Rotary-steerable systems possessclear advantages, both technologi-cally and economically, over manymud motor-based drilling systems.

The service industry is respondingto this by providing operators withnew tools, expanding their ability todrill extended-reach and underbal-anced wells, smooth out well trajecto-ries, and optimize well configurationsfor production enhancement.

Ultimately, motor-based directionalsystems may eventually be replaced byrotary-steerable alternatives, except forthose cases where a downhole motor isneeded for nondirectional attributes.

This first part of a two-part seriesdescribes the technological differencesbetween conventional mud motor-based directional drilling systems androtary-steerable systems. The secondpart defines implementation issues thatare of concern to the operator.

BackgroundThe oil and gas industry relies heav-

ily on directional drilling to developoffshore reserves, facilitate develop-ment in environmentally sensitiveareas, and provide production en-hancement through horizontal andmultilateral completions.

Although directional oil wells werefirst drilled in the California Hunting-ton Beach field in 1933,1 the introduc-tion of a directional drilling system in1962, based on the development of apositive-displacement motor (PDM)and bent-sub assembly, provided thefirst practical capability for developingoffshore fields.

Introduced in California, the tech-nology quickly spread to the Gulf ofMexico and continued to evolve intothe steerable motor system that is usedtoday.2 During the evolution of thesteerable-motor technology, there havebeen many enhancements in materialsand designs (Fig. 1).

Fundamental limitationsSteerable motors provide a capabili-

ty that is essential to the oil industry.

Unfortunately, this technology has sig-nificant limitations and inefficienciesthat affect its ability to continuouslysupport increased operational de-mands.

Drilling with steerable motors is di-vided into two activities: Sliding androtating (Fig. 2). Sliding involves preci-sion guidance of the assembly towardsa prescribed target. Rotating involvesno active guidance of the trajectory.

Problems associated with each ofthese states are listed in Table 1 anddiscussed in greater detail by Warren.3Most of the limitations and inefficien-cies are related to the need to drill aportion of the well without drillstringrotation or concerns related to motorperformance.

Continued evolution of the steerable

motor system is not likely to solve mostof these problems since they are relatedto the fundamental characteristics ofthe steerable-motor directional system.

Rotary-steerable systemsThere is an active effort in the in-

dustry to overcome some of these limi-tations by using rotary-steerable di-rectional-drilling systems. Rotary-steerable systems are directional toolsthat allow the well trajectoryinclina-tion and azimuthto be actively guid-ed while rotating the drillstring.

In fact, the concept for rotary-steer-able directional systems predates thecommon use of mud motors (PDMs).These systems include many of the fun-damental concepts that are being usedtoday.

Fig. 3a shows a system, patented in1955, that used a nonrotating sleeve todirect the bit in a specific direction.4The patent describes the objective ofthe tool as to cause the drill collar toassume a slightly inclined positionwith respect to the axis of the hole sothe bit will be laterally directed.

Fig. 3b shows another systempatented in 1959 that used hydraulical-ly activated guide shoes near the bit tocontrol the drilling trajectory in a simi-lar manner.5 The guide shoes, locatedon a nonrotating housing, were pow-ered by mud pressure and could be ac-tivated and retracted without trippingthe drillstring. The specific objective of

Dec. 21, 1998 Oil & Gas Journal 101

TECHNOLOGY

ROTARY-STEERABLE TECHNOLOGYPart 1

Technology gains momentumTommy Warren Amoco Production & Exploration Technology Group Tulsa

OGJ

Fig. 1

STEERABLE MOTOR SYSTEM EVOLUTION

PDMPositive displacement motorMWDMeasurement while drilling

Steerablemotor

MWD

Wire linesteering tool

PDM andbent sub

Rotarysteerablesystems

1960 1970 1980 1990 2000

Sliding Rotation

Inability to slide V i b r a t i o n s -m o t o r a n dMWD failures

Maintaining orientation Accelerated bitwear

Poor hole cleaning Poor hole qual-ity for logs

Low effective ROP Poor perfor-mance in air

High tortuosity ECD fluctuations Differential sticking Buckling and lock up Build rate formation sensitive

PROBLEMSTable 1

this tool was to eliminate the need totrip in and out of the hole to set whip-stocks for well guidance.

