30 Oilfield Review

Advancing Downhole Conveyance

Mark AldenHouston, Texas, USA

Faisal ArifMatt BillinghamSugar Land, Texas

Njål GrønnerødHydroBergen, Norway

Samuel HarveyBergen, Norway

Matthew E. RichardsChris WestUnocal, Inc.Sugar Land, Texas

For help in preparation of this article, thanks to Hilde Anfindsen, Hydro, Bergen, Norway; Paul Beguin, Joao Felix, Martin Isaacs, Ali Mazen, Steve Pepin and Todor Sheiretov, Sugar Land, Texas; Matt Carmichael,ChevronTexaco, New Orleans, Louisiana, USA; Ron Coleman, D&R Enterprises, Sugar Land, Texas; Dayne Kells, Olav Opøien, Brynjulf Pedersen and Colin Whittaker, Bergen, Norway.DepthLOG, FloScan Imager, FloView, LWF (logging whilefishing), MaxTRAC, MDT (Modular Formation DynamicsTester), OBMI Oil-Base MicroImager, PLT (Production Logging Tool), PS Platform, TLC (Tough Logging Conditions)and Xtreme are marks of Schlumberger.

Specialized delivery systems are available to place equipment in wellbores far from the

surface and to extract information from downhole locations. New forms of conveyance

systems provide reliable access in wells that are deep or have complex trajectories.

Ordinarily, we don’t give much thought to thevehicle that takes us to work and back home. Weget in our car and turn the key to start it, orjump into a taxi or onto a bicycle, bus or air-plane, and we proceed to our destination. Ourassumptions about a vehicle’s ability to get uswhere we want to go rise to full consciousnessonly when that destination is difficult to reachor the equipment breaks down.

For much of the history of the explorationand production (E&P) industry, the means forconveying or transporting tools and equipmentto a subsurface location and back has carriedsimilar unspoken assumptions. When most bore-holes were nearly vertical and not too deep,standard wireline cables were capable of carrying logging tools and other equipment tothe desired depth. Most users left conveyancedetails to a service company.

That situation has changed. Although thereare still a large number of shallow, near-verticalwellbores, many wells now are extremely deep,have high inclination angles, or both. Thechoices in downhole conveyance have expandedto meet these challenges.

For many years, drillpipe has conveyeddrilling bits and drilling equipment, and wire-line cable has been used for downhole loggingmeasurements, perforating and setting comple-tion equipment. Now, measurements are madewhile drilling, coiled tubing drills or conveysequipment downhole, real-time data supportgeosteering while drilling, wireline cables are

stronger and longer, and downhole tractors pullequipment great distances in wellbore sectionsthat are otherwise difficult or expensive toreach. In deep wells with complex trajectories,operators and service companies actively sharein the selection of conveyance methods andplanning of subsequent operations.

This article describes various downholedelivery options. It focuses on two new develop-ments: ultrastrong cables and a downholetractor. Case studies from the Gulf of Mexico andthe North Sea illustrate these latest advances inconveyance technology.

Getting ThereTransporting a piece of equipment for severalmiles or kilometers through a small hole in theground to reach a particular point is a remark-able feat, but one that occurs daily in the E&Pindustry. Once at the required downhole location, the equipment is expected to performcomplex tasks that often need to be monitoredand controlled in real time at surface sites farfrom the well. The conveyance choice is integralto providing these capabilities.

The object being conveyed could be a drillingbit, a logging toolstring, perforating guns, fractur-ing or other fluids, monitoring and recordingdevices or various types of well-completion equip-ment. Some conveyance jobs take the object on around trip, while others may take equipment onlyin one direction, into or out of a well.

Autumn 2004 31

Wireline logging

Pipe-conveyedlogging

Coiled tubinglogging

Tractor-conveyedlogging

Transporting devices is just one of the func-tions of a conveyance system. For example, thesystem must be able to support the weight oftools and equipment into and out of a wellbore,plus the weight of the conveyance system itselfand associated frictional resistance. Thereshould also be a method for determining at leastthe approximate location of the bottomholeassembly. Some forms of conveyance provide anemergency, or contingency, means for detachingfrom stuck tools. As a final example, in anincreasing number of situations, telemetry ofinformation to the surface is required.

A simple steel cable, or slickline, is perhapsthe least complicated form of downhole con-veyance (above). Slickline is often used toconvey completion equipment. Up to its loadlimitation, slickline can convey tools downwardby gravity. A surface motor spools the slicklineback onto a reel. The location of the deviceattached to the bottom of the cable can bedetermined, at least approximately, by thelength of cable deployed into a wellbore, plusthe amount of cable stretch resulting from itsweight and that of the conveyed device.

Wireline cables add further functionality. Anelectrical connection provides power to down-hole devices and can also transmit informationdirectly to the surface. Wireline cables may havea single conductor, a coaxial conductor or multi-ple conductors (next page). The exterior of awireline cable comprises several steel strands,referred to as the cable armor. This armor

carries the load and protects the electrical conductors within the cable.

Wireline is quick to rig up and economical torun. However, since it depends on gravity todeploy tools downward in a wellbore, wirelinecannot be used to convey equipment into high-angle or horizontal wellbores. Typically, wireline-conveyed tools can be delivered in well-bores up to 65° inclination, but in some caseswireline has been used successfully in wells withas much as 75° of inclination.

Both wireline and slickline cables have loadlimitations, but the ultrastrong cables describedlater in this article have a significantly higherlimit. The weight of a conveyed device alongwith the cable is known before entering a borehole; however, the extra force that may berequired to pull a stuck tool free or compensatefor friction during retrieval may exceed thecable load rating. Therefore, a weakpoint isplaced in the tool head between the cable andthe toolstring. This contingency-release device isdesigned to break before the cable does. Thealternative—a wellbore containing a brokencable still attached to the toolstring—makesfishing, or retrieving, the tool difficult.

Oilfield tubulars are also used for con-veyance. Drillers typically use pipe to conveydrill bits into a borehole because it is strongenough to sustain the forces encountered duringthe drilling process. Today, even wellbore casingis being used to convey drilling bits in casing-drilling operations, but the function remains the

same.1 Drillers get a rough location of measureddepth (MD) by counting measured pipe standsand recording their lengths in a pipe-tally book.

