Mike Williams, Schlumberger Oilfield Services

ROTARY STEERABLE DRILLINGsystems were commercialized and intro-duced to the oil and gas industry 6-7years ago, and work using two differenttechniques, either push the bit or pointthe bit. Both work very well and thetechnology has been accepted by mostoil and gas companies to more preciselyand efficiently target their geologicalobjectives, especially in long horizontalwells.

With push the bit systems, pads areused to steer the tool. These are typical-ly close to the bit but there still must besome interaction with the wellbore,which occasionally results in some limi-tations on the tool’s capabilities. Forexample, in openhole sidetracks oncethe sidetrack is started the pad can onlypush so far before it loses contact withthe wellbore. If drilling in a very soft for-mation, the pad might not have anythingto push against because the hole couldbe washed out. Most point the bit sys-tems also require well bore interactionto maintain directional control andmany of the same limitations apply.

I M P R O V E D E F F I C I E N C Y

To overcome these obstacles and chal-lenges, Schlumberger developed itsPowerDrive suite of fully rotating rotarysteerable systems, which keep anyinteraction with the wellbore to a mini-mum, and ensure any interaction is atdrillstring rpm. The fully rotationalcapability makes the system very unob-trusive to the drilling process, and pro-vides the capability for the driller todrill, ream and back ream without anyincreased risk of mechanical or differ-ential sticking.

Fully rotating systems provide addition-al benefits. The flow of drilled cuttingspast the bottomhole assembly isenhanced because annular bottlenecksare not created in the wellbore. Penetra-tion rates are improved since there areno stationary components to create fric-tion. Risk of mechanical and differentialsticking are also reduced since no sta-tionary or slow rotating componentscontact the casing, whipstock or well-bore.

Tool life is enhanced by stationary inter-nal seals that are not exposed to fluidscontaining abrasive cuttings. The steer-ing system operates equally well in alltypes of fluid environments.

O R I E N T I N G T H E B I T

There still was one challenge that need-ed to be overcome, and that was drillingin soft formations and consistently per-forming open hole sidetracks with

rotary steerable systems, which iswhere the PowerDrive Xceed technologycomes into play. With this system, theonly interaction with the wellbore isthrough a normal stabilizer near the bit.No pads are necessary to push againstthe wellbore or slow rotating sectionsneeded to interact with the wellbore forsteering control when drilling in soft for-mations, openhole sidetracks or roughdrilling conditions.

Communication is based around thecompany’s PowerPulse MWD telemetrysystem and includes a full set of surveyelectronics that rotates with the system.When combined with PowerPulse, Xceedprovides real-time toolface, inclinationand azimuth data for reliable, long runs.Based upon this information, commandscan be transmitted to the system tomaintain or change the bit’s trajectory.The electronic survey package constant-ly references itself to the high side of thehole internally without the need for well-bore interaction.

Steering the bit itself is accomplished ina unique but simple manner. The bit ismounted on a short shaft that spins tothe right at drill string rpm. The backend of the shaft is attached to a disc withan eccentric connection between the bitshaft and an electric motor that rotatesto the left at the same rpm as the bit.The disc is also attached to a controllerthat matches the varying rpm of the bitas it increases or decreases rotation asit drills ahead.

To steer the bit, for example, changingthe trajectory of the bit to point upward,a command is transmitted from the sur-face to the electronics. The disc will thenturn itself around so that the offset inthe disc is at the bottom relative to thewellbore, meaning the back end of theshaft is also at the bottom and the bit ispointing upward.

If the system is used in a straight hole inwhat is referred to as neutral mode, thedisc spins at a slightly different rpmenabling the bit to wobble slightly,drilling a straight but slightly overgauge hole. The system always has abend of 0.6 so if a straight tangent isdrilled it will result in 1/8-in. gauge hole,much smaller than a traditional positivedisplacement motor.

The tool OD is 6 ¾-in. and is capable ofdrilling from 8 ½-in. to 9 7/8-in. hole size.The tool can drill in temperatures up to300° F.

L O W E R C O S T W E L L B O R E S

Since the tool does not push against thewellbore in order to build angle, or inter-act with it to reference itself, the Xceedsystem is very effective for steering insoft formations and where over gauge or

Steerable drilling needs minimal well interaction

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The steering section assembly of the Power-Drive Xceed continuously orients the tiltedshaft to control the drilling directions and thedogleg severity of the borehole.

