Jon Ruszka, Baker Hughes INTEQ

JUST FIVE SHORT years ago, rotarysteerable drilling systems were consid-ered an exotic new technology, com-manding what some considered exorbi-tant daily costs, showing poor reliabilityand unproven value. For these reasons,the systems were used only on selectedprojects where benefits from applicationof the technology were very clear. Addi-tionally, there were only very limitednumbers of these tools available andtheir use was almost exclusivelyreserved for projects in high cost, highactivity areas with short supply chains(e.g., North Sea and Gulf of Mexico).

That was five years ago. Today, the pic-ture is much different. The technologyhas matured with a wide range of bene-fits accepted by the industry, valueproven and tremendous improvementsseen in reliability. Rotary steerabledrilling systems are now used the worldover with an estimated 7 million feet tobe drilled using them this year aloneand growing by approximately 50% peryear.

T E C H N O L O G Y O V E R V I E W

Conventional directional drilling tech-niques require the use of bent housingdownhole motors to be oriented in theborehole and “slid” along the boreholewithout rotation of the drillstring toachieve a change in the well’s trajectory.Periods of this “slide” drilling are inter-spersed with periods of rotary drilling toachieve the desired three-dimensionalwellbore trajectory.

Rotary steerable drilling is, as its namesuggests, technology that enables fullthree-dimensional directional drilling

control to be performed while drillingwith continuous drillstring rotation fromsurface. No “slide” drilling is necessary.This capability requires a special BHAcomponent above the bit to direct thewellpath in the desired direction, main-taining the orientation of the drillingtrajectory independent of the rotation ofthe BHA and drillpipe above it. Thiscomponent is the rotary steering device.

How the different available rotary steer-ing devices accomplish their task variesfrom relatively simple gravity-based ori-entation systems to more sophisticatedflexure of internal driveshafts or flexureof the lower portion of the BHA by appli-

cation of forces from pads against theborehole wall. Some systems alsoemploy automatic drilling modes wherethe wellbore is automatically steeredusing closed loop control systems pro-grammed in the downhole tool. Thesesystems (such as the AutoTrak® RotaryClosed Loop System) deliver significantbenefits in wellbore placement and over-all wellbore quality compared to non-automated systems.

In addition to differences in the mecha-nisms, which these systems use to phys-ically steer the well, there are also dif-ferences in the manner in which sys-tems are communicated with from sur-face and also the level of integrationwith the MWD and LWD systemsemployed in the BHA.

Efficient communication from surfaceresults in significant time savings and ahigher level of integration withMWD/LWD systems allows positioningof LWD sensors close to the bit whilesimultaneously minimizing BHA lengthand increasing reliability.

Each different system has its own mer-its with regards to drilling efficiency,predictability of directional control, pre-cision of wellbore placement, physicalhole quality and cost effectiveness on aparticular application. It is important torecognize that while there is a naturaldesire to commoditize this technologyand not recognize the significance of thedifferences between the available sys-tems, these differences are fundamen-tally critical to the cost effectiveness onany particular project.

T H E A D V A N T A G E S

Eliminating the need to drill withoutdrillstring rotation using bent housingmotors to achieve directional drillingcontrol has some immediately obviousbenefits. These include significant timesaving through ROP improvements, con-tinuous effective hole cleaning anddrilling of a hole with lower “tortuosity”.Other accepted benefits include drillingof much more ambitious well trajecto-ries (either complex 3D and/or ERD)with lower technical risk to achievingobjectives. These benefits are consid-ered “tangible” and it is quite easy tocalculate a dollar value for them whenestablishing the cost effectiveness ofapplying rotary steerable technology. Asexperience in the use of these systemshas grown, the less obvious or intangi-ble benefits have gained acceptance andare arguably of more value than theimmediately obvious benefits indicatedabove. These less obvious benefitsinclude, but are not limited to, the fol-lowing:

Safety When drilling with rotary steer-able systems, fewer trips in and out ofhole of the drillstring should berequired. These systems commonly uti-lize fixed cutter bits where previouslyTricone® bits may have been used fordirectional control reasons. The longerlife of fixed cutter bits results in morefootage per bit and thus fewer trips forbit change. In addition, continuous rota-tion at high rotary speeds results in veryefficient hole cleaning, removing theneed for many short trips. Rotary steer-able systems are also much more versa-tile and should be able to drill all of therequired section trajectories (build,drop, tangent, turn) using a single BHAdesign, resulting in fewer trips for “BHA

44 D R I L L I N G C O N T R A C T O R July/August 2003

Rotary steerable drilling technology matures

A higher level of integration with MWD/LWD systems allows positioning of LWD sensors closer tothe bit while simultaneously minimizing BHA length and increasing reliability.

Inclination Pressure Resistivity

Gamma &Azimuthal Gamma

Directional

Vibration & Stick Slip Density Caliper Porosity

change”. This benefit results in lessdrillfloor activity, less handling of tubu-lars, and, ultimately, increased safety.Reduced tripping activity can be meas-ured by comparing the footage drilledvs. the total amount of pipe tripped overthe course of a project. A study in Nor-way showed approximately a 50%reduction in tripping activity followingthe introduction of rotary steerable sys-tems on a development.

