Key Issues in Multilateral Technology

Steve BosworthUnion Pacific ResourcesFort Worth, Texas, USA

Hussein Saad El-SayedDubai, United Arab Emirates

Gamal IsmailZakum Field Development CompanyAbu Dhabi, United Arab Emirates

Herv OhmerMark StrackeChris WestSugar Land, Texas, USA

Albertus RetnantoJakarta, Indonesia

For help in preparation of this article, thanks to LennisCook, Anadrill, Sugar Land, Texas, USA; Chip Corbett,GeoQuest, Houston, Texas; Vladimir Doroshenko,Schlumberger Wireline & Testing, Moscow, Russia; Doug Durst, Weatherford, Houston, Texas; AlexanderMikhailovich Grigoryan, Los Angeles, California, USA; David Hill, Anadrill, Shekou, Shenzhen, China; DavidMalone, Camco, Houston, Texas; Pat McKinley, Anadrill,Midland, Texas; Eric Neme, Schlumberger Oilfield Services,Lagos, Nigeria; and Claire Vishik, Austin Product Center,Austin, Texas.

Drilling, completing and later reentering wells with multiple branches to improve

production while saving time and money are becoming commonplace, but complications

remain, as do the risks and chances of failure. Existing techniques have been applied

and fresh approaches are being developed to overcome technical hurdles, establishing

new standards and a specialized vocabulary for these well types and applications.

In 1953, a unique oil well called simply 66/45was drilled with turbodrills in the Bashkiria fieldnear Bashkortostan, Russia. This well ultimatelyhad nine lateral branches from a main boreholethat increased exposure to the pay zone by 5.5times and production by 17-fold, yet the cost wasonly 1.5 times that of a conventional well.1 It wasthe worlds first truly multilateral well, althoughrudimentary attempts at multilaterals had beenmade since the 1930s. Under the auspices of theSoviet Oil Industry Ministry, another 110 suchwells were drilled in Russian oil fields over thenext 27 years (see The Father of MultilateralTechnology, page 16 ). Not until ARCO drilledthe K-142 dual-lateral well in New MexicosEmpire field in 1980, did another operatorattempt such a feat, for multilaterals were simplytoo difficult and too risky, requiring substantialinvestment of both time and technology.

A multilateral well is a single well with oneor more wellbore branches radiating from themain borehole. It may be an exploration well, aninfill development well or a reentry into an exist-ing well. It may be as simple as a vertical well-bore with one sidetrack or as complex as ahorizontal, extended-reach well with multiplelateral and sublateral branches. General multi-lateral configurations include multibranchedwells, forked wells, wells with several lateralsbranching from one horizontal main wellbore,wells with several laterals branching from onevertical main wellbore, wells with stacked later-als, and wells with dual-opposing laterals (nextpage, top). These wells generally represent twobasic types: vertically staggered laterals andhorizontally spread laterals in fan, spine-and-ribor dual-opposing T shapes.

14 Oilfield Review

INFORM (Integrated Forward Modeling), PowerPak,RapidAccess, RapidConnect, Slim 1, USI (UltraSonicImager) and VIPER are marks of Schlumberger.1. Horizontal Well Technology Unit, Heriot-Watt University

and The Petroleum Science and Technology Institute,Multi-Lateral Well Technology Technical Study (1995): 6-9.

2. Horizontal Well Technology Unit, reference 1: 6-14.

Vertically staggered wells usually target sev-eral different producing horizons to increase pro-duction rates and improve recovery frommultiple zones by commingling production.Wells in the Austin Chalk play in Texas (USA) aretypically of this type (below right). Their produc-tion is a function of the number of natural frac-tures that the wellbore encounters. A horizontalwell has a better chance of intersecting morefractures than a vertical well, but there is a limitto how far horizontal wells can be drilled. Bydrilling other laterals from the same wellbore,twice the number of fractures can often beexposed at a much lower cost than drilling longhorizontal sections or another well.

Horizontal fan wells and their relatedbranches usually target the same reservoir inter-val. The goal of this type of well is to increaseproduction rates, improve hydrocarbon recoveryand maximize production from that zone.Multiple thin formation layers can be drained byvarying the inclination and vertical depth of eachdrainhole. In a naturally fractured rock with anunknown or variable fracture orientation, a fanconfiguration can improve the odds of encoun-tering fractures and completing an economicwell. If the fracture orientation is known, how-ever, a dual-opposing T well can double thelength of lateral wellbore exposure within thezone. In nonfractured, matrix-permeability reser-voirs, the spine-and-rib design reduces the ten-dency to cone water. Lateral branches aresometimes curved around existing wells to keephorizontal wellbores from interfering with a ver-tical wells production.

A successful multilateral well that replacesseveral vertical wellbores can reduce overalldrilling and completion costs, increase productionand provide more efficient drainage of a reservoir.Furthermore, multilaterals can make reservoirmanagement more efficient and help increaserecoverable reserves. But why has it taken so longfor multilateral technology to catch on?

Between 1980 and 1995, only 45 multilateralwell completions were reported; since 1995,hundreds of multilateral wells have been com-pleted and many more are planned over the nextfew years.2 This increased number of multilateralwells is related to a rapid sequence of advancesin the methods for drilling multilateral wellsdirectional and horizontal drilling techniques,advanced drilling equipment and coiled tubingdrilling. However, the levels of well complexityhave remained low due to a lack of comparableadvances in multilateral completion equipmentand designs. As a consequence, the primary risksinvolved in multilateral wells have been in lateraljunction construction and completion rather than

Multibranched Forked

Laterals into horizontal hole

Stacked laterals

Laterals into vertical hole

Dual-opposing laterals

Multilateral Well Configurations

> Common forms of multilateral wells in use today. Wellbore design and configuration are dictated byspecific formation and reservoir drainage requirements.

5700

6100

6500

6900

73000 800 1600 2400 3200 4000 4800

Lateral section displacement, ft

True

ver

tical

dep

th, f

t

Top ofAustin ChalkBentonite

EaglefordFalse BudaBuda

Georgetown

> Typical Austin Chalk well in south Texas, USA. Stacked drainholes target multiple zones to increaseproduction rates and improve recovery by commingling production. Horizontal wells have a betterchance of intersecting natural fractures than vertical wells; production is a function of the number offractures that the wellbore encounters.

15Winter 1998

(continued on page 18)

As with many advances in petroleum technology,the first multilateral well was accomplished by aSoviet drilling engineer. Alexander MikhailovichGrigoryan was born during 1914 in Baku, thecapital of todays Republic of Azerbaijan, then aprincipal center of oil production. After gradua-tion from high school, he worked as a drillersassistant, became an apprentice and ultimatelygraduated as a petroleum engineer in 1939 fromAzerbaijan Industrial Institute (right).

During most of the Soviet era, the official pol-icy was to produce as much oil as possible, sinceit was a strategic commodity and one of the fewexports that could be exchanged for grain andother consumer goods. High quotas wereimposed on drillers to bore as many holes asthey could. The prevailing attitude was that themore holes drilled, the greater the likelihood ofsuccessfully tapping a reservoir and therebyachieving greater production.

