1. Pittard A: “Field Abandonment Costs Vary WidelyWorldwide,” Oil & Gas Journal 95, no. 11 (March 17,1997): 84, 86–91.“Heavy Deck Removal Vessel Under Review for Frigg,Ekofisk,” Offshore 61, no. 10 (October 2001): 88, 90.

28 Oilfield Review

The Beginning of the End: A Review ofAbandonment and Decommissioning Practices

Ian BarclayJan PellenbargFrans TetteroPetroleum Development Oman LLCMuscat, Sultanate of Oman

Jochen PfeifferOklahoma City, Oklahoma, USA

Harold SlaterPanCanadian Energy CorporationCalgary, Alberta, Canada

Timo StaalMuscat, Sultanate of Oman

David StilesCalgary, Alberta, Canada

Geoff TillingPhillips Petroleum Company United Kingdom LimitedWoking, England

Chris WhitneyUnocal CorporationSugar Land, Texas, USA

For help in preparation of this article, thanks to LeoBurdylo, Dan Domeracki, Roger Keese, James Garner, Erik Nelson and Keith Rappold, Sugar Land, Texas, USA;Erick Cunningham, Clamart, France; Alan Salsman, Calgary, Alberta, Canada; and Lisa Stewart, Ridgefield,Connecticut, USA.CemSTONE, FlexSTONE, LiteCRETE, SqueezeCRETE,TubeCLEAN and UniSLURRY are marks of Schlumberger.

The oil and gas industry anticipates growing activity in well-abandonment and plat-

form-decommissioning operations. Technically sound well-abandonment practices

are essential for long-term environmental protection, especially as regulations

become more stringent and complex. Although advanced technologies and tech-

niques bring new meaning to the “permanent” aspects of abandonment work, opera-

tors seek to minimize abandonment and decommissioning costs because these

expenses are not recouped.

The life of a well comprises numerous stages.The discovery of a new accumulation of oil or gasafter months or years of exploration and drillinginvigorates the responsible technical team.Achieving first production is another upliftingmilestone. Successful enhanced-recovery opera-tions can make the waning stages of productionfinancially and technically rewarding. The stagethat no one seems to enjoy facing is the finalcessation of production and the abandonment of wells and production facilities. Althoughabandonment is supposed to mean permanenttermination, the effects of some abandonmentpractices can be felt for many more years than the relatively brief producing life of theaverage well.

Well abandonments are becoming increas-ingly frequent as aging fields reach their productive and economic limits. The cost ofdecommissioning the world’s 6500 offshore plat-forms is estimated at $29 to $40 billion over thenext three decades.1 Onshore, tens of thousandsof wells must be abandoned someday.

Responsible operators now seek to balanceenvironmental responsibilities with the profitdemands of shareholders. Shoddy plugging andabandonment (P&A) operations are expensive toremediate and exact a toll on both the environ-ment and the reputation of the company. LocalP&A mishaps can affect the reputation of theentire oil and gas industry. With those issues inmind, many operators are upgrading their well-and field-abandonment procedures to ensure thatabandoned reservoirs truly are permanentlysealed and facilities properly decommissioned. Inthis article, we review P&A and decommission-ing practices, explain how sound abandonmentpractices protect the environment and discussnew technologies that reinforce the meaning ofthe “permanent” part of abandonment work. Wealso discuss permanent-abandonment challengesand practices, and examine well- and platform-abandonment operations in Oman, Canada, theGulf of Mexico and the North Sea.

50973schD2R1.p28.ps 1/31/02 4:41 PM Page 28

Winter 2001/2002 29

50973schD2R1.p29.ps 1/31/02 4:41 PM Page 29

Wellbore-Abandonment Challenges and SolutionsThe key goal of any well abandonment is the per-manent isolation of all subsurface formationspenetrated by the well. Although sealingdepleted reservoirs is an important concern inP&A procedures, ideal abandonment operationsisolate both producing reservoirs and other fluid-bearing formations. Complete isolation preventsgas, oil or water from migrating to surface orflowing from one subsurface formation toanother. Experts estimate that a high proportionof seals placed in wells may be faulty.2

Leaking seals pose risks to the environment—groundwater resources and the overlying land or sea—and must be repaired, but remedial

plugging operations are difficult and expensive.Sealing a well correctly at the outset is far eas-ier, even if the initial financial outlay appearshigh. Considering well abandonment at the earli-est stages of the well design makes sensebecause the quality of the primary cementbetween the casing and formations is a key fac-tor in successful well abandonment years later(below).3 For decades, oilfield engineers have rec-ognized that Portland cement is the best materialto seal abandoned wells. It is durable, reliable,available worldwide and relatively inexpensive.Complete removal of drilling mud and filter cakeduring primary-cementing operations decreasesthe risk of forming a microannulus or channel inthe cement sheath, and improves bonding

between rock formations, cement and casing.Bulk shrinkage of ordinary Portland cement slurryas it sets can create small cracks and gaps thatcan become flow paths.

Any deficiencies in primary cementing tend toaffect long-term isolation performance. Widefluctuations in downhole pressure and tempera-ture can negatively affect cement integrity orcause debonding. Tectonic stresses also can frac-ture set cement. Regardless of the cause, loss ofcement integrity can result in fluid migration,impairment of zonal isolation or casing collapse,even when high-quality cement is placed prop-erly and initially provides a good seal.

Advanced, flexible cement provides greaterlong-term cement integrity than ordinary Portland

30 Oilfield Review

Wrong density Poor mud- and filter-cake removal Premature gelation Excessive fluid loss

Highly permeable slurry Significant cement shrinkage Cement failure under stress Poor interfacial bonding

> Primary cementing parameters that affect sealing. Incorrect cement density can result in hydrostatic imbalance. Poor mud- and filter-cake removalallows gas to flow up the annulus. Premature gelation leads to loss of hydrostatic pressure control. Excessive fluid loss allows gas to enter the slurry column. Highly permeable slurries provide poor zonal isolation and resistance to gas flow. Significant cement shrinkage and cement failure under stresscreate fractures and microannuli that transmit fluids. Poor bonding at cement-casing or cement-formation interfaces also can cause failure.

