Ensuring Zonal Isolation Beyond the Life of the Well

18 Oilfield Review

Ensuring Zonal Isolation Beyond the Life of the Well

Mario BellabarbaHélène Bulte-LoyerBenoit FroelichSylvaine Le Roy-Delage Robert van KuijkSmaine ZerougClamart, France

Dominique GuillotCambridge, Massachusetts, USA

Nevio MoroniEni S.p.AMilan, Italy

Slavo PastorTyumen, Russia

Augusto ZanchiStogitCrema, Italy

For help in preparation of this article, thanks to MatthewAndruchow, Clamart, France; and Martin Isaacs and Ali Mazen,Sugar Land, Texas, USA.CemCRETE, CemSTONE, CemSTRESS, FlexSTONE, FUTUR,Isolation Scanner, LiteCRETE, PS Platform, SCMT (SlimCement Mapping Tool), SlimXtreme, SlurryDesigner and USI(UltraSonic Imager) are marks of Schlumberger.Fann is a trademark of Fann Instrument Company.

When zonal isolation fails, production or injection efficiency is severely degraded.

In some cases, the well is lost entirely. No less significantly, such failures present

environmental and safety implications since hydrocarbons or previously injected

fluids may flow to the surface or into nearby aquifers. Therefore, it is not sufficient

to obtain good zonal isolation; the resulting seal must last many years beyond the life

of the well.

Placed between casing and wellbore, a cementsheath is expected to provide zonal isolationthroughout the life of a well. But its ability to do sodepends on the proper placement of the cement,the mechanical behavior of the cement and thestress conditions in the wellbore. Even if theslurry was properly placed, changes in downholeconditions can induce sufficient stresses todestroy the integrity of the cement sheath. Overtime, stresses are imposed on the cement bypressure integrity tests, increased mud weight,casing perforation, stimulation, gas production ora large increase in wellbore temperature.1 Any ofthese events can damage the sheath.

Often, damage to the cement sheath resultingfrom these forces manifests as microannuli sosmall as to be nearly impossible to pinpoint andeven harder to repair. Even the smallest micro -annulus can be large enough to provide apathway for fluid migration. Remedial work for such cement failures has been estimated tocost more than $50 million annually in theUnited States.2

Despite changes in downhole conditions overtime—both predictable and unexpected—obtaining long-term zonal isolation is not onlypossible, in today’s fiscally and environmentallysensitive oil industry, it is mandatory. To do sorequires the right technology, processes and

evaluation because drilling a well disturbs long-settled and precariously balanced stresses.Drillers must compensate for this disturbance, tothe degree that it is possible, by using drillingfluids to exert hydrostatic pressure on theformation. However, this pressure may beinsufficient to maintain equilibrium with the far-field stresses, and the formation surrounding theremoved volume will deform.3

Draining fluids from a formation during pro -duction may also change formation pore pressureand related stresses. Within the rock, the result -ing increased loading leads to varying degrees ofdeformation or failure that can cause cement tobreak or debond at the formation interface.Production-induced stresses can also result inreservoir compaction, which may lead to tubularshearing and buckling of completion components.4

An obvious key to long-term zonal isolation isobtaining a good seal in the first place. Todetermine whether that objective has beenachieved, standard sonic and ultrasonic loggingtools have been developed and improved over timein an effort to quantify the cement-to-casing bond.Recent versions of ultrasonic tools can now detectthe presence of channels within the cementsheath through which hydrocarbons can flow.

In this article, we will highlight the mostrecent development of these ultrasonic tools that

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Spring 2008 19

can also indicate casing eccentricity, evaluatethe material in the casing annulus anddistinguish between new lightweight cementsand drilling fluids of similar acoustic impedance.Case histories will also demonstrate the newultrasonic logging tool’s ability to offer improvedcharacterizations of cement-to-casing bonds andannular fill.

This article also examines industry efforts toachieve long-term zonal isolation using speciallyformulated cements as annular sealing material.Of primary interest is a new, long-life, self-healing cement. Developed in response to whathas been called the weak link in zonalisolation—the inability to correct defects afterthe cement has set—the new sealant swells inthe presence of hydrocarbons to close cracks andmicroannuli that may form in cement sheaths asa result of changing downhole conditions. Wealso present laboratory tests and case historiesthat demonstrate the success of this develop -ment effort.

Preparing the Ground Obtaining a good cement sheath demandsadherence to well-established operating practicesof hole preparation, casing centralization andcasing rotation and reciprocation.5 Successfulzonal isolation first requires removingcontaminants—principally drilling mud—fromthe wellbore. Since formation pressure must becontained during this hole-cleaning operation, thedrilling fluids being removed must be displacedwith a fluid of higher density—a spacer—pumpedbehind the mud and ahead of the cement.

1. Le Roy-Delage S, Baumgarte C, Thiercelin M andVidick B: “New Cement Systems for Durable ZonalIsolation,” paper IADC/SPE 59132, presented at theIADC/SPE Drilling Conference, New Orleans,February 23–25, 2000.

2. Cavanagh P, Johnson CR, Le Roy-Delage S, DeBruijn G,Cooper I, Guillot D, Bulte H and Dargaud B: “Self-HealingCement—Novel Technology to Achieve Leak-Free Wells,”paper IADC/SPE 105781, presented at the IADC/SPEDrilling Conference, Amsterdam, February 20–22, 2007.

3. Gray KE, Podnos E and Becker E: “Finite Element Studiesof Near-Wellbore Region During Cementing Operations:Part I,” paper SPE 106998, presented at the SPEProduction and Operations Symposium, Oklahoma City,Oklahoma, USA, March 31–April 3, 2007.

4. For more on formation stresses: Cook J, Frederiksen RA,Hasbo K, Green S, Judzis A, Martin JW, Suarez-Rivera R,Herwanger J, Hooyman P, Lee D, Noeth S, Sayers C,Koutsabeloulis N, Marsden R, Stage MG and Tan CP:“Rocks Matter: Ground Truth in Geomechanics,” Oilfield Review 19, no. 3 (Autumn 2007): 36–55.For more on reservoir compaction: Doornhof D,Kristiansen TG, Nagel NB, Pattillo PD and Sayers C:“Compaction and Subsidence,” Oilfield Review 18, no. 3(Autumn 2006): 50–68.

5. Casing rotation and reciprocation refer to any movementof the casing to help remove drilling fluids from theannulus while cement is being pumped downhole.

