Bond log of light cements and ultrasonic SPE 76713.pdf

Copyright 2002, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE Western Regional/AAPG Pacific Section Joint Meeting held in Anchorage, Alaska, U.S.A., 20–22 May 2002. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract In recent years specialized cements have been developed for use in the oilfield. These cements have ranged from very lightweight to very heavy cements. They have included additives such as glass or ceramic beads to alter their properties. With an increased use of these cements, concerns have risen about the cement evaluation. Quite often the question is asked, “Can we evaluate these cements?” rather than, “How do we evaluate these cements?” This paper will discuss some logging techniques that can be used to evaluate these new cements.

The desire to cement longer casing intervals without exceeding the formation fracture gradient has led to the use of very light weight cements. These cements have low acoustic impedance values that are not much higher than the acoustic impedance values of drilling fluids. The compressive strength of these cements can also vary significantly. Acoustic impedance and compressive strength are the properties that cement evaluation tools use to differentiate between the fluids and the solids in the casing formation annulus. To identify these differences it is critical that the proper logging techniques and input parameters are selected for the sonic and ultrasonic tools. A statistical analysis of acoustic impedance measurements from the ultrasonic tools has also proven to be valuable for the evaluation of these special cements.

Examples from Alaska, and other areas where these types of cements are being used to solve difficult completion problems, will be shown to demonstrate the use of these cement evaluation techniques.

Introduction Since the 1960s sonic logging tools have been used to evaluate the placement of cement for hydraulic isolation of formations in both production wells and injection wells. Since this time, there have been several advancements in the logging tools that improved the ability to evaluate the cement sheath. During the same period of time the cements being used changed little. During the past few years however, there has been an emphasis on optimizing the cementing operation and reducing the overall cost of the completion. To the cementing operation, this meant developing lightweight and specialized cements that would allow setting casing strings deeper without worrying about lost returns. Other gains in efficiency were also achieved using lighter cements while drilling and completing weak formations. Changes in these cements, and their properties, have also brought about the need for re-evaluating the techniques and tools used for the evaluation of these cements with the sonic logging tools currently available.

The Cement Bond Log (CBL), while still in existence

in the same form it was originally developed, has also evolved into several other cement logging tools using the same physics of measurements with improved measurement outputs. The first tool of this type uses two transmitters and three receivers to make a direct measurement of attenuation that compensates for many of the problems observed in the basic CBL measurement of amplitude. This measurement was then put into another tool that could measure the attenuation in six sectors around the circumference of the casing. Since the use of the Cement Bond Log first began, there have been several different versions of the standard CBL tool to measure the amplitude in different sectors around the casing.

Perhaps the most significant development in cement

evaluation technology was the ultrasonic logging tools. These tools were first used in the early 1980s and originally had 8 fixed ultrasonic transducers spaced at 45 degree intervals around the tool. This allowed for higher resolution cement evaluation of the cement sheath. The weaknesses of this tool were the inability to log in high density borehole fluids, and the lack of full radial coverage of the casing circumference. These problems were mostly overcome with the second generation of ultrasonic cement evaluation tools. These tools have a single rotating transducer that provides full coverage of

SPE 76713

The Evaluation of Specialized Cements R.J. Butsch, SPE, and C.W. Morris, SPE, Schlumberger, R. Wydrinski, SPE, BP, and M.J. Kasecky, Schlumberger

2 R. J. BUTSCH, C. W. MORRIS, R. WYDRINSKI, M. J. KASECKY SPE 76713

the circumference of the casing as well as the ability to log in all but the heaviest borehole fluids.

The change in cement properties required for making

the lightweight cements has created the perception that the evaluation of these cements is very difficult for all logging tools. In fact, the current logging tools with only minor changes in the parameters can evaluate these new lightweight cements with few problems. By understanding what the differences are between the tools, and how each of the logging tools is interpreted one can choose the tools and parameters to best evaluate these cements.