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washed out hole is present. This inde-pendence from the wellbore enablesdoglegs and sidetracks to be drilled athigh build rates reliably. In soft forma-tions the system can develop doglegseverity as high as 8°/100 ft, significant-ly higher than systems that depend onwellbore contact to steer. This independ-ence from the wellbore to steer reducesthe tendency for the hole to spiral due tovariations in trajectory.

Steering that is independent of the well-bore also allows for the use of bicenterbits to increase hole gauge. This capa-bility is advantageous in extended reachand deepwater applications wheredirectional drilling techniques are oftenrequired.

The totally enclosed internal steeringmechanism limits tool exposure toextreme wellbore conditions. Addition-ally, the field proven electronics pro-vides significant wear and reliabilityadvantages in hot and high-shock appli-cations, resulting in a system that is lesssusceptible to wear when drilling

through sandstone or other highly abra-sive conditions. The result is lowerdrilling costs and longer tool life forreduced risk and improved economics.The system provides accurate andresponsive steering regardless of theformation or hole gauge, resulting inmore reliable and predictable drilling ofdoglegs and sidetracks.

C A S E H I S T O R I E S

Several case histories show that utiliz-ing the Xceed system can result in fasterdrilling and savings of potentially mil-lions of dollars per run.

North Sea. In one application the fullyrotational system saved Norsk Hydromore than $200,000 by drilling a side-track in one run. The tool was used todrill an 862 m openhole sidetrack in asingle run of 145 pumping hours in thegeologically complex Njord field in theNorwegian North Sea. Directionaldrillers initiated the sidetrack at 91°inclination at 4,246 m measured depthand geosteered the bit to a total depth

using VISION formation evaluation andimaging while drilling real-time densityimages and near-bit data from theXceed system.

The quick communication resultingfrom formation evaluation and imagingdata increased geosteering efficiency.Narrow flow thresholds for downlinkingallowed 90% of the directional controlsettings to be made on bottom whiledrilling ahead, further increasing theefficiency and economics of the sys-tem.As a result of successfully perform-ing the sidetrack, the well contributedhigh net present value from the addi-tional reserves.

Angola. The system was selected for awell in the thin-bed Girassol formationof Angola based upon the need toincrease drilling performance, optimizedrainage and enhance of hole quality forrunning screens and logs. Using conven-tional rotary steerable systems for therequired doglegs in the unconsolidatedformations had been unsuccessful asconventional drilling systems do not

allow azimuthal density neutron loggingwhile sliding. The Xceed system consis-tently delivered doglegs of more than4.5°/30 m. Using the system providedconsistent dogleg capabilities derivedfrom a deflection principle based uponbottomhole assembly curvature.

Use of the system accomplished both therequired dogleg severity and drainplacement while maintaining hole quali-ty. The unique fully rotating technologywas used to respond quickly to changesin formation dip using continuous incli-nation 4 m from the bit.

Indonesia. The system was successful-ly used to drill doglegs that were previ-ously impossible in an 8 ½-in. section ofextremely difficult drilling conditions.Doglegs of 1.8°-2.4°/10 m were achievedwhile building angle from 51.91° inclina-tion to 85° to hit the pay sand. Total welldepth was 4,346 m. The highly interbed-ded formation has highly compressedshale-limestone-sand-coal sequences.To avoid stuck pipe, the coal layers weredrilled in 50 cm steps. The driller oftenhad to backream at 150 rpm when pass-ing through those layers.

Middle East. Footage drilled increaseddramatically in a heavily faulted sand-stone reservoir. Resulting primarilyfrom a 91% increase in drilling efficien-cy per bit run with the Xceed system,$1.5 million reduction (52%) in total wellcosts resulted. This experience con-vinced the operator to replace conven-tional motor technology throughout itsongoing field program.

During the second phase of the multi-well program, the operator neededincreased capability to overcome obsta-cles in the laterally variable sandstonereservoir.

Nearly 90% of the wells require open-hole sidetracks for geological realign-ment. Rotation of the entire drillstringand less wellbore tortuosity resulted ina 68% reduction in trip time, which inturn was a result of improved hole qual-ity.

Using the rotary steerable systemresulted in the longest well drilled in thefield; the first time the sandstone sec-tion was drilled (13,689 ft) in one run;two successful openhole sidetrackseliminating two round trips; run lengthexceeding 191 circulating hours; andmaximum dogleg of 12°. n

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