Cementing a well to achieve zonal isolation, then confirming the integrity of the cement sheath before proceeding to completion, is a basic requirement in the well-construction process.

A competent cement sheath contributes to the safe, environmentally sound and profitable operation of cased oil and gas wells by helping ensure that targeted subsurface geologic zones may be produced without directly communicating with other formations penetrated by the well bore. Strong cement-to-pipe and cement-to-formation bonds, properly positioned in the well bore, also help manage subsurface stresses that can lead to failure of the casing string as a result of buckling, parting, elongation or deformation.

The ability of the cement sheath to achieve zonal isolation can be diminished before a well is placed into service by downhole conditions encountered during drilling and completion, such as low formation fracture gradients, lost-circulation zones, or variable wellbore size due to washouts.

Confirming cement-sheath integrity
Assuring the competency of a primary cementing treatment is essential before the completion of an oil and gas well can proceed. Yet, inaccurate evaluation of downhole cement placement and bonding characteristics can prove costly. For example, if logging data are inaccurately interpreted indicating the cement sheath has isolated the geologic zones intended, problems are sure to arise and costs certain to escalate if the operator attempts to complete the well and place it on production.

Similarly, concluding incorrectly that cementing objectives have not been achieved can lead to unnecessary—and, again, sometimes costly—remedial cementing treatments. According to one estimate, as much as 20 percent of the $200 million spent globally each year to remediate faulty primary cement jobs might be spent unnecessarily because of misinterpretations of available cement logging data.

When considered within the context of the foremost objective in drilling and completing a well and placing it on production—that is, achieving the optimal combination of efficient well construction, high recovery rates, and long production life—confirming the integrity of a primary cement job in an oil and/or gas well is as important as selecting the optimal cement system and properly pumping the chosen treatment.

Standard Presentation for a CBL/CAST-V Combination for a 8 lb/gal slurry. Click image to enlarge and for more information.

CBL tools provide waveform images and amplitudes derived from the acoustic signals, which together help describe cement-to-pipe and cement-to-formation bonding. Typically, any variation in the acoustical waveform that corresponds to changes in the lithology indicates that some cement is present.

Ultrasonic multi-transducer and scanning transducer logging tools provide circumferential images and more detailed information about cement-to-pipe bonding, by the use of the impedance value of the materials in the annular space. In essence, ultrasonic tools discern cement from fluids based upon their impedance values.

While CBL data are omnidirectional, ultrasonic data are azimuthal. That means ultrasonic data not only can detect channels in cement, but also can enable determination of channel orientation.

When used in tandem, CBL and ultrasonic logging tools can determine accurately the integrity of a sheath constructed using conventional oilfield cements. Unfortunately, the standard approaches to evaluating conventional cements using sonic and ultrasonic logging data are far less reliable when applied to low-density, lightweight, or complex cements, which have become the cementing systems of choice for deep, highly deviated, horizontal, or high-temperature/high-pressure oil and gas wells, and other high-cost, cutting-edge applications. Difficulties arise because many of the additives used in lightweight, high-performance cementing systems alter the acoustic properties of the cement.

In fact, the acoustic impedance values of foam cements can be less than those of water or drilling or spacer fluids, and can even approach the impedance of free gas. Ultrasonic data and images exhibiting low impedance values can be misinterpreted to indicate fluid is behind casing rather than cement, even when complete cement bonding has been achieved.

New cement evaluation method
Halliburton in the 1990s developed and introduced a new evaluation method that reinterprets data already being generated by standard CBL and ultrasonic logging tools and procedures to provide quick, accurate information about the integrity of lightweight, foamed or complex cement sheaths. Known as Advanced Cement Evaluation (ACE™), the technique employs statistical variation processing (SVP) to distinguish foamed cement from fluid, even when both have identical impedances.

CAST-V With Segmented Curves For An 8-Lb./Gal Slurry. Click image to enlarge and for more information.

Although the ACE process was developed specifically for lightweight, high-performance cements, results of lab tests and experience in the field have shown that it provides a superior method for interpreting and evaluating any type of cement, including conventional, complex, lightweight or foam slurries. As a result, Halliburton recommends using ACE to analyze cement data any time there is a question about the integrity of the cement sheath.