Oil and Gas

Surface SedimentsSurface Casing

Shale

Shale

Sandstone

Shale

Productive Formation

Fresh Water

Salty Water and Sandstone

Intermediate Casing

Cement Sheath

Limestone

Dolomite

Limestone

Shale

Hole

Production Casing

Annulus

Packer

Productive Formation

Production Tubing

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The spacer is designed to keep the drillingfluids and cement apart while the cement isbeing pumped through the casing and into theannulus, and is generally formulated with aviscosity close to or greater than that of thedrilling fluid. Besides maintaining well control,the spacer also serves as a chemical wash toclean leftover drilling mud from the casing-casing and casing-wellbore annuli. If the spacerleaves drilling fluids behind, or if it allows themto mix with the cement, then good bondingbetween cement and formation or casing isunlikely. Since these contaminants remain in a

liquid state, they are liable to form channels ofcommunication between zones along theborehole or casing (left).6

Efficient borehole cleaning is not the onlyrequirement for good zonal isolation. A poorlydrilled hole, for example, may have washed-outareas that are difficult to clean and that maycontain pockets of gelled drilling fluids. Thesegelled fluids can be pulled into and contaminatethe passing cement slurry. Poor casing centrali -zation can contribute to a poorly placed cementsince it can be difficult to remove fluids from theside that is closest to the borehole wall ineccentrically positioned casing. Since the 1940s,research and development efforts have gone intodeveloping recommended standards forcentralizer placement along the casing string tobe cemented. Those practices are now beingtested by new cement evaluation tools thatprovide casing eccentricity measurements. Thesemeasurements can be compared with traditionalcalculated standoff values that rely on unlikely assumptions such as a perfectly in-gauge wellbore.7

To avoid leaving behind a heavy filtercakethat is impossible to remove, the properties ofthe drilling fluid must be altered to match thosemore suited to hole cleaning. For best results,the mud density, yield stress, plastic viscosity andgel strength should all be reduced.

Mud rheology may be reduced by addingwater or dispersants to the system andcirculating the fluid until its properties reach thedesired range. This requires circulation of at

least one borehole volume and, when possible,should be performed before removing thedrillpipe to prevent mud from gelling while it isstatic during pipe-pulling operations.

Mechanical steps are also recommended tohelp remove contaminating fluids prior tocementing. Moving the casing frees mud trappedin narrow sections of the annulus. Attachingscratchers, scrapers and wipers to the casingalso helps remove gelled and dehydrated mud asthe casing is rotated and reciprocated.

The optimum wellbore for cementingpurposes, then, is one with controlled subsurfacepressures and minimum doglegs, is in-gauge,stabilized and free of drill cuttings, and has athin dynamic filtercake across permeable zones.8

Sound TechnologiesFollowing industry best practices does notguarantee that the resulting cement sheath willbe up to the tasks—casing support, corrosionprotection and, most critically, zonal isolation—for which it was designed. Determiningcon tami nation, continuity and bonding quality ofthe cement behind the pipe is thereforetantamount to protecting the asset and theenvironment by recognizing the need forremedial operations before the well is brought on production.

Finding the top of the cement behind pipewhere expected is a reasonable indication thatthe volumes displaced match those calculatedand that the annulus is filled with the correctamount of cement. Since cement hydration is an

20 Oilfield Review

> Failed isolation. Problems that occur whilerunning casing and cementing can createconditions that may lead to loss of zonalisolation. Among the most common of these iscasing eccentricity from poor centralizerpositioning. Cement, like all fluids, seeks the pathof least resistance and so flows to the more openside of the casing, creating a narrow spacebetween the casing and the formation that canbecome fluid-migration paths (A). Inadequateslurry density can also allow formation gas (red)to enter the wellbore (B) and create weak pointsor gaps within the cement that fail when stressesare imposed on the cement sheath by changingdownhole temperatures and pressures. Thegeometry of washed-out areas (C) often resultsin inefficient flow rates during wellbore cleaningoperations that leave drilling fluids behind. Thesecontaminants also lead to weak spots in thecement sheath and, if large or numerous, cancreate channels through which formation fluidsmay flow.

Centralizer

Good cementingwhere casingis centered

Casing

A

B

C

> Cracks and microannuli. Over time, as downhole stress conditions change, primarily in response totemperature and pressure changes, even a successful cementing operation can fail. Large increasesin wellbore pressure or temperature and tectonic stresses can crack the sheath and even reduce it torubble. The interplay of tangential and radial stress changes may be caused by displacement ofcasing as a result of cement bulk shrinkage or temperature or pressure decreases (left). These stresschanges can cause the cement to fail in tension or compression, or to debond from the casing orformation, creating microannuli (right).

Tangentialstress Tensile

failure

Compressionalfailure

Microannuli

Radialstress


Spring 2008 21

exothermic reaction, this can be done using atemperature survey. This method, however,reveals little else about the results of thecementing operation.

Hydraulic testing—a common test of zonalisolation—applies internal pressure along theentire casing string. But pressure can expand thecasing, causing the cement sheath to experiencetensile failure. This may lead to radial cracks andlocal debonding of the cement and casing inareas where the cracks are near the casing wall(previous page, bottom).

Because of the limitations of the othertechniques, acoustic logging has become theindustry’s tool of choice for detecting cementbehind casing and assessing the quality of thebonds between casing, cement and formation.Acoustic logs help indicate nonintrusively thedepth interval at which cement has been placedaround the casing, measure acoustic impedanceof the cement bonded to the casing, and quantifythe percentage of pipe circumference bonded to the cement.

These characteristics inform the operator offaults in the cement sheath that may requireremedial measures—commonly a squeezeoperation in which cement is pushed throughperforations into the annulus to fill gaps alonginterfaces at the casing, formation or within theannular material itself.

Cement bond logs (CBLs) and variable densitylogs (VDLs) are acquired using a sonic logging tool(right). Standard CBL tools, which comprise thosethat measure signal amplitude or attenuation, havea common theory of measurement andinterpretation. The principle behind them is tomeasure the amplitude of a sonic signal producedby a transmitter emitting a 10- to 20-kHz acousticwave after it has traveled through a section of thecasing as an extensional mode.9

Measurements are displayed on the CBL login millivolts (mV) or decibel (dB) attenuation, orboth. Increased attenuation indicates betterquality bonding of the cement to the outer casing

6. For more on hole cleaning: Abbas R, Cunningham E,Munk T, Bjelland B, Chukwueke V, Ferri A, Garrison G,Hollies D, Labat C and Moussa O: “Solutions for Long-Term Zonal Isolation,” Oilfield Review 14, no. 3(Autumn 2002): 16–29.

7. Guillot DJ, Froelich B, Caceres E and Verbakel R: “Are Current Casing Centralization Calculations ReallyConservative?” paper IADC/SPE 112725, presented at theIADC/SPE Drilling Conference, Orlando, Florida, USA,March 4–6, 2008.