50973schD2R1.p30.ps 1/31/02 4:41 PM Page 30

Winter 2001/2002 31

cement because it resists stress cracking andmicroannulus formation. Introduced in 2000,FlexSTONE cementing technology incorporates anoptimized distribution of flexible particles into thecement matrix to adapt to temperature and pres-sure variation, providing zonal isolation for the lifeof the well and beyond.4 The corrosion resistance,low permeability, flexibility and posthydrationlinear-expansion capabilities of FlexSTONEsystems make them ideal primary cements andabandonment-quality cements (right).

If fluids are migrating from a well that mustbe abandoned, then the first challenge is tolocate the fluid-migration path. Typically, sub-surface fluids migrate through completioncomponents, leaky plugs, deficient cementsqueezes, or flaws in the primary cement sheathor the caprock—the relatively impermeable for-mation that encloses the reservoir. The caprockmight be compromised by natural fracturing or byfracture-stimulation treatments. When multiplereservoirs exist, identifying which one is leakingenables targeted remediation.5 Knowing thecondition of primary and secondary cement isvitally important. Personnel involved in abandon-ment must understand the geology, wellboregeometry and accessibility, downhole equipmentand its condition, reservoir pressure and poten-tial fluid-migration paths to abandon a wellsuccessfully (right).

2. While the estimates of the proportion of leaky seals varywidely from one region to another, in a survey initiated in1993 in the Lloydminster area, western Canada, Husky Oilreported that 45% of surveyed wells suffer from gas-migration problems. On the basis of their research, thecompany estimated that remediation of these wells mightcost $15,000 to $150,000. For more information: Schmitz RA,Cook TE, Ericson GMJ, Klebek MM, Robinson RS and Van Stempvoort DR: “A Risk Based ManagementApproach to the Problem of Gas Migration,” paper SPE35849, presented at the International Conference onHealth, Safety & Environment, New Orleans, Louisiana,USA, June 9-12, 1996.

3. Primary cement is the initial cement sheath placedaround a casing or liner string. The main objectives ofprimary cementing operations include zonal isolation toprevent migration of fluids in the annulus, support for thecasing or liner string, and protection of the casing stringfrom corrosive formation fluids.For more on primary cementing: Bonett A and Pafitis D:“Getting to the Root of Gas Migration,” Oilfield Review 8,no. 1 (Spring 1996): 36–49.

4. For more on flexible cements: Le Roy-Delage S,Baumgarte C, Thiercelin M and Vidick B: “New CementSystems for Durable Zonal Isolation,” paper IADC/SPE59132, presented at the IADC/SPE Drilling Conference,New Orleans, Louisiana, USA, February 23-25, 2000.Thiercelin MJ, Dargaud B, Baret JF and Rodriguez WJ:“Cement Design Based on Cement MechanicalResponse,” paper SPE 38598, presented at the SPEAnnual Technical Conference and Exhibition, SanAntonio, Texas, USA, October 5-8, 1997.

5. Isotope analysis of gas from two formations in theLloydminster area demonstrated that the shallowerformation was more prone to gas-migration problems.For more information about this example: Schmitz et al,reference 2.

Nonproducingreservoirs

Production packer

Perforations

Caprock

Cement sheath

Completion

< Well-abandonment considerations. Abandon-ment designs should address geological consider-ations, such as the type and condition of reservoirand caprock formations. The design also shouldaddress the condition and configuration ofcement, perforations, tubulars and downholeequipment. Caprocks, cement and completionequipment are common fluid-migration pathsthat must be identified and sealed for effectivelong-term isolation.

Line

ar e

xpan

sion

, %

0.00.1

Saltcement

Foamedcement

0.0

Plastercement

FlexSTONEcement

T/E ratio

Conventional cementFlexSTONE system

Bond strength,MPa after 4-week

set time

Permeability,µD

0.7

3.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Wel

l-iso

latio

n pr

oper

ties

0

4

6

8

10

2

12

> Properties of isolation-quality cement. New, flexible cement systems offer higher linear expansionthan other expanding cement systems (left). FlexSTONE systems also offer improved critical well-isolation properties, including tensile strength-to-Young’s modulus ratio (T/E), bond strength andlow permeability (right).

50973schD2R1.p31.ps 1/31/02 4:41 PM Page 31

Another challenge in P&A procedures is thatrelevant documents detailing the life of the well,such as well logs and schematic diagrams, mightnot be accessible. Information about the geologymight be lost or unavailable because of the timethat elapses between first production and wellabandonment. This time frame can be decades.Also, it is common for wells to change ownership.

Operators also must strictly adhere to localwell-abandonment regulations (below). In someregions, regulators grant permits for specificabandonment procedures and observe key stagesof the operations. Compliance requires carefulplanning and coordination, which, for some oper-ators, may be facilitated by specialized databasesand software.6 Regulations have changed consid-erably over the years, and keeping track of themrequires engineering, environmental, legal andsafety expertise.

In many regions, there are rules and regula-tions in place that constitute the requirementsfor well abandonment. In areas where theregulatory authorities do not supply minimumstandards, operators tend to follow their owninternal standards. Most of these standards aresimilar because many originated in the NorthSea, where environmental-protection goalssignificantly influence operations.

Rigless Wellbore AbandonmentPreparation is a key ingredient in well abandon-ment, including a thorough assessment of thenear-wellbore geology and the unique mechani-cal condition of the well. In a straightforwardcase, well abandonment begins with cleaningthe production tubing and cementing, or squeez-ing, the production perforations. After the tubingabove the production packer is perforated,cement is circulated between the tubing and cas-ing. At shallower casing-shoe levels, multiwallperforations are shot and cement is circulated inall open annuli to achieve a wall-to-wall cementbarrier. Finally, the tubing is perforated at a shal-low depth—perhaps 150 m [490 ft]—and asurface cement plug is placed. When all cementplugs have been placed and tested, the wellheadand casing stump are removed.