ACE processing is used to determine the derivative activity level of the impedances of materials behind the casing. Homogeneous materials, such as water, show very little variation in the activity level, while solids, such as cement, are nonhomogeneous and therefore show a high rate of activity. The derivative impedance-activity values determined by ACE processing allow detection of minor differences in the composition of gases, liquids and solids in the well, enabling users to easily identify the presence and position of cement.

Perhaps most importantly, ACE processing gives clear answers to the most urgent cementing questions: How good is the well's zonal isolation? And: Does this well need a remedial cement squeeze job?

Evaluating cement using ACE
The ACE process yields its greatest benefit when applied to CBL and ultrasonic data logged in tandem. This technique enables an engineer to provide a detailed zonal isolation study, complete with reinterpreted CBL waveform and impedance data augmented by the derivative impedance values calculated with SVP.

Reprocessing conventional CBL waveform data with the ACE method highlights information not previously used to evaluate cement bonding, particularly responses of collars on the cemented casing strings.

Advanced Cement Evaluation Display For The 8-Lb./Gal Slurry Click image to enlarge and for more information.

Evaluating cement bonding within concentric strings of pipe has been notoriously difficult because of the excessive noise reflected by outer strings and the extremely thin cement sheath that is often unavoidable at collars of the inner strings. With ACE processing applied to CBL waveform data, distinct sets of collars may appear twice on the CBL variance log, as wedges or arrowhead shapes. The first response is from the inner casing, and the second collar response is due to the outer casing. Such outer casing collar responses are possible only if there is acoustic coupling between the two strings, indicating the presence of a bonded cement sheath between the two strings of casing.

ACE processing uses the original ultrasonic impedance data along with the derivative impedance-activity data to provide an image that improves the certainty of determining pipe-to-cement bonding. Combining data yielded by two different analytic methods improves the confidence in the accuracy of the amount of cement bonding indicated, or lack thereof.

With the introduction of ACE for scanning transducer logging, derivative impedance activity levels are calculated directly, enabling creation of a new foam cement image based upon the results of both the raw impedance image and the derivative image. In addition, single-transducer ACE processing enables the calculation of several new curves based upon the number of sample shots per depth, including the normal bond index from the scanning tool without any further processing, the bond index from the derivative image, and the bond index from the foam cement image.

Ultrasonic data from scanning transducer tools can be arranged into segments, with each segment representing vertical data points at a given position on the circumference of the well bore. This approach enables quick evaluation of data at specific circumferential positions in the well and allows useful curve data to be traced back to the original image.

In addition to Halliburton sonic and ultrasonic cement evaluation tools, ACE processing has been adapted to work on every cement evaluation tool currently being deployed by other major oilfield service companies. So Halliburton cement engineers can evaluate cement even in places around the world where the company offers no logging services. In fact, Halliburton can provide an accurate evaluation of the cement sheath no matter the type of cement, the type of cement evaluation tools used, or location of the well.

In 2002, Halliburton adapted ACE processing to image attenuation data generated by another service company. In 2003, the company modified ACE to account for variations in casing surfaces and tailored the software to fit the specific needs of a client.

Advanced Cement Evaluation Display Using BP Option Click image to enlarge and for more information.

This year, ACE processing technology has been adapted for radial bond logging tools manufactured by outside vendors and available to many service companies. Halliburton is the only service company that can process radial bond data using knowledge gained from previous adaptations of ACE for other cement evaluation tools.

The process has been used successfully all over the world to reliably evaluate every type of cement presently in use in the upstream oil and gas industry—including conventional, complex, lightweight, or foam slurries—based upon data obtained with literally every cement logging tool used by major oilfield service companies. Under the right circumstances, ACE typically can deliver answers right away—in real time when Halliburton data are used and in hours when evaluating third-party cementing data. In one instance, Halliburton processed third-party ultrasonic data, completely analyzed the results, and thoroughly discussed the findings with the customer, before the original logging company provided the basic interpretation.

ACE saves time and money because it works with existing, standard logging data, helping minimize rig time by avoiding additional logging runs. It is reliable whether processing data from new wells or reprocessing data from existing wells acquired weeks, months or even years earlier.

ACE also has helped eliminate countless unnecessary squeeze jobs and associated expenses, by confirming the presence of cement behind the casing when conventional processing techniques could not provide a clear interpretation. Avoiding unnecessary remedial cement operations not only contributes to the bottom line by reducing operating expenses, it also diminishes the possibility of casing or formation damage, which can enhance both production rates and ultimate recovery.