Devices are often placed behind the bit tomake measurements while drilling. Informationis transmitted to surface by sending pressurepulses through the drilling mud in a borehole.

Drillpipe can also be used in other con-veyance applications when a drilling rig andsufficient pipe to reach the downhole destina-tion are available. However, pipe conveyance isslow, since each stand of pipe has to be connected, or made up, when running into theborehole and unmade when running out. Thecost of rig time and equipment makes this anexpensive method of conveyance. However, it can be successful even in extreme conditions—of depth, deviation or boreholeenvironment—that exceed the capabilities ofother conveyance systems (see “Conveyance inExtreme Situations,” page 34).

The use of coiled tubing (CT) as a con-veyance device continues to expand.2 CT is usedto transport fluids to a given MD in a wellbore. Itis also used to convey drilling and completionequipment. Since it does not rely solely on gravity to go into the wellbore, it can be used inhigh-angle and horizontal wellbores. A wirelinecable can also be run inside the CT, affordinggreater protection to the cable than is availablewith the TLC Tough Logging Conditions system,although pipe-conveyed logging has a higherload capacity than CT logging. CT has the

32 Oilfield Review

> Conveyance methods.

Conveyance for logging

Slickline

Conventional wireline

TLC Tough LoggingConditions pipe-conveyed system

LWF logging whilefishing system

Coiled Tubing (CT)

Tractor

MaxTRAC tractor

Advantages Disadvantages

Cost-effectiveFast

Gravity dependentNo electrical connection

Gravity dependentCost-effectiveFast

Highly successfulIndependent of environmentMaintains well-control setup

Allows conventional loggingattempt with backup of LWF runIndependent of environment

Requires rig, drillpipe and associated personnelSlow; uses rig time

Requires rig, drillpipe andassociated personnelCut-and-thread fishingSlower than TLC system

CT logging unit must be mobilizedRequires extra personnel forCT operationLimited reach due to helical lockup

Primarily for cased-hole operationNot suited for every well

High success rateRig not requiredMaintains well-control setup

FastStandard field crew

Can log down with all standardPS Platform services

•

•

•

•

•

•

•

•

•

•

•

•

•

•

FastStandard field crew

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

Primarily for cased-hole operationNot suited for every well

•

•

Autumn 2004 33

capacity to push long toolstrings past obstruc-tions and doglegs that might cause conventionalwireline cable to hang up. Although CT is occasionally used in openhole logging, most ofits use is in cased-hole logging. In openhole situations, a TLC operation is usually warranted.

The alloys used to manufacture CT havesome stretch, which amounts to a few feet per10,000 ft when loaded, although this can varydepending on wellbore temperature, pressureand well deviation. A universal tubing lengthmonitor (UTLM) measures the length of coiledtubing that comes off the surface reel and thetube ovality. If the imprecision due to tubingstretch is insignificant, the UTLM can be usedfor depth measurements. The DepthLOG CTdepth correlation log is recommended whengreater precision is required, such as for criticalperforating or cement placement jobs. TheDepthLOG device locates casing collars as itpasses them, giving a more precise depth measurement than the surface UTLM.

CT conveyance has a high rate of success anddoes not require a drilling rig. This methodrequires mobilization of a CT unit and sufficientpersonnel, but the surface well-control systemallows operations to be performed under pressure, that is, in live-well conditions.

CT conveyance has a limited reach. As thetube unspools, it passes through a gooseneckand chain-driven injector head that causes thecontinuous tubing to exceed its yield strength.This operation helps remove the residual curva-ture that the string developed while on the CT reel.3 However, the tubing retains a smallamount of curvature. This curvature, coupledwith bends and deviations in the wellbore, putsthe CT string in contact with the wellbore wallin many places. The contacts generate frictionalresistance, and as more tubing is pushed intothe wellbore, the string wraps against the wall ina long helical loop. The force required to pushCT into the well increases as more of the stringcontacts the wall. Eventually, frictional resis-tance builds to the point that the CT cannot bepushed farther into the wellbore.4 This conditionis called helical lockup.

In high-angle and horizontal wellbores wherewireline cable is not appropriate and CT mayexperience helical locking, a relatively new formof downhole conveyance, a tractor, works well. Astandard field crew can rig up a tractor systemquickly. It is designed primarily for cased-holeuse, although tractors have been used occasion-ally in open hole. Tractors are typically conveyedon wireline, but some tractors work with CT.

Tractors are discussed in greater detail later inthis article (see “Tractor Drive Technology”).

The well trajectory, borehole condition andknown constrictions should be evaluated and dis-cussed in advance to be sure that use of a tractoris appropriate. Basing the choice of conveyanceon generalized rules can lead to problems.Schlumberger tool-planning software guidesselection of the form of conveyance and theweakpoint to use. Input to the software includesthe wellbore trajectory and completion informa-tion, along with details about the toolstring.

Program output provides critical parametersfor planning conveyance. Output plots are gener-ally presented on the basis of the MD of thetoolstring as it moves up and down the borehole.These parameters include dogleg severity, maxi-mum possible tension on the tool head withoutexceeding the cable rating at the surface, normal tension at surface, tension at surfaceneeded to break the weakpoint, and tractorforce required to move the logging toolstringdownwards. The software also calculates tensionalong the length of the cable, for any depth ofthe toolstring. Examples demonstrating the useof this software in field situations are includedin the case studies presented below.

Longer, Stronger Wireline Cable SystemCables are an important and reliable way to gettools and equipment into and out of a well. Logging the very deep wells now commonlydrilled in the Gulf of Mexico, offshore USA, andelsewhere, requires special wireline cables thatextend more than 24,000 ft [7,300 m]. New systems have been developed that handle up to40,000 ft [12,200 m] of wireline cable.

Schlumberger introduced the ultrastrong 7-48Z US cable in 2001. Worldwide, 7-48Z UScables have completed more than 200 descents.In December 2003, a record depth was achievedfor wireline logging in the ChevronTexaco Tonga #1 well in the Gulf of Mexico, about 150 miles [240 km] southwest of New Orleans.The ultrastrong 7-48Z US cable conveyed a high-pressure OBMI Oil-Base MicroImager tool andan Xtreme high-pressure, high-temperature welllogging platform to 31,824 ft [9,700 m], transmit-ting data to surface in real time.

