well-trajectory and casing cotilguration, drillstring and drillingparameters, based on a continuous elastic ti columntheory. From the vast array of state-of-the-art analyticalsolutions, the relevant solutions for Euler, sinusoidal, helicalbuckling and post buckling behavior, drillstring torsion andload displacement hysteresis in buckling mode transition wasthe focus in his application [2].

Availability of computational tools facilitate fast andaccurate iterations required in drillstring design andoptimization work.

The Tubular Buckling Theones Compared:The available theoretical foundations on which tubular

buckling has been developed can be grouped into twocategories, namely conservative and extended models, Themodels that can be classified as conservative criteria consistsof the combined work of Lubinski, Dawson/Paslay,Chen/Cheatham and, Sextro and He/Kylinstad[l].

The recent developments by Wu/Juvkam-Weld qualflesas an extended criteria model [1]. The differences between thetwo classes of criterion is enumerated in terms of scope ofapplication and impact on modeling.

Conservative Theories: Criticrd buckling loads predicted

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4 YEMI ADEWUYA, SON PHAM lADC/SPE 39321

by the Dawsotiaslay equation are much lower than actual oroperating critical loads. In addition, the equation representsthe mechanical behavior of long finite tubdar elements andproduces erroneous results for short elements [15].

The critid buckling load limits predicted by Chenindicated a 40% increase in load during sinusoidal to helicalbuckling transition and an 18% increase in the magnitude ofcritical load required to initiate buckling.

Extended Criteria Theories: A full understanding of thepremises on which the buckling theories proposed by Wu andJuvkam-Weld is important to recognizing their relevanee toresdting load behavior of modeled drilling assemblies.Directional wells with long tangent sections and holeinclination approaching critical angles with respect to frictionare stereotypical of the parameters which validate thesuitability of the extended buckling theories. At lowinclination angles where the contribution of tubular weight toaxial compressive force is greatest, the Wu and Juvkam-Weldbuckling equation [12] stilces with the critical length andaxial load term. The He & Kyllinstad work contributed theeffect of wellbore curvature to tie development ofmathematical basis for assessment of critical buckling loads.In essence, the normal forces due to curvature as anadditiomd resistance modulus is added to the force term [15].

Impact on Modeling: A broad comparison of the twoclasses of criteria can be summarized in terms of commonfactors namely the normal force and the stiffness terms.Invoking the conservative buckling criteria assesses bucklingloads based on a quotient of unit s~ess and normal force,while the extended buckling criteria assumes higher indicesfor moUlers to stiffness and normal force terms.

In summary, the reason for enumerating the d~ereneesbetween the conservative and extended buckling assessmentapproaches is to draw attention to the quantitative quality ofanalytical work based on these models. In practice, factorssuch as hole friction, wellbore inclination and curvature affectthe initiation of buckling and un-buckling discriminatorily.

Extended-reach wells by virtue of design and requiredtubtiar configuration tiest loads at higher thresholds andare best analyzed with models based on extended criteria.Fr~uent occurrences of drillstring failures or completionstring collapse would have dogged ERD save that there arefavorable interplay of itiuencing factors which make currenttheories poor predictors.

Process for achieving Analytical Robustness withWell-path and Torque and Drag Modeling

Well-path ModelingTo account correctly for the degree of variation of dogleg

severity infinite course lengths tsvo approaches was examinedby the authors. One approach uses a user-s~ified maximumrelative noise amplitude based on a scale of O -10 to producerandom net well-path tortuosity nominally ranging from 0.0-

2.0 deg/100 fi. [1], while the definition given by Dr. RapierDawson suggest the correction of the well-path by theaddition of a sinusoidal variation to the inclination andazimuth angle over 1,000 R. course lengths [17], Differentmethods of applying tortuosity to a well-plan may result inthe same average dogleg severity [see Figure 3] but from ourobservation, of the drilling operation, applying a randomnoise factor is more representative of a tortuous well pathcompared to a cyclic factor applied by the tortuosity equation[see Figure 3, Equation 3-A]

The Genealogy of a Robust Torque and Drag ModelingApproach

Multiple Analytical stations:Traditionally the drillstring design process tended to focus

on meeting minimum safety requirements in the string, forexample design based on mechanical ratings, size, drillingmode, casing points and relative component function. Also,emphasis was placed on drillstring applicable ody at TD,whereas in most cases stations such as KOP, casing points,whipstock exits and build-turn sections present greaterdrilling challenges.

