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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.

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.

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.

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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.

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) THEORYIn 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 pipe-amplitude 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

Steiles, 2012, SPE, ref 3

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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.

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.

Sonic wave signatures (against time) in various materials).3

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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 165-171. 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.

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“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

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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.”

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.

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”.

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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, wellheadpressure, 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