Grigoryan was an innovator and inventor.Upon graduation, he began working as an oil-field driller and soon was attached to theMinistry of Oil. Believing that he could producemore oil by following a known oil sand than bymerely penetrating it with a number of bore-holes, he drilled one of the worlds first direc-tional wellsBaku 1385in 1941, nearly 20years before anyone else attempted such a feat.Without a whipstock or a rotating drillstring, heused a downhole hydraulic motor to penetrateoil-bearing rock and significantly expand reser-voir exposure and production. It was the firsttime that a turbodrill was used for both verticaland horizontal sections of a borehole.1

Grigoryans pioneering work in horizontaldrilling technology led to scores of other suc-cessful horizontal wells across the USSR and his elevation to department head at the All-Union Scientific-Research Institute forDrilling Technology (VNIIBT). He was not, however, satisfied with these accomplishments.He developed a new borehole sidetrack kickofftechnique and a device for stabilizing and

controlling curvature without deflectors. But allof these innovations were in preparation for hismajor contribution to drilling technology.

Inspired by the theoretical work of Americanscientist L. Yuren, who maintained thatincreased production could be achieved byincreasing borehole diameter in the productivezone, Grigoryan took the theory a step furtherand proposed branching the borehole in theproductive zone to increase surface exposure,just as a trees roots extend its exposure to thesoil. In 1949, he took his ideas to notedRussian scientist K. Tsarevich, who confirmedthat branching a well in a productive zone withuniform rock permeability should yield anincrease in oil production in proportion to thenumber of branches.

Grigoryan put this new theory into practice inthe Bashkiria field complex in what is todayBashkortostan, Russia (right). There, in 1953,he used downhole turbodrills without rotatingdrillstrings to drill Well 66/45, the first multilat-eral well. Bashkiria field complex lies in south-ern Bashkortostan (next page, left). Late

Carboniferous carbonate reefs built by rugosecorals trap vast oil reserves (next page, right).The fields had been in production since before1930, and most wells produced low volumes atthe time Grigoryan first attempted a multilat-eral well. 2

Grigoryan chose to drill Well 66/45 inBashkirias Ishimbainefti field, which evidencedan interval of Artinskian carbonate rocks withgood reservoir properties and wide areal distri-bution. His target was the Akavassky horizon, an interval that ranged from 10 to 60 m [33 to 197 ft] thick.

Grigoryan drilled the main bore to a totaldepth of 575 meters [1886 ft], just above the payzone. From that point, he drilled nine branchesfrom the open borehole without cement bridgesor whipstocks; the window configurationenabled insertion of tools on drillpipe into the

The Father of Multilateral Technology

> Alexander Mikhailovich Grigoryan. Now 84years of age, Grigoryan immigrated to theUnited States in the 1980s and became anAmerican citizen. He kindly granted OilfieldReview an interview and made documentsabout his technique available.

Russ ian F

edera t

i o n

Ufa

Neftekamsk

Salavat

Sterlitamak

Oktyabrski

Ishimbai

B as h

k o r t o s t a n

Beley

aRi

ver

> Map of Bashkortostan inset in a map of the Rus-sian Federation. The first multilateral wells weredrilled in the Ishimbai region in the south-centralregion of the republic.

16 Oilfield Review

sidetracks without instrumentation. He drilledby touch alone, slanting away from the verticalbore like roots of a tree, each branch extendingfor 80 to 300 meters [262 to 984 ft] in differentdirections into the producing horizon. 3

Grigoryan allowed the drill bit to follow the payzone into the most productive zones, thebranches curving automatically to the plannedtrajectory. Drilling speed and penetration ratedepended entirely on the hardness of the rockand downhole motor capabilites.

When completed, Well 66/45 had nine produc-ing laterals with a maximum horizontal reachfrom kickoff point of 136 meters [447 ft] and atotal drainage of 322 meters [1056 ft].

Compared with other wells in the same field,66/45 penetrated 5.5 times the pay thickness. Its drilling cost was 1.5 times more expensive,but it produced 17 times more oil at 755 B/D[120 m3/d] versus the typical 44 B/D [7 m3/d].4

Under the auspices of the Soviet Oil IndustryMinistry, another 110 multilateral wells weredrilled in Russian oil fields over the next 27years, with Grigoryan drilling 30 of them him-self. About 50 of these first multilaterals wereexploratory, the remainder were for delineationof reefs and channel structures.

223

425455

473

490

521

617 TD

506

595 TD595 TD605 TD

617 TD

613 TD

660 TD

582 TD

627 TD

Artinski limestone

Well 66/45

25

50

75

100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

475

500

525

550

575

Mea

sure

d de

pth

,m

> An early multilateral well. Drilled in Bashkiria, now Bashkortostan, one of Russias most prolific regions, the first multilateral well had nine lateral branches that tapped the Ishimbainefti field reservoir.

250

260

270

280

290

300

310

320

330

340

350

360

370

380

390

400

410

Pale

ozoi

c

Perm

ian

Devo

nian

Carb

onife

rous

Upper

Late

Lower

Early

UpperLate

Middle

Lower

Early

Upper

Late

Middle

Lower

Early

Thuringian

Saxonian

Autunian

OrenburgianGzelian

Kasimovian

Moscovian

Bashkirian

Serpukhovian

Strunian/Etroeungtian

Couvinian

ZlichovianPragian

Lochkovian

Millionyears

> Bashkiria stratigraphic column. The first multilateral well target waswithin the Akavassky horizon, in the center of the lower Bashkiriansequence, middle Carboniferous era. [Adapted from Haq BU and Van Eysinga FWB: Geological Time Table, 4th ed. Amsterdam, The Netherlands: Elsevier Science BV, 1994.]

1. Gaddy D: Pioneering Work, Economic Factors ProvideInsights into Russian Drilling Technology, Oil & GasJournal 96, no. 27 (July 6, 1998): 67-69.

2. Boisseau T, Chuvashov B, Ivanova R, Maslo A, Masse P,Proust J-N, Vachard D and Vennin E: EtudeSedimentologique et Biostratigraphique du Stratotype duBashkirien (Oural du Sud, Russie), Bulletin, CentresRecherche Exploration-Production, Elf Aquitaine 20, no. 2(December 1996): 341-365.

3. Bakke D: Russia Gears Up Offshore Activity for BiggestProduction Gains in Its History, Offshore 35, no. 5 (May1975): 303-306.

4. Horizontal Well Technology Unit, Heriot-Watt Universityand The Petroleum Science and Technology Institute,Multi-Lateral Well Technology Technical Study, 1995: 6-9.

17Winter 1998

drilling. Of the hundreds of multilateral wellsdrilled, most have been simple openhole comple-tions in hard rock; many have been reentries tosalvage wells or boost output from old fields, butan increasing number represents new, develop-ment wells seeking to maximize drainage ofknown reservoirs.

Regardless of the level of complexity, multi-lateral wells today are drilled with state-of-theart directional drilling technology. Even so, thedrilling of multilateral wells involves certain risksranging from borehole instability, stuck pipe andproblems with overpressured zones to casing,cementing and branching problems. And therecan be a high risk of drilling or completion forma-tion damage and difficulties locating and stayingin the productive zone while drilling the laterals.

Multilateral technology may be at about thesame level of development that horizontal anddirectional drilling were 10 years ago. Horizontaland reentry multilateral drilling has increased50% over the past five years and is expected togrow another 15% a year through 2000.3 Thisrapid growth is attributed to operators realizingthat the advantages of multilateral systemsincreasingly outweigh the disadvantages.

