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Cement Evaluation Methods to Prove Isolation of Barriers in Oil and Gas Wells:
Should a Cement Bond Log (CBL) Be Run or Required in Every Well?
12 July 2012
George E. King, Apache Corporation
Objective
The purpose of this work is to establish whether a specific method or tool is effective in proving cement
isolation of a zone, and if the method or tool should be required on the surface casing cement or other
cement strings as a part of initial well construction (or, if not required, when it may provide useful
information).
This report also looks at the cement evaluation methods with intent to determine the best investigation
tools and their limitations.
Conclusions
1. The only cement test method that can confirm zone-to-zone isolation is a pressure test.
2. To provide an effective seal and isolation of a zone, only part of the total cement column must
be channel free. Cement channels may be present in parts of the cement, but as long as there
are one or more significant, continuous sections of channel-free cement, isolation of the zone
along the wellbore will be adequate.
3. Cement bond logs can give a reasonable estimate of bonding and a semi-quantitative idea of
presence or absence of larger cement channels, but will not certify pressure or fluid isolation of
a zone. Cement bond logs have been proven to miss a percentage of smaller channels in
cement, even under ideal conditions and interpretation.
4. Bond logs have failed to show bond in many wells that proved to be well isolated in a
differential pressure test. Error within the application and interpretation of cement bond logs
has resulted in numerous workovers to repair cement that was not faulty, resulting in high
workover costs and a decrease in the well integrity by unnecessary perforating and attempts to
block squeeze cement.
5. Top of cement (TOC) can be established by several methods. Temperature logs are functional
within a time-window for determining top of cement. Low-strength natural radioactive sources
(e.g., raw uranium ore) mixed in the cement are useful for non-time-dependent determination
of cement tops (although the process has not been used in decades). Density logs may show
cement tops, but will confuse formation fill with cement. Bond logs may provide cement top
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information but can be fooled by highly compacted cuttings, barite, formation collapse in the
annulus and formations with fast sonic travel times.
6. With the exception of a pressure test, requiring a specific tool on every cement job appears to
be a poor choice, with possible detrimental economic and structural consequences for squeeze
cementing attempts made on wells with cement suspected by CBL investigation but proven
effective by a pressure test.
Discussion
Cementing is an integral part of well construction. Cement provides the seal, protection and support for
the casing to maintain the strong barriers that isolate the well. The benefits of cement are well known
and a compendium of knowledge on cement design and durability has been building since the
development of engineered cement application beginning in the 1906 to 1922 time period.
To achieve effective isolation, cement needs to fill the area around the pipe and produce a channel-free
section of cement over a length of the cement column suitable to isolate zones and prevent leakage into
or out of a hydrocarbon productive zone. In many published case histories of cement bond studies and
several multi-well studies, logs of cement quality show channels over short zones, even where isolation
has been proven by decades of production. Channels probably exist for short intervals in many
cemented intervals that are still effectively isolated. Unless the channels extend through the entire
length of the cemented column, the isolation potential of a cement column is still acceptable. Even a
few feet of high quality cement where channels are absent is adequate to form a seal. Typical cement
column lengths of hundreds of feet sharply reduce the potential for an isolation failure, even under
adverse cementing conditions.
Most channels in the cement occur from inability to displace mud from around the pipe and from gas
migration into the annulus as the cement starts to gel. The amount of channel free cement required for
pressure and fluid isolation is small, typically less than about 50 ft, while the amount of cement used in
primary or overlap cementing operations is 200 to 600 feet or more. These long cemented intervals in
primary or overlap cementing build in a significant safety factor that takes potential for cement
channeling into account.
Additionally; centralizers, pipe movement (rotation and/or reciprocation), preliminary mud
displacement flushes, swelling cement, gas migration prevention and other placement steps sharply
lower potential for channels in the cement. In vertical sections of well, casing centralization and gas
migration actions are usually sufficient to minimize channels. The highest incidence of channels within
the cement column occurs in the deviated section of wells (in the 30o to 60o range) due to the
combination of Boycott Settling effect of circulating dispersions, the effects of casing weight (flexing the
casing to the low side of the drilled hole, and density difference segregation of gas to the top of the
hole.