A common assumption is that the analysis at TD of thewell-plan will yield the limiting parameters for the drillingapplications of the entire well-path - which neglects thevarying tool size utilized and changing geometry of the well-bore. Due to the weight and complex load bearingcharacteristics of different sized drillstring components it isnecessary to perform computational analysis for each holeinterval to better understand and optimize on the well-bore/drillstring interactions.

Correct interpretation of the drilling program enableseff~ve drilling meehanics analysis of the drillstring and thequality of results approach close approximations.

Reflective of Actual/ Changing Hole Conditions:The initial torque and drag modeling allows us to

systematically develop a thorough understanding of theinteraction between well-bore, drillstring components andoperational parameters [see Figure 4]. By discretely selectingmodeling stations or evaluation intervals, drilling parameters(ROP, WOB, RPM) that best describe hole conditions(lithology, temperature, hole cleaning, mud properties, etc.)can be applied for a representative model that approachesactual drilling condition.

It also enables narrowing the drillstring design searchdomain and improves ability to test sensitivities of drillstringwell-bore interaction in the following modes: rotary drilling,slide drilling and tripping. By closely evaluating the differentdrilling modes we can determine safe drilling limits.Operational limits consist of the applicable WOB withoutbuckling the drillstring, tripping capabilities and frictionaltolerances.

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On a scale of si~lcance, friction factor and WOB aredominant mntributors to the torque and drag effects ondrillstring application.

Trade-Offs:Primarily, the objeetive of Heavy Weight Drillpipe

(HWDP) application is to contribute weight to the drillingassembly with minimal increases in stiffness. However wellcurvature impose a limit on the fictional relevance of theweight property of HWDP. String weight in the curveproduces greater normal loads and contact forces.

As hole friction changes, the ability to maximize HWDPfictional performana is affected by the rate at which acompressive state or a tension state is approached ormaintained. In cased hole, when friction bmes a propertyof the contacting materials and imposed loads, the function ofHWDP ean be exploited to a higher degree.

A seeondary mechanical characteristic of HWDP is theinherent capacity to withstand relatively higher compressiveloads [12]. By strategically utilizing this structural loadbearing characteristic of HWDP we can meet complex andchallenging drilling objectives which would not otherwise besucces~ with normal drillpipe application.

The inverted drillatring configuration is now anestablished arrangement of drillstring assemblies. An inverteddrillstring arrangement places the HWDP above regulardrillpipe.

The common belief supported by static force analysis ofweight-derived axial for~ indicates that half the amount ofthis force is available at hole inclinations greater than 60 i.e.the weight of HWDP element x COS(60)= weight of HWDP x0.5. Although this guideline is genemlly acceptable for non-critical applications, advanced well-bore construction requiresmethodical computatiomd drillatring analysis which takesinto account friction factor, trip analysis, WOB and otherdrilling optimization and constraining parameters.

In drilling work of horizontal wells with long laterals, theeffective application of HWDP is becoming more of a sciencethan a convention. This emerging fictional use of HWDPfor sustainable transfer of weight to the bit is becomingcritical to achieving target lateral length displacements andreaching total depth.

Optimization requires an evaluation of the load and dragdistribution of the well based on the selected drillstring. Inaddition, optimal use of HWDP requires corr~t assessment ofrequired length, location in the borehole and balance ofperformance in mitigating buckling while maximizingtransmission of the weight to bit.

Presentation of Model Analysis:Graphid representation and summ~ tables simplfies

complex data sets for quick and accurate inte~retations andserve as an invaluable communications tool. The extensive

knowledge mptured from the modeling process needs to becommunicated to all team members.

When used as a monitoring or look ahead tool on thefield, deviation from predicted outcome ean be flagged earlyand corrective measures taken. In the next aeetion theapplication of this approach on the field is discussed using theexample well. Logical presentation of data allows theoperational team to easily and quickly assemble feedbackinformation facilitating easy understanding of complexrelationships between modeled and output variables.

Execution of retits and recommendations isstraightforward and less prone to misinterpretation by field orimplementation staff because of the graphical highlighta thatlimit additional processing.

Example Step-through Modeling ProcessIn this section the application of the components of the

robust modeling thesis enumerated thus far as it applies to thedifferent phases involved in the design and eventualsucce~ drilling of the well is presented. This methodoloWwas first used in an extended reach well with an MD/TVI)ratio of 2.9 and a lateral displacement of 6000 h.