In reality, most abandonment operations aremuch more complicated. Multiwell onshore-abandonment programs demonstrate both thecomplexity of the operations and the gains inefficiency and cost savings achieved when usinga coiled tubing unit instead of a workover ordrilling rig.7

Although pioneered on land in 1983 in thePrudhoe Bay field, Alaska, USA, rigless well-workover procedures using coiled tubing units

have been adapted for abandonment operationsworldwide.8 Rigless abandonment operationsalso have been performed offshore for more thana decade, but removing production facilitiesgenerally requires mobilizing heavy lifting equip-ment (see “Field Abandonment and PlatformDecommissioning,” page 37).9

There are clear advantages to rigless aban-donment using a coiled tubing unit for a multi-well abandonment program. The equipmentcosts less offshore and often is much easier tomobilize; onshore, its value lies in time savingsover a comparable hoist operation. Coiled tubingallows precise placement of cement plugs, evenin deviated wellbores. Also, coiled tubing opera-tions can be performed without killing the well orremoving the production tubing or wellhead.

In several depleted oil and gas fields onshoreOman, Petroleum Development Oman LLC (PDO)initiated a multiwell abandonment program withSchlumberger (next page). The primary concernwas to properly abandon all producing zones andprotect aquifers while minimizing cost and risk.

PDO began its rigless abandonment project inNovember 2000 following a research report onpermanent plugging materials and rigless appli-cations. After an initial literature study, a reviewof the PDO well inventory revealed 60 redundantexploration wells located throughout their con-cessions. PDO decided to begin the 60-wellabandonment project in southern Oman, whereformation pressure regimes are mainly at orbelow hydrostatic levels.

PDO prepared the scope of work and equip-ment requirements for the contract tender, andsoon realized that the complexity and variety ofthe abandonment activities required an inte-grated services approach. A key element of thisrigless abandonment program would be coordi-nating all well services to maximize efficiency.Ideally, the lead contractor would perform atleast 80% of the work, and preferably could per-form the two most critical activities, perforatingand cementing.

PDO developed five main criteria for optimalrigless abandonment performance:• Supermobile equipment—All equipment,

including a mobile camp, would be mounted onwheels to allow faster relocation, given the factthat the units must move every four to six days.

• Self-supported operations—The contractortakes care of nearly all activities, such as sup-plying materials, transport and subcontractorservices, with minimal operator involvement.PDO supplies only programming, air transport,communication facilities, some mud chemicalsand a site representative.

32 Oilfield Review

Examples of P&A Regulations

Alberta, Canada2

Openhole and cased-hole abandonment require: Regulatory preapprovalPlans that meet regulatory standards, includingspecial requirements in oil sands and areashaving gas-migration problemsCompliance with timing and notificationrequirements

Well abandonment regulations specify:

Texas, USA1

Written notice of intent to plug and proposedproceduresCommencement of plugging operations withinspecified time limitsPlugging by the Railroad Commission of Texas andreimbursement by the operator under certain conditions

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Both you and the third person would be coapplicant on the loan. B

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Thank you and have a great day,

FCCU

Texas Railroad Commission, http://204.65.105.13/texreg/archive/January42002/adopted/16.ECONOMIC%20REGULATION.html#372Alberta Energy and Utilities Board (EUB): Well Abandonment Guide, March 1996.http://www.eub.gov.ab.ca/bbs/products/guides/g20-1996.pdf

1

2

> Examples of well-abandonment regulations. Designed to protect water andhydrocarbon resources, some regulations include standards for the type ofcement used and the locations of cement plugs in the well, how deeply belowsurface the casing must be cut, and how the well location should be marked.Returning the surface to a pristine condition also is part of the job. In areasnot subject to regulation, operators generally follow their own guidelines.

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Winter 2001/2002 33

• Dry location concept, also known as zero dis-charge—No fluids are drained on or near thewellsite. This eliminates the possibility ofrepairing or rebuilding waste pits at abandon-ment locations. Although fluid circulation anddumping are inevitable during the job, all fluidsare stored in tanks. Dry locations speed up theabandonment and restoration job by approxi-mately ten days per well because no time isspent cleaning a waste pit or waiting for thelocation to dry. Previously, this required asmuch as several months.

• One-stop job—Each wellsite is visited onlyonce and the entire abandonment job must becompleted at that time. Any return to the sitesubstantially delays the next operation.

• Minimum mileage—Equipment moves areoptimized to reduce move time and optimizetransport. With the PDO concessions cover-ing approximately 41,000 square miles[110,000 square kilometers], executing themoves in accordance with health, safety

and environment (HSE) guidelines is a crucial requirement.

From September through December 2001, 18 wells were abandoned, with typical savings of30% over previous abandonment procedures.The wells were up to 25 years old and producedfrom sandstone and carbonate reservoirs of vari-ous geologic ages, so the abandonment of eachwell had to be planned and executed accordingto its unique attributes.

Although the lessons learned from each wellwere incorporated into subsequent operations,procedures could change significantly from onewell to the next. Typically, the need for thesechanges became clear only after the first entryinto a well. Thus, abandonment work differsgreatly from new-well engineering, where for-ward planning is possible. Abandoning old wellsrequires an initial plan plus constant commun-ication between field and base once the jobcommences, since the status of the well at sur-face and downhole differs for each well. For

these reasons, seasoned site representativesand experienced, dedicated contractor staff arerequired to cope with the constant changesdictated by well conditions.