Other examples include a widerange of mechanisms used to alter theborehole trajectory, including mechan-ically activated guide shoes, hydrauli-cally activated guide shoes, eccentricnonrotating sleeves, nested eccentriccam sleeves, and intermittently activat-ed paddles.6-12

These tools were based on conceptsvery similar to modern rotary-steerabletools, but none were commercially suc-cessful. Apparently the lack of effectivedownhole sensors and control systemsimpeded development of these tech-nologies, more so than a lack of trajec-tory control ideas.

Steerable systems vs. motorsThe lack of commercial success of

the first pre-PDM rotary-steerable toolsdid not mitigate the attractiveness ofactively steering the bit while continu-ously rotating the drillstring. Aftermore than 30 years of dominance by di-rectional drilling mud motors, a combi-nation of market forces, as follows, hasrecently initiated a resurgence of inter-est in the rotary-steerable concept.

The complexity of many direc-tional wells increased.

Extended-reach wells exceededthe range with which motors could beoriented.

The benefits of horizontal wellsbecame better understood and madethe economics for redevelopment of re-serves from existing platforms more at-tractive.

Expertise for running fixed stabi-lizer assemblies slowly disappearedand operators began to rely upon steer-

able motors for drilling the entire well. Improvements in bit technology

provided aggressive PDC (polycrys-talline diamond compact) bits with ad-equate durability to drill a wide rangeof formations. Unfortunately, these bitswere incompatible with motors.

Interest in simultaneous drillingand underreaming has increased, pro-viding more-flexible casing programs.In some cases, this technology couldnot be efficiently implemented withmotors.

Thus, the point was finally reachedwhere the most challenging operations,using directional motors, could notcontinue the aggressive developmentof reserves. At the same time, the capa-bility to design downhole monitoringand control systems had dramaticallychanged since the rotary-steerable sys-tems were first investigated.

Thus, the demand for increased di-rectional capability and improved abil-ity to design more-sophisticated toolscombined to provide a fertile groundfor the resurgence of the rotary-steer-able concept.

ReemergenceOver the past few years, several

companies have started work on ro-tary-steerable systems aimed at partic-ular niche markets. The high-cost, ex-tended-reach market was one areawhere rotary-steerable technology wasperceived to provide an enablingtechnology, marketable at a premiumprice.13 On the other end of the spec-trum, work was undertaken to providea very cheap system aimed at onshoremarginal properties.14

Competition to provide viable ro-

tary-steerable systems has accelerated,and now there are several commercialsystems, including Baker HughesInteq, Camco Drilling & Service, andCambridge Drilling Automation Ltd.Others in the prototype-testing phaseinclude Schlumberger Anadrill andSperry-Sun Drilling Services.15

Another tool type, sometimesclassed as a rotary-steerable system, in-cludes downhole adjustable stabilizersthat can be used for inclination control.It is reported that about 90% of thedownhole trajectory corrections are infact inclination corrections with the im-plication that for most cases there is lit-tle need for the ability to make azimuthchanges.

While it is probably true that mosttrajectory changes are inclinationchanges, it is also true that rarely is adirectional run made where the opera-tor can guarantee that an azimuth ad-justment will not be needed.

Thus, there is certainly a place forthe downhole-adjustable stabilizersbased on capability, reliability, cost,and directional requirements, but inthe following discussions, rotary-steer-able systems will be considered as toolscapable of making both inclinationaland azimuthal changes.

Current conceptsThere is a wide variation in the de-

signs of the systems entering the mar-ket as compared to steerable motors(Fig. 4). Steerable motors direct the bitalong a particular path by providing arelatively rigid three-point geometrythat biases the bit to drill along the arcof a circle.