8. Nelson EB and Guillot D: Well Cementing, 2nd ed. Sugar Land, Texas: Schlumberger, 2006.

9. Morris C, Sabbagh L, Wydrinski R, Hupp J, van Kuijk Rand Froelich B: “Application of Enhanced UltrasonicMeasurements for Cement and Casing Evaluation,”paper SPE/IADC 105648, presented at the SPE/IADCDrilling Conference, Amsterdam, February 20–22, 2007.

> Traditional sonic cement bond log tools. The slim array sonic tool (SSLT) isa digital sonic tool that provides conventional openhole sonic measurements,standard cement bond log (CBL) amplitude and a variable density log (VDL).The SlimXtreme slimhole well logging platform provides the samemeasurements as the SSLT for evaluation of the cement bond quality in high-pressure and high-temperature environments. The digital sonic logging tool(DSLT) uses the sonic logging sonde to measure the cement bond amplitudeand provide a VDL display for evaluation of the cement bond quality. Thehostile environment sonic logging tool (HSLT) provides the samemeasurements of the cement bond amplitude and the same variable densitydisplay as the SSLT in standard wellbore sizes. The SCMT Slim CementMapping Tool is a through-tubing cement evaluation tool combinable withthe PS Platform new-generation production services tool. It is sized so that itmay be used to evaluate the cement behind casing in workover operationswithout having to first pull the tubing.

HSLT

SCMTtool

DSLT

SlimXtremetool

SSLT


wall. In simple cases, the interpreted logresponse can provide good information aboutcement quality (above).

About 25 years ago, engineers developedcased-hole ultrasonic imaging tools that used ahigh-frequency pulse-echo technique (next page,top left).10 More recent versions of these

ultrasonic imaging tools, such as theSchlumberger USI UltraSonic Imager, use arotating transducer that emits a broadbandultrasonic wave perpendicular to the casing wallwith a frequency that can be adjusted between250 and 700 kHz (next page, right). The effect is toexcite a casing resonance mode at a frequency

dependent on casing thickness and with anamplitude decay dependent on the acousticimpedances of the media on either side of thecasing. The cement acoustic impedance is thenclassified as gas, liquid or cement based on thethresholds set for acoustic impedanceboundaries between these materials.

Strengths and WeaknessesThese sonic and ultrasonic logging tools havehad shortcomings. The traditional sonic CBL-VDLtool does not provide radial or azimuthal information to differentiate among channels,contaminated cement, microannuli and tooleccentricity; this makes confident data interpre-tation difficult.11

Ultrasonic imaging tools that are based on thepulse-echo technique are limited when logging inhighly attenuative muds because of low signal-to-noise ratios. Their radial probing power is limitedto the cement region adjacent to the casing.12

Because of the high acoustic impedancecontrast between steel and the surroundingmaterial—mud inside the casing and cementoutside—the signal dies away so rapidly thatechoes arising from acoustic contrasts outside ofthe casing are typically undetectable unless theyare very close to the casing and stronglyreflective surfaces.

Additionally, the pulse-echo technique hasdifficulty differentiating between a drilling fluidand a lightweight or mud-contaminated cement ofsimilar acoustic impedance. Even under favorableconditions, the acoustic impedance contrastbetween drilling fluid and cement typically mustbe larger than 0.5 Mrayl for the pulse-echotechnique to distinguish between them.

To overcome tool limitations, and depending onwell conditions, an utrasonic and standard CBL-VDL tool may be run together. But even then,experience from various wells around the worldhas shown that an unambiguous conclusion aboutthe quality of the cement bond may be elusive. Thisis particularly true in the case of lightweight andcontaminated cements.

This issue has become increasingly urgentwith the proliferation of lightweight cements indeepwater wells and in sealing across formationswith low pore pressure. To deal with thisproblem, Schlumberger has developed ameasurement technique that is the basis of theIsolation Scanner cement evaluation service. Thetool combines the classic pulse-echo techniquewith an ultrasonic imaging technique thatprovides more effective imaging of the annularfill including reflection echoes at thecement/formation interface.

22 Oilfield Review

10. Sheives TC, Tello LN, Maki VE Jr, Standley TE andBlankinship TJ: “A Comparison of New UltrasonicCement and Casing Evaluation Logs with StandardCement Bond Logs,” paper SPE 15436, presented at theSPE Annual Technical Conference and Exhibition,New Orleans, October 5–8, 1986.

11. Coelho de Souza Padilha ST and Gomes da Silva Araujo R:“New Approach on Cement Evaluation for Oil and GasReservoirs Using Ultrasonic Images,” paper SPE 38981,

presented at the SPE Latin American and CaribbeanPetroleum Engineering Conference and Exhibition, Rio de Janeiro, August 30–September 3, 1997.

12. Van Kuijk R, Zeroug S, Froelich B, Allouche M, Bose S,Miller D, le Calvez J-L, Schoepf V and Pagnin A: “A Novel Ultrasonic Cased-Hole Imager for EnhancedCement Evaluation,” paper IPTC 10546, presented at theInternational Petroleum Technology Conference, Doha,Qatar, November 21–23, 2005.

> Sonic logging tools. Cement bond logs (CBLs) and variable density logs(VDLs) are acquired using a sonic logging tool with a monopole transducerand two monopole receivers placed at 3 and 5 feet [0.9 and 1.5 m] from thetransmitter (left). The monopole sonic transmitter sends an omnidirectionalpulse at relatively low frequency (10 to 20 kHz) that induces a longitudinalvibration of the casing. The recorded amplitude of the first positive peak(E1) of the sonic waveform received at 3 ft and the full waveform receivedat 5 ft represent the average values over the circumference of the casing(top right). In well-cemented pipe, the sonic signal in the casing is attenuated,and the CBL E1 amplitude is small. In free pipe, the casing arrivals are strong.The transit time is the time it takes the wave to travel from transmitter toreceiver. It is used for quality control of the tool centralization and to setparameters for material detection. In partially cemented pipe (bottom right),casing, formation and mud arrivals may be present and can occur in thepresence of a microannulus at the casing/cement interface. The VDL (bottominset) provides visualization of arrivals that propagate in the casing asextensional waves and in the formation as refracted waves.

Transmitter

3-ft receivergives CBL

5-ft receivergives VDL

Casing

Bonded cement sheath

Sonic pulse path

Ampl

itude

, mV

Detection level

Early Signal Arrival

Partially Cemented Pipe

E 1

Transit time

Transmitterfiring Time

Ampl

itude

, mV

Transmitterfiring

Casingarrival

Formationarrival

Mud

Time

E1

arrival

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Sounds of SuccessThe Isolation Scanner tool includes a rotatingsubassembly supporting four transducers (left).A normally aligned transducer for generatingand detecting the pulse echo is positioned onone side of the tool. The other three transducersare on the opposite side of the tool and arealigned obliquely. One of these transducerstransmits a high-frequency pulsed beam of about250 kHz to excite a flexural mode in the casing.