Cementing: Forming an effective Barrier
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Construction of the well is a step-by-step process and the well design is a function of the geologic
character of the area. A simplified schematic is shown in Figure 1, where different casing strings and
accompanying cement is used to form a section of a well.
Figure 1
This is a section of the well where
casing had to be set to control either
formation fluid pressures or formation
stability.
In a case, where isolation is need to
protect the casing against an exterior
corrosive salt water, cement is placed
over the entire interval.
In other cases, cement may or may
not need to extend over the entire
casing string.
The cement used in well construction may vary slightly from high-quality construction grade cement
since it must be modified by additives to be placed across hot or cold zones and must remain in a fluid
state until it has been completely placed behind the pipe.
Strength of the cement increases as it sets and will reach strengths of several hundred psi in a few hours
and several thousand psi after a few days. Even low strength, partially cured cement is capable of
supporting the casing and preventing flow of fluids.
An effective cement job around steel casing placed in a drilled hole through rock is provides many
critical requirements for environmental and well protection.
1. Cement completes the isolation step by sealing the annulus (area between the outside of the
casing and the drilled hole wall), preventing unwanted fluid movement out-of or into a well.
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2. A cement sheath around the pipe has the hardness and durability of rock, with much lower
ability to pass fluid than even most seal rocks. The cement from the primary cement job and the
cement added during abandonment becomes part of a permanent plug and abandonment
design.
3. Cement is useful for preventing fluid movement behind the pipe (including gas migration from
gas charged but un-economic sand, shale or coal stringers).
4. Cement reduces exposure of the pipe exterior to corrosive influences such as salt water and
acid gasses (CO2 and H2S). Exterior corrosion of the casing is common in a few areas where
naturally low pH (acidic) salt water inhabits some formations. Cement forms a coating when in
contact with many forms of natural acids that prevents further reaction and insures stability of
the cement. These acid reactions are usually short term and generally pose no risks that are not
easily controllable with additives.
5. As the liquid cement sets, strength is developed that supports the casing, preventing buckling
and joint failure. A cement sheath also increases the resistance of the pipe to burst and collapse
forces.
Cement Monitoring Methods – What They Do and Their Limitations
Pressure testing of the wellbore after a cement job provides a recorded record of whether the wellbore
between the top and bottom exposure points will hold a test pressure equal or greater than the highest
test pressure that the well will see in any subsequent operation. This is the most reliable test, quickly
separating adequately isolated casing strings from those that need repair to pass isolation tests.
Pressure tests are a standard during well construction and, if the test pressures are more than
subsequent operational pressures, the test offers documentation of isolation. The area that is tested in
this manner is only the zone in contact with the pressuring fluid, which is typically the bottom section or
casing “shoe”.
Cement evaluation behind the pipe began with the calculation of cement tops. Properly run
temperature surveys can identify the TOC, but distribution of cement-e.g., vertical isolation through
zones of interest-is difficult to ascertain. The most critical factor in the evaluation technique is the timing
of the survey. Heat dissipates rapidly from the exothermic reaction as cement sets, thus temperature
measurements must be taken within a few hours. Laboratory tests indicate a return to near normal
temperatures in 24 hours. Early surveys also showed the effects of time, bottomhole temperature,
circulating time, and thermal conductivity of the surrounding formations on the temperature profile.
The temperature log remains an economical and effective way to determine the TOC and intervals of
larger cement accumulation. It is not a direct measure of vertical isolation across hydrocarbon-bearing
zones; however, if a four-arm caliper is available, it is possible to infer good cement displacement if the
TOC agrees closely with the calculated top (and full returns were present during pumping and
displacement of cement). Top of cement investigation are usually run only in the first few wells in an
area until cementing operations are understood.
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Radioactive tracer surveys were run in the late 1930's to determine cement tops, but are not run today.
Carnotite (a low grade uranium ore with a low radioactive level) was mixed in the lead cement slurry
and cement tops were determined with a gamma ray log. Tracer surveys had the same limitations as
temperature logs (shallow investigation and limited to vertical wells) but were not time-sensitive.