Challenges encountered to date includedheavy, thick crude oil in the production tubing and“A” annulus—the space between the tubing andthe first string of casing—that made it impossi-ble to run wireline. Corrosion of the outer casingof some wells required additional pluggingthrough the outer annulus of each well. Somewells had more than one annulus, requiringperforations through overlapping, concentriccasing strings. On the other hand, some wellshad good injectivity, which simplified downholedischarge by bullheading the cleaning slugs intothe formation.10 Bullheading saved time andeffort compared with handling the cleaning slugsat surface. All these situations created a need fordetailed programming for each individual well.

Common to all abandonments, however, is the requirement to perform the operationflawlessly the first time and to protect the envi-ronment at all stages of the operation.Accordingly, all locations are kept “dry” to speedrestoration to their natural state. All waste fluidsmust be removed for safe disposal at PDO-designated areas, except for excess cement,which is nonhazardous. Fluids used on site arestored in tanks. Since there are no rigid govern-mental abandonment directives in place, theapplicable PDO abandonment policy and stan-dards are developed in line with the UnitedKingdom Offshore Operators Association (UKOOA)standards set for North Sea operations and similarDutch government-accepted standards.

6. Woody F: “Streamlining Abandonments for CostReduction,” paper SPE 66497, presented at theSPE/EPA/DOE Exploration and Production EnvironmentalConference, San Antonio, Texas, USA, February 26-28,2001.

7. Bigio D, Rike A, Christensen A, Collins J, Hardman D,Doremus D, Tracy P, Glass G, Joergensen NB andStephens D: “Coiled Tubing Takes Center Stage,” OilfieldReview 6, no. 2 (October 1994): 9–23.

8. Harrison T and Blount CG: “Coiled Tubing CementSqueeze Technique at Prudhoe Bay, Alaska,” paper SPE15104, presented at the 56th California Regional Meeting,Oakland, California, USA, April 2-4, 1986.

9. Hoyer CWJ, Chassagne A, Vidick B and Hartley IP: “A Platform Abandonment Program in the North SeaUsing Coiled Tubing,” paper SPE 23110, presented at theSPE Offshore Europe Conference, Aberdeen, Scotland,September 3-6, 1991.

10. Bullheading refers to the technique of forcibly pumpingfluids into a formation. In addition to fluid-disposaloperations, bullheading may be performed whenformation fluids have entered the wellbore during a well-control event or when normal circulation can-not occur, such as after a borehole collapse. Duringbullheading operations, the fluid usually enters theweakest formation.

0

0 150 300 km

100 200 miles

Muscat

Jisr field

UNITED ARAB EMIRATES

OMAN

A r a b i a nS

ea

G

u l fo f O m a n

A r a b i a n

G u l f

> Location of Jisr field, Sultanate of Oman.

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Abandonment of the Jisr-1 well, located insouthern Oman, represents an average degree ofdifficulty for the PDO well-abandonment program(right). The operation began as a coiled tubing rigwas moved on location (next page, top). Becausethe well was 12 years old, all valves on theChristmas tree were backed up with new valves,and coiled tubing blowout preventers wererigged up to ensure well control. Gas-detectionand other safety equipment was installed beforeentering the cellar.11

Next, the tubing-hanger plug, used for tem-porary well suspension, was removed. Theproduction tubing and “A” annulus were cleanedby jetting cleaning fluids down the tubing and upthe annulus. The cleaning fluid contains surfac-tants and acids that remove sludge, oil andparaffin. Cleaning is critically important becauseseals within the wellbore can shift if sludge orother material moves after setting cement plugs.Also, cement will not form a perfect hydraulic sealwith materials that are coated with hydrocarbon.

The production tubing and 95⁄8-in. casing sumpwere cleaned with a high-pressure jetting toolrun on coiled tubing. The tubing and the “A”annulus were then displaced with 11.4-kPa/m[0.5-psi/ft] salt brine. High-pressure jetting hasproved to be an effective, environmentallyfriendly method for cleaning the tubing andsump, as waste generation is kept to a minimum.In cases of heavy-oil contamination, light crudeand TubeCLEAN slugs are pumped and bull-headed through the perforations where possible.These were not required on Well Jisr-1, where a2-m3 [13-bbl] surfactant wash, with a 10-minutecontact time, was considered sufficient to cleanthe “A” annulus.

With coiled tubing, the operations team set abentonite spacer on bottom to serve as a base forthe cement plug. On Jisr-1, the perforations wereshot 342 m [1122 ft] above the base cement plug.The PDO requirement is to set the reservoir-isolation plug from 50 m [164 ft] below the lowerperforation to 50 m above the top reservoir. Tocomply with this requirement at minimum cost, a280-m [920-ft] bentonite spacer was spotted onbottom as a filler. The first cement plug was setthrough coiled tubing across the perforations. Asecond plug was set higher in the wellbore,opposite the 133⁄8-in. casing shoe, after a bridgeplug was set inside the 31⁄2-in. production tubingusing coiled tubing. The 31⁄2-in. tubing and 95⁄8-in.casing were perforated and a wall-to-wallcement plug was placed. Next, a bridge plug was set at 155 m [508 ft], and the tubing wasperforated at 150 m. Finally, the surface cement

plug was pumped. In contrast to procedures forthe previous cement plugs, PDO abandonmentstandards do not require pressure-testing of thesurface plug.

In most of the wells in this abandonmentprogram, the cellars are about 2.6 m [8.5 ft] deep. Once the wellhead was cut 50 cm [1.6 ft]above the cellar floor, a 10-mm [0.4-in] thick steel plate was welded to the stump, and a smallpole with the well number was installed to markthe location above the surface. Then, the cellarwas temporarily filled with sand until final siterestoration. The operation concluded with riggingdown the coiled tubing unit, and removing thesevered wellhead assembly and all junk from thelocation (next page, bottom). This operation took

five days, including a two-day rig move. Placingcement plugs consumed much of the remainingtime. Rig moves for closely spaced wells typicallyrequire 6 to 10 hours. Moves greater than 15 km[9.3 miles] require relocating the work camp.