For years, because there were so few reliableand sophisticated examples of successful multi-lateral applications, few such wells were drilledbecause operators lacked benchmarks by whichto determine whether prospects were suitablecandidates for multilateral development (right).There were concerns about higher initial costsand the risk of possible interference of lateralswith each other, crossflow and difficulties withproduction allocations. An increased sensitivityto and concern about reservoir heterogeneitieslike vertical permeability deterred multilateraldevelopment. The prospect of complicateddrilling, completion and production technologies,complicated and expensive stimulation, slow andless effective cleanup, and cumbersome well-bore management during production also madeoperators cautious.

As more multilaterals were drilled success-fully, however, even the simplest wells demon-strated the potential of this emergingtechnology. The main benefits of these success-ful wells have been increased production,increased reserves and an overall reduction inreservoir development costs.

Production from known reserves has tradi-tionally been expanded by drilling additionalwells to increase drainage and sweep efficiency.As a consequence, both capital expenditures andoperating costs have also increased with every

new well. To counteract these cost increases,multilateral technology is now being employed toincrease borehole contact with the reservoir,improve operating efficiency and reduce wellcosts. These goals are achieved primarily bydrilling the main trunk and overburden from sur-face to the reservoir only once and by reducingsurface equipment to a single installation at asignificant cost-savings. Furthermore, this can beachieved in both offshore platform and subseasituations where a limited number of slots isavailable and in onshore locations where surfaceinstallations are expensive or where the leasehas an irregular configuration.

Multiple lateral penetrations in the samereservoir or in independent reservoirs not onlyproduce significant cost-savings, but increaseproduction rates appreciably (next page). Suchpenetrations are commonly used to increase theeffective drainage and depletion of a reservoir,particularly when reservoirs have restrictedhydrocarbon mobility due to low permeability,low porosity or other characteristics that limitproduction flow. When independent reservoirsare targeted, production can either be commin-gled into a single production tubing string or pro-duced separately in multiple production tubingstrings. Multilateral wells are also an economical

YesNo

YesNo

YesNo

YesNo

YesNo

YesNo

YesNo

YesNo

YesNo

YesNo

Does the reservoir contain hydrocarbons in small or isolated accumulations?

Is there an accumulation of oil abovethe reservoir's highest perforations?

Is the reservoir separated into low-transmissibilityvertically stacked segments?

Is the reservoir naturally fractured or does it havehigh permeability only in one direction?

Does the reservoir have numerouslens-shaped pay zones?

Are there two different, or distinct sets ofnatural fractures in the reservoir?

Does the reservoir require waterflood?

Does waterflood of the reservoir cause a breakthrough inhigh-quality zones before low-quality zones are swept?

If offshore, is the platform unable to accomodate anadditional well that is needed to drain additional fault blocks?

Are future rigless completions plannedfor additional zones?

Drill a conventional verticalor horizontal well.

Consider amultilateral well.

> Determining if multilateral technology is applicable.

18 Oilfield Review

3. Longbottom J and Herrera I: Multilateral Wells CanMultiply Reserve Potential, American Oil & Gas Reporter40, no. 9 (September 1997): 53-58.

way of rapidly depleting a reservoir, effectivelyaccelerating production, shortening the field lifecycle and reducing operating costs.

Multilateral wells are often able to overcomethe shortcomings of both horizontal and conven-tional wells, particularly if there are geologicalfactors like thinly layered formations or a signifi-cantly fractured system, and in specific enhancedoil recovery scenarios such as steam-assistedgravity drainage. In addition, the application ofmultilateral technology can result in decreasedwater and gas coning.

Because of the capability to more thoroughlydrain reservoirs vertically and horizontally, recov-erable reserves per well and per field areincreased considerably while both capital andoperating costs per well and per field are mini-mized. In fact, the cost of achieving the samedegree of drainage with conventional wellswould be prohibitive in most cases, especiallysituations like deepwater subsea developments.Multilateral wells allow costs to be amortizedover several reservoir penetrations and in somecases have eliminated the need for infill drilling.In heterogeneous reservoirs with layers, com-partments or randomly oriented natural fractures,more pockets of oil and gas can be exploited andan increased number of fractures can be inter-sected by drilling multilateral wells.

In anisotropic formations with unknowndirections of preferred permeability, drillingmultibranched wells can reduce economic risk.Lateral branches can balance the nonuniformproductivity or injectivity of different layers.Multilateral wells provide extensive informationabout the reservoir and can be useful for explo-ration and formation evaluation in addition totheir capability to efficiently and economicallydrain reservoirs.

TAML ClassificationUntil 1997, there was considerable confusionregarding multilateral technology. Few terms thatdescribed the technology were universally agreedupon, and a classification of multilateral wells bydifficulty and risk was lacking. As a consequence,under the leadership of Eric Diggins of Shell UK Exploration and Production, a forum calledTechnology AdvancementMulti Laterals(TAML) was held in Aberdeen, Scotland, in theSpring of 1997. Its goal was to provide a moreunified direction for multilateral technology devel-opment. Experts in multilateral technology fromleading oil companies shared experiences andagreed to a classification system that ranks multi-lateral wells by complexity and functionality.Today, multilateral wells are referred to by levelof complexity from Level 1 through 6S, anddescribed with a code to represent type and func-tionality (see, Classifying Multilateral Wells,page 20).

The three characteristics used to evaluatemultilateral technology are connectivity, isolationand accessibility. Of these, the form of connec-tivity or junction between the main trunk and lat-eral wellbore branches is not only the mostdistinguishing feature, but also the riskiest andmost difficult to achieve. For this reason, about95% of multilateral wells drilled worldwide havebeen Level 1 or 2. Some 85% of 1998 multilater-als have been Levels 1 to 4, with 50% of thoseLevels 1 and 2. But the race is on; virtually allmajor operators and drilling service companiesare developing multilateral connectivity, isolationand accessibility capabilities. In addition, newjunction systems are emerging to facilitateincreasingly higher levels of difficulty.

Level 1 is essentially a simple openhole side-tracking technique, much like the first multilater-als drilled in Russia. The main trunk and lateralbranches are always openhole with unsupportedjunctions. Lateral access and production controlare limited.

In Level 2 wells, the main bore is cased, butthe lateral junction remains openhole, or possiblywith a drop-off linercasing placed in lateralsections without mechanical connection orcementingto provide full-opening main well-bore access and improve the potential for reentryinto the lateral.

Anadrill performs Level 1 and 2 multilateralconnections throughout North America and theMiddle East (see, Multilaterals in the MiddleEast, page 24). Drilling is usually carried outwith either short-radius or medium-long radiusdrilling assemblies. The Anadrill RapidAccesssystem and third-party casing exiting serviceslike those of Smith International are used to pro-vide support. Milling can also be carried out inexisting wells using conventional retrievablewhipstock or cement plug techniques. Otherproviders can supply similar systems or junctionswith windows precut.

Level 2 wells commonly require a window, orhole, to be cut in the casing with a milling assem-bly. Generally, this level of multilateral consistsof whipstock sidetracks from existing casing

Shallow ordepleted reservoirs

Layered reservoirs

Fractured reservoirs

> Enhancing productivity with multilateral well configurations. In shallow or depleted reservoirs, branched horizontal wellbores are often most efficient,whereas in layered reservoirs, vertically stacked drainholes are usually best. In fractured reservoirs, dual-opposing laterals may provide maximum reservoir exposure, particularly when fracture orientation is known.