Cement bond logs (CBL), which may be sonic or ultrasonic, range from simple averaging instruments
similar to those that came out in 1960 to the more sophisticated models that investigate 60o segments
of the cemented annulus around the tool.1-6
A properly run and executed CBL or CET (Cement Evaluation Tool) can provide some information on
cement fill behind the pipe, going past area of the cemented shoe and along the wellbore where a
pressure test may not reach.
Steiles, 2012,
SPE, ref 3
The bonding investigation theory behind the CBL is basically good but application and investigation
problems along with the inability of any CBL to find all of the small channels in cement are problems.
CBL signal interpretation expertise is easily the most important part in using a CBL.3
TRADITIONAL CEMENT BOND LOG (CBL) THEORY
In conventional CBL tools, a transmitter is pulsed to produce an omni-directional acoustic signal that
travels to a set of receivers along various paths through the borehole fluid, pipe, cement and formation.
The logging system records the receiver waveforms and displays them on the log along with a pipeamplitude curve. Interpretation of the CBL signals uses these two measurements to indicate two bonds;
the pipe-to-cement bond and the cement-to formation bond. An additional measurement is the sound
travel time, which confirms cement presence, tool centralization and is an indication of cement-to-pipe
bond.
The pipe amplitude curve displays the amplitude of the acoustic signal that has traveled through pipe,
but not through cement and formation, to arrive at the receivers. Conventional cements will have an
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amplitude measurement of less than 10 mV for good bond. Foamed, nitrified cements and cements
containing light or heavy weight components will affect the amplitude measurements of the CBL.
Ultrasonic tools provide the most beneficial data when evaluating cement placement and bonding.
Instead of a separate source and receiver, the ultrasonic source and receiver are packaged together as a
transducer.
When a signal emitted by a transducer encounters an acoustic interface (for example, between casing
and annular material outside casing), some of the signal energy is reflected at the interface, and some is
transmitted across the interface. The fractional amounts of reflected and transmitted energy depend on
the acoustic impedances of the materials at the interface. The signal strength received at the tool, the
acoustic impedance, or Z, is a function of the bulk density of the material through which the sonic waves
travel.7
This CBL presentation3 shows the five logging tracks that make up a good CBL report. The raw transit
time data helps distinguish between cement measurements and interruptions from “fast formations”
(faster sound travel than steel pipe). The gamma ray is a gamma emission measurement log that
reports natural radioactivity of a formation and is a good depth correlation. The CCL is a casing collar
locator log that correlates the thicker steel collar locations (affects sonic travel and cement thickness).
The amplitude is the strength of the signal after loss of signal due to attenuation of the transmitted
sonic or ultrasonic signal. The VDL or variable density log is a recording of the bond log interpretation.
Amplitude is the magnitude or loudness of the signal when dealing with sound waves. Attenuation is
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the loss of energy during transmission of the signal. Density of cement and formation are two of the
variables.
The difference in the speed of the sonic or ultrasonic signal in the specific media (formation, cement,
steel, mud, etc.) must be known to make the bonding calculation accurate. Errors in the information
from variances in the materials or inaccurate measurements create significant error potential in the CBL
measurement.
Sonic wave signatures (against time) in various
materials).3
Newer tools such as the Segmented Bond Tool (SBT) and ultrasonic imager are definite improvements,
but the small channel detection problem remains. Other developments may include more sophisticated
tools, such as the cement volumetric scan tool. Each logging technique currently in use has limitations
and none will measure isolation like a pressure test.4,5,6
Frisch described the industry problem with cement investigation tool with this statement: “Previous
conventional cement evaluation techniques that rely on combined data from a traditional acoustic
cement bond logging (CBL) tool and modem ultrasonic tools can be problematic. It is important to
accurately evaluate the downhole placement and bonding characteristics of any type of cement to
ensure zonal isolation of economic fluids from undesirable fluids. Inaccurate evaluation can lead to
unnecessary and expensive remedial cementing operations. It is estimated that the industry spends
about $200 million per year on remedial cementing. Of this amount, between $30 to $40 million per
year is wasted because of misinterpretation of cement evaluation logs.”7
Limitations and Problems
Field performance for a properly run and calibrated CBL is about 90% in finding channels of 10% or more
of total annular space.16 Smaller annular channels are not easily identifiable to a bond log because of
variations in cement composition that create density differences in the cement. These channels may or
may not compromise cement seal (isolation) integrity depending on their extent and connectivity along
the annular cement sheath.