The Logging Tools It is not the intent of this paper to discuss in great detail the various logging tools and all of the parameters, inputs and outputs, associated with each tool. This information can be found in the references. The paper will discuss the general principles of the different type tools and how the differences in the tools and their interpretation can be a benefit or detriment in the evaluation of lightweight cements. The CBL type tools, which include all tools that measure amplitude or attenuation, have common theory of measurement, interpretation principles, strengths, and weaknesses. The principle of measurement of these tools is to measure the amplitude of a sonic signal, produced by a transmitter emitting a 20 kHz acoustic wave, after it has traveled through a section of the casing. This amplitude is then converted into attenuation by either using a ratio of multiple transmitter and receiver amplitudes, or using chart book conversions. The percent of the circumference of the casing bonded is then computed by calculating a Bond Index (BI) where: BI = Measured Attenuation / Attenuation in 100% bonded casing At this point the interpreter has to select a value for the attenuation of a 100% bonded interval. This can be done based on the CBL data collected in the well or it can come from the cement properties provided by the cementing company. The value for the attenuation in a 100% bonded interval is the key to the interpretation of this type of log. Cement properties are the key to the interpretation of the CBL type tool. It is important to make accurate estimates of the properties if they are not known from the cementing company. These tools also provide a qualitative indication of bond to the formation through the use of a VDL waveform. The ultrasonic type tools are designed to measure the acoustic impedance of the material on the outer surface of the casing. This is accomplished by using a transducer to project a short pulse of acoustic energy with a bandwidth of 200 kHz to 700 kHz toward the casing. The transducer then becomes a receiver and measures the returning echo from the casing. The analysis of the returning wave can be performed in several

different ways with the outputs being acoustic impedance of the material on the outside surface of the casing and thickness of the casing. Early tools of this type had eight transducers spaced at 45 degree intervals around the tool. Current versions of the ultrasonic tools have a rotating transducer that provides full radial coverage of the casing circumference. These tools have far better vertical and radial resolution than the CBL type tools. The interpretation of the cement bond for the ultrasonic tools is determined by the selection of a liquid/cement threshold for acoustic impedance. Where the acoustic impedance is greater than the liquid/cement threshold, the casing is interpreted as cemented. Where the acoustic impedance is less than the liquid/cement threshold, the casing annulus is interpreted to be fluid. An image of the acoustic impedance measurements and an image of the interpreted data is then created to provide a visualization of the cement in place.

The parameter that is key to the interpretation of the ultrasonic tools is the selection of the liquid/cement threshold, which is based on the acoustic impedance of the liquids that could be outside the casing, rather than the cement that might be in contact with the casing. Therefore, it is not really necessary to know the acoustic properties of the cement in great detail when logging with ultrasonic tools. It is always good to be able to compare the measured response of the acoustic impedance with the expected acoustic impedance, but these properties are not needed for the interpretation. There are several approaches used by the service companies to enhancing the interpretation of ultrasonic logs where gas-contaminated cement, gas-filled microannulus, or liquid filled microannulus are present.

Comparing the different type cement logging tools,

the main advantage the CBL type tools have over the ultrasonic tools is the small affect of the borehole fluids. These tools can be run in very heavy mud. The ultrasonic logs cannot be run in very heavy drilling mud, but heavy mud will rarely be used in wells utilizing lightweight cements. The CBL tools provide qualitative information about the bond to the formation through the use of the VDL waveform, where the ultrasonic logs provide no information about the cement to formation bond. The ultrasonic tools have two main advantages. First, the ultrasonic logging tools are capable of much better vertical and radial resolution. This provides the interpreter an improved “picture” of what the cement sheath looks like. Second, the interpretation of the cement is not dependant on the acoustic properties of the cement. The ultrasonic interpretation is sensitive to the acoustic properties of the fluids. The only requirement is there must be at least a 0.5 MRayl difference between the acoustic impedance of the fluid and the acoustic impedance of the cement. This is considered to be the accuracy of the acoustic impedance measurement.

SPE 76713 THE EVALUATION OF SPECALIZED CEMENTS 3

The Cement The development of lightweight cements has been aimed at allowing more efficient and economic completion operations especially in low strength formations. By using lightweight cements, casing could be set deeper or a casing string could be eliminated. These new lightweight cements exhibit outstanding performance at densities as low as 10 lbm/gal. For the proper evaluation of these lightweight cements, the changes in acoustic properties must be considered for each evaluation tool type.

