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Through-tubingcement bond logging

Memory Slim Cement Bond Logging Tool

Applications■ Evaluation of circumferential

cement bond between casingand formation

■ Free-pipe and free-casing indi-cation without need for surfacetorque application

■ Estimation of cement compres-sive strength

Benefits■ Efficiency of cementing opera-

tion confirmed through tubing

■ No need to pull tubing

■ Tool conveyance flexibility

■ Surface-readout systems notrequired

■ Fast, easy, economical operation

Features■ Compact 111⁄16-in. logging plat-

form with simultaneous record-ing of gamma ray, casing collarlocation, pressure, temperature,and waveforms

■ Can be run in 27⁄8- to 75⁄8-in. casing

■ Full waveform digitizationdownhole

■ Cement bond log values corrected for pipe size andweight, temperature, and fluidproperties

■ Conformance to rigorousSchlumberger logging-while-drilling-tool shock specifications

■ Resistance to corrosion

■ Processing identical to electric-line-conveyed surface-readoutservice

■ Multiple data output and inter-pretation formats

■ Integrated collapsible tool centralization

■ Over 40 hr of combined toolrunning time possible

■ Over 16 hr of continual wave-form recording time available

■ Suitable for logistically chal-lenging operations

Through-tubing cement evaluationThe memory slim cement bond loggingtool provides 3-ft cement bond log (CBL)and 5-ft Variable Density* log measure-ments. Because of its slim size (111⁄16 in.),the tool can be run into the zone ofinterest without removing the tubingfrom the well. All measurements arerecorded from a single pass. The mem-ory slim CBL tool can be run with othermemory PS Platform* production loggingtools for complete well and reservoirevaluation in one descent.

The principal application of the tool is to provide an evaluation of cementquality and integrity around the casing.This information can be used to ascer-tain cement quality and devise solutions,such as cement-squeeze programs,when necessary.

Fast, highly efficient operationThis memory tool service deliverscement bond and Variable Density logs of the same accuracy and quality as thosefrom surface-readout logging configura-tions when logistical, operational, ordownhole constraints prevent use of thesurface-readout system.

The compact size and portability ofthe memory slim CBL package facilitaterapid deployment. One Schlumbergerengineer can quickly set up all equipmentrequired for the job. All tools are certifiedas transportable by passenger aircraft.

Tools and sensors can be conveyed inthe borehole by drillpipe, coiled tubing,slickline, or unintelligent tractor. A stan-dard 15⁄16-in. sucker-rod connection pro-vides a crossover between the tools andconveyance system.

Flexibility and reliabilityThe portable surface system is designedfor maximum flexibility. Using a surface-system computer and software, theSchlumberger wellsite engineer pro-grams the tool to acquire the requireddata series. Waveforms are fully digi-tized downhole; PS Platform softwareperforms onsite data processing—orpostprocessing when necessary—andprepares the log presentation.

Depth-recording systems are avail-able for both hazardous and nonhaz-ardous environments. Optional externalsensors enable recording of cable andline tension or surface pressure. The

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The standard presentation shows correlation curves and travel times in Track 1, 3-ft CBL curves in Track 2, and a 5-ft Variable Density log display in Track 3. Good bonding is represented by the shadedarea (XX76 ft to XX92 ft).

A. Gamma ray

B. Well temperature

C. Cable velocity

D. Cement bond

E. Cement bond * 10

F. Well pressure

G. 3-ft travel time

H. 5-ft travel time

AB

CD

E

F

G

H

combination of leading-edge technologyin the surface system and the advanceddownhole instruments provides memoryproduction logging data that are indistin-guishable from data acquired in surface-readout operations.

Cement bond logging technologyAcoustic waves emitted by the tooltransmitter travel different paths to the receivers. The waveform obtainedat the receiver is a conglomerate of thearrivals from each path. Sound travelsfastest through solid materials, so thefirst arrival is usually the sonic wavetraveling outward from the transmitter,through the borehole fluid, up the casing, into the borehole, and to thereceiver.

The CBL measurement is the ampli-tude of the casing first arrival (E1) at the3-ft receiver. E1 is a function of the atten-uation caused by the shear coupling ofthe cement sheath to the casing. Theattenuation rate depends on the cementcompressive strength, cement thickness,casing diameter, pipe thickness, andpercentage of bonded circumference.