The geometry of the tool prescribesthe position of the bit relative to the

102 Oil & Gas Journal Dec. 21, 1998

TECHNOLOGY

OGJ

Fig. 2

STEERABLE MOTOR ACTIVITIES

Sliding Rotating

OGJ

Fig. 3

EARLY CONCEPTS

Nonrotatingsleeve

Hydraulicallyactuated

guide shoes

a b

two noncutting, upper-contact points.The side-cutting capability of the bit al-lows it to move along the circular arctrajectory that continually minimizesthe side force on the bit.

With the steerable motor system, thebit axis may be aligned with the holeaxis; but in most cases, it is not. Thebending moment applied to the upperstabilizer by the bottom-hole assembly(BHA), positioned above the motor,tends to provide additional curvatureby increasing the deflection of the bit.

This effect must be considered whenestimating the curvature for a particu-lar motor BHA. For a typical 634-in.steerable motor, the curvature that canbe achieved ranges between 0 and15/100 ft, using typical tool configura-tions and bent sub angles (up to 3).Higher curvatures may be obtained byother BHA designs such as adding asecond bend above the motor or articu-lating the motor to place the upper twocontact points closer to the bit.

Steerable tool designCases 1-3 in Fig. 4 illustrate rotary-

steerable tool applications where thetrajectory is determined by the samethree-point geometry that controlssteerable-motor trajectory. The ideal

situation is to align the bit axis with thewell path arc, as determined by thethree control points. For the toolsshown as Case 1, the bit will always bepointed to the outside of the curve.

For Case 2, the bit will still general-ly be pointed to the outside, but to alesser degree than Case 1. For Case 3,the bit may be pointed either to the in-side or outside portion of the curve, de-pending on the particular tool design.

In terms of trajectory control for au-tomated guidance systems, the exactdirection of bit pointing may not be tooimportant as long as the parameters re-main in the active range of the controlsystem. However, increased bit mis-alignment may increase sensitivity tobit design and formation properties, re-sulting in a tendency for increased bitvibrations.

In Case 1 the controlling geometricalconstraints are provided by one or morepads, positioned near the bit, that inde-pendently extend outward to apply aforce against the borehole wall. Thepads may extend semistatically from anonrotating housing (OGJ, Mar. 2, p.65), such as in the Baker Hughes InteqAutoTrak system, or dynamically froma rotating housing such as in the CamcoSteerable Rotary Drilling (SRD) system.

The curvature that can be obtaineddepends on the geometry of the partic-ular tool, the bending moment from theassembly above, and the amount ofpad extension. The pad extension, acritical determinant of directional con-trol, may be directly or indirectly con-trolled by adjusting the force applied toit, limited by design considerations.

In either case, effective pad exten-sion, hole gauge, and bit-side cuttingforces control the trajectory. In general,the maximum build rate from thesetools is less than can be obtained froma bent-housing motor.

Case 2 shows a tool where a contin-uous rotating drive shaft is deflectedinside a nonrotating housing (Cam-bridge Automatic Guidance System).Again, the bit trajectory is determinedby the three-point geometry, but the bitaxis tends to be more closely alignedwith the centerline of the hole.

In general, it is difficult to get asmuch curvature with this type of toolas with a Case 1 design because of geo-metrical and strength considerations.However, it can more conveniently ac-commodate a wider range of hole sizeswith the same tool.

Case 3 illustrates another systemwhere the bit axis is maintained in atilted position relative to the well direc-tion by actively rotating the upper endof the bit shaft counterclockwise and insynchronization with the clockwise ro-tation of the drillstring (DirectionalDrilling Dynamics Co.).

This system can provide a relativelyhigh degree of bit tilt, does not requirenonrotating contact with the borehole,and can align the bit axis with the bore-hole axis. This system lags behind thefirst two cases in terms of commercialimplementation.