These record depths require ultrastrongcables. A cable’s load strength is provided by thesteel armor surrounding the conductor or con-ductors. The armor comprises two layers ofhelically wound metal strands, one layer winding

> Simple wireline cables. A monocable has one conducting wire, protectedby insulation, a jacket and two layers of armor (left). The armor layers wind inopposing directions, so that a twist that opens one layer will tighten the otherlayer. A coaxial cable is a monocable with a coaxial shield between theinsulation and the jacket (center). A heptacable has seven conductors (right).The heptacable has filler strands to give it a rounder shape and an interstitialfiller to prevent air pockets and to make the core more rigid. A jacket and thetwo armor layers complete the outer layers. Conductor 1 here has a greenjacket to distinguish the conductors when making connections.

Armor wire

Jacket

Insulation

Conductors

InterstitialfillerShieldFillerstrands

1. Shepard SF, Reiley RH and Warren TM: “Casing Drilling:An Emerging Technology,” SPE Drilling and Completion 17,no. 1 (March 2002): 4–14.

2. Afghoul AC, Amaravadi S, Boumali A, Calmeto JCN,Lima J, Lovell J, Tinkham S, Zemlak K and Staal T: “Coiled Tubing: The Next Generation,” Oilfield Review 16,no. 1 (Spring 2004): 38–57.

3. Afghoul et al, reference 2.4. Adnan S and Chen YC: “An Improved Prediction of

Coiled-Tubing Lockup Length,” paper SPE 89517, presented at the SPE/ICoTA Coiled Tubing Conferenceand Exhibition, Houston, March 23–24, 2004.

The TLC Tough Logging Conditions system is apipe-conveyed method that uses specialequipment to connect logging tools todrillpipe. This system is used to log wells withdifficult borehole conditions such as highdeviation angle, multiple doglegs or washouts.It is also used with toolstrings whose weightapproaches the limit of a logging head weak-point. Logging tools on the string can beprotected from the drillpipe weight by contin-uously monitoring tool compression.

A TLC docking head (DWCH) attaches thetoolstring to the drillpipe (right). Stands ofpipe are run into the borehole to the top of thelog interval, typically just above the bottom cas-ing shoe. The logging cable is threaded througha cable side entry sub (CSES). The CSES allowsthe cable to cross over from inside the drillpipebelow the CSES to outside the drillpipe abovethe CSES. Next, a TLC wet-connect sub(PWCH) is attached to the cable, and it ispumped downhole inside the drillpipe. Afterthe PWCH latches into the DWCH, an electricalconnection between the logging unit and tool-string is established, and the toolstring ispowered up. The system can then log downwardsby adding drillpipe.

The CSES typically does not go into uncasedborehole, because doing so increases thechance of damaging the wireline cable outsidethe pipe above it. This usually restricts TLClogging in open hole to a distance equal to thelength of cased borehole.

An enhanced TLC system is available foruse in high-pressure, high-temperature envi-ronments. This system can operate at 500°F[260°C] and 25,000 psi [172 MPa] and hasimproved chemical, electrical and mechanicalstability at high temperatures. Additional wellcontrol is provided by inclusion of two checkvalves in the system to isolate the pipe fromthe annulus. These valves can hold up to a20,000-psi [138-MPa] differential pressure.

34 Oilfield Review

Conveyance in Extreme Situations

> Pipe-conveyed logging processes. A TLC Tough Logging Conditions operation and a LWF loggingwhile fishing operation are similar, but use some different equipment. Both are pipe-conveyed loggingmethods. In the TLC system, the pipe and toolstring are made up together, with a docking head(DWCH) connected to the toolstring (bottom right). Pipe is run until the toolstring reaches the lowestcasing shoe. Then the cable is threaded through a casing side entry sub (CSES) (middle right). A wet-connect sub (PWCH) connected to a wireline cable is pumped down to establish electricalconnection to the toolstring through the DWCH (bottom right). More pipe is run while logging. In anLWF operation, the tool is stuck in the borehole at the beginning of the operation. The cable is cut atsurface, and it is threaded through pipe that is run into the borehole. When the string of pipe reachesthe lowest casing shoe, a cable cutter tool (CCTS) and a CSES are added (middle left). More pipe isrun until it reaches the tool. A grappler at the bottom of the pipe takes hold of the tool, and tension isput on the wireline to assure connection (bottom left). The cable is reconnected to the logging unitusing a double-ended LWF torpedo (top left), and logging continues. The CCTS unit is needed at theend of the LWF job to cut the cable and retrieve the toolstring from the grappler.

Pipe

LWF torpedo

LWF Operations TLC Operations

CSES

CCTS

Grappler

Toolstring

CSES

DWCH

Toolstring

PWCH

Cable

Autumn 2004 35

In situations where wireline logging isoften successful, but risky, the LWF loggingwhile fishing system provides an alternativeto a TLC operation. In difficult logging situa-tions, the LWF service greatly increases thechance of obtaining a log. The toolstringincludes a tool to monitor compression on thetoolstring in the event the LWF procedure isused, and a standard wireline-logging run isattempted. If it is successful, the operatorsaves the time and expense of a TLC run. If atool becomes stuck in the borehole, the LWFservice provides a means of retrieving thetool and continuing logging as a TLC job.

An LWF operation is a cut-and-thread fish-ing procedure. A special tool securely gripsand cuts the wireline at the surface. The cutend of the wireline is threaded through afirst stand of drillpipe. While the pipe hangsin the wellbore, the wireline is threadedthrough another stand of drillpipe, which isscrewed onto the stand in the wellbore. Thisprocess is repeated. The need to thread thecable makes it a slower operation than a TLCjob until the pipe reaches the point forinstallation of a CSES. A cable-cutting tool(CCTS) is installed just below the CSES.Beyond that point, the process runs at thesame speed as a TLC job, and has the samedepth restrictions related to keeping theCSES within the casing.