This multiwell abandonment program willcontinue through much of 2002. The most com-plicated abandonment operations are scheduledlate in the program to capitalize on the experi-ences of the previous operations. Otherchallenges continue to be addressed by theoperations teams. Because unit moves consumea substantial part of the operational time, achiev-ing the “supermobile” equipment goal and usingmultifunctional, fit-for-purpose equipment will

34 Oilfield Review

Depth,m

249

395

637

986

988

994

1105

1223

1315

13371382

1425

1778

Dammam

Rus

Upper Umm el Radhuma

Middle Umm el Radhuma

Shammar

Aruma

Natih

Nahr Umr

Mesozoic clastics

Upper Gharif

Middle Gharif

Basal sands

Al Khlata 9 5⁄8-in. casing

13 3⁄8-in. casing

13 3⁄8-in. casing

9 5⁄8-in. casing

Topfill, 20 mPlug #3, 150 mTubing punch, 155 mBridge plug, 155 m

Plug #2, 797 to 947 mMultiwall perforations,944 to 947 mBridge plug, 952 m

Plug #1, 1255 to 1460 m3 1⁄2-in. kill string, 1377 mPerforations, 1401 to 1408 mBentonite spacer, 1460 to 1743 m

Before Abandonment After Abandonment

> Jisr-1 wellbore schematic before (left) and after abandonment (right).

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Winter 2001/2002 35

increase efficiency. In addition, the major, dailyactivity is placing cement plugs in the wellbore,so there is considerable interest in developingshort but safe cement-setting times.

In an operation such as rigless abandonment,where the benefits are in the time saved, longwaiting-on-cement (WOC) times are a majorhurdle. Slurry recipes are modified frequently toreduce pumping and thickening times as fieldexperience increases. Current conventionalthrough-coil slurries have 3-hour pump times andhave been tagged, or contacted, in 11 hours.UniSLURRY formulations are being considered toreduce this time even more. UniSLURRY systemscan be used for all cementing operations over awide temperature and density range, addressingmost oilfield cementing requirements. TheUniSLURRY family consists of solid and liquidfluid-loss additives and liquid retarders. Theirversatility simplifies the logistics of cementingoperations by reducing the number and quantityof additives that have to be transported and,eventually, stored at the wellsite. The additivessynergistically reduce overall additive concentra-tions while maintaining slurry quality.

As the operation teams prepare to abandonthe most challenging wells, PDO is consideringthe use of flexible systems from the CemSTONEfamily. FlexSTONE long-term zonal isolation tech-nology is expected to increase resistance tocracking under changing field conditions andprovide an abandonment-quality plug that willlast longer than ordinary cement plugs. Anotheradvantage of the FlexSTONE slurry system is thatit can be designed to expand, which eliminatesany possible bulk shrinkage that might lead toloss of isolation. The expansion and flexibilitywill ensure excellent bonding with the casingand prevent the development of a microannulusbetween casing and cement plug so the wellremains properly abandoned over time. An addedimprovement is faster compressive-strengthdevelopment for slurries based on optimizedparticle-size distributions, such as FlexSTONEsystems, than for conventional slurries, reducingthe WOC time for pressure-testing the plugs.Laboratory tests have confirmed the faster com-pressive-strength development. The FlexSTONEsystem is one of many new systems that will beemployed over the course of the PDO abandon-ment project.

11. The cellar is a pit located below the drilling rig that holdsthe casinghead and casing spools. The depth of thecellar allows access to the master valves of theChristmas tree from ground level.

> Typical P&A site in Oman. Mobile equipment, such as the coiled tubing rig (orange), allows P&A jobsto be completed in about five days (top). Camels visit the sites from time to time (bottom).

> Typical restored desert location in Oman after well abandonment.

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Reduced-density LiteCRETE slurries are apossible solution for interaquifer isolation, whereheavy fluid losses make the use of conventionalslurries impractical.12 The use of dropped prop-pant and pelletized bentonite to isolate wellsections that are below the reach of conventionalcoiled tubing, either because of borehole restric-tions or depth, is also being investigated. The useof resins to seal 1⁄8-in. pressure-transmitting sys-tems and control lines has been successfullyyard-tested and is awaiting a suitable candidatewell for a full field test.13 This procedure will per-mit wells with control lines to be abandonedwithout a rig, rather than using a rig to pull thecompletion. The development of resins also maylead to short, low-volume plugs with quick set-ting times that will replace expensive inflatablepackers and mechanical bridge plugs.

Remediation of Imperfect AbandonmentsIn some cases, the initial well-abandonment pro-cedures fail to seal the reservoir completely orpermanently, and remedial operations must beundertaken.14 This is particularly troublesome ingas wells because gas can pass through micron-scale leaks easily. Even high-quality primarycements sometimes fail to seal microannuli atpipe-cement or cement-formation interfaces.Remediation is essential for protection ofgroundwater resources.

Perhaps the most persistent efforts to sealsmall gas vents in onshore wells are occurring in western Canada.15 PanCanadian EnergyCorporation, for example, is working steadily to improve all aspects of its well-cementingoperations; recently, it has focused on optimiz-ing remedial cementing for permanent wellabandonment. Working with Schlumberger,PanCanadian has sealed vent flows in abandonedgas wells in Alberta using an ultralow-ratesqueeze-cementing technique.16

In a well in the Killam North field, twosqueeze attempts failed to stop gas migration tosurface. Another well in Bantry field also hadbeen squeezed twice without success (aboveleft). Both wells were permanently abandonedafter successful ultralow-rate squeeze opera-tions with advanced cementing technology.

A key ingredient in sealing the gas vents wasthe SqueezeCRETE remedial cementing solution,which uses optimized particle-size distributionsto penetrate and fill minute gaps.17 The extremelylow set-cement permeability and resistance to cracking enhance the performance ofSqueezeCRETE technology.