19Winter 1998

Classifying Multilateral Wells

1

2

3

4

5

6

6S

< Multilateral well complexity ranking (Level 1 to 6S).This general classification is based on junction complexity. Level 1 is an openhole sidetrack or unsupported junction. Level 2 has a cased and cementedmain bore, or trunk, with openhole lateral. Level 3 is acased and cemented main bore with lateral cased, but notcemented. Level 4 has both main bore and lateral casedand cemented at the junction. Level 5 pressure integrity isachieved at the junction with completion equipment. ForLevel 6, junction pressure integrity is achieved with casingand without the assistance of or dependence on comple-tion equipment. In the subcategory Level 6S, a downholesplitter, basically a subsurface dual-casing wellhead,divides a large main bore into two equal-size laterals.

Single Bore

Dual Bore

Concentric Bore

> Multilateral well descriptions. Inaddition to criteria such as the num-ber of junctions and well typepro-ducer with or without artifical lift,injector or multipurposethe com-pletion type, whether single, dual or concentric, has a major impact onthe type of equipment that is neededat the junction.

NRNo selective reentry

PRReentry by pulling completion

TRThrough-tubing reentry

NONNone

SELSelective SELSelective

REMRemote monitoringRMCRemote monitoring and controlSEPSeparate

Accessibility

Flow Control

> Junction types. The categories of accessibility are no selective reentry, reentry by pulling completion andthrough-tubing reentry (top). Flow control (bottom) is the degree to which fluid flow across a junction can be adjustedno control, selective or separate control, and remote monitoring or remote monitoring and control.

20 Oilfield Review

(right). Premilled window casing subs are alsoused frequently to avoid the higher risk task ofmilling. Although retrievable whipstocks areemployed to drill laterals, their removal alongwith the packer assembly from the main wellboremakes locating laterals and reentry accessalmost impossible. Accurate positioning of sub-sequent guide assemblies and azimuthal orienta-tion are also difficult if not impossible. For thisreason, the Anadrill Level 2 RapidAccess multi-lateral completion system was enhanced byadding a mechanical connection with a fullborecasing profile nipple for positioning and orientingwhipstocks or other assemblies to provide selec-tive drainhole access.

The Level 2 RapidAccess construction wasengineered with robust simplicity to be transpar-ent to the drilling operation, while retainingoptions for higher level multilateral completions.RapidAccess couplings do not require orientationor special procedures during installation and arecemented using conventional equipment and pro-cedures. These couplings are full opening, per-manent reference points from which multiplebranches can be constructed and reentered fromthe main wellbore. Since orientation prior tocementing is not required, casing movement dur-ing primary cementing helps ensure a successfulcement bond. Multiple RapidAccess couplingscan be installed in casing strings to allow numer-ous reservoir penetrations for optimum fielddevelopment. Depth and orientation of each cou-pling can be determined by measurements-while-drilling (MWD) survey after cementing andby wireline or coiled-tubing conveyed USIUltraSonic Imager surveys (right).

Level 3 multilateral technology offers bothconnectivity and access. The main trunk and lat-erals are cased; the main bore is cemented, butlaterals are not. Until recently, only premilledwindows were used at this level if access intoeach lateral needed to be maintained. Lateral lin-ers are anchored to the main bore by a linerhanger or other latching system, but cementing isnot required. There is no hydraulic integrity orpressure seal at the lateral liner and main casingjunction, but there is main bore and lateral reen-try access.

The Level 3 RapidConnect system will providemechanical connectivity to both the lateral andmain wellbore and high-strength junctions forunstable formations. This enhancement is criticalwhen sands or shales become unstable over theproductive life of a well. Completion options thatmay be required by the reservoir depletion planallow upper laterals to be isolated at the junctionwhile producing from lower laterals. Selectiveaccess to laterals is made possible by placing anoriented diverter at the junction.

The most common completion performed inLevel 2 and 3 wells is uncemented, predrilled orslotted liners and prepacked, but not gravel-packed screens. Anadrill uses a drop-off linercompletion design in which the top of the liner inthe lateral is immediately released outside theexit from the casing through a hydraulic sub.External casing packers are often used in thedrop-off liner completion assembly to isolatezones, anchor the liner top and facilitate reentryaccess to the liner.

Another mid-tier approach to multilateralcompletion offers only individual hydraulic isola-tion of a lateral. In this case, laterals are drilledusing whipstock sidetracking procedures and ifany completion is performed in the lateral, it usesa drop-off liner. Conventional casing packers inthe main casing with tubing between themstraddle packersare used to isolate each of

Step 1

Run multilateral packer onstarter millassembly

Step 3

Completemilling ofwindow

Step 2

Set packer

Shear startermill

Begin millingwindow

Window from USI log

Index casing couplingfrom USI log

Window toICC spacing

< Window orientation,depth and quality. AUSI UltraSonic Imagerlog can determine theorientation and depthof a cemented coupling relative tocasing collars andgamma ray (GR) logs.A USI log can also be used to providefeedback about window quality dur-ing well construction.These images showan index casing coupling (ICC) and a window milled in 7-in., 26-lbm/ft casingusing a downholemotor. This log wasrun to verify thelength of a full-gaugewindow. A USI logcan be run in mostcommon drilling fluids.

>Window milling. Lateral openings are cut into the casing wall with whipstock and milling equipment.The whipstock packer is run and set on a mill assembly. The starter mill is then sheared off the top ofthe whipstock and a window is cut into the wall and formation to begin a lateral drainhole branch.

21Winter 1998

the laterals hydraulically. Production from the lat-erals is controlled with sliding sleeves and otherflow-control devices. This is an inexpensive andrelatively straightforward multilateral completionmethod that was proven in the North Sea and isnow being adapted for deepwater subsea wells.

The critical technology in these completionsis operation of flow-control devices downhole.Schlumberger Camco intelligent well technologyis now capable of activating and controllingthese flow control devices remotely.

Level 4 multilateral wells have both the mainbore and lateral cased and cemented at the junc-tion, which provides a mechanically supportedjunction, but no hydraulic integrity. The lateralliner is, in fact, cemented to the main casing. Themost common sidetracking procedure relies onwhipstock-aided milling of casing windows,although premilled window-casing subs are alsoemployed. There is no pressure seal at the junc-tion interface of the lateral liner and the maincasing, but the main bore and the laterals havefullbore access. This level of multilateral technol-ogy, although complex, high risk and still indevelopment, has been successful in multilateralwells worldwide.

A Level 5 multilateral well is characterized byeither the Level 3 or Level 4 lateral connectiontechnique with addition of completion equipmentto provide a pressure seal across the junction ofthe lateral liner and main casing. The main well-bore is fully cased and the junction is hydrauli-cally isolated; cement is not acceptable as thehydraulic isolation. Reentry access to both themain bore and the laterals is available. Hydraulicisolation is achieved with the use of auxiliarypackers, sleeves and other completion equip-ment in the main casing bore to straddle the lat-eral junction with production tubing.