> Cased-hole ultrasonic tool basics. An ultrasonic tool’s transducer sends aslightly divergent beam—an acoustic wave generated by a transducer whenelectrical power is applied to it—toward the casing to excite the casing intoits thickness resonance mode. The USI UltraSonic Imager tool scans thecasing at 7½ revolutions per second to render an azimuthal resolution of 5 or10 degrees. This yields 36 or 72 separate waveforms at each depth. These areprocessed to yield the casing thickness, internal radius and inner wallsmoothness—from the initial echo—as well as an azimuthal image of thecement acoustic impedance—from the signal resonance decay (top). Theacoustic impedance of the cement (essentially the quality of the cementsheath) can be derived from the resonance decay (bottom). A good casing-cement bond results in immediate resonance decay, while free pipe rings(generates echoes) for an extended period.

Echo amplitude

Internal casingcondition

Transit time

Internal radius

Resonance frequency

Casingthickness

Resonance decay

Cement acousticimpedance

Time

Resonancedecay

Casing resonance

Transducer Metal plate

Acousticbeam

Cement

FormationCasing

Tranducer Mud Casing Cement Formation

Rotatio

n

> USI tool. The Schlumberger USI tool improvedon earlier versions of the ultrasonic imaging toolby using a single rotating transducer mounted onthe bottom of the tool (A).

A

> Isolation Scanner subassembly. The Isolation Scanner sub combines thetraditional pulse-echo technique using an acoustic transmitter and receivernormal to the casing (A), while adding flexural-wave imaging with onetransmitter (B) and two receivers (C) aligned obliquely. This configurationexcites the casing flexural mode (D). The subassembly, mounted on the sameplatform as the USI tool and with updated signal generation and acquisitionsoftware, is the basis for Isolation Scanner tool.

C

D

B

A


As it propogates, this mode radiates acousticenergy into the annulus; this energy reflects atinterfaces that present an acoustic contrast,such as the cement/formation interface, andpropagates back through the casing predomi-nantly as a flexural wave to reradiate energy intothe casing fluid. The two receiving transducersare placed to allow optimal acquisition of thesesignals (above left).

This new technique is termed pitch-catch.Processing of the resulting signals providesinformation about the nature and acousticvelocity of the material filling the annulus, theposition of the casing in the hole and thegeometrical shape of the hole.

The first aim of processing Isolation Scannerlogs is to obtain a robust interpretation of thematerial immediately behind the casing. Theinputs to this processing sequence are the

24 Oilfield Review

0 0 180 0

0 0 1 2 3 410.5–5,000 5,0000

0180 0 180 0 18050 50100 100

FlexuralAttenuation,

dB/cmImpedance,

MraylChannel

mapChannelwidth, %

X,440

X,450

X,460

X,470

X,480

X,510

X,540

Measureddepth, m

CBL Sonic VDL

SLGmap

X,490

X,500

X,520

X,530

L

GS

> Isolation Scanner and CBL-VDL measurements. In 2003, the IsolationScanner prototype was tested in an In-Salah Gas vertical well. The 95⁄8-in.casing was cemented in a 12¼-in. hole using the low-density (low-impedance) LiteCRETE slurry system. The CBL (Track 1) and VDL (Track 2)show a nearly free-pipe response with strong casing arrivals in the VDL. Thepulse-echo impedance map (Track 5) shows fluid with patches of solid.Obtaining an adequate interpretation from both measurements was madedifficult by the low-impedance LiteCRETE cement. The flexural-waveattenuation map (Track 4), on the other hand, provides a correct diagnosis ofthe solid behind casing. It also reveals the existence of a fluid-filled channelbetween X,465 and X,485 m. The solid-liquid-gas (SLG) map (Track 3) supportsand simplifies this information. The azimuthal and axial extent of the channelis reported automatically in Tracks 6 and 7.

> Geometrical interpretation of USI measurementsand Isolation Scanner flexural-wave imaging.Shown here is a geometrical ray interpretation ofthe propagation of the signal for the pulse-echo(top, red) and from the transmitter (T) to areceiver (R) for the flexural-wave pitch-catchtechniques (top, blue). A typical waveform fromthe latter technique comprises an early echo,called casing arrival, and third-interface echoes(TIEs) (bottom, blue). The attenuation of thecasing arrival amplitude is used to complementthe pulse-echo measurement (bottom, red) indistinguishing unambiguously between fluid andsolid behind the casing. The properties of the TIEprovide an enhanced characterization of thecased-hole environment, indicating the acousticproperties of the material filling the annulus, theposition of the casing within the hole and thegeometrical shape of the hole.

Time, µs

80 90 100 110 120 130 140 150 160 170

Isolation Scanner tool flexural-wave imagingUSI tool pulse-echo imaging

TIECasing arrival

Annulus

Formation

Casing

R

T

USItool


Spring 2008 25

cement impedance, as delivered by the pulse-echo measurement, and the flexural-waveattenuation computed from the amplitude of thecasing arrivals on the obliquely aligned receivers.

These two inputs are independent measure -ments linked through an invertible relation tothe properties of the fluids inside both thecasing and annular fill. The inputs are firstcombined to eliminate the effect of the insidefluid, thus obviating the need for specifichardware for fluid-property measurementsrequired by the USI tool.

The output of the Isolation Scanner service isa solid-liquid-gas (SLG) map displaying the mostlikely material state behind the casing. The stateis obtained for each azimuth by locating the twomeasurements, corrected for the effects due tothe inside fluid, on a crossplot of attentuationand acoustic impedance, giving the areaencompassed by each state (above left). Themeasurement plane can be mapped out indifferent regions with three colors correspondingto the different states (previous page, top right).

The white-colored areas in the SLG mapcorrespond to locations with nonsolvableinconsistencies between measurements, such asmight appear at the casing collars.

In addition to evaluating the material behindthe casing, a second objective of processing is toextract relevant information from the annulus-formation reflection echo or echoes and furthercharacterize the annulus between the casing and formation.

First, the software detects the echoes on thewaveform envelope following the casing arrivaland then measures their time of arrival andamplitude. From the time differences betweenthe reflection echoes and the casing arrival—provided enough echo azimuthal presence isavailable in the data—it is straightforward todetermine how well the casing is centered withinthe borehole. This is presented as a percentagein which 100% represents perfect centering, and0% is casing in contact with the formation wall.Additionally, if the borehole diameter is known,the time-difference processing can be furtherconverted into a material-wave velocity and

displayed as an annulus-velocity map or cementazimuthal thickness.