The trajectory for Case 4 is not con-trolled by the three-point rigid geometry,but instead is determined solely by bit di-rection. The bit is designed to minimizeside cutting with a flexible joint that iso-lates the bit from the drillstring bendingmoment and resulting lateral bit forces toprovide a controlled trajectory.

This trajectory control mechanism al-lows much higher build rates in smallerhole sizes than the other mechanisms,but is not as tolerant of over-gaugehole conditions as the rigid three-pointsystems that have been implemented onmanually oriented systems.

Control mechanismsThe tools designed around the Case

1-3 tool design concepts are generallyautomated guidance tools, meaningthey have adequate onboard sensors,power systems, and control systems todynamically adjust well trajectory ac-

Dec. 21, 1998 Oil & Gas Journal 103

TECHNOLOGY

OGJ

Fig. 4

TRAJECTORY CONTROL MECHANISMS

Steerable motor

Case 1

Case 2

Case 3

Case 4

cording to a particular command.Commands are generally of two

types:1. Closed loopEmployed by the

AutoTrak and the AGS systems. Thesesystems direct the tool to drill to a par-ticular inclination and azimuth at a cer-tain build rate.

2. A fixed-command system that di-rects the tool to drill in a particular ori-entation relative to the high-side toolface at a given build rate.

None of the tools have downholedistance measurement sensors neededto actually determine the true buildrate. Thus, the build rate generally be-comes a surface-specified fraction ofthe tools theoretical maximum rateand is adjusted by using surfacedownlink commands once tool perfor-

mance becomes known.The build rate is most often controlled

by alternatingon a short-duration timeschedulefrom a three-point geometrythat provides a maximum build rate toone that drills a straight hole.

A few tools, including the Auto-Trak, alter the three-point geometry toprovide continuous build-rate adjust-ment. Most of the new tools coming tomarket are being introduced with thesimplest control system, but have capa-bilities built in for upgrading.

From a trajectory control stand-point, there is little difference betweenany of the above control systems aslong as a directional driller is em-ployed to manage the system. Themajor differentiating feature will be re-liability of the control system.

Market perspective

The introduction of rotary-steerabledirectional systems will clearly benefitoperators, but the situation is some-what more complicated for servicecompanies investing in the new tech-nology. The rewards that accrue toeach depend on the particular path thatthe technology commercializationprocess takes.

The following discussion identifiesthe most important applications for ro-tary-steerable systems and indicatessome of the issues facing both opera-tors and the service sector as the direc-tional market shifts from motors to ro-tary systems.

Economic opportunitiesRotary-steerable technology can sig-

nificantly impact an operators wellcost. Directional wells tend to be moreexpensive than straight holes primarilyfor two reasons. First, the process of di-rectional drilling with motors is less ef-ficient than drilling a straight hole. Es-pecially since the distance drilled to aparticular horizon is greater, and addi-tional equipment and services are re-quired.

Second and more importantly, thedaily operating costs for directionalwells tend to be higher because theyare used in places where surface accessand support costs are high, includingoffshore and environmentally restrict-ed locations. Thus, the incentive to re-duce the total number of days on thewell may have a greater impact on theoverall economics than the cost ofusing directional equipment.

Fig. 5 shows the break-even pointfor a rotary-steerable system whoseequipment cost is two to three timesthat of the alternative steerable-motorequipment. Assuming the rotary-steer-able equipment is as robust as the al-ternative, it would require only anoverall time savings of 20% to be costeffective for most offshore operations.

Operational experience with the ro-tary-steerable systems over the last fewmonths demonstrates an efficiency im-provement much more than this inmany cases (Fig. 6), but also indicatesthat reliability may not yet be quite asgood as that for steerable motors andmeasurement and logging-while-drilling systems.

