Advances in Cement Evaluation Tools and Processing Methods Allow Improved Interpretation of Complex Cements
Abstract: TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractEvaluation of cement bonding and zonal isolation is a challenge that the oil and gas industry continues to face as wells are drilled deeper within more hostile environments. The complexity of this task has increased as these wellbores have more challenging trajectories and being drilled in formations for which there is little drilling and completions experience. In addition, cement slurries have become more complex with the addition of inert gases, micro-sphe… Show more
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Because of increasing rig rates, the costs associated with the cement bond evaluation are now subject to scrutiny. Traditionally, the cost of the service and interpretation were linked to the value of the information provided, i.e. the quality of data gathered was directly related to the necessity of zonal isolation. The daily rig cost must now be considered, specifically costs directly related to the operating time required to acquire the cement evaluation data. The cost of operating rig time is often an order of magnitude higher than that of the cement bond logging service and should be a critical factor in the determination of the service provider and type of equipment run. The correct logging tool and evaluation method provides the best process for evaluating the cement sheath. Several wellbore factors should be considered when selecting the proper tool for optimal results, including casing size, well deviation, fluid type, and fluid weight during the logging operations. However, the reservoir and the chosen completion should also influence the selection of the cement evaluation tool. Reservoir properties to consider include water contacts in or near zone of interest, drive mechanisms and isolation between production zones. The completion design, including perforation strategies, stimulation method, and sand control, should also influence the cement sheath evaluation process. The challenges of a cost efficient and complete cement sheath evaluation required the development of a new cement evaluation tool. The FASTCAST™ tool expands the capabilities of CAST-V™ and existing analysis software. The new ultrasonic tools increase the logging speed up to five times, significantly reducing logging time, rig time, and costs. The tool provides cement evaluation and casing evaluation during the same logging pass. The existing software packages for cement and casing evaluation were updated to provide a complete analysis package for the new tool. The paper presents case histories to provide details about the economic advantage from using this new technology and processed logs showing both casing and cement evaluation. Traditional Cement and Casing Evaluation Tools Traditional cement evaluation uses standard tools and methods to determine the presence and integrity of the cement sheath. These tools include the sonic or standard cement bond log (CBL), and the newer ultrasonic tools, which consist of the multi-transducer ultrasonic tools and the new generation of rotating, single-transducer ultrasonic tools. Several previously published papers in the bibliography Frisch et al.12345 provide more detail about tool theory and applications. When combined, these two types of tools provide an accurate determination of the cement sheath and cement to pipe and cement to formation coupling. Unfortunately, the standard logging speed of cement evaluation tools are 15 FPM to 30 FPM. Since the cost of a typical remedial cement operation can be as much as $2,000,00 in Deepwater US, it is time to seriously evaluate cement evaluation tools and techniques to reduce needless remedial cement expenses and cement evaluation logging time. Cement Bond Log (CBL) Tools In conventional CBL tools, a transmitter is pulsed to produce an omnidirectional acoustic signal that travels along various paths through the borehole fluid, pipe, cement, and formation, to a set of receivers. The logging system records the received waveforms and displays them on the log along with a pipe-amplitude curve. Interpretation of the CBL uses these two measurements to evaluate two bonds; the first bond measures the cement-to-pipe bond, and the second measures the cement-to-formation bond. These tools include cement bond (CBL), segmented bond (SB), and the radial bond (RB). Traditionally the pipe amplitude curve has been used to determine the quality of the pipe to cement bond, while the waveform display is used to determine both the pipe to cement bond and the cement to formation bond. The classic interpretation of the waveform display is that straight traces indicate there is no cement in the borehole, while any variation in the acoustical waveform indicates that some cement is present.