Wellpath Planning of Example WelIPreliminary well design requirements was developed by a

mtitidisciplinary team composed of the operator and servicepersonnel.

The example well [see Figure 5] consists of a 20 drive-pipe set at* 300 ft., an initial drill-out 12-1/4 hole kicks-offbeginning at 3/100 fi, and end-of-build reached at 1,000ft.,MD with final heading of 341.23. The 12-1/4 hole is re-entered and opened to a 17-1/2 conductor hole to be drilledwith a 5/100 ft. build rate, building to a 40 inclination at1,500 ft.,m.

Beyond the planned 13-3/S conductor set depth, curvebuilding wotid be continued at 5/100 ft. to an intermediateend-of-build inclination of 83 at 2,373 ft.,MD. The 83inclination is held to the end-of-hold depth at 6,317 fi.,h4D.

A MO section drop was designed to intersect a target sandfor which the complete coordinates (orientation and depth)definition was unknown. The two seetion drop wotidfacilitate a slide and search drilling operation and acontrolled drop rate of 1.5/100 fi. to reach the bottom holelocation.

Fit for Purpose Well Design: The combination build-rateof 3/100 ft. and 5/100 ft. used in the kick-off after drivepipeis installed was chosen after carefil evaluation of its potentialtorque and drag implications. The curvature produced by thestrategically chosen combination build rates is intended toprovide a less aggressive trajectory thereby reducing normalforces and lateral loads that affect the drag distribution in thebore-hole drillstring interface.

The magnitude of build-up rate used in curved sectionsfollow a scheme that locates the smaller Bm 3/100 ft. at

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6 YEMI ADEWUYA, SON PHAM IADCISPE 39321

the beginning of the curve section and the larger B~ 5/100ft. at the end of the curve. By following this scheme theability to maintain WOB and stable string is ensured andlateral load in the curved ~tion of the hole is evedydistributed.

Surface Casing Location: The choim of set depth for thesurfaw casing was informed by the following reasons:

provide a cased hole to place HWDP for effectivetransmission of WOB for the drilling operation in thetangent section and on towards total depth.

. provide a reduced friction channel to minimize dragand torque.

to enable rotation of the drillstring in the tangentseetion, producing a smooth bore-hole with leasdogleg severity, manageable tortuoueity and increasethe ability to drill to target depth.

introduce a significant hole-cleaning advantageoffered by placing the surface casing about midway ofthe tangent section, since half the section is cased-off,the hole cleaning requirement for the tangent ~tionis halved.

Production Hole Design for Minimum Rotary drilling andDrilling Assembly drop tendency: This hole ~tion co~is~of a long 4000 R. tangent section and a 20 drop ininclination at the end of drop smtion in order to search for thetarget sand.

The challenging aspect of this work like moat ERDdrilling work is the ability to maintain sufficient WOB in tiesliding mode while retaining drillstring stability. Byplanning on the natural drop tendency of the drillingassembly we can minimi= the need for slide drilling andtherefore maximize operational success,

Torque and Drag Analytical Stations:The analysis for the example well was performed for all

hole intervals (surfam, protective and production). For eachscenario defined by well-bore design, select BHA andtillstring, drilling parameters and imposed loads, the analystmust model all possible configurations and must take intoaccount the combination of several interacting or relatedfactors. The reality of such undertaken is that modeling is astudy of nondeterministic events, a phenomenon amplifiedby the number of cases required to test the influence of eachfactor. Inarguably, a guided search to test sensitivities offactors is indispensable, providing a prmise experimentaldelineation to reduce the number of iterations.

Due to space constraints we use the analysis of the TDpoint of the production interval to highlight the robustmethodology. The selection of the 8-1/2 production intervalclearly demonstrates slide drilling and tripping concerninherent in all ERD.

For this cycle of evaluatio~ we will scrutinize the resdtsin the form of Summary Data Tables and the Drilling,Tripping and Frictional Sensitivity Analysis. As will be

demonstrated, the format of the data presentation leads to athorough and logical interpretation of the modeled retita.

Drilling Sensitivity Analysis: In this scenario we willisolate WOB to determine its effti on the drillatring duringthe drilling of specific intervals. The modeling consists ofvarying the WOB while holding constant pertinent frictionalfactors, drilling assembly and other rig parameters.