Directional toleranceThe growth of horizontal drilling for

production enhancement has placedgreater requirements on directionaldrilling capability. A well path that in-cludes long intervals of simultaneousinclination and direction changes is

104 Oil & Gas Journal Dec. 21, 1998

TECHNOLOGY

OGJ

Fig. 5

BREAK-EVEN POINT

MTBFMean time between failure

20,000

Tim

e r

ed

uct

ion

, %

50,000 80,000

Rig spread cost, $

110,000 140,000

50

40

30

20

10

0

Assumptions:Motor + MWD = $12,000Rotary steerable = $35,000

Same MTBF for both systems

OGJ

Fig. 6

EFFICIENCY IMPROVEMENTSNORWAY WELL

Source: Baker Hughes Inteq

Tim

e r

ed

uct

ion

, %

35

20

15

10

5

0

Offsets

AutoTrak(rotary steerable)

Effective ROP Number of trips Number of days

often required to place the horizontalsection within the optimal reservoir po-sition and orientation.

Drilling these paths requires an in-creased amount of sliding. The combi-nation of increased sliding and higherfrictional drag makes changes in orien-tation more difficult with PDM steer-able motors. While it is often possibleto drill these paths with steerable mo-tors, their efficiency decreases with thecomplexity of the well and makes posi-tioning the horizontal relative to thegas cap or water level difficult.

Often, only a few feet in elevation atthe entry point or a few dips in eleva-tion can make a significant differencein the ultimate production before gasor water breaks through. Rotary-steer-able systems have demonstrated thecapability to provide infinitely more in-clination control than steerable motorsbecause of their ability to provide con-tinuous automatic orientation.

Capital costsRotary-steerable technology may

also reduce the number of platformsand wells that are needed to drain anasset by increasing the achievable de-parture for extended-reach wells and al-lowing increased complexity for design-er wells. Capital costs and delayed pro-duction associated with setting a plat-form are often the greatest economicdriver for developing an offshore field.

In addition, because many produc-tion areas are defined by stacked payzones where designer wells are neededto optimally encounter each zone, a wellpath may require multiple direction andinclination changes that are difficult toachieve with steerable motors.

In both of these cases, the use of ro-tary-steerable technology has the po-tential to leverage capital cost muchgreater than the cost of employing thenew technology.

Problem formationsThere are cases where formation

considerations make operating steer-able-motor systems particularly trou-blesome. One example includes situa-tions where the operating window be-tween lost circulation and maintaininghole stability becomes very narrow.

Even though the actual directionaltrajectory may not be very challenging,the equivalent circulating density (ECD)variations between sliding and rotatingthe steerable motor may become greatenough to make it tedious, if not impos-sible, to stay within the window.

It only takes a few incidents of pack-ing-off or lost-circulation problems topay for the additional ECD stability af-forded by rotary-steerable systems.

Underbalanced drillingUnderbalanced drilling (UBD) has

historically been used in formations toimprove penetration rates. However,UBD has recently gained prominenceas a way to reduce formation damagein horizontal wells, particularly in ma-ture areas where the reservoir pressureis low.

The primary method of providingan underbalanced fluid column is todrill with air or aerated mud. In manycases, nitrogen is used instead of air.The use of these highly compressiblefluids causes two types of problemswith mud motors.

First, the compressibility of the airsignificantly affects the efficiency ofthe motor and causes high rotationalspeeds when the bit load is removed.This can affect the motor by over heat-ing the stator and is further com-pounded by absorption of the gas intothe stator rubber, causing it to swell.The compressibility of the fluid alsomakes it difficult to keep the motorproperly loaded and oriented, particu-larly as the reach of horizontal wellsincreases.

Second, the combination of themotor over-speeding and reduced fluiddamping significantly increases lateralvibration problems that are very detri-mental to the bits, motors, and steeringequipment.

So far, rotary-steerable tools havehad limited UBD applications with asingle closed loop tool run in Italy(Case 2) and several runs with Case 4tools. However, rotary-steerable direc-tional systems have the potential toeliminate many of the problems that re-sult from running a PDM with a highlycompressible fluid.