Lightweight cements are achieved by either adding low density materials to the cement such as glass or ceramic beads, using gases to create a foamed cement, or by controlling and proportionally mixing the size of graded aggregates within the cement. Table 1 provides a comparison of the common acoustic impedance values for many of the materials and cements. The data from this table illustrates the wide range of values in acoustic impedance for various cements. Since many CBL interpretation charts still use compressive strength rather than acoustic impedance, Ref. 4 provides an empirical conversion from acoustic impedance to compressive strength for standard cements.

The variation in acoustic impedance or compressive

strengths of different cements can cause problems with the evaluation for those tools that depend on knowledge of the cement acoustic properties to interpret the log data. The CBL type tools require the interpreter to determine a maximum attenuation or minimum amplitude for the interpretation of the cement sheath. This is based on the acoustic properties of the cement and is a function of the casing size and thickness. A selection of the wrong acoustic properties of the cement would cause the interpretation to be in error. The interpretation of the ultrasonic tools is based on the acoustic properties of the fluid rather than the cement. Since +/-0.5 MRayl is considered to be the accuracy of the ultrasonic tools in this part of the measurement range, all that is required is that there be at least 0.5 MRayl difference between the fluid acoustic impedance and the cement acoustic impedance. Cements that may not have sufficient difference with the fluids are usually foamed cements or have some gas-contamination. These cements are best evaluated using techniques such as discussed in Ref. 2. Examples Lightweight cement with microannulus. A USIT∗/CBL log combination was run in a well cemented with 12.0 lbm/gal mixed aggregate lightweight cement. The drilling mud system in the wellbore was 10 lbm/gal brine. The measured acoustic impedance of the cement solids is in the range of 3-5 MRayl and the USIT could easily distinguish solid, liquid, and gas behind casing. Fig. 1 shows the USIT and CBL log results

∗ Mark of Schlumberger

across one interval with good zone isolation. The logs were run with the wellbore under zero applied wellhead pressure and with 2000 psi applied pressure to remove possible micro-annulus conditions.

There are three bond index curves shown in Fig. 1. On the USIT presentation, the bond index (A) from the CBL field analysis (obtained under pressure) is shown. The CBL presentation gives a bond index (B) from both a wellbore pressure applied pass and a no pressure pass. The CBL interpretation requires some knowledge of the cement properties and a pick of the 100% cement bond amplitude. The difference between the two bond index curves obtained under pressure represents the result of different 100% bond end point picks used to perform the calculations. The two bond index curves from the CBL show that the measurements are affected by a micro-annulus at zero applied pressure, whereas, the USIT measurements are not affected. Gas-generating cement mixture. This job was performed in a well with 7-5/8”, 33.7 lbm/ft casing cemented with an anti-gas migration agent. This cement is designed to produce gas from the cementing material as it cures, creating a positive pressure in the annulus space to prevent formation gas migration from contaminating the cement. It is perceived by some that this type cement sheath has bubbles and/or voids that will affect the measurements. However, the gas bubbles, under downhole conditions, are very small (below the level of resolution for any ultrasonic tool).

The USIT and CBT* logs were run in 9.9 lbm/gal brine without pressure being applied to the borehole. The results across one interval are shown in Fig. 2. The gamma ray curve indicates the oil production interval with a short 10 ft gas cap at the top. Good zone isolation was achieved during this operation as indicated by both tools. The USIT shows a general cement acoustic impedance of 4-7 MRayl and 100% bond. The gas zone shows some limited cement contamination, but the micro-debonding processing shows solid material around the circumference of the annulus. The CBT also indicated good casing bond with an attenuation range of 8-9 DB/ft and a bond index from 80-90%. The VDL display shows no pipe ringing and good formation arrivals. The gas zone is well isolated from the oil zone. Gas-contaminated cement. This well was completed with a 12.2 lbm/gal mixed aggregate lightweight cement. The wellbore was constructed of 7”, 29 lbm/ft casing in an 8-1/2” borehole. Fig. 3 shows the USIT log results over an interval of about 200 feet with a known gas sand across the top section. The log is presented in two parts – the left part is a standard cement evaluation and the right part is the micro-debonding processing of the same data. The cement, being a higher density mixture, shows an acoustic impedance range from about 4-6 MRayl and a bond index near 100%. The upper section shows the response of a cement sheath that has been severely gas contaminated by the migration of formation