Level 5 and 6 wells are distinguished from themid- and lower tier levels by hydraulic isolation ofthe laterals as well as connectivity and accessi-bility characteristics. The most difficult aspects ofmultilateral technology are hydraulic isolationand integrity at high pressure, and most providersare still seeking ways of improving these.

Level 6 multilateral systems incorporate anintegral pressure seal in the junction of the lat-eral liner and the main casing. A pressure-tightjunction, achieved with an integral sealing fea-ture or a monolithic formed or formable metal

design, is the goal and will be valuable in deep-water offshore and subsea installations.

Schlumberger first evaluated Level 6 multilat-eral technology in 1995 with a system developedby Anadrill, Camco and Integrated DrillingSystems. With multilateral technology develop-ment transferred from Anadrill to the CamcoAdvanced Technology Group, Schlumberger isevolving these techniques into newer systemsrather than proceeding with this particular version. The company is continuing developmentof multilateral technology with a new Level 6 design.

Level 6S, a generally recognized Level 6 sub-level, uses a downhole splitter, or subsurfacewellhead assembly, that divides the main boreinto two smaller, equal-size lateral bores.

Positioning MultilateralsRegardless of the design level or multilateraltechnology used, for lateral branches to achievethe desired contact with productive intervals,borehole direction must be an integral part ofwell plans. Determining these trajectoriesdepends on reservoir properties, the rock stress

1 2 3 4 5 6 7

ICC

Slim 1MWD

Gelledfluid

SLT

ICC

> The Level 2 multilateral process.

1. The main wellbore casing is run with indexcasing couplings (ICC) as integral compo-nents. The ICC, normally of standard couplingOD and pipe ID sizes, does not need to be oriented when run. It can be placed below,above or in angle-build sections.

2. The main bore casing is cemented using stan-dard procedures and casing wiper plugs.

3. The lower branch is drilled, completed andisolated with a retrievable bridge plug.

4. The coupling orientation is determined fromthe USI log or by running a Selective LandingTool (SLT) with Slim 1 MWD in the UniversalBottomhole Orienter (UBHO). During this tripthe coupling can be cleaned with a specialjetting tool and a gel pill may be spotted in thekickoff section to suspend debris.

5. The whipstock face is then properly alignedwith the landing tool orientation key and runinto the well. This assembly automaticallyaligns and latches in the appropriate coupling. The milling tool is then releasedfrom the whipstock.

6. A casing window and short pilot hole into theformation are cut with a special millingassembly powered by a downhole motor, inthis case, an XP series PowerPak motor.

22 Oilfield Review

4. Ehlig-Economides CA , Mowat G and Corbett C:Techniques for Multibranch Well Trajectory Design inthe Context of a Three-Dimensional Reservoir Model,paper SPE 35505, presented at the European 3-DReservoir Modeling Conference, Stavanger, Norway,April 16-17, 1996.

5. Roberts M, Kirkwood A and Bedford J: Real-TimeGeosteering in the Tern Field for Optimum MultilateralWell Placement, paper SPE 50663, presented at the 1998SPE European Petroleum Conference, The Hague, TheNetherlands, October 20-22, 1998.

Allen D, Dennis B, Edwards J, Franklin S, Livingston J,Kirkwood A, Lehtonen L, Lyons B, Prilliman J and Simms D:Modeling Logs for Horizontal Well Planning andEvaluation, Oilfield Review 7, no. 4 (Winter 1995): 47-63.

regime and the geometries of productive reser-voir units. Laterals can be vertical, inclined orhorizontal, as can the main wellbore, butbecause production from several laterals can becommingled in the main wellbore, it is possibleto drill more drainholes in the reservoir thanwould be feasible with a conventional well.4

Trajectories for the main wellbore and later-als are determined using various informationsources, including 3D surface and borehole seis-mic data, well logs and core analyses, formationand well testing, and other data like fluid proper-ties and production histories. Predrill planningalso ideally includes geological and petrophysi-cal forward modeling with tools like INFORMIntegrated Forward Modeling software to helpidentify risks and the value of logging-while-drilling (LWD) measurements. Such modelingprovides initial petrophysical descriptions alongproposed trajectories by using imported geologi-cal models (above). Thereafter, 2D and 3D LWDtool response functions are generally used toproduce synthetic log datasets to complete for-ward models.5

Well path designs begin in the producing for-mation where the optimal lateral location isdetermined. From the farthest point in the later-als, the design proceeds to the main bore, thenupward to the surface or seafloor wellhead. Bothpermeability and stress anisotropy are important

considerations when selecting an optimal wellpath orientation in three dimensions. Productionand perhaps drainage volume can be severelyrestricted by pressure gradients associated withconverging flow in formations. Productivity canbe enhanced if laterals are oriented to takeadvantage of permeability differences in produc-ing zones or across an interval of different layers.For this reason, slanted and horizontal lateralsare most productive when oriented perpendicularto natural fractures. When vertical permeabilityis much less than horizontal permeability, slantedlaterals are best.

Closely spaced lateral branches increase thepossibility of accelerated production andimproved recovery efficiency in large reservoirswith thin zones or in thick zones underlain bywater or overlain by gas. In reservoirs with struc-turally or stratigraphically isolated zones, multi-lateral wells are able to target the various layerswith several laterals.

While multilateral wells are from the bottomup, risks involved in actually drilling drainholesdevelop from the top down. The best drillingand completion strategy is to construct lateralsfrom the deepest branch up. This isolates risksat the lowest point and ensures that developingproblems leave the wellbore above that pointfree of difficulty.

Drilling MultilateralsThe majority of multilateral wells drilled since1953 have been Level 1 and 2 openhole com-pletions in hard rock. Much of this drilling usedrelatively simple technologies, but as openholecompletions with limited functionality give wayto higher level multilaterals to meet therequirements of complicated reservoir and geo-logical conditions, standard directional drillingis being replaced by increasingly complex tech-nologies (previous page and below).

m 4m 3m 2

m1

Trunk

> Subsurface models. Petrophysical descriptionsalong proposed well trajectories can be gener-ated using imported geological models. Here fourlaterals (m 1, m 2, m 3 and m 4) branch from a mainwellbore in a vertically stacked configuration.

8 9 10 11 12 13

RDT

SLT

7. After a lateral is drilled to depth, it may be leftopenhole or a simple cemented or drop-offliner may be run. The landing tool is releasedand the entire assembly is retrieved from thewell. The hole is cleaned out and the bridgeplug is retrieved.

8. The process is changed for a cemented linerby replacing the full-size whipstock with asmaller diameter reentry deflection tool (RDT)that is run and latched into an ICC.

9. The bottomhole assembly (BHA) is run and alateral branch is drilled.

10. A liner is run into the lateral and possiblycemented back into the main casing.

11. The liner running tool is released, the holecleaned up by reverse circulating, and thenthe liner running tool is pulled out of the hole.

12. After the lateral is completed, the RDT isretrieved by releasing the selective landingtool (SLT), and both the RDT and SLT arepulled from the well.

13. The lower wellbore section is cleaned out, theisolating bridge plug is retrieved and the mainbore is ready for completion.

23Winter 1998

(continued on page 27)

Since multilateral drilling began in the MiddleEast during the mid-1990s, it is estimated thatmore than 200 horizontal wells have beendrilled in the region. In the United ArabEmirates, Zakum Field Development Co.(ZADCO) and its operating company Abu DhabiMarine Operating Co. (ADMA-OPCO) are devel-oping one of the largest Middle East oil fields.The experience of ZADCO with various aspectsof multilateral horizontal drilling is typical ofthe state of this technology.