The longer 5-ft spacing is used torecord the Variable Density waveformfor better discrimination between cas-ing and formation arrivals. The VariableDensity log is generally used to qualita-tively assess the cement-to-formationbond, and it helps detect gas.

In fast formations, sound travels fasterthrough the formation than it doesthrough the casing. The amplitude of a first arrival will reflect losses incurredwhile traveling through the casing andborehole fluid.

The next arrivals will usually be soundwaves traveling through the formationrock to the receiver. The amount of fluidor gas behind the pipe affects whetherformation arrivals will be seen at all in the waveform.

The importance of cement evaluationIt is imperative that every well withhydrocarbon production potential beproperly cemented to hydraulically iso-late the zone of interest. In the idealcase, good cement-to-casing and cement-to-formation bonds will exist through-out the perforated zones so that thereis no hydraulic communication betweenthe zone of interest and other zones.

Broadly speaking, there are two pri-mary causes for poor cement jobs.

■ Flow problems of mechanical origin,such as poorly centralized pipes indeviated wells, washed-out boreholes,inefficient preflushing, and incorrectflow regimes, lead to incompletemud removal in the cement annulus.

■ Degradation of the cement slurryduring the curing stage is caused bya difference between the cement porepressure and the formation pressure.When a standard cement dehydratesand starts shrinking and the porepressure becomes less than the for-mation pressure, the cement qualitycan be degraded by formation fluidsor, even worse, by inflow of gas.

Laboratory measurements have shownthat a typical well-cured cement has apermeability of approximately 0.001 mD,with a pore size less than 2µ and poros-ity around 35%. However, when gasmigrates within the slurry before it iscompletely cured, the pore structure is partially destroyed, and the gas cangenerate a network of tubular poresthat reach 0.1 mm in diameter and leadto permeabilities as high as 1 to 5 mD.This compromised cement can supportthe casing, but it is unable to preventmigration of gas from the formation.Additives are available to prevent thismigration and ensure zonal isolation of gas-bearing intervals.

A comprehensive cement evaluationprogram can determine the quality ofthe cementing operation or the need forrepair, as well as the cause of failures,so that the cementing program can beimproved for future wells.

CBLs—especially those able to iden-tify cement channels—can distinguishzones that are properly isolated.

Channeling of mud in cement probably caused by slight deviation and eccentered casing

Gas invasion into cement matrix

Eccentered casing touching formation; all cement passed in the free side

Poor cementing in washed-out zone; incomplete mud displacement caused by inefficient flow rate

Good cementing where the casing is centered

Cement

Casing

Microannulus

Centralizer

Casing

Cement slurry displacement problems.

04-PR-086 ©Schlumberger

January 2005 *Mark of Schlumberger

Produced by Marketing Communications, Houston

www.slb.com/oilfield

One-trip cement evaluation and production logging in a deviated wellRunning a PS Platform service in thestring below the memory slim CBL toolin West Africa saved significant time andexpense, while providing high-quality data.

Water production from a well hadincreased to 97%, and cement evaluationwas specified to determine whether theupper two producing zones were effec-tively isolated from a wet interval in alower formation. The temperature read-ings and CBL confirmed that most of thewater being produced was flowing behindthe casing from a lower zone to theupper zones.

The lower zone was perforated usingthe eFire-Slickline* electronic firing headsystem and was sealed with a cementsqueeze. Water cut measured at the sur-face was reduced from 97% to 15%.

Memory slim CBL tool.

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The Variable Density log service deployment on coiled tubing (A-F) delivered time and cost savings along withsurface-readout-quality data (G). Both logs identify poor cement quality at XX54–XX57.

Memory Slim Cement Bond Logging Tool Specifications

Max. operating temperature 300˚F [150˚C]Max. pressure 15,000 psi [103.4 MPa]Length 23.4375 ft [7.15 m]Weight 100.2 lbm [45.45 kg]Casing sizes 27⁄8–75⁄8 in.Tool OD 1.6875 in. [43 mm]

A. Gamma ray

B. Travel time

C. Casing collar locator

D. 3-ft CBL

E. 3-ft CBL * 5

F. Initial memory Variable Density log

G. Subsequent surface-readout Variable Density log

A B

C

D

E

F G