36 Oilfield Review

Edmonton

Calgary

Bantryfield

KillamNorth field

ALBERTA

> Bantry and Killam North fields, Alberta, Canada.

Transparentplate

Porousplate

Syringe containing the slurry to be injected

Spacer medium,delimitinga “channel”

Filter paper

Injection point Injection point

Standard microcement slurry SqueezeCRETE slurry

> Slurry testing apparatus and results. Cement slurries are injected into one end of a test apparatus(top). The slot between two plates represents a channel or imperfection to be repaired. Standard micro-cement slurry (bottom left) formed a bridge 30 mm [1.2 in.] into the narrow slot. SqueezeCRETE slurry(bottom right) penetrated the entire 225-mm [8.9-in.] length of the 120-µm slot to provide a complete seal.

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Prior to use of new cementing technology in the field, laboratory testing of ordinary microcement and SqueezeCRETE systemsdemonstrated that the ordinary microcement lostwater rapidly and penetrated only a short dis-tance in the narrow slot before forming a bridgein the testing apparatus.18 In contrast, theSqueezeCRETE system evenly penetrated the full225-mm [8.9-in.] length of the apparatus withoutfingering or bridging (previous page, bottom).

The ultralow-rate squeeze process requiredpumping rates as low as 5 L/min [0.03 bbl/min] tolimit friction pressure and place as much slurry inthe gap as possible. The SqueezeCRETE watercontent is much lower than typical Portlandcement slurries, so the solid particles fill the voidsreadily without having to apply moderately highpressure to force water from the slurry.Maintaining a relatively low pressure reduces thepotential for casing or tubing to expand as slurryis pumped through and then to relax as pressureis released. Even minor casing-shape changesduring cementing operations can form a micro-annulus. Pumping continued as the cement set sothat gas could not migrate into the cement.

Subsequent gas-migration tests of the KillamNorth and Bantry field wells confirmed that the gas vents had been sealed and that the wellsmet regulatory requirements for abandonment.Because tens of thousands of gas wells must beabandoned in western Canada alone, and manyothers worldwide, innovative cement-remediationtechnology and techniques may become increas-ingly important for successful permanentabandonment of wells with gas leaks.

Field Abandonment and PlatformDecommissioningOnce individual wellbores have been pluggedand abandoned, the pipelines, facilities and other

structures in the field must be decommissionedand removed. The surface site must be returnedto a pristine condition. These operations can bechallenging onshore; in offshore environments,especially in deep water, P&A and decommis-sioning procedures can become monumentalendeavors that require careful coordination ofseveral specialized crews.

Offshore production-platform decommis-sioning is subject to extensive regulation worldwide.19 Decisions about when and how todecommission platforms involve complicatedissues of environmental protection, safety andcost. Limited availability of heavy-lift equipmentnecessitates significant advance planning toremove platforms. Operations typically arescheduled to avoid rough weather.

Offshore field abandonment and platformdecommissioning encompass abandonment ofeach well in the field. Permeable subsurface formations are isolated permanently from eachother and the surface. Each well is plugged andthe casing is cut at some depth below theseabed, as specified by local regulations.Pipelines also must be decommissioned andremoved. The pipelines may be reused, sold asscrap or treated as waste.

Next, surface facilities and other structuresare decommissioned, which may involve partialor complete removal or toppling in place. Thismay begin with removing the platform deck ortopsides, followed by removing the supportingstructure, known as the jacket, or the entirestructure may be removed in one piece.Depending on the method selected, extensivediving operations may be necessary to cut thestructure into pieces. Finally, the seabed must be remediated.

Typically, platform removal is the mostexpensive part of decommissioning operations

because of the expensive lifting equipment thatmust be mobilized.20 Ongoing advancements inlifting technology should make platform removalsafer, quicker and easier.21 Most offshore plat-forms are customized, so decommissioningoperations are tailored for the specific configura-tion and conditions.

Maureen Platform RefloatingThe Maureen platform, installed in the UK sectorof the North Sea in 1983 by operator PhillipsPetroleum Company United Kingdom Limited andpartners, was designed with recycling in mind.Because of the marginal reserves of the Maureenfield, the platform was built to be refloated,moved and installed to produce oil from anotherfield after depleting the Maureen field (above). In 2001, after eight years of planning and

12. For more on ultralightweight cementing: Al Suwaidi A,Hun C, Bustillos JL, Guillot D, Rondeau J, Vigneaux P,Helou H, Martínez Ramírez JA and Reséndiz Robles JL:“Light as a Feather, Hard as a Rock,” Oilfield Review 13,no. 2 (Summer 2001): 2–15.

13. A yard test involves simulation of an oilfield operationusing actual field equipment rather than laboratoryequipment. For example, yard testing of cementingsystems typically entails mixing and pumping a smallbatch to assess slurry characteristics before mixing and pumping into a wellbore.

14. For more on remedial cementing: Marca C: “RemedialCementing,” in Nelson EB: Well Cementing. Sugar Land,Texas, USA: Schlumberger Dowell (1990): 13-1 – 13-28.

15. For more on well abandonment in Alberta, Canada:http://www.eub.gov.ab.ca/bbs/products/catalog/g1-pubs.htm#guides.

16. For more on the ultralow-rate squeeze technique: Slater HJ, Stiles DA and Chmilowski W: “SuccessfulSealing of Vent Flows with Ultra-Low-Rate CementSqueeze Technique,” paper SPE/IADC 67775, presentedat the SPE/IADC Drilling Conference, Amsterdam, TheNetherlands, February 27-March 1, 2001.

Amsterdam

Aberdeen

Maureen fieldblock 16/29a

N o r t hS e a

0

0 150 300 km

100 200 miles

UK

> Location of Maureen field, North Sea.