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This major limitation of the CBL in identifying small channels practically eliminates it as a reliable test for
isolation. Many of the early and current problems with CBL’s came from poor running and interpreting
techniques as well as mistakes in selecting correct time and place to run the logs. Early work indicated
several false signals, particularly in thinner cement sheaths and hydrocarbon contaminated cement
(pockets). In multiple well studies, the cement bond log often indicated poor bonding when well
performance and zone pressures were clearly isolated by cement. This finding was proven by long-term
production without problems, water-free well performance (water isolation) and pressure
measurements over time.2
Data in these field tests showed many wells with effective isolation even though the percentage of
acceptable bond ranged from 31% to 75%. Even a few sections of good bond established isolation are
adequate seals between zones, as proved by pressure readings and long term water avoidance that
were only tens of feet away. 2, 17
Field examples of cementing practices showed a correlation between mud removal operations and
better bonding, to the point where a good cementing program was more important, and more reliable,
than running and trying to interpret a cement bond log. The bond log was highly useful in measuring
bonding trend improvements in cementing application where there were demonstrated problems with
cement isolation and are used in some areas to measure improvements in cementing operations. The
information that is needed to assess isolation is whether a significant portion of the wellbore is channel
free and the cement fill, bond and strength are sufficient to contain pressure. A CBL will not reliably
answer those questions as a stand-alone piece of data. In one of the best early field case histories and
assessments of CBL tools, Walter Fertl, a recognized bond log expert, described CBL tools with the
following language: “The validity of Cement Bond Log (CBL) interpretation has been a subject of
controversy since its introduction; and the CBL, despite its great potential, is probably one of the most
abused, misused, and misunderstood logs run in the oil field today. Miss-calibration, inadequate
information, and a severe lack of standardization are enough to push petroleum engineers into a morass
of bewilderment.”4
Well cementing technology in both relatively straight and high-angle directional holes has advanced
dramatically since the first casing was cemented in 1903.4 Besides the everyday cementing needs in
"problem-free" boreholes, recent engineered improvements successfully deal with cementing of arctic
wells, ultra-deep and hot holes, water-sensitive formations, and proper placement opposite
incompetent, fractured, or proper placement opposite incompetent, fractured, or highly permeable
formations. The basic requirements for obtaining a successful primary cement job have been known for
years. Good design characteristics are based on a knowledge of formation, cement, and pipe properties,
and controlled placement techniques that consider fracture gradients. Also important is an
understanding of (1) minimum practical mud density and viscosity, (2) cement type, (3) turbulent flow
conditions, (4) the optimum size of preflushes, (5) centralizing of casing, the use of scratchers, and the
handling of pipe, and (6) the proper choice of casing.
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Basically, the utility of cement bond logs is in determining the presence of cement and information on
cement the bonding across the zone of investigation. It will not predict or confirm pressure isolation.
Technical References
1. Grosmangin, M., Kokesh, P.P., Majani, P.: “A Sonic Method for Analyzing the Quality of
Cementation of Borehole Casings,” SPE Journal of Petroleum Technology. Vol. 13, No. 2, pp 165171. Feb, 1961.
2.
Flournoy, R.M., Feaster, J.H.: “Field Observations on the Use of the Cement Bond Log and Its
Application to the Evaluation of Cementing Problems,” SPE 632, Society of Petroleum Engineers,
New Orleans, LA, October 3-6, 1963.
3. Steiles, D.: “Challenges with Cement Evaluation: What We Know and What We Don’t,” SPE
Webinar, July 11, 2012.
4. Fertl, W. H., Pilkington, P. E., Scott, J.B.:”A Look at Cement Bond Logs,” Journal of Petroleum
Technology, Vol. 26, No. 6, June, 1974, pp 607-617.
5. Pilkington, P.E., Fertl, W.H.: “Field Tests of Cement Bond Logging Tools,” The Log Analyst, Vol.
XVI, No. 4. July-Aug, 1975.
6. Pilkington, P.E.”Cement Evaluation – Past, Present and Future,” Journal of Petroleum
Technology, Vol 44, No. 2, Feb 1992. Pp 132-140.