Since the operation calls for the use of a water baaed fluidsystem the frictional values of .25 and .30 was chosen, basedon offset well tieldderived historical database, for casing andopen-hole sections respectively. The model results lead to anoperational WOB boundary based on rdistic frictional factorestimation.

Seleetion of WOB is baaed on tools specifications as wellas operational parameters. The operational WOB expectedfor an 8-1/2 hole aeetion will range from O-25 klbs. Themodeling take points will anal@ at O, 15, 25, and 50 klbs.WOB in order to view the dynamic condition reflative of theoperational performance.

We will now interpret the actual data grouped in a tableand graphical format. The resdt summary table allows us toeasily compare the model resdts with the s~ification of the5 drillpipe. The initial analysis was performed on adrillstring consisting solely of drillpipe [see Table 1]. Wequickly learn that any WOB above 10 klbs will redt in anegative Hook Load at date and pushing the drillingassembly into the helical buckling regime at the top 500 ft. ofthe well-bore, as seen from the graphical representation [seeFigure 6].

A comparison of the analysis using the preliminarydrillstring with the same sensitivity analysis of the m~leddrillstring [see Table 2] further demonstrates the value of thesimple method in integrating complex variables.

The strategic placement of 4,000 R. of HWDP in themodified drillstring was not derived from the DrillingSensitivity Analysis alone, but also from the TrippingSensitivity Analysis which informed effative length.

Tripping Sensitivity Analysis: These sets of analysis usessimilar parametem as set for the Drilling Sensitivity Analysiswhich was performed using tie preliminary drillstring. Themain objective for this type of analysis is to determine thelocation and amount of HWDP needed (if any) in order to tripto TD while still having ticient WOB available toovercome any ledges.

The graphical retita [see Figure 7] indicates that byinterpolating between the Oand 15 klbs trip curve, any ledgerequiring WOB over approximately 5 klbs cannot besurpassed with the current drillstring. Contingency planrequires the force of at least 15 klbs for any ledgesencounted during the drilling process and thereforemodifications must be made to the prelimina~ drillstring.

The information captured from the trip analysis leads tothe replacement of the 5 drillpipe with 4,000 ft. of HWDP atthe top seetion of the preliminary drillstring. Subsequently,

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the selection of Me amount of HWDP leads to the strategicdecision to set the 13-3/8 casing string at 4,000 fi.,MD toenwmpass the heavy weight drillpipe in a stable well-boreand therefore reducing the overall torque and drag affects.

Friction Sensitivity Analysis: This final sensitivityamdysis completes the torque and drag evaluation of the 8-1/2 production hole interval. Determining the tolerablefrictional factor operating range assists in making a decisionon the @ of mud Systew lubricious additives, drillpiperubbers, hole citing equipment stringent fluid parameters,etc. to incorporate into the drilling program,

The graphical retits in this ease [see Figure 8] showsthat adequate load trantier and helical buckling can befeasibly mitigated by modification of the drillstring designrather than upgrade to the more wstly oil-based mud system.The economical approach, from a tiictional perspective, is adrilling program which incorporates the use of the water-based fluid system with a stringent solids removal programand the addition of lubricious additives as contingency.

Interpretation and Field Implementation:In all modeling and simulation work, generation of retita

accomplishes 75A of the required engineering work. A mostimportant find step beyond modeling is eorreet interpretationof the retits and translation of such retits into cause-effeet-recognition-action schema for field implementation. Briefinterpretation of example re~ts for this work appends theanalytical disassion, however during the real work exerciseinterpretation of the results are presented to the project teamin tilcient detail cl~ng gray areas tinerable to varyingviewpoints,

In rmnt publications and papers it has been implicitlyexpressed that there are discrepancies in the analytical retitaand recommendations put foward by drilling servicecompanies, and the expectations of operators during crucialdrilling operations or at planning stages, Presentation ofretits following interpretation requires for ease ofunderstanding and validation of data a disclosure of softwarecapability, model set-up and parameters, definition of inputs,distinction between user inputs and calculated data, errorsources and magnitude, and known algorithm idiosyncrasies.This disclosure clarities the discrepancies that manifest fromresults produced by different computational engines,modeling methods and software sophistication.