A polar plot of the flexural waveforms fromthe variable density log provides a picture of thegeometry of the casing within the borehole(above right).

New Cements Even the most sophisticated logs present only asnapshot of the cement condition and its abilityto provide zonal isolation. Over the long life spanof a well, changing downhole conditions remainthe enemy of cement sheaths and may causeeven well-placed sheaths to fail over time.

Throughout the industry’s long history ofusing cement in well construction, addressingthese failures first focused on placement of theslurry and later on its chemical makeup. Duringthe 1980s, engineers and scientists began toconsider ways to deliver specific set-cementproperties with the aim of increasing the

Flex

ural

-wav

e at

tenu

atio

n, d

B/cm

Pulse-echo impedance, Mrayl

1.6

1.4

1.8

1.0

1.2

0.8

0.6

0.4

0.2

0

–0.2–0.2 0 2 4 6 8 10

GasLiquidSolid

>Waveform polar plot across the fluid-filledchannel at a depth of X,477 m on the IsolationScanner log (previous page, top right). Thecurvature of the annulus-formation echo revealsthat the casing is slightly eccentered in theborehole and that the channel is located on thenarrow side (direction of blue arrows). Theabsence of a third-interface echo across thecement azimuth may be due to a low acousticcontrast between the cement and formation.

150

100

50

0

–50

–100

–150–50–100–150 0 50 100 150

Time, µs

Tim

e, µ

s

Casing

Channel

Formation reflectionwithin channel

> Solid-liquid-gas mapping of the measurement plane for a Class G cement.Once the expected impedance values are defined for the cement, liquid andgas through a laboratory-measured database and the material selection isconverted into acoustic properties, the next step is to predict themeasurements from the expected acoustic material properties. Then, multiplerealizations of the measurement noise are added to generate three clouds ofpoints (solid, liquid and gas) in the bidimensional measurements plane.


likelihood of attaining good zonal isolation. Fromthis strategy came the idea of reducing cementdensity through the injection of nitrogen into theslurry while pumping, and of introducing ceramicmicrospheres into the cement blend. The latterdesign was the precursor to the SchlumbergerCemCRETE concrete-based oilwell cementingtechnology, including the LiteCRETE slurrysystem, and CemSTONE advanced cementtechnology. These innovations allowed engineersto increase or decrease slurry density withoutsignificantly affecting the permeability of the set cement.

The new cement systems were accompanied bydevelopment and deployment of software toanalyze and improve fluid displacement behindcasing and simulate stresses on the cement overthe life of the well (left). Beginning in 2000,continual improvements to cementing softwareprovided engineers with a tool to tailor slurriesbased on gas-migration risks and wellbore stresses.

In 2002, Schlumberger introduced FlexSTONEadvanced flexible cement technology to handlechanging stresses imposed on cement sheathsover time. Expected stress changes from drilling,production and abandonment activities arepredicted by numerical modeling. The system’smechanical properties are customized usingFlexSTONE trimodal particle-size distributiontechnology. The resulting mechanical flexibilityallows these cement systems to resist failurethrough a variety of changes that may occurduring the drilling, production and abandonmentcycles of a well.13

While such methods increase the resistance ofthe cement matrix to physical stresses, they areineffectual once the cement sheath fails. Even ifthe sheath is intact during the well’s lifetime, theincreased emphasis on environ mentalresponsibility dictates that hydrocarbon-bearing formations remain sealed for many yearsafter the asset has been plugged and abandoned.This extended period of service significantlyincreases the chances that even the mostappropriate and resilient cement sheath may fail.

In response to these concerns, Schlumbergerengineers have taken another step in theevolution of zonal isolation systems with theintroduction of self-healing cement (SHC). Asthe name implies, when cracks or microannulioccur at the interface between the cementsheath and the casing or formation, self-healingcomponents within the set-cement matrix swell

26 Oilfield Review

> Designing cement systems. Cementing experts can use the CemSTRESS cement sheath stressanalysis software to analyze the radial and tangential stresses imposed on each casing string duringevents such as treating and pressure testing. In addition to indicating cement sheath performance incompression, tension, or both, the software has the ability to establish parameters, including set-cement flexibility, support and standoff. It can also be used to identify both inner and outer microannuliand show their size and development over time. CemSTRESS software uses a three-stagemethodology to aid in selecting and designing a cement system that can extend well life. In the firststage of the method, a cement expert determines whether the well requires a conventional cementsystem or a specialized system. This provides direction for the next two stages. The second stage ofthe methodology analyzes scenarios to design a cement system whose Young’s modulus is below thestress level that the software predicted would induce failure. In the third stage, Schlumbergercementing engineers use proprietary software, such as SlurryDesigner cement blend and slurrydesign software, to optimize the cement slurry design.

Old wells New wells

NoYes

Appropriate?

Laboratory tests

Cement properties

Pressure andtemperature history

Formation andcasing properties

Laboratory tests

SlurryDesignersoftware

NoYes

Appropriate?

Predicts Young’smodulus of newcement systemfor input toCemSTRESSsoftware

Continue to operatewell within safestress limits

Mitigateconsequences offailed cement sheath

Blend, test andpump cement

Database of properties

Cement properties

Pressure andtemperature prediction

Pressure uncertainties

Robustness criteria

Cemsoftware

STRESS


Spring 2008 27

to close the gaps without any outside inter -vention. This FUTUR active set-cementtechnology reacts specifically to the presence ofhydrocarbons. When the integrity of the cementsheath is compromised and zonal isolation isbreached, the cement reacts to the presence ofhydrocarbons by swelling. This effectively closesthe gap and shuts off formation fluid movement.

Except for its self-healing abilities, FUTURcement is similar to traditional cement.Successful placement requires adherence to thesame best practices as any oilfield cementingoperation, and the cement itself requires nospecial mixing or pumping equipment. FUTURslurries are compatible with all standardadditives and spacers. Standard mixing andslurry tests of rheological properties, free fluid,sedimentation, fluid-loss control, thickeningtime and development of compressive strengthall apply.

Once placed in the well, FUTUR cementbehaves in the same way as classic cements whennot in the presence of hydrocarbon, and its set-cement properties are equivalent to those oftraditional cements (right).

Laboratory WorkFUTUR cement technology is designed to providewell integrity for the very long term. Therefore,laboratory testing to replicate downhole sheathfailure was critical in proving that the cementwould indeed heal itself and that it wouldcontinue do so for years after placement. Thecement also had to be checked for any problemsits self-healing characteristic might create.