7. Frisch, G., Graham, Griffith, J.: “A Novel and Economic Processing Technique Using Conventional
Bond Logs and Ultrasonic Tools for Enhanced Cement Evaluation,” SPWLA 41th, Annual Logging
Symposium, June 4-7, 2000.
8. Bigelow, E.L.: “A Practical Approach to the Interpretation of Cement Bond Logs,” SPE 13342, JPT,
1985
9. Smith, R.C.: “Successful Primary Cementing Can Be a Reality,” Distinguished Author Series, SPE
JPT, Nov 1984, pp 1851-1858.
10. Beirute, R.M., Wilson, M.A., Sabins, F.L.: “Attenuation of Casing Cemented with Conventional
and Expanding Cements Across Heavy-Oil and Sandstone Formations,” SPE Drilling Engineering,
Sept 1992, 2000.
11. Goodwin, K.J., Crook, R.J.: “Cement Sheath Stress Failures,” SPE Drilling Engineering, December
1992, pp 291-296.
12. Jutten, J., Toma, I., Morel, Y., Ferreol, B.: “Integration of Cement Job Data for Better Bond Index
Interpretation”, SPE 21690, OKC, April 7-9, 1991.
13. Gui, H., Summers, T. D., Cocking, D.A., Greaves, C.: “Zonal Isolation and Evaluation for Cemented
Horizontal Liners,” SPE Drilling and Completion, December, 1996.
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14. Frisch, G. O’Mahoney, L., Mandal, B.: “Examination of Cement and Casing Evaluation Logs,”
IADC/SPE 77212, Jakarta, Indonesia, 9-11 Sept, 2002.
15. Thornhill, J.T., Benefield, B.G.: “Injection Well Mechanical,” Report 625/9-87/007, USEPA,
Washington DC, 1987.
16. Albert, L.E., et.al.: “A Comparison of CBL, RBT and PET Logs in a Test Well With Induced
Channels,” JPT (Sept, 1988) 1211-16.
17. Author’s personal experience on isolation of three to five thousand psi between zones
separated by 50 ft of high quality cement. Tests on Tuscaloosa wells in Louisiana run by Amoco
Production Company in 1990’s.
Examples of Papers and Presentations on Cement Evaluations.
“Ultrasonic tools normally require an
impedance contrast in the materials
behind pipe to differentiate between
cement and fluids. The impedance of
foam or complex cements can be lower
than that of water, drilling mud, or spacer
fluid, and can even approach the
impedance of free gas. Because of low
acoustic impedance, the data and images
may indicate fluid behind casing rather
than cement even when zonal isolation is
achieved. Unfortunately, the standard
approach of cement evaluation does not
provide the most accurate method for
determination of zonal isolation. New
cement additives, fluids, and nitrogen - all
change the acoustic properties of the
cement, which affect the zonal isolation
assessment. Incorrect interpretation
often leads to an unnecessary remedial
cement treatment.”7
11
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The cement bond log has been controversial since its inception.8 Despite its potential, it is possibly the
most maligned logging service available to the industry. Effective zone isolation between permeable
intervals in a well requires a cement sheath over an appreciable vertical interval. It is necessary for the
annular cement sheath to provide an effective hydraulic seal to withstand subsequent completion and
production operations. The oil industry has used wireline well logs to detect the presence or absence of
cement behind pipe for more than 20 years. Users have attempted, not always successfully, to evaluate
the effectiveness of cement bond to both pipe and formation with cement bond logs. Cement bond logs
do not misIead. Poor interpretation habits mislead. Knowledge of the well completion and tie inherent
physical restraints placed on the log measurements is needed to evaluate the log properly.8
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“The interpretation of cement bond logs is controversial for three primary reasons: (1) dependence on
and oversimplified use of the pipe amplitude curve, (2) lack of an understanding of the full acoustic
waveform, and (3) failure to compare tie physical restraints of the well completion to the log
measurements. Most misinterpretations are caused by, any or all of these reasons.”
Note – this bond log shows channels in some areas, but a
section of effectively cemented pipe in others. This may still
be an effectively isolated area, but only a pressure test can
confirm the isolation.
Cement displacement is generally good but sometimes not
perfectly uniform due to hole washouts, caving formation
and anomalies in the transition between formation layers.