Field implementation of results and recommendations wascarried out using operational data for real time correction ofmodel parameters to refine input variables and improve trendmatching of torque, drag magnitudes and derived fictionfactors. Unfortunately rigorous monitoring cannot besubstituted with predictive modeled restits since analyticalmodels do not provide predictions for continuum of eventsand rd time implementation do have some drawbacksespecially the mechanical behavior of tubulars under dynamicand combined loading states which models cannot simulate

accurately.Perhaps if the current trend of real time use of torque and

drag arudytical retita are sustained in terms of accuratecorrelation and minimized drilling mishaps, drillingengineers may bring the use of modeling and retits furtherdo~ in the design process to improve drillingperformance as well as design verification.

ERD Drilling Limitations And Impetus For State-of-the-art Drilling Technology:

The frictional drag in the drill ahead direction in the well-bore relative to the string poses a limitation to the ability toslide. The severity of this frictional drag is dependent on well-bore profile, traversed formation ~ and bore-hole gmmetry.Note the tremendous reactionary load ~erenee between thesliding and rotating drilling model [w Table 1 & 2].

Recently, torque reducers have seen prolflc use in solvingthe problem by isolating wodd-be mntact points between thetool-jointidrill-sem md the well-bore. Torque reducers canbe defined as active if they rotate relative to drillpipe orpassive if non-rotating.

However, in ERD wells the severity of fiictiond drag issuch that contact points become pseudo-fixd points along thedrill-string producing increasing sensitivity to WOB. Thefollowing approaches have bmn touted as succes~l antidotesfor excessive drag[4],

Increased mud lubricity Low fiction drill-pipe protectors Running DC or Heavy-Weight Drill-Pipe (HWDP) in

near vertid well sations Boost and control weight transfer with Thrusters for

smooth WOB application. Use extended power section motors to increase

stalling resistance.And, recently Rotary Closed Loop Drilling Systems , an

advancement over the Variable Gauge Stabilizer emerged as apanacea for overcoming critical drag limitation in ERD wells.Otherwise, drilling mechanics practitioners emphasizequalifying drillstrings and well-sections for ro@tion, that mayotherwise present drag limitation[4, 19].

ConclusionScrupulous modeling and representation of system states intorque and drag analysis tend to be tedious requiringparameter selectio% case study design and several iterationsofien terminating with divergent inferences. Subtlecomplexities are introduced when well geometric properdesand imposed loads must support drilling assemblyperformance under applied drilling load.

Simulating a tortuous wellpath using correctionalgorithms allows the application of noise amplitudes thatproduce the degree of tortuousity closely representative ofactual well-path. Wellpath design revisions using torque andfrictional drag analysis data must follow a scheme that ranks

249

8 YEMI ADEWUYA, SON PHAM lADC/SPE 39321

the significance of explicitly defined design spaceconstraints.

The critical design effort in drillstring torque and draganalysis is the ability to satisfy the applicable criteria defininglimits of desirable operation. Tubdar buckling load criteriaqualified as extended thmries allow for evaluation ofdrillstring behavior at higher loads and operating marginswithout necessarily compromising safety and feasibility,especially in extended-reach drilling o~rations.

From the genealogy of the robust methodology discussedin this papr, the importance of multiple analytical stationsstands out as most effective in evaluating anomalous behaviorof the modeled system across transition points that may notnecessarily fall at well-path geometric variations, drillstringand casing configuration change points.

Case study design used in a guided seareh to testsensitivities of dominant drilling parameters in few iterationsrequired the combined use of retits from Drilling, Trippingand Friction Sensitivity Analyses. The *uat case studydesign yielded a recommendation for the effative length ofHWDP for multifunctional deployment and ftiest easing setdepth necessary, within associated risk and ast limits.

Completing the template of a robust torque and dragmodeling work is the translation of the inferences obtainedfrom the arudytical results into usefid communication toolsfor well-plan design revisions or inclusion into drillingprograms for field implementation.

AcknowledgmentsThe authors wish to thank the management of Baker

Hughes INTEQ for permission to prepare and publish thispaper. The occasional question and answer sessions betweenthe authors and the following individuals, Raymond Jackson,Keith Fisher, Thomas Dahl, and Steve Dearman inspired thiswork. The support of the following people are gratetilyacknowledged during the initial ~ges and final preparationof this work: Pat Havard, Spencer Harris, Les Shale andDavid Gaudin.