Once the pipe reaches the stuck tool, agrappler on the base of the pipe connects toit. A small downward movement of the pipeincreases tension on the cable, ensuring thatthe grappler holds the tool. The cable isreconnected at surface using a double-endedLWF torpedo. The tool is powered up and thelogging job continues. After logging, the pipeis pulled out of the borehole. The CCTS cutsthe cable so the cable and tool can beremoved separately.

clockwise and the other winding counterclock-wise. Ultrastrong cables, such as the 7-48Z UScable, have armor that is made using a coldforming process to give the cables a higherstrength-to-weight ratio than other cables (below). These cables are rated to 18,000 lbf[80 kN]. The next strongest type of cable can

support 15,500 lbf [69 kN]; conventional cablessupport lower loads.

Service companies use ultrastrong cablesavailable from several manufacturers to log deepvertical wells. However, stronger sheaves, a special capstan and an improved weakpoint areneeded to obtain the full benefits of this additional load capacity (bottom).

> Force at the tool head for various cables. The ultrastrong cable 7-48Z US (blue) has a maximum pullof 18,000 lbf [80 kN] at surface, more than conventional (XS) and extrastrength (XXS) cables. Theamount of pull available decreases with depth. The slope of each line is determined by the weight ofthe cable in mud.

0 10,000 20,000 30,000 40,000 50,000

4-51AK US7-48Z US 7-48Z XXS 7-52 XXS7-52 XS 7-46 XS7-46 XXS 4-46 XXS

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

Forc

e at

tool

hea

d, lb

f

Depth, ft

> Surface equipment for the ultrastrong cable system. To achieve the full benefits of the ultrastrongcables, additional equipment is necessary at surface. The sheaves are made of specially developedcomposites, and the lower sheave has a high-strength tie-down. The wireline dual-drum capstan(WDDC) provides tension relief between the cable coming out of the borehole and the logging unit.Recommended distance between the capstan and the logging unit is 50 ft [15 m] without a specializedsetup, and 10 ft [3 m] with one. Angular deviation between the capstan and the lower sheave isrestricted to 2°.

Sheave wheel

Wireline dual-drum capstan

Offshore logging unit

Conventional wireline cable supporting lowloads typically runs from the logging unitthrough two sheaves, or pulleys, and into a well-bore. To support greater loads up to 24,000 lbf[107 kN] of line tension, heavy-duty sheavesmade from specially developed composite materials are used. The bottom sheave must besupported by special, strong tie-downs.

The rig and heavy-duty sheaves can support acable tension of 18,000 lbf, but the winch on thelogging unit can only apply up to 8,500 lbf[37.8 kN] sustained force when spooling and canpull only to the maximum limit of the unit’sengine, typically about 12,000 lbf [53.4 kN].

To reduce or step down the cable tensionbetween the drill rig and the logging unit, awireline dual-drum capstan (WDDC) can beplaced between them. A capstan relies on fric-tional contact between its drums and a cable todecrease tension in the cable. A larger contactarea yields a greater decrease in tension. Thecable passing through the Schlumberger WDDCwraps several times around two large drums,relieving 20,000 lbf [89 kN] or more of tension atlogging speeds up to 20,000 ft/hr [6,100 m/hr].The primary use of this capstan system has beenin the Gulf of Mexico. The Larose, Louisiana,USA, district used the WDDC for more than 400

descents of extrastrength 7-48Z XXS cables andultrastrong 7-48Z US cables between May 2002and August 2004 (top right).

Stepping down the cable tension beforerespooling it also prolongs cable life. If a cable iswrapped onto a drum under high tension, itdeforms (top left). Too much deformation cancause breakage within the cable, potentially creating shorts or contacts between conductorsor with the cable armor. Therefore, high tensionshould be relieved before spooling ultrastrongwireline cables onto a drum.

Schlumberger periodically examines allcables (above). High-strength cables are spooledoff, cleaned and retorqued within a week to amonth of doing a job, depending on the cable andmud used. The cables are examined for loosearmor and checked for ductility and wear. Strictguidelines are used to qualify the cable for itsnext use.

Ultrastrong cables undergo this qualificationprocedure even more often—within a week afterevery job is complete. Note that a job often

36 Oilfield Review

> Capstan use in the Gulf of Mexico. The wireline dual-drum capstan (WDDC) has been used for morethan 500 descents (red) in the Gulf of Mexico. More than half the jobs exceeded measured depth of20,400 ft [6,220 m].

Dept

h, ft

Desc

ents

08-F

eb-0

2

15-J

ul-0

2

05-N

ov-0

2

06-F

eb-0

3

11-M

ay-0

3

24-A

ug-0

3

01-D

ec-0

3

01-M

ar-0

4

09-M

ay-0

4

15-S

ep-0

4

35,000

30,000

25,000

20,000

15,000

10,000

5,000

Date of descent

0

600

500

400

300

200

100

0

> Cable maintenance. Ultrastrong cables use a capstan to relieve cabletension before it is spooled onto a drum. To ensure that the cable torque isrelieved, Schlumberger cables are retorqued and respooled after jobs. Thecable runs through a twister and a capstan unit. The twister is constantlyturned to remove the twist that is inherent in the stressed cable. This twistingresets the armor windings. The cable also runs through two cable-cleanerunits to remove foreign materials, and if required, to add oil to the cable armor.

Drum

Twister

Capstan

Cable cleaners

> Cable deformation. A typical seven-conductorcable (top) contracts when stressed (middle). Theouter armor layer loosens somewhat, and theinner armor layer tightens. The conductors andfiller compress. If a cable is spooled onto a drumunder high tension, it can also deform, perhapsenough to eventually fail (bottom). The deformationis exaggerated in this figure.

Autumn 2004 37

involves multiple trips into a well. So long as thecable has not exceeded its 18,000-lbf rating, it canbe used for repeat runs in the same operation.

In addition to cable deformation whenrespooled under high tension, cable stretchtends to tighten one helical armor layer and toloosen the other. The Schlumberger qualityassurance system includes twisting the ultrastrong cable while unspooling to reset thearmor windings.

A second feature needed to work with highcable tensions is a specialized weakpoint. If atool gets stuck in the borehole, it is highly desir-able to be able to separate the cable as close tothe stuck tool as possible. The alternative, a broken cable chaotically twisted in the borehole,makes a fishing operation to retrieve the stucktool more difficult. To avoid this, a weakpoint ispart of the tool head that is placed between thetoolstring and the cable. The weakpoint breaks

when pulled beyond its rating, thereby detach-ing the cable from the tool and allowing theentire cable to be pulled out of the well.