Zakum field, discovered in 1963, is situatedoffshore in the Arabian Gulf about 80 km [50 miles] northwest of Abu Dhabi. The producing formation is a large Cretaceous limestone with various layers in three mainstacked reservoirs (above). Development beganin 1977 with conventional drilling. Horizontaldrilling was introduced in 1989 and extensivemultilateral drilling commenced in 1994 as a

result of improvements in horizontal technology.The first multilateral well was completed inMarch 1995. Encouraged by a significant pro-duction increase, ZADCO decided to develop thestacked reservoirs using horizonal and multilat-eral drilling. To date, 39 dual-lateral and 45 mul-tilateral wells have been drilled and completed,and more are planned.1

During initial development, the complex ofreservoirs was penetrated by a deviated well-bore and then by a single horizontal drainholethrough most of the layers. These two tech-niques increased borehole exposure to thereservoir and allowed oil to be produced fromthe highest permeability layers, but oil in lesspermeable layers was left behind with subse-quent substantial loss of reserves. Drilling sepa-rate drainholes for subzones provides a betteropportunity for stimulation and enhanced pro-duction because each horizontal hole is con-nected directly to the main wellbore.

Drilling MultilateralsLevel 2 multilateral wells at Zakum field beginwith a deviated section. After surface and inter-mediate casing are cemented, wells are deep-ened to 958-in. production casing or 7-in. linerdepth just above lower reservoir targets withmaximum inclination of 55 to facilitate wire-line operations. Using a retrievable whipstock, acasing window is milled near the top reservoirand the upper drainhole is drilled using inter-mediate- and short-radius techniques. The whip-stock is removed so that multiple openholesidetracks and laterals can be drilled. The nexthorizontal hole is kicked off below the produc-tion casing string. Wellbore inclination isincreased to horizontal and a lateral is drilledinto the reservoir. A new deviated section isdrilled from the last kickoff point and anotherlateral is drilled using the same procedures.Specialized or custom profiles, like stair-steps tomaximize footage in certain intervals, can alsobe used (next page, top).

Curves are drilled with dogleg severity rang-ing from 6/100 ft [31 m] to 10/100 ft depend-ing on reservoir requirements and whethermedium- or short-radius techniques are used.Horizontal sections are typically 750 to 3000 ft[229 to 396 m] and the common hole size is 6 in. Position and direction in thin oil layers are achieved using measurements-while-drilling(MWD) and logging-while-drilling (LWD) tokeep well trajectory within the required reser-voir target interval.

Successful multilateral drilling depends onseveral factors, including zonal insolation, win-dow milling, drilling dense barriers, early waterbreakthrough, low-departure targets, low-per-meability zones, staying within targets, multipleholes from a single casing window and stimula-tion of multilateral openholes (next page, bottom left).2

Multilaterals in the Middle East

0 100 km

0 63 m

Zakumfield

Bahrain

United ArabEmirates

Qatar

SA

UD

IA R A B I A

less than5 mD

less than5 mDWest East

I1, I2I3I4I5I6I7

IIIA

IIIBIIIC

IIIDH

IIIJ

IIAIIBIICIID

IIEIIF

AbuDhabi

> Zakum field location and geology. Locatednorthwest of AbuDhabi in the ArabianGulf, Zakum field pro-duces from threestacked reservoirswith various layers in a large Cretaceouslimestone. Threemajor producing reser-voirsI, II and IIIare subdivided according to lithology.

1. Siddiqui TK, El-Khatib HM and Sultan AJ: Utilization Of Horizontal Drainholes In Developing MultilayeredReservoir, paper SPE 29879, presented at the SPEMiddle East Oil show, Bahrain, March 11-14, 1995.

2. El-Khatib H and Ismail G: Multi-Lateral Horizontal DrillingProblems & Solutions Experienced Offshore Abu Dhabi,paper SPE 36252, presented at the 7th Abu DhabiInternational Petroleum Conference and Exhibition, Abu Dhabi, UAE, October 13-16, 1996.

24 Oilfield Review

Zonal isolation The ability to achieve isola-tion is key to multilateral well success. Betweenupper laterals and lower drainholes, zonal isola-tion is extremely important due to pressure dif-ferential between the two reservoirs. Cementadditives and operations were optimized toimprove primary cement bond in addition to theuse of external casing packers (ECP) on the pro-duction casing in some wells (above).

> Drilling Zakum field multilateral wells. The drilling sequence for a Level 2 Zakum field multilateral is as follows: A. Surface and intermediate casing are set and wells are deepened to production casing or liner depth just above the reservoir targets. Maximum inclination is 55 to facilitate wirelineoperations. B. A window is milled in the casing and the upper drainhole is drilled using intermediate- and short-radius techniques. C. The next horizontalhole is kicked off below the production casing string. D and E. New deviated sections are drilled from previous kickoff points so that more laterals can bedrilled. F. Multilaterals with stair-step, traverse or other profiles can be drilled to minimize drilling in tight barriers, maximize horizontal footage in pro-ductive intervals and delay water breakthrough.

(dense) (dense) (dense)

(dense)

(dense)

IAIIIIA

IIBIICIID

133/8-in. casing 30-in. casing

95/8-in. casing

133/8-in. casing 30-in. casing

95/8-in. casing

133/8-in. casing 30-in. casing

95/8-in. casing

133/8-in. casing 30-in. casing

95/8-in. casing

IAIIIIA

IIBIICIID

IA

IIA

IIBIICIID

IAIIIIA

IIBIICIID

A B C

D E

II

IA

IIAIIB

IIC

IID

IIE

II(dense)II

F133/8-in. casing 30-in. casing

133/8-in. casing 30-in. casing

95/8-in. casing

95/8-in. casing

81/2-in.hole

IAII (dense)IIAIIBIIC

IIEIID

133/8-in. casing

95/8-in.casing

200010000

-1000-2000-3000

IA

IIDEIIBIIC

Departure, ft

IAIIAIIBIICIIDIIE

II (dense)

81/2-in. hole

95/8-in. casing shoe 6-in. hole

6-in. hole6-in. hole

dense

-1000

0

-2000-3000-4000

1000

2000

3000

4000

Departure, ft

Latit

ude,

ft

IIIJ branch 1IIIJ branch 2

IIIF IIIG

95/8-in. casing shoe

95/8-in.casing shoe

III

IIIF

IIIG1

IIIJ Branch 1 Branch 2

7-in. liner shoe

IIA

IIA-D

IIG-F

IIA

95/8-in. casing

95/8-in.whipstock

133/8-in. casing

New 7-in. liner shoe

1000

-100

0

-200

0

-300

0

2000

3000

Latit

ude,

ft

Departure, ft

IA

IA

IICDIICD

IIEF

IIEF

Sidetrack

Original hole

-1000

0

-2000-3000-4000

1000

2000

3000

4000

Departure, ft

Latit

ude,

ft

IA IIC

IIFIIDIIE

Branch 2 Branch 1

Branch 1

Branch 1

Branch 2

Branch 2

TH.IATH.IIATH.IIBTH.IICTH.IIDTH.IIE

TH.IIFTH.III (dense)

60inclination

35 inclination 75 inclination

7-in.liner shoe

A B

C D

(dense)

> Multilateral wellbore profiles these can include: A. Hook shapes for low-departure multilateralwells. B. Two branches in thin and tight reservoirs. C. Two opposing branches for target centralization.D. Multilateral holes from one window to minimize casing cement bond failure.