17. For more on SqueezeCRETE technology: Boisnault JM,Guillot D, Bourahla A, Tirlia T, Dahl T, Holmes C, Raiturkar AM, Maroy P, Moffett C, Pérez Mejía G,Ramírez Martínez I, Revil P and Roemer R: “ConcreteDevelopments in Cementing Technology,” OilfieldReview 11, no. 1 (Spring 1999): 16–29.Farkas RF, England KW, Roy ML, Dickinson M, Samuel Mand Hart RE: “New Cementing Technology Cures 40-Year-Old Squeeze Problems,” paper SPE 56537, pre-sented at the 1999 SPE Annual Technical Conferenceand Exhibition, Houston, Texas, USA, October 3-6, 1999.

18. The laboratory testing and apparatus are described fullyin Slater et al, reference 16: 2.

19. For more on offshore platform decommissioning:Anthony NR, Ronalds BF and Fakas E: “PlatformDecommissioning Trends,” paper SPE 64446, presentedat the SPE Asia Pacific Oil and Gas Conference andExhibition, Brisbane, Australia, October 16-18, 2000.

Byrd RC and Velazquez ER: “Partial Removal Best Optionfor Large US Pacific Deepwater Platforms,” Offshore 61,no. 10 (October 2001): 84, 86, 160.Griffin WS: “Evolution of the Global DecommissioningRegulatory Regime,” SPE Production & Facilities 14, no. 2 (May 1999): 83–87.

20. Chapman LR, Coats A and Lajaunie C: “Containing FieldAbandonment Costs in the Gulf of Mexico,” Offshore 55,no. 11 (November 1995): 54-55, 101.

21. “Customized Barges, Hinged Arms Form Single-LiftRemoval Concept,” Offshore 61, no. 10 (October 2001): 92.“Heavy Deck Removal Vessel Under Review for Frigg,Ekofisk,” Offshore 61, no. 10 (October 2001): 88, 90. Bradbury J: “Find the Right Solution for Cessation,”Hart’s E&P 74, no. 11 (November 2001): 45.

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preparation, the platform was refloated and relo-cated successfully to Norway, where it awaitsreuse or dismantling.22

Unique among North Sea production platforms,the Maureen platform rested on a steel-gravitystructure rather than the more common concrete-gravity structure; its three storage tanks servedas legs for the platform.23 The storage tanksdouble as ballast tanks when the platform istowed. Its facilities integrate all equipment nec-essary to drill, produce and store oil while alsoaccommodating personnel. The main platformcomponents—the steel-gravity structure and thedeck—were constructed onshore and matednear the shore before being towed to the field.

An articulated loading column (ALC) wasinstalled to move oil into tankers because apipeline to shore was uneconomic.

The platform supported 13 production wells and seven water-injection wells in theMaureen field, which produced 223 million bbl[3.5 million m3] of oil between 1983 and 1999.24

In addition, a single subsea well in the Moirafield was tied back to the Maureen platform. Asproduction declined, Phillips began to studydecommissioning options.

Platform decommissioning typically involvesmultiple operations.25 Before any operationsbegin, regulatory agencies require proof that theabandonment plans will meet environmental andsafety standards. The first step is to cease produc-tion and abandon each well. Next, the platform

is decommissioned, after which it may bedismantled and removed. The size, water depthand structural condition of the platform stronglyinfluence dismantling and removal plans. Themost common options for offshore platformdecommissioning include total or partial removal,toppling in place or reuse. As in most offshoreoperations, equipment availability and weatherare critical factors. Following decommissioningoperations, the site must be surveyed to ensurenavigational safety and environmental protection.

Total removal of offshore facilities leaves theseabed free of debris, which is desirable for fish-ing but tends to be expensive. Partial removalreduces cost, but necessitates careful surveyingof the remaining structure to ensure navigational

38 Oilfield Review

> Refloated Maureen platform.

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safety. This option currently is open only tostructures greater than 10,000 metric tonnes[11,000 tons] in the OSPAR Commission area.26

Toppling a platform in place, which will be dis-cussed later, is considerably less expensive thanremoval, but is illegal in North Sea and northeastAtlantic Ocean areas under OSPAR regulations.The operation would have to ensure that the plat-form is devoid of environmental hazards and fallsas intended. Some well-maintained platformsare left in place or moved to another location forpossible reuse.

Studies of the Maureen platform revealedmore than 60 possible removal options. Of these,six were considered achievable, and each of thoserequired refloating the platform before subsequentdisposal operations. Computer simulations ofrefloating operations, including issues such asmetal fatigue, confirmed that the operations couldbe performed safely.27 The studies confirmed thatreusing the platform was feasible, even after morethan 18 years of production, because of the excel-lent maintenance of the facilities.28

All the wells were abandoned and the con-ductors and risers cut before refloating theplatform. Seawater in the storage tanks waspumped out to provide buoyancy. Refloating oper-ations began with water injection below theplatform to lift the structure from the seabed.Aker Offshore Partner performed the refloatingoperation in about 60 hours without incident(previous page). Operational arrangementsincluded preparing extensive contingencyplans—particularly to provide additional buoy-ancy if necessary—and constant monitoring ofascent rate and tank pressures to achieve thedesired draft for towing.

Despite marketing the platform worldwide forseveral years, Phillips has been unable to identifya full reuse option for Maureen that satisfies itsfive criteria—regulatory, technical, commercial,environmental and scheduling. Partial reuseoptions also have been evaluated, with the mostviable being a proposal from Aker to turn thebases of the platform and parts of the ALC into adeepwater quay at Stord, Norway.

Toppling an Offshore PlatformIn some regions, platforms may be abandoned inplace to form an “artificial reef.” Toppling a plat-form in place requires significant preparatorywork to ensure safety and care for the environ-ment. In the Main Pass area, Block 254, in theGulf of Mexico, the production platform wastoppled in place in August 2000 (above right). The

LOUISIANAMain Pass Block 254

MISSISSIPPI ALABAMA

FLORIDA

G u l fo f

Me

xi c

o

> Location of Main Pass 254 platform, Gulf of Mexico.