The tool-planning, or conveyance-planning,software predicts loads on a wireline cable. In adeep borehole, the downhole tension required toovercome a mechanical weakpoint may exceedthe maximum stress that can be applied at sur-face. If that happened, the weakpoint could notbe broken when a tool became stuck, and over-pull could break the cable instead.

This problem led Schlumberger to developan electrically controlled release device (ECRD)to replace the mechanical weakpoint in the toolhead. With this device, tension at the tool headcan reach 8,000 lbf [35.6 kN], which is the loadlimitation of the ECRD.5 Until it is activated, anECRD weakpoint can withstand high cable-tension pulls and large shocks.

If an operator needs to release the cablefrom the toolstring, the ECRD is activated by aspecific electrical signal. Then, the ECRDreleases easily, separating the cable from thetool head (top left).

ECRD weakpoints are rated to operate up to400°F [204°C] and 20,000 psi [138 MPa]. Thereis also a version capable of operating to 500°F[260°C] and 30,000 psi [207 MPa].

Challenging the Depths Unocal reentered a deepwater well in the Gulf ofMexico, USA, to drill to a greater depth. Offsetwells had encountered hydrocarbons in deeperstrata. The original well was not designed forcontinued drilling, so the borehole size of thenew section was restricted to 61⁄8 in. The MD tobottom of the planned section was about 29,100 ft [8,870 m].

Unocal worked with Schlumberger to deter-mine the best way of evaluating the newopenhole section. They had to address chal-lenges that included the extreme depth of thewell and the severely restricted borehole diame-ter, as well as accommodating the size andweight of well logging strings that the companywanted to run. Day rates for the drillship werequite high, so efficiency was important.

The well was essentially vertical, but the new openhole section included about 1,000 ft[300 m] in which the deviation increased toabout 15°, and dogleg severities in that sectionwere as high as 4°/100 ft [4°/30 m] (left).

5. Note that the weakpoint does not require the same 18,000-lbf rating as the cable, since the weakpoint isdownhole while the cable has to support the higher tension at the surface.

> Electrically controlled weakpoint operation. An electrically controlled release device (ECRD) isactivated by a specific electrical signal. Once assembled, an actuator rod holds a latch in place,preventing the unit from pulling apart. The actuator rod is held against a spring by a four-part bobbin,which in turn is held together by a thin beryllium-nickel (Be–Ni) sheet. Solder on the outside joint ofthe thin sheet holds it in place around the bobbin. When an electric activation signal is sent to theECRD, the heater melts the solder, releasing the Be–Ni sheet and allowing the four parts of the bobbinto separate. The spring-loaded actuator can move down, freeing the latch. A slight tug separates thelatch from the ECRD housing, releasing the cable from the tool.

Spring

O-ring Be–Ni-wrappedbobbin

Lower head

Heater

Actuator rodLatch housing

Latch

> Dogleg in a Unocal well. The maximum dogleg that would be experiencedin the extension of the Unocal well was 4°/100 ft [4°/30 m]. This doglegoccurs in an openhole section with a deviation that reaches about 15°.

Wellbore deviation, degrees

Dogleg severity, degrees/100 ft

100

543210

Deviation

806040200

Dept

h

5,00

0 ft

Dogleg severity

In planning this job, Schlumberger used itsproprietary tool-planning software that indicatedan ultrastrong cable system could deliver tools tothe required depth in this small-diameter bore-hole. Data from earlier logging runs in the samewell verified the tool-planner predictions. Each ofthree runs modeled the cable and toolstring actu-ally used. Cable-tension data from these previouslogging runs agreed with the predictions from thesoftware (below). The results gave Unocal confi-dence that the software output covering the newopenhole section would be accurate.

Unocal and Schlumberger worked togetherto design logging runs for the new well section.A WDDC with heavy-duty sheaves and tie-downswas used on surface, and the string included anECRD weakpoint. Both were to allow full use ofthe 18,000-lbf rating of the cable.

Eleven of the twelve logging descents usedthe same 7-48Z US cable. No down-time wasrequired between runs to swap or recondition

the cable. The logging sequence included severalchallenges. The longest toolstring extended147 ft [44.8 m]. The heaviest string weighedabout 4,000 lbm [1,800 kg] in air. Seven of thesedescents included the MDT Modular FormationDynamics Tester, which is a heavy toolstring. Thelargest diameter tool used in the 61⁄8-in. diameterborehole was 51⁄2 in., leaving a clearance aroundthe tool of only about 0.3 in. [0.8 cm].

The cable tension at surface reached 17,000to 18,000 lbf [76 to 80 kN] on more than 20 occa-sions during the 11 logging runs. Such high pulltensions could not have been achieved withoutthe complete ultrastrong system, including theWDDC and specialized sheaves at surface.

The third logging run proved to be trouble-some. In its first run, the MDT tool spent7.5 hours on station performing an interferencetest, after which the tool became stuck in theborehole. The crew pulled the cable to its maxi-mum rated level of 18,000 lbf, but could not

retrieve the tool. Either the cable was stuck in akeyseat, or borehole pressure exceeded forma-tion pressure and a mudcake had formed aroundthe cable as drilling mud was forced into the for-mation, a condition called differential sticking.6

Unocal activated the ECRD weakpoint torelease the cable from the tool. Then theyapplied a surface tension of 18,500 lbf [82.3 kN]that freed the cable. Before fishing for the tool,Unocal ran a vertical seismic profile logging job,using the same ultrastrong cable.

A few days later, an LWF trip retrieved theMDT tool. When the tool was reconnected to apower source downhole after spending morethan three days in the high-pressure borehole, itpowered up and began sending signals to surface.

Logging tools conveyed using an ultrastrongcable with the WDDC at surface provided Unocalwith reservoir information that otherwise wouldhave been difficult or impossible to obtain.Alternative cables or less robust surface systemswould have required shorter and lighter toolstrings to remain within safe operating tolerances. Either this would have made Unocalchoose an abbreviated formation evaluation pro-gram, or it would have prolonged the loggingoperation. A longer operation would have beenan expensive proposition since it involved a Discoverer class drillship.

Since the beginning of its deepwater drillingprogram, Unocal has been an active user of thenewest cables. The company has kept two of thestrongest cables on board its drillship at alltimes, along with a WDDC unit.