gas into the annulus. The micro-debonding processing2 (green color image) clearly indicates the gas contaminated solids and produces a bond index very similar to the CBL log measurement (black curve in bond index track). Zone isolation has been achieved across this interval. Zone isolation evaluation. Fig. 4 shows log data from a USIT run to evaluate the primary cement job of a 7” 26 lbm/ft production casing. The cement was a mixed aggregate lightweight cement of 11 lbm/gal. The fluid in the wellbore while drilling and cementing this well was an 8.8 lbm/gal oil base mud system. The average deviation throughout this section is approximately 73 degrees. The cementing procedure was to first inject an annular flush to remove drilling fluids and foreign materials in the 8-1/2” borehole to 7”casing annulus prior to cementing. The next step was to pump 40 barrels of diesel, followed by 40 barrels of mud displacement fluid. This was followed by 92.4 barrels of 11.0 lbm/gal cement and the plug was bumped with 8.8 lbm/gal oil base mud.

The USIT log was run 4 days after pumping the cement. Of primary concern was a comprehensive cement evaluation to diagnose if remedial cementing was necessary to provide hydraulic isolation of the production perforations from the water-bearing reservoir below it. The gamma ray curve in Fig. 4 shows the location of the oil productive interval, a shale break and the water sand at the bottom of the interval. The proposed production perforations were all the way to the lower portion of the oil zone. The USIT shows an adequate cement sheath in the 7” casing annulus across the shale section to provide hydraulic isolation from the water reservoir below. The acoustic impedance measurements are in the 3-5 MRayl range. Nearly 100% cement sheath coverage was obtained with no continuous channels up to the base of the production perforations. Production test results showed no water production, confirming zone isolation. Two-slurry cement operation. Fig. 5 shows log data from a USIT run to evaluate the primary cement job on a 7-5/8” production casing cemented with a two-slurry single stage cement job. This portion of the log is the upper section of the well across the permafrost layer. The cement slurries designed for use in permafrost are a unique blend of lightweight cements; these cements ultimately develop compressive strengths in the range of 500 to 800 psi. Due to the lower compressive strength, these cements have much lower acoustic impedance properties and can be extremely difficult to evaluate using conventional sonic bond logging techniques. To date, the USIT has been used with very favorable results in instances where a quantitative evaluation of these cements is required. The casing is a 7-5/8”, 29.7 lbm/ft casing and the well was drilled with an 8-1/2” drill bit. The drilling fluid in the well during the drilling and cementing process was a 9.6 lb/gal lignosulfonate mud. The deviation in this section of the well is nearly vertical. The cement job was designed for an annular flush of the 8-1/2” borehole to 7-5/8”

casing annulus to remove drilling fluids and drill cuttings. This was achieved by pumping 20 barrels of wash and 40 barrels of 10.5 lbm/gal mud displacement spacer. Immediately following the annular flush was 504 barrels of the lightweight special blend cement and 70 barrels of 15.8 lbm/gal class G cement. The plug was bumped with a 9.6 lbm/gal lignosulfonate mud.

The USIT was run several months later during the completion process. The primary objective was to provide a quantitative evaluation of the two slurry, single stage cement job. Prior to pumping this particular cement job in a single stage, the common cementing practice had required more complex multi-stage operations. These previous operations required more rig time as well as additional cementing hardware. The primary cement job showed full returns to surface. No remedial cementing operation was undertaken prior to the drilling rig moving off the well. The top part of Fig. 5 shows the interval where the port collar is located. This port collar was run such that it could be shifted open and more cement could be pumped if remedial cementing operations were required. The log shows cement with no continuous channels from lower intervals towards the surface indicating good hydraulic isolation. This condition meets the regulatory requirements for surface casings with continuous cement from the shoe to surface. At some point during the completion operations the port collar was somehow shifted open. In preparation for the hydraulic fracture stimulation of the production interval, the 7-5/8” casing and 3-1/2” fracture string annulus was pressure tested. This pressure test showed that the port collar had been shifted open. Although they were able to pump into the annulus at the port collar, no pressure response was seen in the 7-5/8” casing-to-conductor annulus, confirming the isolation between the port collar and surface.