22. Bradbury J: “Majestic Maureen Makes It,” Hart’s E&P74, no. 8 (August 2001): 75, 76, 78, 80.For more on the Maureen field:http://www.phillips66.com/maureen/.

23. Tilling G: “Refloating Maureen Oil Platform (110,000Tonnes) for Re-Use in Waters Away From the NorthSea,” paper SPE 29938, presented at the SPEInternational Meeting on Petroleum Engineering, Beijing, China, November 14-17, 1995.

24. The production proved to be substantially higher thanthat predicted at the time of discovery. See Tilling, refer-ence 23: 6.

25. For more on facility removal: Della Greca A: “OffshoreFacility Removal: How to Save Cost and MarineResources,” paper SPE 36936, presented at the SPEEuropean Petroleum Conference, Milan, Italy, October22-24, 1996.

26. The OSPAR Commission for the Protection of the MarineEnvironment of the North-East Atlantic, formerly the Oslo and Paris Commissions, works to prevent andeliminate marine pollution. For more information:http://www.ospar.org/.

27. Denise J-P and Tilling GM: “Interactive Hydrodynamic/Structural Analysis for Refloating a Very Large NorthSea Structure: Maureen Alpha Platform,” paper SPE36937, presented at the SPE European PetroleumConference, Milan, Italy, October 22-24, 1996.

28. Tilling, reference 23: 2.

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40 Oilfield Review

> Toppling a platform in place. These time-lapse photographs show the controlled sinking of the platform,which took 37 seconds from first motion to complete submergence.

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operations, which took several months of plan-ning and coordination by operator Unocal,involved contractors for marine operations, engi-neering and diving services.

From the start, Unocal worked with severalagencies to be sure that all operations wouldcomply with existing regulations. The companyalso decided at an early stage not to use explo-sives to topple the platform because of theabundant marine life in the area. Next, the teamhad to prepare for all aspects of decommission-ing and toppling operations.

The platform, which sat in 280 ft [85 m] ofwater, was installed in 1975. Six wells producedto tanks mounted on the decks, so there was nopipeline to abandon. However, the companysought to topple the deck and jacket together, anoperation not attempted previously in the area.

The abandonment operations began withdecommissioning and removal of productionequipment and piping, including recovery andproper disposal of all fluids. Next, each of theproduction wells was abandoned according torules of the US Minerals Management Service(MMS). During well abandonment, divers cuteach well and leg of the platform. Cutting theabandoned wells eliminated the possibility ofdamaging cement plugs below the mudline bytransmitting stresses along the casing duringtoppling. Cuts in the legs created weak points tofacilitate toppling. Before completely severingthe pilings, the crews installed rigging to providethe force necessary to topple the platform.Personnel stayed on the platform throughout theoperations to ensure safety once navigationalaids were removed. Divers worked for 205 hours,completing their work in 26 days.29

At that point, the platform was ready fortoppling with horizontal force provided by ananchor-handling vessel equipped with two 500-ton [181-tonne] winches. The initial attemptsfailed because of problems with the rigging sys-tem, so the crew deballasted subsea drill-watertanks to reduce the horizontal toppling force

required. The third attempt succeeded in topplingthe platform, which submerged completelywithin 37 seconds (previous page). Divers sur-veyed and videotaped the toppled platform toconfirm its location and a buoy was installed tomark its location (below left).

There has been at least one case in whichwells and a platform were abandoned after acargo ship collided with a platform. In an acci-dent in the Gulf of Suez in 1989, the wellheadsand the platform were damaged to the degreethat they were declared a total loss.30 The opera-tor’s key objectives were to control the wells,minimize pollution and decommission the dam-aged structures safely. Scaffolding erected overthe mangled platform allowed workers to safelyaccess valves, so oil spills were halted withinone week of the accident. After the top deck ofthe platform was removed, a drilling rig wasmobilized to support well-abandonment opera-tions. Finally, two and a half years later, the restof the platform was decommissioned. Thisexceptional event underscores the need for con-tingency planning in case wells or platformsmust be abandoned prematurely.

Abandonments AheadThe conceptual “life of a well” clearly extendsbeyond the production phase. Ideally, modernwell-abandonment procedures isolate subsur-face formations forever. Oil and gas producersrecognize the importance of true permanentabandonment, which begins with well design,continues through primary cementing and con-cludes with fit-for-purpose well-abandonmentprocedures. Creating a common budget for eachof these operations at the beginning of a projecthelps ensure that they are carried out properly.

Field abandonment, which typically involvesmore than one well, requires close coordinationof many different operations to ensure subsur-face isolation of each well, removal of surfaceequipment and facilities, and restoration of thesurface to a pristine condition. With new tech-nologies for primary and remedial cementing,perforating, and cement evaluation in addition tocoiled tubing and slickline intervention,Schlumberger and E&P companies are ready totackle the many well- and field-abandonmentprojects ahead. —GMG

100 ft

115 1⁄2 ft 20 ft 129 1⁄3 ftToppled section Main structure

Buoy

> Toppled platform. This diagram, prepared after a post-topple survey, shows the stable resting posi-tions of the main structure and toppled section more than 100 feet [30 m] below the sea.

29. For more on the Main Pass 254 abandonment: WhitneyCD: “Toppled Platform In-Place Creates Reef in US Gulf,”Oil & Gas Journal 98, no. 45 (November 6, 2000): 53, 54,56, 58, 59.

30. El Laithy WF and Ghzaly SM: “Sidki Well Abandonmentand Platform Removal Case History in the Gulf of Suez,”paper SPE 46589, presented at the SPE InternationalConference on Health, Safety and Environment in Oil andGas Exploration and Production, Caracas, Venezuela,June 7-10, 1998.

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