To test swelling properties, the cement wasplaced in an annular expansion mold. These testssimulate normal setting of the cement matrix inthe well, followed by an invasion of hydrocarbonsuch as would be expected when cracks orcreation of a microannulus causes a loss of zonalisolation. The FUTUR cement was cured in waterfor seven days prior to immersion in oil, andidentical temperatures and pressures were used in water and oil. Results showed that thelinear swelling increased with temperature atconstant pressure.

To evaluate the FUTUR system’s self-healingproperties, engineers at the SchlumbergerRiboud Product Center in Clamart, France,developed a flow loop to simulate downholeconditions and installed an SHC cell designed to

13. For more on new cements: Abbas et al, reference 6.

> Slurry designs. Laboratory tests determined the properties of three FUTUR slurry designs. Designs 1and 3 were tested at 60°C, and Design 2 was tested at 25°C. All designs used Class G cement andwere prepared with fresh water. The slurry rheologies were measured with a Fann 35 viscometer after mixing at ambient conditions and after 20 minutes of conditioning at the bottomhole circulatingtemperature (BHCT). The plastic viscosity (PV) and the yield value (Ty) were calculated using theBingham plastic model. Thickening times of these systems were controllable, and no free water wasobserved. For all three designs, a compressive strength of 3.44 MPa [500 psi] was achieved in lessthan 48 hours, as measured using an ultrasonic cement analyzer. The compressive strengths of thedesigns ranged from 4.5 to 20 MPa.

*gal/sk = gallon of additive per sack of cement***%BWOC = by weight of cement

**%BWOB = by weight of blend****Bc = Bearden’s unit of consistency

Formulation and Properties Design 3Design 2Design 1

BHCT, °C [°F]

Density, kg/m3 [lbm/galUS]

Antifoam, L/t [gal/sk]*

Dispersant, L/t [gal/sk]

Retarder, L/t [gal/sk]

Gas-migration control additive, %BWOC***

Fluid-loss control agent, %BWOB**

Gelling control agent, %BWOB

60 [140]

1,870 [15.8]

2.66 [0.03]

6.22 [0.07]

2.66 [0.03]

25 [77]

1,700 [14.2]

0.2% BWOB

4.2 [0.05]

0.77

60 [140]

1,400 [11.7]

4.2 [0.05]

0.5% BWOB

0.7

0.5

Ty, Pa [lbf/ft2]

PV, MPa [thousand psi]

10-s gel, Pa [lbf/100 ft2]

10-min gel, Pa [lbf/100 ft2]

1-min stirring gel, Pa [lbf/100 ft2]

API free fluid at 60°C [140°F], mL

API free fluid at 25°C [77°F], mL

API fluid loss at 25°C [77°F], mL

API sedimentation test, lbm/galUS

5.7 [11.8]

151 [21.9]

5.1 [10.7]

13.7 [28.5]

Traces

0.2

9.0 [11.9]

148.7 [21.6]

18.2 [38]

10.8 [22.6]

11.4 [23.7]

0.5

30

–0.2

7.8 [16.3]

60 [8.7]

7.4 [15.4]

10.2 [21.4]

58

–0.15

API Rheology

Temperature, °C [°F]

Compressive strength, MPa [thousand psi]

Young’s modulus, MPa [thousand psi]

60 [140]

20 ± 5 [2.9 ± 0.7]

6,500 ± 500 [940 ± 73]

25 [75]

10 ± 0.8 [1.5 ± 0.1]

2,800 ± 400 [400 ± 60]

25 [75]

4.5 ± 0.5 [0.65 ± 0.07]

1,300 ± 300 [190 ± 44]

Mechanical Properties of SHC Matrix After 7 Days Curing in Water at Atmospheric Pressure

Time to reach 50 psi at BHST, h:min

Time to reach 500 psi at BHST, h:min

24-h compressive strength, MPa [psi]

9:23

35:44

2.5 [363]

6:00

11:16

4 [637]

Compressive Strength Development

At BHCT, h:min

Time 30 to 100 Bc****, h:min

6:16

0:54

8:05

1:09

6:33

4:13

Thickening Time

Ty, Pa [lbf/ft2]

PV, MPa [thousand psi]

1.9 [4.0]

237 [34.3]

11.4 [23.9]

202.7 [29.4]

5.9 [12.3]

55 [8.0]

Mixing Rheology


evaluate self-healing capability in an annularconfiguration (above). Oil was injected throughsamples to test both cracks and microannuli. Inone test, a microannulus of 100 microns wascreated between the casing and cement insidethe SHC cell. While a conventional cement systemtested in this apparatus allowed oil to flowthrough the sample, the FUTUR system reacted tooil invasion with an efficient closure of themicroannulus in less than six hours (above right).

The self-healing system has also been testedin a cyclic failure scenario. Successive micro -annuli were created and repaired with the SHCat a differential pressure of 1.4 MPa/m[62 psi/ft]. Using the same flow loop, the flow ofoil through successively generated cracks wasrepeatedly shut off by the SHC. The same testusing a conventional neat cement system did notshow any decrease in flow (next page, top left).

FUTUR cement also can handle oil flows athigher pressure. A pressure increase to 3 MPa/m[133 psi/ft] did not diminish the self-healing

capabilities of the system, which maintainedintegrity and continued to block the flow of oil athigh pressure. The tests were repeated withdifferential pressures up to 5.3 MPa/m, and thecement’s self-healing property was confirmed inevery test.

A specialized testing system was devised tostudy self-healing properties in dynamicconditions with dry gas under realistic reservoirconditions. Test results highlighted the efficiencyof the FUTUR system when exposed to naturalgas under dynamic conditions. In less than onehour, the self-healing cement caused asignificant decrease of flow rate from425 mL/min [26 in.3/min] to 0.52 mL/min[0.03 in.3/min] (next page, top right).

Finally, researchers investigated thedurability of the self-healing cement. Thedurability—or aging—test consisted of twoparts. The first part, using a swelling test, was tocheck that the self-healing property ismaintained over time. The second was toevaluate whether matrix integrity is maintainedwhen the cement is immersed in oil for a longperiod of time.

Swelling tests performed after prolongedimmersion of FUTUR cement in water confirmedthat the self-healing properties were maintained.In this test, cement placed in an expansion cellwas cured in water for several months, thenimmersed in oil at 60°C. Results showed that thereactivity of the self-healing matrix remainedeffective even after resting dormant for a year.Testing of the matrix integrity, after one year ofexposure in oil, also showed no indication thatthe integrity of the matrix was deteriorating. Themechanical properties remained within the samerange even after immersion in oil for a year.