Examples of well isolated cement are known when the bond
log interpretation reported “no cement”.
Amplitude is the magnitude or loudness of the signal when dealing with sound waves. Attenuation is
the loss of energy during transmission of the signal.
The amplitude measurement is representative of the first detected arrival at tie receiver. It is the
measurement from which quantitative numbers of cement compressive strength and the bond index are
derived. The generality accepted qualitative interpretation of the amplitude curve is illustrated in Fig. 2,
and is as follows.
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1. High amplitude indicates that the pipe is relatively free to vibrate; hence, it is poorly bonded or
unsupported.
2. Lower amplitude indicates that tie casing is more confined or bonded. The confinement causes
adsorption of the wave energy, and hence, lower amplitude.
3. Amplitude readings between maximum and minimum values are logarithmic functions of the percent
of bond. This single measurement, and the oversimplified interpretation of it, is the source of most of
the tales created about cement bond logs.
Amplitude can be measured with electrical accuracy, but the physical restraints of the logging
instrument and its relationship to casing, borehole, cement, and formation, and their physical
relationship to one another, complicates the measurement. In cement bond logging, mechanical energy
(transmitter) is transformed into acoustical energy during transmission to the receiver.
A number of physical conditions can lead to erroneous amplitude interpretations. Some of these, along
with reference sources relating to them, are as follows.
1. Amplitude detection method-fixed gate or floating gate. Erroneously high amplitudes can occur
with floating gates.
2. Fast formation. This condition occurs earlier than, or at the same lime as, pipe arrival.
Amplitude reading is questionable at best.
3. Tool eccentering (tool out of center of the wellbore). This condition reduces amplitude.
4. Insufficient curing time for cement. This condition increases amplitude.
5. Cement sheath < ¾ in. [<2 cm]. With either well centered or poorly centered casing, this will
increase amplitude.
6. Micro-annulus. This condition increases amplitude.
7. Gas bubbles. Gas bubbles in the borehole fluid will decrease the acoustic signal.
8. Void spaces in the cement sheath. These will increase amplitude.
9. Pipe thickness. Changes in pipe thickness from one joint to another will cause different
minimum and or maximum amplitude values.
10. Cement may be bonded to the pipe, but not to the formation. This results in low pipe
amplitude but poor cement integrity.
In addition to these factors, comparison to cement bond logs on adjacent wells can be misleading
because (1) the equipment-transducer type, transmitter-receiver spacing, transmitter frequencies, etc.,
varies, and (2) the operational techniques – tool centering, logging speed, calibrations, etc. vary.
Good Bond to Pipe; No Bond to Formation. Casing periphery can be totally surrounded by a reasonably
thick ~> ¾” [>2 ml) hardened sheath of cement, which is not in contact with the formation. The
condition is not uncommon, but often goes unrecognized. It can be expected to occur across the face of
permeable zones, since mudcake is a natural enemy of cement. Cement does not bond to mudcake. As
mudcake dries, it shrinks away from the cement, creating a void between cement and formation.
The resultant void space presents unfavorable conditions for acoustic coupling because very little
acoustic energy will remain in the casing. The cement will attenuate the transmitted energy. Because of
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the poor acoustic coupling to the formation, any energy transmitted into and received from the
formation will be very weak.
The most important operation performed on a well is the primary cementing job at the time of
completion. It must achieve complete zonal isolation in the wellbore; that is, obtain a hydraulic seal of
cement to casing and cement to formation while at the same time eliminating mud or gas channels
within the cement sheath.9
To monitor a cementing operation properly, many
data must be collected and analyzed during the
actual cementing operation. These data include
pump rate, annulus mud return rate, wellhead
pressure, density of fluids pumped (radioactivity
devices or equivalent), cumulative displacement
volume, and hookload during casing
reciprocation. Some high-pump-rate jobs will
require two pump rate meters, two return rate
meters, two radioactivity densometers, and two
totalizers. To facilitate the data collection process,
recorder vans and treatment monitoring vans
(TMV) are being used more widely. Recorder vans
plot data on various real-time devices as the job
progresses. The computerized TMV's record data
on magnetic tape, disks, and paper, plot selected
data on an X-Y plotter, and display selected data
on a CRT. This enables the job supervisor to
observe the entire operation at one location and
permits timely decisions as necessary.