References

1. Baker Hughes ~Q Torque and Drag v.4. 1. Program,Users Guide

2. Baker Hughes ~Q, Drilling Engineering So*arev.3.20, Marketing Documentation

3. Batchelor, B. J., and Moyer, M, C., Selection andDrilling of Recent Gti of Mexico Horizontal Wells:OTC!8462 (May 1997)

4. Payne, M. L., and Abbassian, F., Advanced Torque-and

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

-Drag Considerations in Extended-Reach Wells: SPE35102 (March 1996)Ruddy, K. E., and Hill, D., Analysis of Buoyancy-Assisted casings and Liners in Mega-Reach Wells,IADC/SPE 23878 (February 1992)Guild, G. J., Hill T. H., and Summers, M. A., Designingand Drilling Extended Reach Wells, Part 2 , PetroleumEngineer International (January 1995)McKo~ G. K., Drillstring Design Optimization forHigh-Angle Wells, SPE/IADC 18650 (February 1989)Maurer Engineering Inc., Horizontal TechnologyManual - DEA 44 September 1994Payne, M. L., Duxbury, J. K., and Martiw J. W.,Drillstring Design Options for Extended-Reach Drillingoperations; PD-VO1. 65, Drilling Technology, ASMEETCE, 1995Callin, J. K., and Hatton, P., Drillstring Considerationsand BHA Design for Horizontal Wells, InternalEastman Christensen Paper(now Baker Hughes INIEQ)Che% Y. C., Li@ Y. H., and Cheathw J. B., Tubingand Casing Buckling in Horizontal Wells, JPT, p140-191, February 1990Morris, E. R, Heavy Wall Drill Pipe A Key Member ofthe Drill Stem, Presented at the Joint PetroleumMechanical Engineering and Pressure Vessels and PipingConference, Mexico City, Mexico, September, 1976Ww J. and Juvkam-Weld, H. C,, Buckling and Lockupof ~bdars in Inclined Wellbores, PD-VO1.56, DrillingTechnology, ASME ETCE, 1994BreK J. F., Becke& A. D., HOILC. A,, and Smim D. L.Uses and Limitations of a Dnllstring Tension andTorque Model to Monitor Hole Conditions, SPE 16664,September 1994McCann, R. C. and Suryanarayana, P. V. R.,Experimental study of Curvature and Frictional Effectson Buckling, OTC 7568API Bulletin D20, Directional Drilling SurveyCalcdation Methods and Terminology, AmericanPetroleum Institute, D-mber 1985Maurer En~nmring DDWG8 Torque and Drag UsersManualHill, T. H., Summers, M. A., and Guild, G. J.,Designing and Qual~ng DrillStrings for Extended-Reach Drilling, SPE DRILLING AND COMPLETION,June 1996, Vol. II, No. 2, Pg. 111-117Aro~ J. S., Introduction to Optimum Design;McGraw-Hill, Inc., 1989

250

Table 1: Result Summary - Analysk of Preli

IREFERENCE

1

Weight on Bti [klbs] .

Max. Tot. Eqv. Stress (MTES) [psi

Location d k4TES [ft, MD]

ModeI of MTES

YM k [p$il 135,000

Torqw - Drilling [ft-1~

Torque - Rot-off-w. [Ibfl

MakeUp Torque [ft-iw 24,645

T-1 Y* [ti-!~ 63,W

Hook Load - Drilling [Ibfl .

Hook Load - Rot. Off Bet. [1~

Hook Loed - Pick-UP [1~

Hook Load - Slack-Off[Ibfl

Max.Allow.Hk W @ Min. Yld [ibfl 5$0,764 Neutral Point [ft, MD M bit]

Table 2: Result Summaw -

RESULTS

rictii Factors [cs@oh]

Neight on Bt [klb]

U=. T@. Eqv. *S (MTES) [W

.ocatii of MTES [ft, MD]

Wodaof MTES

ri stress @qrorque - Drilling [ft-1~

orq~ - Rot-Off-Bot. fl~

tlake-Up Torqw [ft-1~

OrsionaiY* [ft-lw

{ook Load- Drilling [lbfJ

iook Load - Rot. Off Sot. flbfl

imk Load - Pti-Up [Ibfliook Load - Slack~ [1~

flax. Allow. Hk Ld @ Min. Yld fl~,

~nalysi9of Opt]REFERENCE

I

.

135,000 55,m

24,645 29,400

63,406 51,375.