28 Oilfield Review

> Test cell results. The SHC cell was installed in a flow loop to investigate FUTUR self-healingefficiency. Oil was injected through samples inthe SHC cell at pressures up to 0.4 MPa [58 psi],corresponding to a differential pressure of 5.3 MPa/m [234 psi/ft] across the sample. In onetest, a microannulus of 100 microns was createdbetween the casing and the cement inside theSHC cell. The neat cement system (green)allowed oil to flow through the sample, whereasthe SHC system (blue) responded to oil invasionwith an efficient closure of the microannulus inless than 6 hours.

Time, days

Time, days

100

80

40

20

60

0

100

80

60

40

20

00

0 1 2 3

0.5 1.0 1.5

Mic

roan

nulu

s w

idth

, µm

Nor

mal

ized

flow

rate

, %

Neat systemSelf-healing cement

> Healing check. An SHC cell with two concentric cylinders simulates anannular volume. The outer cylinder, or ring, is a thin, steel sleeve (green). Theinner cylinder (purple) is made of a deformable elastic material into which aradially expandable core assembly (gray) is inserted, allowing expansion ofthe inner sleeve in a controlled manner. Top and bottom plugs sealing theannular volume are equipped with fittings that allow fluid to enter and toescape the cell. With the inner cylinder expanded by a core assembly, thecement is injected into the annular space. Once the cement has set, theinner-cylinder expansion is released. The inner cylinder shrinks back to itsoriginal shape, generating a microannulus of a controlled size. Radial crackswithin the cement are created by expanding the inner core assembly afterthe cement has set.

Expandable andretractablecenter assembly

Inner ring

Outer ring

Cement


Spring 2008 29

Surface SolutionsThe self-healing nature of FUTUR cement overtime, as demonstrated in the laboratory, makes itparticularly well-suited for long-term zonalisolation. That same ability also means SHC is agood solution for immediate or chronic gas-migration problems.

For instance, because of highly varied geologyand several shallow, gas-bearing coal seams,wells in the foothills of the Rocky Mountains ofAlberta, Canada, present a particular set ofcementing challenges. The coal seams may emitgas that eventually migrates through the casingannulus and manifests as surface casing ventflows (SCVF). Depending on the extent of theleak, operators may be required to shut in, repairor even abandon their afflicted wells.

Remediation of SCVF on these wells costs fromUS $250,000 to $1 million per well—a figure thatdoes not include the loss in production orpotential loss of the well.14

To address the problem, the operator of adeep gas field in the west-central Alberta GrandeCache area turned to FUTUR cement technologyfor zonal isolation in two new wells. The self-healing system was chosen to complementcementing practices implemented to reduce therisk of SCVF, which occurs in approximately 10%of Grande Cache area wells.

Both Well 1 and Well 2 required cement to thesurface. The operator had a particular concernabout Well 1; a similar well located about 500 m[1,640 ft] away had experienced SCVF. Lossesencountered while drilling Well 2 required theuse of a stage tool to ensure cement placement tothe surface across known nuisance gas zones.15

The SHC slurries were mixed and pumped usingstandard oilfield equipment, easily achievingcontinuous mixing rates of up to 0.95 m3/min[6 bbl/min].

Immediately after cementing of Well 1, somegas pressure was observed in the inter mediatecasing annulus. However, gas pressure was notevident after the well’s completion, suggestingthat the SHC had activated to contain a leak. Well 2 displayed no leaks in the 12 monthsfollowing cementing operations. While that mayseem a short observation period, SCVF in this

area typically occurs within days or weeks of thecementing operation.

During completion and production, the twowells were subjected to various downhole stresses,including downhole pressure of 64 MPa [9,282 psi]applied to test the completion and stressesrelated to cyclical temperature changes caused bya heater string in the top 600 m [1,968 ft] of thewell. Throughout and after these events, nosurface casing vent flows were detected.

Elsewhere in central Alberta, another fieldwas also plagued by SCVF from zones above the target formation. To complement othercementing technologies already in use, theoperator selected FUTUR self-healing cement foruse in two wells. In the first, cement density wasconstrained to a maximum 1,380 kg/m3

[11.5 lbm/galUS] because fluid losses had beenobserved during drilling. With a nearly oppositeproblem on the second well—nuisance gasdetected by mud loggers during drilling—drillingfluid density was increased to 1,470 kg/m3

14. Roth J, Reeves C, Johnson CR, De Bruijn G, Bellabarba M,Le Roy-Delage S and Bulte-Loyer H: “InnovativeHydraulic Isolation Material Preserves Well Integrity,”paper IADC/SPE 112715, presented at the IADC/SPEDrilling Conference, Orlando, Florida, March 4–6, 2008.

15. Cementing stage tools allow slurry to be placed atspecific depths along the casing through sliding sleeves.They are used when the hydrostatic pressure of the fullcolumn of cement threatens to overcome the wellborefracture gradient beneath the stage tool.

> Repeated healing. The self-healing systemswere also tested in a cyclic failure scenario.Successive microannuli were created andrepaired with the SHC system (top). At adifferential pressure of 1.4 MPa/m, the flow of oilthrough successively generated cracks wasrepeatedly shut off by the SHC system, while thesame test using a conventional system did notshow any decrease in flow. At increasedpressure, the FUTUR slurry design reacted tostop the invasion of oil through a 100-microncrack in less than 20 minutes (bottom).

Time, min

Time, min

0

0 10 20 30 40 50 60 70 80

10 20 30 40 50 60 70 80 90 100

60

50

120

100

80

60

40

20

0

40

30

20

10

0

Mic

roan

nulu

s w

idth

, µm

Crac

k w

idth

, mm

Neat systemSelf-healing cement

> Testing SHC with gas flow. SHC seals in the same way for hydrocarbon gasas it does for oil. A test cell containing cement was cured in such a way thata microannulus of an arbitrary size was present. With SHC in the holder, theflow rate through the cement drops significantly in less than one hour fromthe time the fluid was switched from inert nitrogen to natural gas (left).Traditional cement tested in the same fashion experienced almost no loss of flow rate in that time (right). The specialized testing system is based on aHassler sleeve-type core holder to prevent gas passing around the outside of the core.