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Filter cake and formation differences may confuse attenuation measurements leading to poor judgment
of the cement quality when a good cement sheath had been placed. 10
Casing expansions created by excessive internal casing pressures can create radial stress cracks in the
set cement in the annulus. These cracks, which cause loss of annular zonal isolation, generally are
created in the lower one-quarter to one third of the well.11
Typical causes of such cracks include pressure testing the casing after the cement has attained high
compressive strength. The same type of failure has been observed on wells that used expendable
perforating guns to shoot high-density patterns with large holes.
Generally, low compressive strength cement is more ductile.
Cement sheath cracking as the result of excessive temperature are in the upper 1/3rd to upper ½ of the
well.
Cement job evaluation has always been a major problem. Acoustic and ultrasonic logs are widely used to
assess the quality of the cement job but even experts experience difficulties in locating cement on a
bond log.12
The 30 years of field practice which have followed demonstrate that the CBL interpretation has many
limitations. The CBL interpretation method described has inherent limitations, either related to the
physical nature of the CBL measurement, or due to the lack of integration of the cement job information
and well data in the log interpretation process
.
Several parameters are known to influence the CBL amplitude to some variable and often unpredictable
extent12:
1. Measurement set-up, such as the position and the width of the amplitude measurement
window which must be fine-tuned to specific well conditions, is extremely important.
2. Downhole conditions are known to have an effect on the CBL signal. In 1981, Nayfeh et al.
presented charts showing the influence of temperature, wellbore fluid density and wellbore
fluid type on the CBL amplitude in free pipe. One of the most striking results presented was that
CBL amplitude in a 7" casing increases by 70% if the casing is filled-up with a 11.5 lb/gal CaCl2
brine instead of water.
3. Microannulus is when a small gap exists between the casing and the cement; the casing vibrates
freely and the CBL signal gives a wrong indication of a poor cement job. Microannuli generally
appear after the cement job due to pressure or temperature changes. One of the most common
source of microannulus is the replacement of the wellbore fluid by a lighter one.
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4. Fast formations cause sound to propagate faster in the formation than along the casing resulting
in the early part of the sonic waveform to be affected by direct energy path through the
formation leading to high CBL amplitude in well bonded casing. On~. of the most common open
hole logs is the sonic log, which clearly identifies fast formations: the transit time is below 57
us/ft. Having a sonic log before the CBL is run permits one to anticipate discrepancies.
5. Cement thickness influence14 is when the cemented annulus is too thin, and reflection of sound
energy at the formation interface may interfere with the first peak E1 (Jutten et al.) .which often
leads to higher CBL amplitude especially in well cemented concentric casings. This can be
anticipated when annular thickness and sound velocity through the cement are known. Accurate
hole size is generally known from a caliper log and an annular thickness can be calculated.
Therefore, when the sound velocity through the cement is known, one can expect problems and
recommend the use of a shorter amplitude measuring window for running the CBL. Generally,
cement impedance variations are due to poor control of slurry density while mixing, or short
elapsed time after the cement job in the case of extended slurries. Jordan et al. , proposed a
method to determine the waiting on cement time prior to running a CBL. Such a method would
sometimes lead to excessive waiting-on cement times, especially in the case of extended
cements.
6. The main cement parameter which influences directly the CBL signal is the acoustic impedance
of the cement (the product of density times sound velocity). Knowing this parameter at the time
of logging eliminates the need for excessive waiting on cement times.
Horizontal sections13 may be cemented to improve pipe support or as an initial isolation in the zone to
be fractured so that adjacent fracturing jobs. These sections of wellbore do not need to be pressure
tested since the entire section is within the hydrocarbon producing formation. In multi-frac completions
in naturally fractured formations, for example, the natural fractures within a pay-zone formation, like
some shales, will communicate fluids and pressure along the commonly shared formation and below an
effective common fluid migration barrier. Cement, therefore, will be an isolation mechanism only
between the casing and the wellbore; cement cannot affect fluid migration or travel through natural
pathways in the formation.
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For accurate cement evaluation, it is necessary to have at least ½” of cement sheath around the casing.
A cement sheath of less than ½” will affect the log response.14