TH.IA reservoir

TH.II (dense)

TH.II reservoir

95/8-in. casing shoe

Perforations

95/8-in. Externalcasing packer (ECP)

81/2-in. hole

> Zonal isolation. In addition to optimizingcement slurries to improve primary cement jobs, external casing packers (ECP) are sometimes used to separate certain intervals.

25Winter 1998

(continued on page 26)

Window millingUsing retrievable whip-stocks and removal of these assemblies are criti-cal to successful drilling of multilateral wells.More than 40 horizontal wells have been side-tracked with retrievable whipstocks. New single-trip whipstocks reduce the number of trips andthe time necessary to exit the casing (right).

Drilling dense barriersBecause Zakumfield porous layers are separated by tight reser-voir rock, different techniques were adopted tominimize drilling in these dense, low-permeabil-ity barriers and maximize horizontal footagewithin specific reservoir zones to improve oilrecovery. The technique of stair-step drillingthrough various reservoir layers is operationallydifficult because of low angles of incidencewhen trying to cross barriers. Another tech-nique, drilling separate drainholes for eachreservoir zone, resulted in postdrilling problemsassociated with production monitoring and stim-ulation of individual drainholes.

Early water breakthroughMultilateralwells are drilled to avoid or delay water break-through by selecting the horizontal section posi-tion and length within desired layers based onspecific reservoir requirements.

Low-departure targetsAnother challengewas drilling multilateral wells with targets lessthan 1000 ft [305 m] from the platform well-heads. Various options were considered to drillthe deviated sections of these low-departuremultilateral wells, but a hook-shaped profilewas found to be operationally and economicallythe best. This well profile can be designed tohave sufficient inclination to use previously suc-cessful medium-radius drilling. Several hook-shaped multilateral wells with four drainholesfrom the main bore were successfully drilledand completed.

Low-permeability zones One benefit of amultilateral approach is the ability to exploitthin reservoirs. Developing stacked low-perme-ability limestone oil reservoirs is typicallyunattractive because of anticipated early waterbreakthrough in vertical or deviated wells. Oneof the fields reservoirs that held substantial oilin place was a 8 ft [2.5 m] thick zone with 6-mDpermeability. Two branches were drilled in dif-ferent directions to increase the drainage areaand improve production. The number and geom-etry of the branches were dictated by reservoircharacteristics.

Staying within targetsAnother challengefor drilling multilateral wells is to correctlyposition and maintain horizontal sectionswithin existing sweep patterns. Since branchesdrilled in opposing directions were found to be

optimum, severe left- and right-turning trajec-tories must be drilled to achieve the requiredreservoir exposure. A significant increase inproduction rates was observed in wells drilledin this manner.

Multiple holes from a single casingwindowSeveral drainholes were successfullydrilled from the same main borehole after exit-ing casing in reentry and new wells. This proce-dure can avoid the time and expense of multiplecasing exits, but does limit the ability to moni-tor and stimulate laterals.

Stimulation of multilateral openholesZADCO uses openhole completions that com-mingled production from reservoirs I and II.Production from these two main reservoirs iskept separate using dual-tubing completion.Because of the inability of current through-tub-ing stimulation systems to access each drain-hole selectively, common practice is tobullhead stimulation treatmentspump downthe production tubing from suface. When possi-ble coiled tubing was run through the produc-tion tubing to selectively treat individualopenhole laterals in the main reservoirs.

Permeability variation in each productivelayer requires that acid be diverted across allintervals where coiled tubing is unable toreach total depth. Techniques using divertingadditives and procedures integrally combinedwith stimulation acid treatments are successfulin increasing the productivity of some multilat-eral wells, but in many wells these diversiontechniques cannot effectively stimulate the

desired number of laterals. Production logs arebeing used to further evaluate stimulationeffectiveness as well as design and proceduralmodifications.

Future MultilateralsMultilateral drilling in Zakum field provided anopportunity to improve recovery and managefield production more efficiently. Some 84 newand reentry Level 1 and Level 2 multilateralwells, from single and dual laterals up to sevenlaterals, were drilled and completed success-fully in the last four years. Multilateral horizon-tal drilling brought new life to the fields thin,low-permeability reservoirs where developmentby deviated or vertical wells had not been effec-tive. Horizontal wells with branches in opposingdirections were the optimum solution. Thefuture challenge is to conduct independentoperations in each lateral and overcome zonalisolation difficulties.

After comparing drilling costs for differenthorizontal well typesmedium, intermediate-radius and short-radiusZADCO determinedthat short-radius drilling is more expensive thanmedium-radius wells, but short-radius wells arebetter in terms of production compared withvertical wells. Through rapid growth in short-radius drilling technology, the cost per foot ofhorizontal drilling was reduced by 30% afterdrilling 27 horizontal sections in ten wells.Lower costs, resulting from steerable drillingtechnology, encouraged ZADCO to continuedrilling multilateral horizontal wells.

4.8

2.6

1.6

0

1

2

3

4

5

6

Mechanical

Hydraulic

Single-triphydraulic

Win

dow

mill

ing

dura

tion,

day

s

Mechanical Hydraulic Single-triphydraulic

> Retrievable whipstock and anchor window milling performance.

26 Oilfield Review

Short-radius wells, small-diameter wells andmultiple radial slimholes are now being drillednot only in the Texas Austin Chalk region, butalso in areas like the Middle East and SoutheastAsia. In Alaska, USA, for instance, BP and Camcohave drilled multilaterals with build angles ofaround 1.8/ft, changing the well from vertical tohorizontal in approximately 50 ft [15 m]. Thissteep build rate produces less formation damage,requires less time for drilling to target, uses lessdrilling fluid, and is generally more economical.

Small-diameter boreholes are drilled toreduce cost, and multiple slimhole horizontalreentries can be drilled from small-diameterwells to further increase reservoir exposure.Coiled tubing is also employed to drill multipleradials from the main bore. Coiled tubing drillingis frequently used to remove near-wellbore for-mation damage to increase reservoir flow poten-tial, but in the Snorre field, Norway, for example,has also been used for drilling drainholes toreplace perforations.

Multilateral reentry is not the sidetrackingtechnique used for decades to salvage old well-bores that would otherwise have to be aban-doned. Rather, it is an evolving technology forproducing from and working over both the mainbore and the laterals. Determining the right tech-niques for reentering multilateral wells to per-form stimulation, acidizing, perforating or anyother fluid pumping operation is a key problemconfronting the oil industry today. As well config-urations become more complex, the degree ofdifficulty increases.

Two major challenges are reentering a singlebranch at a specific depth and reentering multi-ple branches at the same depth. In addition, com-pletion type, whether openhole or cased, thehole size and the vertical-to-lateral build raterepresent primary factors involved in the selec-tion of proper reentry techniques. The need tohydraulically isolate laterals impacts the choiceof solution as well.