1,000

100

10

1

Flow

rate

, mL/

min

Self-healing cement Neat cement

Flow rate of nitrogen through cement

Flow rate of natural gas through cement


30 Oilfield Review

> Analysis of cement evaluation logs in the Cortemaggiore 155dir underground gas-storage (UGS) well. The brown color of theSLG map (Track 1) of the ultrasonic flexural attenuation logging tool indicates solid (cement), resulting from a measuredacoustic impedance (Track 2) of about 5 Mrayl (close to the expected cement value) and high flexural attenuation. The CBL(Track 7) is in agreement, showing 100% casing to cement bonding (average CBL value of 5 mV) and strong formation arrivalson the VDL (Track 8), which is an indication of excellent cement-to-formation bonding. The optimal cement bonding is alsorelated to the fact that the liner is fairly well-centralized, as shown by casing centering (Track 4) and third interface short- andlong-axis outputs (Tracks 5 and 6). The casing centering curve is above 80% for most of the interval shown except near a depthof X,720 m, where the casing nearly touches the formation. This is also indicated in the vanishingly small cement thicknessalong the short axis. The horizontal features visible on the SLG map, cement map and flexural attenuation map (Track 3) are thecasing joints about every 14 m [45 ft] and two casing centralizers per joint. Together the logs show optimal cement bonding tothe casing and to the formation, providing assurance of effective hydraulic isolation across the permeable injection zones ofthe UGS well.

X,650

X,675

X,700

X,725

MeasuredDepth, m

SLGMap

AcousticImpedance

CementMap

0 0 10.521 3 4

CasingCentering

%00 180 100

Casing andThird-Interface

Long Axis

in.5 2.5

FlexuralAttenuation

Map CBL

mV0 50Variable Density

200 1,200

AmplitudeMin MaxCasing and

Third-InterfaceShort Axis

in.5 2.5S G

L


Spring 2008 31

[12.25 lbm/galUS]. This meant cement densityfor that well had to be increased to 1,550 kg/m3

[12.9 lbm/galUS] to meet the requirements of themud-removal plan.16

Zonal isolation was achieved with the newcementing system in both wells despite difficultconditions—a low-pore-pressure zone in Well A,and a narrow pore-pressure-fracture windowacross a gas-influx zone in Well B. Cementreturns to the surface in Well A alsodemonstrated that SHC can be applied in asingle-stage cement job in wells prone to lostcirculation. During and after subsequent drilling,stimulation and completion operations, therewas no indication of annular gas flow.

Gas StorageSustaining self-healing characteristics over timeholds special attraction for engineers chargedwith sealing underground gas-storage (UGS)wells. Because these wells are used to both injectand produce, the wellbores are repeatedlysubjected to considerable temperature andpressure changes—often in short cycles—thatcan induce stress-load changes on the casing and cement.

Additionally, in contrast to producing wellsthat have a life expectancy of perhaps 20 years,UGS well plans are likely to include a production-injection life span of 80 years or more. As aconsequence, zonal isolation failure inunderground gas storage wells is a significantand ongoing operator concern (see “IntelligentWell Technology in Underground Gas Storage,”page 4). In many UGS facilities, poor hydraulicisolation is caused by drilling fluid channeling asa result of eccentric casing or through thedevelopment of a dry microannulus.17

A combination of these factors hadhistorically resulted in poor zonal isolation inUGS wells in an Eni S.p.A.-operated, depleted gasfield in northern Italy. The challenges facing Eniand Schlumberger engineers included obtainingseals across gas-injection zones and gas-tightcement sheaths across deviated (49°), washed-out sections.

For a new well in this field, Eni subsidiaryStogit chose to cement the production casingwith FUTUR self-healing cement. The plan alsoincluded proper placement of centralizers, a gas-migration analysis software package and

Isolation Scanner logging tools to evaluate thecement bond.

The solution to persistent problems ofsustained casing pressure in UGS wells drilled inthis area involved a multipronged approach:• more centralizers to improve standoff• a liner across the zone of interest to facilitate

casing rotation during cementing• use of a software advisor tool for gas-migration

prediction and prevention during cementhydration

• software to tailor the cement system to associ-ated risk

• use of FUTUR SHC• a full suite of postcementing logging tools that

included all ultrasonic and flexural-wave measurements.

In 2007, the SHC cementing operation wasperformed on the Cortemaggiore 155dir UGSwell. The team used a computer-aided design andevaluation software program that optimized mudremoval and standoff, and fine-tuned the spacerand slurry characteristics. A separate softwareprogram was used to evaluate the risk of theseverity of gas migration based on the pressure-decay limit—a measurement of how far thehydrostatic pressure of the slurry will fall duringhydration before it is below the pore pressureand might allow gas migration into the annulus.

Finally, a mechanical-stress modelingsoftware tool that simulated pressure andtemperature variations during the life of the wellwas used to evaluate cement sheath integrityover time. The software modeled the threemechanisms of cement sheath failure—traction,compressional failure and both internal andexternal microannulus development.

Once the cement was placed, CBL-VDL andIsolation Scanner tool analyses all indicatedoptimal cement bonding at the casing andformation interfaces (previous page). Theultimate success of the SHC system is beingmonitored over time as gas is cyclically injectedinto and produced from the well.

In another recent FUTUR cement appli -cation, the technology was applied inenvironmentally sensitive areas of the CanadianRocky Mountains. Following the suspension ofdrilling because of surface casing leaks andresulting environmental concerns, one USoperator reevaluated its well constructionprocedures in the area and then resumedoperations. Despite the revised strategy, three ofseven wells drilled had obvious gas leaks whilethe other four were suspect; possible leaks weremasked by heavy cement poured around the

casing at the surface. The company then addedFUTUR cement to its drilling and completionprogram. Of the 13 area wells drilled andcompleted since then, only two wells haddetectable leaks; one was due to an operationalfailure unrelated to the SHC and a second was barely detectable.

Time Will TellLong-term performance of its self-healingcharacteristics is key to the success of FUTURcement. Laboratory work shows that this SHCwill continue to close the pathways throughwhich gas migrates without interventionthroughout the life of the well and beyond. Intime, operators pressured by environmentalregulators—internal and external—will insistthat the cement sheaths in their wells preventhydrocarbons from escaping formations longafter the well has been plugged and abandoned.The ability of FUTUR cement to react to andrepair the channels through which formationfluids travel to the surface makes it an idealanswer to such demands.

Operators, especially those in gas-migrationprone areas, will also come to expect an improvedview behind casing to eliminate other costly zonalisolation tests in the face of conflicting orambiguous CBL-VDL logs. In drilling environ -ments with proximate pore pressures andfracture gradients, lightweight cements that posea significant challenge to traditional sonic loggingtools are required. The Isolation Scanner tooloffers a clear solution to these and other currentzonal isolation challenges. —RvF

16. Cavanagh et al, reference 2.17. Moroni N, Panciera N, Zanchi A, Johnson CR,

LeRoy-Delage S, Bulte-Loyer H, Cantini S, Belleggia Eand Illuminati R: “Overcoming the Weak Link in CementedHydraulic Isolation,” paper SPE 110523, presented at theSPE Annual Technical Conference and Exhibition,Anaheim, California, USA, November 11–14, 2007.


Documents

Ensuring Zonal Isolation Beyond the Life of the Well