560,764 691,1M

Jeutral Point [ft, MD from bit] I

linary Drillstnng6-1/2Hole SUe 5DP to Surface

ORIENTED ROTARY= 100rpm ORIENTED

.2W.30 .2s.m .25/.30 .2W.30 .251.m

o

I,w l,W 1,630 mPick-up Drilling Drilling Drilling

0 708 1,181 2,361

10,360 10,360 10,360 Io,w .

107,959 107,959 107,959 107,959

15,322 15,322 15,322 15,322.

650530530 1,210 1,060 9(Drilling Drilling DrilIiw Drilling Piik-up Dfil~m

10,335 10,193 10,722 13,943 1,181 1,18

10,360 10,360 10,360 10,360 14,W 17,m.

53,735 36,764 28,762

36-54,091 54,091 54,091 54,031 54,091 54,09

107,959 107,959 107,959 107,959 139,324 l~,08!

15,322 15,322 15,322 15,322 +,624 -39,02[

0

ized Dfitring6-1/2 H* Ssize 5DP to 9-7/6 Casing_ and5H~P to Su-

ORIENTED ROTARY= 100rpm ORIENTED

1

.25/.30 .25/.30 .23.30 .25/.30 .251.30 .25/.30 .25/.30

o 15 25

28.163 28,798 27,478

6;797 4;323 4,323 4,% 4,053 4,053 4,053 7,458 4,323 4,323

Drilling Drilling Drilling Drilling DrilIing Drilfiw Drilling Drilling Drilling Drilling .

0 708 1,181 2,361 13,877 14,~ 14,~ 14,291 1,161 1,18113,889 13,889 13,889 13,889 13,889 13,669 13,889 13,889 19,006 24,126

1ll 1 1 1

45,305 28,547 +3,124

199,227 84,245 74,246 49,202

1

99,717 99,717 99,717 99,717 99,717 99,717 99,717 99,717 99,717 99,717

185,682 185,882 185,@2 185,662 165,882 185,882 185,882 165,882 204,021 253,452

45,305 45,305 45,305 45,3# 45,% 45,305 45,305 45,305 15,085 -28,911

0 7,019 7,344

NO*: - Cfilcal mutts

Referenm limits and specific.atiins

251

. . .. .

F~ure 1: WellPlanningandEngineeringAnalysis Flowchart. --.- .... . . .. .. ._ .._ ..-.. ..--.-.- -- . ..

~~-W~tiT~

-m~-=~d

-=-~

~-------;

TY=

~-------i -------

~-------;

252

Figure 2: Critical Inclination Curve - Simple Static Anaysis... ..... . .-.

1.00

0.90

0.80

0.70

Sting forces in the X direction yields

the followingsimplerelationship:

8= Inclination[deg]p = Friction Factor

dx

0.30

0.20

0.10

45 50 55 60 65 70 75 80 85 9CCritical Inclination [deg]

.--= -=.

?igure 3: Tortuosity Comparison Chart . -..

10.0 , - J

9.0

8.0

7.0

I

To?tuosify Number - Developed ( )Tortuosi~ = T. sin 2. n. ~(Eqn. 3 -A)

by Dr. Rapier Dawson to applytortusity to the well-bore by

where:

varying inclination and azimuth in T = Tortuosity Number [deg]

a sinusoidal manner MD = Measured Depth [ft]X=3.14159

/ 1~ 6.0 ~~

3 Noise Leve/ - Ramdonlyapplied DLS within a speoified.-

0z 4.0- ~

3.0- -

2.0- ~

1.0- ~

*Noise Level +Tortuosity Number 10.0 , r

1 11 !

5.0

4.5

4.0

3.5

83.0$

3

2.5: ,-m

2.0:

~1.5

1.0

0.5

0.0

0.00 0.10 0.20 0.30 0.40 0.50 0.60

Average DLS Induced on Well Plan [deg/100l,.._=

I

=3

---

253

~ire 4: TorqueandDragAnalysisFlowchart+ ,.,..4+.

A thorouti evaluation of the d=]~roamm-fill include this cycle o?~ 1

.-

!, .