Tractor Drive Technology While wireline is one of the oldest means ofdownhole conveyance, downhole tractors areone of the newest. A tractor pulls the con-veyance string down the borehole; that is itsfundamental difference from other means ofconveyance. A tractor can be placed to eitherpush or pull a toolstring, but the toolstring isshort in comparison to the cable connecting thetractor and toolstring to surface. Putting themotive force near the front of the conveyancestring enables it to move tools and devices alongextended horizontal sections. Even in locationswhere wellbore deviation exceeds 90°, the trac-tor pushes the toolstring uphill.

Friction is an enemy of conveyance, an addi-tional force to be overcome as conveyed objectsrub against the wellbore wall. However, a tractoruses friction to its advantage, pressing againstthe borehole wall with sufficient force so thatfriction keeps it from slipping back along thewall as it pulls itself and its load forward.

38 Oilfield Review

> Comparing actual and predicted tension. Tension measurements madeduring three logging runs (data points) in the original well compare favorablywith predictions of the tool-planning software (lines). The first logging runcovered about 15,000 ft [4,600 m] of the cased well (top). The second and thirdruns were over shorter distances, one inside casing (middle) and the otherbelow casing (bottom). The software predicts tension for both moving downinto the borehole (blue) and pulling up out of the borehole (red). The step intension is due to a slower logging speed when the toolstring is outside ofcasing. The tensions differ among the three cases because of the differenttoolstrings used in the three logging runs.

Dept

h

Tension, lbf

0 5,000 10,000

Wellbore deviation, degrees

0 20 40 60 80

Maximumsafe pull on cable

Deviation

Down

Up

5,00

0 ft

Autumn 2004 39

The drive system has to provide enough trac-tion force to overcome the drag of the load,including the conveyed toolstring and the logging cable or other devices needed for conveyance. In addition, the frictional forceobtained by pressing against the borehole wallmust exceed this traction force. Therefore, thefrictional resistance that can be achieved is onelimit on the force a tractor can effectively apply.

Two basic tractor-drive mechanisms are currently available to obtain this resistance:continuous drive or reciprocating grip. Continu-ous-drive designs rely on wheels, corkscrewwheels, chains or tracks that contact the well-bore wall. A suspension system is necessary tokeep the drive in contact with the wellborewhen the tractor encounters small diameterchanges. Large changes in wellbore diameterare difficult for continuous-drive tractors, particularly when the diameter decreases: thedrive mechanism must both push outward tohold against the borehole wall and pull in toaccommodate the decreasing diameter.

A reciprocating-grip tractor requires at leasttwo drive units that alternate between drivingand resetting. The drive unit presses its gripagainst the wall, locking in place. It then movesthe toolstring forward with respect to the lockedgrip. Meanwhile the resetting unit releases fromthe wall, and it moves itself forward with respectto the toolstring. At the end of the stroke, thesetwo units exchange functions.

Although reciprocating-grip tractors can bemade that use only one motor, the toolstringmotion is discontinuous, creating a large inertialload as the string starts and stops. Using separate motors provides overlapping motion atthe end of each stroke so that the toolstring continues to move at a constant speed.

Unlike continuous-drive models, the recipro-cating-grip design does not require a suspensionsystem to move across small changes in wellborediameter, because the locked grip is stationary. Inboth continuous and reciprocating designs, thedrive mechanism retracts into the tractor bodyfor transport before and after use. Both designsshould retract their arms in case of power failureto avoid getting stuck in the borehole.

Moving the tractor and its load requires powerthat is supplied by wireline from surface. Sinceconducting cables transmit a limited amount ofpower, downhole tractor power consumption isalso limited. The power expended is the productof the speed of motion and the load the tractor ispulling (top). This means that a tractor caneither pull a heavy load or move quickly; it cannotdo both simultaneously.

Unfortunately, not all the power provided atsurface goes into moving the tractor and pullingthe load. Resistive losses occur in the wirelinecable. In addition, tractor drive systems are not100% efficient in converting electrical powerinto motion. Many tractors deliver only 10% to20% efficiency, with the remaining energy con-verted to waste heat.

A Cam-Grip TractorThe MaxTRAC downhole tractor system is areciprocating-grip downhole tractor that delivers more than 40% efficiency. This high-efficiency tractor has power available for higher

speeds or greater loads than other tractors for agiven surface power.

A central feature of this tractor is its cam-gripdesign. Three sets of arms centralize each tractorunit in a wellbore. Each arm presses a cam andtwo wheels against the wellbore wall. The cam isa heart-shaped device that can rotate about itsaxis (above). As the cam rotates, the distance D

6. A keyseat is a narrow channel that a cable or drillpiperubs into an openhole borehole wall as it moves up anddown. This can be the result of a sharp change in direc-tion of the wellbore—a dogleg. This may also occur if ahard formation ledge is left between softer formationsthat enlarge over time. When the cable or pipe is pulledto the surface, it passes through the slot, but a tool or bitwith larger diameter may stick at that point.

> Tractor speed and available push force. There is a trade-off between speedand load for tractors. For a given amount of power supplied to the tractordownhole, the tractor can either move fast or convey a heavy load. TheMaxTRAC tractor (blue) is much more efficient than a conventional tractor(red), allowing it to move faster or convey greater loads.

Tractor force, lbf

0

500

1,000

1,500

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0 200 400 600 800 1,000

Spee

d, ft

/hr

MaxTRAC tractorConventional systems

> Principle of a cam grip. The rotating part of the cam grip is heart-shaped (insets). The verticaldistance D from the axis to the wall can change continuously as the cam rotates. When the unitbegins its power stroke, the rightward push of the toolstring causes a leftward reactive movementagainst the casing. This rolls the cam toward a larger D, and its teeth grip the wall (top). The tractorarms are locked, so this increase in cam diameter has the effect of holding the arms in place againstthe pull force FP, resisting backward slippage as the unit moves the toolstring (not shown here) to theright (middle). In a unit’s return stroke, a force FR tends to roll the cam to a smaller diameter, so itslides along the wall (bottom). A small spring force FS holds the cam against the casing, but the camteeth do not grip.

Return stroke

Power stroke

Gripping

FP

D

FR

FS

D

from the axis to the outside edge that is in contact with the wall can change continuously.