Reentry is a two-step operation: recognize theentry point and enter the lateral. One recognitionmethod is accomplished by running a tool oncoiled tubing that rotates to reentry depth. Thetool has a bend on the end that provides a surfaceweight change indication when the bend enters alateral opening. The Schlumberger coiled tubingVIPER system also has a bottom orientation subthat is used to locate and access laterals.

Mechanical methods are another way ofachieving lateral entry. In a minimum of threeruns, a whipstock diverting device is set; coiledtubing work is performed; and the diverter isretrieved. The tool carrying the diverter controlsdepth as it lands on a predefined tubing or casingprofile nipple. The nipple provides tool orientationand allows the diverter to be located accurately atthe lateral opening. This technique is used withcompletion equipment designed specifically forthrough-tubing reentry into laterals.

Reentry technology is evolving towards viableand reliable systems most likely based on a cas-ing profile nipple or a tubing nipple and portedtubing sub aligned with the casing window, towhich a bottomhole assembly will attach. An ori-entation locator coupled with upper and lowerpacker assemblies will find the orientation nippleand align the lateral access joint in the correctdirection. A landing nipple plug will be used toisolate the lower packer or window joint for test-ing. An orientation device to accept a coiled tub-ing-conveyed diverter will facilitate access to thelateral opening for reentry.6

Wellbore ManagementIn production engineering and operation of multi-lateral wells, the key considerations are whethera well needs artificial lift and the degree to whichimposed formation pressure drawdown isaffected by frictional pressure drop inside thewell. For example, short opposed laterals arepreferable to a long, single horizontal well in onedirection if drawdown is about the same as pres-sure drop in the wellbore. Conversely, if draw-down is several hundred psi, or more, a singlehorizontal leg may be adequate.

Selective wellbore control is provided bythree basic completion configurations: individualproduction tubing strings tied back to surface,commingled production, and commingled produc-tion from individual branches that can be reen-tered or shut off by mechanical sliding sleeves orplugs. These options relate directly to reservoirmanagement because the need for selective con-trol increases as wellbores are opened to moreareas of the reservoir. For example, laterals thatdrain multiple layers or different formationsrequire selective management if pore pressuresand fluid properties differ widely between zones.The degree of communication between thedrainage areas of individual laterals may be themost important reservoir engineering issue inmultilateral applications.

There are logistical and operational issues incompleting certain well systems that may bedictated by obvious reservoir exploitation strate-gies and schemes. Currently, multilateral wellscan be constructed with connectivity, isolationand access. Numerous completion choices areavailable. The following three configurations arecommon: Drain several stacked layers that may not be in

communication Drain a single layer in which areal permeability

anisotropy is critical Drain geologic compartments that may not be

in communication.Draining stacked layers favors a vertical

main bore, but heterogeneous and com-partmentalized reservoirs favor a singlehorizontal well, dual-opposing laterals, or multi-branched wells. Commingled production fromstacked laterals is analogous to commingledproduction from two or more layers in a verticalwell. The two main advantages of stacked later-als are that each lateral has greater productivitythan a conventional vertical completion throughthe same layer, and that control of verticalinflow, or conformance, is facilitated becausethe productivity of each lateral is approximatelyproportional to its length. Vertical flow confor-mance avoids differential depletion under pri-mary production and uneven water or gasbreakthrough under secondary production.7

Future Multilateral TechnologyOptimal multilateral connectivity will depend onthe development of reliable junctions betweenthe main bore and laterals as well as new com-pletion strategies to connect more lateral well-bores with productive reservoir intervals. A firststep will be the improvement of casing windowsto facilitate efficient drilling and reentry of multi-ple lateral drainholes. Many in the industrybelieve that a technique must be developed toseal casing window connections. Considerableeffort is being expended to perfect a reliablemechanical seal or new chemical sealants forTAML Level 6 wells to provide pressure integrityat the junction. Others maintain that the vastmajority of multilaterals exit the main bore intothe same reservoir, where the pressure differen-tial at the junction is negligible. They advocatethat, rather than pressure integrity, priority begiven to developing fit-for-purpose junctionintegrity to increase production and the ability tomanage laterals over the life of a well.

27Winter 1998

6. Turcich TA: Pressure-Control Engineering, presented atthe SPE Fourth European Coiled Tubing Roundtable,Aberdeen, Scotland, November 19-20,1997.

7. Ehlig-Economides et al, reference 4.

Downhole construction of lateral junctionshas associated problems such as generatingdebris and lack of cementing options. Surfaceconstruction, as in Level 6S wells, which can bedone for new wells only, is debris-free, but lim-ited to shallow wells.

There are also opposing opinions about con-struction of casing windows. Construction down-hole favors milling standard casing byreferencing inexpensive casing profile nipples orpackers. Multiple nipples can be designed intocasing strings, permitting operators to choose

sidetrack locations when they are ready and pro-viding a reentry sleeve reference as well.Another possibility is to run a composite casingsection with a profile nipple below it from whichthe drilling whipstock and lateral entry systemsleeve can be spaced. Although there is nomilling, casing strength is compromised (left).

Premilled windows or casing stock that hasremovable sleeves or is encased in drillable mate-rial are promoted by many to provide tensilestrength without having to mill downhole. As withcomposites, the whipstock and lateral entry sys-tem sleeve are deployed through casing nipples.Generally, lateral casing is allowed to protrudeinto the main casing, where it is cemented inplace and then milled or washed over to restorefull main bore diameter. Both mechanical andpressure-tight tie-backs are being developed.8

Other technical issues need to be resolved aswell, including the management and monitoringof production. Downhole control of flow withremotely operated chokes and other flow devicesthat independently optimize individual lateralsand selectively shut-off zones to block water andgasintelligent completionswill aid produc-tion management. Downhole permanent gaugesfor each lateral are also on the drawing board tomonitor changes in pressure, temperature, flowrate, and water and gas cut. When connected tosurface systems, these advances will permitadditional surface measurement and eventually,the allocation of flow from each lateral. Selectivereentry will permit servicing of these devices andsensors, and allow batch treating of each lateral.

Future multilateral wells will involve fewertrips into the well, incorporation of sealed lateraldevices, and a full range of downhole controlsand sensors to regulate flow, pressure and multi-phase differentials. Downhole fluid separationand injection will be accomplished with surfacecontrol, and expensive rig interventions will bevirtually eliminated by electrohydraulic control ofdownhole functions. This trend will reverse therisk-reward ratio offshore, where risks are highand reserves are large, in favor of multilaterals(left). Ultimately, multilateral well technologywill be the basis for the intelligent completionsthat will one day yield remotely operated subter-ranean and subsea factories with oil and gas asthe finished products. DG, DEB, RR, MET

41/2-in. upper tubing 133/8-in. casing

Top of liner3680 ft, TVD

M-seal sealant

Window bushingassembly

Top of window3858 ft, TVD

7-in. composite joint

Hollow whipstockOrienting latch

Multilateral packerOrienting nipple

Retrievable packer

TD 8439 ft, MD

41/2-in. predrilledcasing

61/8-in. open hole61/8-in. open hole

9 5/8-in. casing 7-in. liner 4798 ft, MD

TD 8439 ft, MD

> Composite casing section with a nipple profile.

> Intelligent completions. Future multilateral completions may involve many processes, from formationdrainage to downhole separation, inflow control, injection and reservoir monitoring.

28 Oilfield Review

8. Turcich, reference 6.