... .- .+

.. . ~. .... A

7iaIIrp %: Pint nf Plsn vs. ActIIsl Weii.Path

o

m

1,OQo

2,000

2,500

Planned ------ Actual

Planned TD

d 20 DrivePiw @ 375fnd

12-1/4; 17-i12 Hole Size

13-3/6 Casing @ 1,Wrnd-2,s00 -2,m -1,500 -1,000 -500 0

c weat [H]

9-5/6 Casing @ 4,000tnd

Actual TD: 7 Casing @ 7,075ti -

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

3,0000 1,000 2,000 3,0i)o 4,W 5,m 6,000

Vertical Section [ti]

I Figure 6 Torque and Drag - Drilling Sensitivity Analysis. , ----- ..- ......-,. .

.. .

TORQUE 6 DRAG ANALBIS

HELlCAL BUCKLING& DRILLING LOADS

Preliminary Drillstring ~ Modified Driiistring

n~1 Critical3 Region1 ,=l_[

1-=--z- _ I->:

_.\- - I

+x ---= -,.._ r-__m-.

-.

?_-----

.- I

1,000

2,000

>__>

~,ooo - _-.e.- .. . ..2 :* ?.>>II

I

L.

>I

>.>>>..>

+

L I I1 1A. -.:.8,000 tI

Hel. Buckling Load [Ibfl -.= ..... . .-. .-= ..-

Drlng Load 2 (WOB=l 5klbs) [Ibfj Eti. Hel. Buckling Load [Ibfl Drlng Load 3 (WOB=25klbs) [Ibq

Drlng Load 1 (WOB= Oklbs) [ibfl Dring Load 4 (WOB=50klbs) [Ibq,, . .-..-. ..-.-. ... ..-..,. . .. . .4!.-., .

255

. ..

Figure 7: Torque and Drag - Tripping Sensitivity Analysis

TORQUE & DRAG ANALYSIS O Critical Trip DepthHELlCAL BUCKLING& TRIP LOADS

HOLE SIZE: 8-1/2MODE: ORIENTED

FRICTION FACTOR (CSG/OH): .25/.30 Modified DrillstringPreliminary Drillstring

o 0 LOADS [Iboo 0

s. o0 ~;

g om o-y o-0

01 1 U)*

m 11tI11

I11

1I11IIII1

--- h

I 1I 1

II II I1 I1 I1 II 11 1I II 1

III

IIr--- p.= ,-

,.I I1 II II I

C*1 lncliJationHWDP

Placement*---- ~---

I

---- ---- ---IIIIIII1IIIIIIIIII[IIIIII1111IIIIIIII1I11IIIIII1I1II1

i

-. ,;,-:-..,.,.-......z.

. . ... . ...>

3,000 .:-;-. ..-.-=.,,

IIIII1I11IIIIIIII1IIIIIII11IIII1II1111IIIII1III11

III

IIIIIII1

11!IIII,

I11IIIIII11

II

II

111

III

I

=

7,000 ~I

I11I111

I

I

.8,000 t ! :II

Trip Load 1 (WOB= Oklbs) [Ibfl

[1, II

11I

Hel. Buckling Load [Ibfl Ext. Hel. Buckling Load [Ibfl Trip Load 2 (WOB=I 5klbs) [Ibfi

Pick-Up Load [Ibfl Trip Load 3 (WOB=25klbs) [Ibfl

Slack-Off Load [Ibfl Trip Load 4 (WOB=50klbs) [Ibfl

256

~Fgure 8: Torque and Drag - Friction Factor Sensitivity Analysis--- - .-

-. ... . .

TORQUE & DRAG ANALYSIS

HELlCAL BUCKLING& DRILLING LOADSwith Varying Friction Factom

Modified DrillstringPreliminary DrillstringI J

L ,

IIIII

0

1,000

2,000

3,000

4,000

[ 1 1

1I

I

HWDP

Placement

I \\ 4

k- ransitiind Inclination------ ----- --- *~

I--1II +

---- ---- ---.- 1

,\II1IIII1IIIIII1IIII11IIII1IIIIIIIIIII111IIII111IIIIII

II

II1

I

III

11

1[1III111111

5,000

6,000

7,000

8,000

I1II

I1

III

11

I 1 1I 1 1I I t r , ,

Hel. Buckling Load [Ibfl. _. ....=..

Drlng Load [Ibq ff=.35/.4O [csg/oh]

Ext. Hel. Buckling Load [Ibfi Drlng Load [Ibfl ff=.45/.5O [csg/oh]

Drlng Load [Ibfl ff=.25/.3O [csg/oh]I . . -.- .-

-.. =-:

257