When the arms are deployed but the unit isnot moving the toolstring, a small, radialhydraulic force holds them against the wellborewall, but they are not locked in place. The armsride along the wall on wheels. In this mode, thearms can move in and out to follow changes inwellbore diameter.

The edge of the cam that contacts the wallhas gripping teeth. The cam teeth are heldagainst the wall by a weak spring, but the for-ward motion of the tool tends to roll the camtoward an edge with a shorter D. The cam slidesalong the borehole wall without gripping.

Just before the tractor unit begins its powerstroke, the hydraulic system changes to hold thearms at a constant extension, pressed against thewall. As the drive system of the tractor unit startspushing the toolstring forward, a reactive forcetries to slide the arms backward with respect tothe wellbore wall (right). This causes thetoothed cam to roll to a larger D. Since there arethree locked arms keeping the tool centralized,this larger D can only force the cam more firmlyagainst the wall. The teeth grip securely, prevent-ing the unit from sliding backward. Thismechanism makes this cam design a naturalmechanical amplifier. It automatically rotatesenough to provide the minimal radial forcerequired to prevent the tractor from slipping.

With the drive unit held firmly, its motor pro-pels the body of the tractor and its load forward.This continues to the end of the stroke, whenanother tractor unit takes over the drive duties.The drive motor reverses to reset its position forthe next stroke. This process rolls the cam in adirection to decrease D, releasing the gripagainst the wall and allowing the unit to slide.

The cam system provides a constant contactpressure, within the borehole inner-diameter(ID) limitations of the tool design. The MaxTRAC unit can apply its full driving force inany size borehole within an ID range from 2.4 to9.625 in. [6.1 to 24.4 cm]. The tool can passthrough a 2.21-in. [5.6-cm] restriction withoutchanging tool parts. The tractor grips at three dis-crete points along the wellbore every 2 ft [0.6 m],reducing the prospect of casing damage. Althougha MaxTRAC system can drive a toolstring backwards, this is normally done for only a shortdistance, such as to back out of a restriction, toavoid damaging the cable. The wireline cable isused to pull the toolstring out of the borehole.

This tractor system is suited for use in con-solidated formations in openhole conditions. Byusing more than two tractor units, the systemcan also drive through washouts with diametersbeyond the reach of the tool arms, so long as thetractor sections can be placed far enough apartthat at any time at least two units can grip thewalls. Up to four drive units can be included inone tractor assembly.

The MaxTRAC system is fully compatiblewith the PS Platform new-generation productionservices platform. It has the same telemetry sys-tem, so the toolstring can log while tractoringdown. This ability is a marked advantage for theMaxTRAC system. In poor logging conditions,logging down—toward the end of the well-bore—may be the only opportunity to acquirecrucial data. The MaxTRAC tractor is compatible

40 Oilfield Review

> Reciprocating cam-action tractor. Two tractor sections reciprocate to move the string forward(bottom). With the back tractor in a power stroke (black), the cams on its arms (red) grip the wall.While the arms remain stationary, the drive mechanism propels the toolstring forward (1 to 3).Meanwhile, the front tractor resets, with arms unlocked and cams rotated to avoid gripping, and thetractor drive motor moves its arms to the front of the section. When the back unit reaches the end ofits movement, the front unit takes over, providing continuous movement (4). Now the front unit gripsand propels the string while the back unit resets (5 to 6). Then the cycle repeats (7). The photograph(top) shows a MaxTRAC section with its arms extended and one with its arms retracted.

1

2

3

4

5

6

7

Autumn 2004 41

with production logging tools for logging down,and is compatible with many other tools whenlogging back up to surface.

The MaxTRAC system also provides signifi-cant real-time feedback about its operatingconditions. Its system can transmit motor current, motor torque, computed speed for eachtractor section, cable-head tension, casing-collarlocations, deviation and relative bearing.

Several operators in the North Sea have usedMaxTRAC conveyance in their operations. Afterseveral PLT Production Logging Tool jobs wererun for Statoil in the Heidrun field, the operatordispensed with a drift run in advance of the primary logging job.7 With a MaxTRAC system,the front tool in the logging suite can be acaliper that identifies a problem area before therest of the toolstring advances into it, saving thecost of the drift run.

Production logging is more effective whenlogging down, as probe holdup tools need a posi-tive differential between the fluid velocity andthe logging velocity of the probes. This is espe-cially relevant in wells or sections of wells with low flow rates. The combination of the MaxTRAC tractor and a FloScan Imager sondeprovides an excellent way to determine flowrates and holdup in horizontal or deviated wells(see “Profiling and Quantifying Complex Multiphase Flow,” page 4).

Tractor into a Y-CompletionHydro wanted to run a production log in a Bragefield well to determine the oil and water produc-tion coming from the main wellbore and from alateral section. This field, which lies about125 km [78 miles] west of Bergen, Norway, in theNorth Sea, is a mature field that has producedabout 85% of its estimated ultimate recovery.

The well was completed with a Y-junction(left). The operator recognized a potential prob-lem in getting past this junction between themain wellbore and a lateral wellbore. Situated ina horizontal section of the wellbore, the Y of thejunction was oriented vertically, with the small-diameter continuation of the main wellboredirectly below the other lateral.

Hydro engineers wanted to be sure that thelogging sonde could be conveyed past the Y-junction and into the main wellbore before committing to a job. A duplicate of the Y-connection, which had been supplied originally by Halliburton, was set up at the service company’s base camp in Stavanger,Norway. The Schlumberger MaxTRAC tractorand a third-party tractor were both tested atthis surface site. Only the MaxTRAC unit wasable to pass through the junction, and it did sowithout difficulty.

The toolstring included three MaxTRAC grip-ping units to convey a logging string with theFloScan Imager sonde. Engineers prepared forthe job using tool-planning software, based onthe known well trajectory and tool configuration.The program indicated that the system couldperform the job safely. The tractor forcerequired was determined to be within the capa-bilities of the tool (left).

7. Drift is the maximum diameter of a tool that can fit insidea wellbore. A drift run pulls or pushes a cylinder of knownoutside diameter through the wellbore to assure thattools of that diameter can pass through the wellbore.