Environmental Drilling with rotarysteerable assemblies results in a morein-gauge hole than drilling with steer-able motor systems. This fundamentalbenefit results in lower volumes ofdrilled cuttings waste and lower drillingfluid losses. Additionally, because thecuttings are fresher as a result of con-tinuous hole cleaning, the cuttings areeasier to clean and less drilling fluid islost. For example in a 12¼-in. section,should the hole be drilled over gauge toan average diameter of 14-in., this rep-resents approximately a 30% increasein cuttings waste and correspondingly30% lower annular velocity compared todrilling the section “in gauge”. Environ-mental impact and waste disposal costsare both reduced where in-gauge hole isdrilled. Where “skip & ship” or cuttingsre-injection systems are in operation,the constant stream of cuttings coupledwith minimized waste volumes from useof rotary steerable systems is a recog-nized benefit.

Stuck Pipe Continuous pipe rotation,smoother and less “tortuous” trajectoryand overall improved hole gauge qualityreduce stuck pipe and lost in hole (LIH)incidents. A study comparing LIH inci-dents of a rotary steerable system andconventional BHA’s showed the rotarysteerable system LIH rate was only 15%of the conventional systems.

Production Benefits The ability tomore precisely land and position well-bores within the reservoir gives animmediate production benefit. The moresophisticated rotary steerable systems,which have automated closed loop con-trol of the steering response, are able toconsistently position wells more pre-cisely than even the very best direction-al driller could achieve using conven-tional technology. This ability to landand navigate wells precisely within thebest production zones has an immediatebenefit for improving the productionperformance of the well. A recent proj-

ect in China showed how the precisedeviation control attainable fromINTEQ’s AutoTrak System allowed mul-tilateral wells to be drilled in exception-ally tight tolerances, increasing produc-tion by whole magnitudes.

Studies have also shown that later in awell’s life, when single-phase fluid pro-

duction changes to multi-phase flow, themore precisely a lateral production holeis drilled, the lower the pressure draw-down. As an example of the level of devi-ation control achievable, in a North Seawell, the lateral section was nearly 5,000ft long and only deviated +/-8 inchesfrom the desired TVD along its entirelength. This would be impossible toachieve using any other drilling system.

Reservoir Access & Drainage In areaswhere three-dimensional directionaldrilling control is troublesome, rotarysteerable systems can provide a muchwider range of well trajectory designoptions at low operational risk.

This has proved beneficial in severalfields where the lack of directionaldrilling control had limited well designsto simple two-dimensional wells whichlimited reservoir access and fielddrainage patterns. With the introduc-tion of rotary steerable drilling tech-niques to these fields, produciblereserves are increased throughimproved reservoir access and drainagepattern.

Bit Selection It’s an integral part of theD&E system. No discussion on rotarysteerables can exclude the bits. It hastaken time and considerable effort toestablish what bit design features are

required to obtain the best performance(drilling performance, steerability,dynamic stability and physical holequality) out of each different rotarysteerable system. As use of these sys-tems started to grow, there was a beliefthat rotary steerable systems requiredvery short and aggressive gauge bits tosteer effectively.

These bits undoubtedly do steer verywell and are useful where top end steer-ing capability is required, but there canbe, unfortunately, a price to pay. Thesebits can create unnecessary vibration inthe BHA and can also drill a slightlyimperfect hole, which may degradesome logging data sets. Research intobit design has made significant progressin this area.

It is established that there is no suchthing as a generic “rotary steerable bit”design. The operating principles of dif-ferent rotary steerable devices vary, andtherefore, require different bit designattributes.

T H E F U T U R E

The considerable benefits of usingrotary steerable technology have beenembraced by the industry and reflectedin the continuation of exponentiallygrowing demand, irrespective of busi-ness cycle. The initially obvious benefitsof using these systems has now grown toinclude a whole host of “less tangible”benefits, which are probably of greaterreal value than the tangible ones. Bittechnology has grown to keep pace withthe need to obtain the best performanceout of each of the very different avail-able systems. It is becoming increasing-ly common to tailor-design bits to pushperformance limits.

So, what’s in store for the future? Thetechnologically leading systems domi-nate the market with closed loop controland systems integration being keys tosuperior performance. While enhance-ments in functionality will continue tobe introduced, the focus for the timebeing is on delivering ever increasingtotal system reliability.

Further in the future, more closed loopcontrol will be introduced to the BHA.An example of this may be automateddrilling dynamics and downhole pres-sure control systems to deliver the nextstep change in drilling performance. �

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Steerable Motors AutoTrak RCLS

LIH Incident Rate (Incidents per 100,000 ft drilled)

Continuous pipe rotation, smoother trajectoryand overall improved hole gauge quality reducestuck pipe and lost in hole (LIH) incidents.