Deep hot wellsDirectional work in deep, high-tem-

perature wells is limited by the capa-bility of Moineau motors to operateunder such conditions, especiallywhere oil-based mud is used. This lim-itation affects the ability to effectivelyuse horizontal completions in thesewells.

In many cases, the pressure environ-ment and subsequent casing programsdictate that this work be done in asmall-diameter hole and often at rela-tively high build rates. Alternative mo-tors such as turbines and other all metalmotors may operate more effectively athigher temperatures,16 but they donothing to reduce the problems of ori-enting small diameter motors in deepwells.

Both the motor durability issues andorientation issues may be addressedwith one of the rotary-steerable sys-tems being marketed.17

References1. Brantly, J.E., History of Oil Well Drilling, Gulf

Publishing Co., Houston, 1971.2. Garrison, E.P., Downhole Motor Cuts Direc-

tional Drilling Costs, Petroleum Engineer,January 1965.

3. Warren, T.M., Trends Toward Rotary-steer-able systems, World Oil, May 1997, pp. 43-47.

4. Giles, M.L., and Wells, C.L., DirectionalDrilling Tool, U. S. Patent 2,712,434, issuedJuly 5, 1955.

5. Page, J.S. Sr, Page, J.S. Jr, Antle, W.H., andKnickerbocker, F.B., Directional DrillingTool, U. S. Patent 2,891,769, issued June 23,1959.

6. Brow, G.E., Deflecting Tools, U. S. Patent2,730,328 issued Jan. 10, 1956.

7. Zublin, J.A., Apparatus for Drilling Wells ofLarge Radii Curved Bores, U.S. Patent2,745,635, issued May 15, 1956.

8. James, W.G, Frisby, T.M. and Hamman, J.A.,Deflecting Tool, U. S. Patent 2,819,040, is-sued Jan. 7, 1958.

9. Sims, D.L., Deflectionl Drilling Tool, U. S.Patent 2,919,897, issued Jan. 5, 1960.

10. McNely, B., Means for Controlling Direction-al Deviation in a Well Bore, U. S. Patent3,043,381, issued July 10, 1962.

11. Tighe, R.E., Oriented Drilling Tool, U. S.Patent 4,076,084, issued Feb. 28, 1978.

12. Baker, R.E., Bit Guidance Device andMethod, U.S. Patent 4,416,339, issued Nov.22, 1983.

13. Barr, J.D., Clegg, J.M., And Russell, M.K.,Steerable Rotary Drilling with an Experi-mental System, SPE/IADC paper 29382,presented at the SPE/IADC Drilling Confer-ence in Amsterdam, Feb. 28-Mar. 2, 1995.

14. Warren, T.M., et al., Short Radius LateralDrilling System, Journal of Petroleum Tech-nology, February 1993, pp. 10815.

15. Von Flatern, R., Steering Clear of ProblemWell Paths, Offshore Engineer, October1998.

16. Stewart, D., and Susman, H., New MotorsSolve UBD and HPHT Problems, PetroleumEngineering International, August 1997, pp3134.

17. Warren, T.M., Slimhole Rotary-steerable sys-tem Broadens Applications,World Oil, Sep-tember 1997, pp. 8397.

Dec. 21, 1998 Oil & Gas Journal 105

TECHNOLOGY

Tommy War-ren is a special re-search associate forAmoco Production& ExplorationTechnology Group.He has more than25 years experi-ence for Amocoworking withdrilling technologydevelopment in theareas of drill bit me-chanics, directionaldrilling, and drillings systems.

Warren was the 1997 recipient of theSPE Drilling Engineering Award and is cur-rently serving as the program chairman forthe 1999 SPE Annual Technical Conferenceand Exhibition. He is also an SPE Distin-guished Lecturer on the topic of rotary steer-able systems. Warren holds BS and MS de-grees in mineral engineering from the Uni-versity of Alabama.

Warren

THE AUTHOR