The lower part of Fig. 5 shows the second stage cement interval. The class G cement, with the higher acoustic impedance values (5-7 MRayl), is clearly channeling through the annular space. This indicates that the higher density cement did not efficiently displace the light cement/fluids in the annulus. However, there does appear to be zone isolation with no continuous fluid channels across the interval. Time lapse logging. Because lightweight cements normally have acoustic impedance properties that are in the 3-5 MRayl range, it is important that the cement be well cured in order to differentiate between the cement and other fluids. The temperature, pressure and the additives in the cement mixture determine the curing rate of the cement mixture in the wellbore and ultimately the acoustic impedance values at the time of logging. The following example provides a comparison of cement evaluation logs obtained over several different times.

This well was completed with 7”, 26 lbm/ft casing using a mixed aggregate lightweight cement of 10.0 lbm/gal. The fluid in the wellbore while cementing this well was an 8.6


lbm/gal brine. USIT and CBL logs were run 12, 18 and 27 hours after the cementing operation. Fig. 6 shows one representative interval. The USIT (after 18 hours) shows some cement is present in the annulus and a calculated bond index of 50-60%. But there are clearly short sections within the interval with potential for liquid channels. The CBL showed generally poor bond with an amplitude of about 50 mv. The VDL display shows some formation signal arrival but with strong casing signals. With time, the cement continues to cure and increase in acoustic impedance and compressive strength. The last USIT log, obtained at 27 hours, shows that the cement has improved to a 60-80% bond index and most of the fluid channels are reduced or eliminated from this interval. The last CBL log (not shown) indicated no change in amplitude attenuation or in the VDL response. Conclusions The quantitative evaluation of lightweight and specialized cements is possible by logging tools currently available. Ultrasonic logging tools are more qualitative than CBL type tools because their interpretation is based on the fluid properties rather than the cement properties. The CBL type tools compliment the ultrasonic tools but are less quantitative because of the uncertainty of the cement properties. References

1. Hayman, A.J., Hutin, R., and Wright, P.V.: “High-Resolution Cementation and Corrosion Imaging By Ultrasound”, SPWLA 32nd Annual Logging Symposium, June 16-19, 1991 paper KK.

2. Butsch, R.J.: ”Overcoming Interpretation Problems of Gas-Contaminated Cement Using Ultrasonic Cement Logs”, SPE paper 30509, SPE Annual Technical Conference and Exhibition, Dallas, TX, October 22-25, 1995.

3. Morris, C.W., Zanutto, S.L., Dacres Jr., W.G.,: “Lightweight Cement Evaluation Using Ultrasonic Measurements”, SPE paper 62517, SPE/AAPG Western Regional Meeting, Long Beach, CA, June 19-23 2000.

4. “Cement Sheath Evaluation”, American Petroleum Institute, Technical Report 10TR1, First Edition, June 1996.

Table 1 – Common Acoustic Impedance Values3 Material Acoustic

Impedance (MRayl)

Fresh Water 1.5 Free Gas 0.1 Steel 46.0 12 lbm/gal Drilling Mud 2.16 15 lbm/gal Drilling Mud 2.70 17 lbm/gal Drilling Mud 3.06 9 lbm/gal Foamed Class C (250 psi) 2.19 9 lbm/gal Foamed Class C (1000 psi) 2.69

10.5 lbm/gal LiteCRETE 3.4 12 lbm/gal LiteCRETE 3.6 13 lbm/gal Cement (500 psi) 3.37 13 lbm/gal Cement (2000 psi) 4.42 16.5 lbm/gal Cement (500 psi) 4.38 16.5 lbm/gal Cement (2000 psi) 5.62


Fig.1 – Cement evaluation of a lightweight cement mixture with a microannulus.

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Fig. 2 – Cement evaluation of a gas-generating cement mixture.

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Fig. 3 – Gas-contaminated lightweight cement evaluation.

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Fig. 4 – Production zone isolation from water sand.

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Fig. 5 – Evaluation of a two-slurry single stage cement operation.



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Fig. 6 – Time lapse evaluation of a lightweight cement job.

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USIT – 18 hr. CBL – 18 hr. USIT – 27 hr.

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Bond log of light cements and ultrasonic SPE 76713.pdf