> Brage field Y-junction. The wellbore divides into two parts at a Y-connector(bottom). In the cross section, the lower opening is the continuation of themain wellbore, and the upper leads to a lateral (top right). The photo wastaken for the system test at surface (top left).

0.876 in.

3.437 in.

3.437 in.

7.74 in.

> Tractor force for a Brage well. The wellbore deviation (blue) exceeds 80° nearthe foot of the well, where the MaxTRAC tractor began conveying the toolstring.The required tractor force (green) was low throughout the operation.

Depth, m0 500 1,000 1,500 2,000 2,500 3,000

0

100

200

300

400

500

600

700

800

900

0

10

20

30

40

50

60

70

80

90

Trac

tor f

orce

requ

ired,

lbf

Wel

bore

dev

iatio

n, d

egre

es

Tractor force required

Maximum tractor force available

Deviation

The toolstring was lowered into the well untilthe inclination angle was too great for gravity toovercome frictional resistance. At that point,MaxTRAC tractors took over the conveyanceduties. The second and third tractor sectionsconveyed the toolstring; the first tractorremained in a closed or retracted state until thestring reached the Y-junction.

A knuckle, or flexible joint, had been placedbetween the FloScan Imager tool and the firsttractor (above). As the toolstring approachedthe Y-connector, the production-logging sondewas lowered to the bottom of the horizontalborehole. The Y-connector shoulder guided thesonde into the lower lateral, followed by the firsttractor, still in a retracted state.

After the first tractor was inside the lowerlateral, its gripping arms were set against thewalls of the lateral, and the gripping arms of thesecond tractor were retracted. The first and

third tractor units alternated as drive units tomove the toolstring forward until the secondtractor was fully within the lateral. The processcontinued with the third tractor retracted andthe first two pulling the toolstring forward. Thenthe first tractor was retracted and the job continued with the second and third tractorsproviding the motive force.

About 40 m [130 ft] beyond the junction, thelogging engineer set the tractor grips to obtain aFloScan Imager log while stationary. The firstproduction log at this station was performedwhile the well was shut in at surface. Althoughthere was no net flow to surface, this shut-in logindicated crossflow from the main lateral intothe other lateral.

The well was opened to flow. The tool-planner software indicated the flow would notgenerate enough lift, or upward force, to movethe tool, but this was assured by the grip of the

tractor cams against the wellbore wall. Anothersurvey was completed at this same station. TheFloScan Imager results showed that all the fluidflowing at the station was water.

Using pressures and flow rates of these tworuns—shut-in and flowing—a selective inflowperformance analysis determined that there wasa 16-bar [232-psi] pressure difference betweenthe two laterals that caused the shut-in crossflow.

The tractor arms retracted and the toolstringwas pulled out using the wireline cable, until the assembly reached a position far above the Y-connection. At this point, the tractor gripswere again set against the borehole wall to stabilize the logging string. Another FloScanImager log measured flowing rates and holdup atthis station (next page). About 6% of the produc-tion in the wellbore above the junction was oil.Since all of the production into the main lateralbelow the Y-junction was water, Hydro determined

42 Oilfield Review

> Tractoring into the Y-junction. A 111⁄16-in. toolstring including a FloScan Imager sonde (green) and three MaxTRAC units (red) was successfully conveyedinto a difficult Y-junction. Here the casing is shown moving to the left, but the tool actually is moving to the right. The two back tractors conveyed thetoolstring through the 7.74-in. bore, to the junction (1). They pushed the FloScan Imager tool and the front tractor into the lower 3.437-in. bore at the junction(2 to 3). The middle tractor was retracted and the front one enabled (4). The first and last tractors conveyed the middle tractor into the smaller bore (5). Nowthe last tractor was retracted, and the first and second tractors pulled the toolstring into the small bore (6). Once all tractor sections were inside the smallbore, the first one retracted, and the second and third provided motive force (7). Other parts of the logging toolstring are not shown for simplicity. Spacersbetween the tractors were required for rigging up the string. In this figure, the vertical scale is exaggerated.

1

2

3

4

5

6

7

Autumn 2004 43

that the main wellbore could be shut off at thejunction to improve productivity.

Including the Y-junction, the tractor pulledthe toolstring through 18 changes in ID over a325-m [1,066-ft] distance, traversing thesechanges without difficulty.

Without the MaxTRAC system, Hydro may not have been able to determine that no oil was flowing from the main wellbore below the Y-junction. This North Sea case study indicates the advantage gained from new conveyance technology.

Driving AheadAdvances in other areas of the E&P industry willprovide new challenges for conveyance systems.Discoveries will be made in deeper locations, andhigh-deviation wells will reach greater distances.

The load limitation on current deepwaterdrillships typically limits drilling depth to35,000 ft [10,670 m]. As new equipment isinstalled on drillships to extend this limit, operators will expect similar expansion of logging capabilities.

Even today, operators are asking for wirelinecables that can operate downhole at 450°F[232°C] and 35,000 psi [240 MPa]. Serviceproviders will have to develop new cables thatcan work under these conditions, as well as theassociated surface systems to support the loads.

Power delivery downhole is another area thatwill see development in the future. New cableswill have larger power conductors that supplymore power downhole. These cables will allowuse of increasingly power-hungry toolstrings.They can also deliver increased power for a tractor system to operate with a higher load orgreater speed.

Tractors currently are used with caution inopen hole. Even a reciprocating-grip tractor likethe MaxTRAC unit needs a relatively clean bore-hole to work successfully. This is a challenge forfuture development. Operators want openholetractors that can operate in soft formations.

The days of casually assuming a simple andinexpensive conveyance system is available forany job are over, at least in the frontier areas ofdeep and deviated wells. Operators and servicecompanies now routinely work together to besure the proper vehicle is selected for a job.With access to predictive and diagnostic tools,such as the Schlumberger tool-planner software,operators are increasingly assured that whenthe key is turned on, the vehicle will take us towork and home again. —MAA

> Real-time holdup and velocity results above the Y-junction. The FloScanImager InFlow Profiler output indicates a fairly flat velocity profile and constantholdup profile (top). About 6% of the flow is oil and the remainder, water. Theelectrical FloView holdup measurement tool probes measured small droplets of oil in water, as indicated in the FloScan Imager Monitor Box (bottom).