Because of increasing rig rates, the costs associated with the cement bond evaluation are now subject to scrutiny. Traditionally, the cost of the service and interpretation were linked to the value of the information provided, i.e. the quality of data gathered was directly related to the necessity of zonal isolation. The daily rig cost must now be considered, specifically costs directly related to the operating time required to acquire the cement evaluation data. The cost of operating rig time is often an order of magnitude higher than that of the cement bond logging service and should be a critical factor in the determination of the service provider and type of equipment run. The correct logging tool and evaluation method provides the best process for evaluating the cement sheath. Several wellbore factors should be considered when selecting the proper tool for optimal results, including casing size, well deviation, fluid type, and fluid weight during the logging operations. However, the reservoir and the chosen completion should also influence the selection of the cement evaluation tool. Reservoir properties to consider include water contacts in or near zone of interest, drive mechanisms and isolation between production zones. The completion design, including perforation strategies, stimulation method, and sand control, should also influence the cement sheath evaluation process. The challenges of a cost efficient and complete cement sheath evaluation required the development of a new cement evaluation tool. The FASTCAST™ tool expands the capabilities of CAST-V™ and existing analysis software. The new ultrasonic tools increase the logging speed up to five times, significantly reducing logging time, rig time, and costs. The tool provides cement evaluation and casing evaluation during the same logging pass. The existing software packages for cement and casing evaluation were updated to provide a complete analysis package for the new tool. The paper presents case histories to provide details about the economic advantage from using this new technology and processed logs showing both casing and cement evaluation. Traditional Cement and Casing Evaluation Tools Traditional cement evaluation uses standard tools and methods to determine the presence and integrity of the cement sheath. These tools include the sonic or standard cement bond log (CBL), and the newer ultrasonic tools, which consist of the multi-transducer ultrasonic tools and the new generation of rotating, single-transducer ultrasonic tools. Several previously published papers in the bibliography Frisch et al.12345 provide more detail about tool theory and applications. When combined, these two types of tools provide an accurate determination of the cement sheath and cement to pipe and cement to formation coupling. Unfortunately, the standard logging speed of cement evaluation tools are 15 FPM to 30 FPM. Since the cost of a typical remedial cement operation can be as much as $2,000,00 in Deepwater US, it is time to seriously evaluate cement evaluation tools and techniques to reduce needless remedial cement expenses and cement evaluation logging time. Cement Bond Log (CBL) Tools In conventional CBL tools, a transmitter is pulsed to produce an omnidirectional acoustic signal that travels along various paths through the borehole fluid, pipe, cement, and formation, to a set of receivers. The logging system records the received waveforms and displays them on the log along with a pipe-amplitude curve. Interpretation of the CBL uses these two measurements to evaluate two bonds; the first bond measures the cement-to-pipe bond, and the second measures the cement-to-formation bond. These tools include cement bond (CBL), segmented bond (SB), and the radial bond (RB). Traditionally the pipe amplitude curve has been used to determine the quality of the pipe to cement bond, while the waveform display is used to determine both the pipe to cement bond and the cement to formation bond. The classic interpretation of the waveform display is that straight traces indicate there is no cement in the borehole, while any variation in the acoustical waveform indicates that some cement is present.
Cement is a critical component in an oil well because it provides sealing and zonal isolation within the well. To help ensure proper performance of a cementing operation, various evaluation techniques/tools have been developed and used over the years, including some based on ultrasonic pulse echo technology. Ultrasonic pulse echo evaluation tools can play a key role in determining in-situ cement performance and if any remedial cementing is required. As the oil and gas industry continues to migrate toward more challenging reservoirs with increasingly harsh conditions, and coupled with more stringent regulatory requirements, significant technology improvements are helping ensure well integrity and safety. For example, ultrasonic tools are being reengineered for the capability to penetrate thick-walled casings (>0.75 in.) commonly used in complex situations, such as deepwater and high-pressure/high-temperature (HP/HT) wells. Meanwhile, there is also great demand for better optimized validation techniques to confirm the operational performance of these new evaluation tools. This paper focuses on investigating the use of oilwell cements as validation media of current and advanced ultrasonic tools designed for cement and casing evaluations. The evaluation system developed in this study consists of a combination of various validation media with a wide range of acoustic impedances (1.5 to 8.5 MRayl) and a multicasing fixture with casing wall thicknesses ranging from 0.4 to 1.4 in. This system could be used for the evaluation of tool performance in the presence of various muds and across a range of temperature and pressure conditions. Performance effectiveness was evaluated by measuring the acoustic impedance of the different media bonded to a multithickness casing and comparing the results to those acquired by direct acoustic measurement performed on the same media. The evolution of acoustic properties over time, directly related to cement hydration, was also monitored. An advanced pulse echo technology tool was used to perform preliminary validation using the newly developed evaluation system. This resulted in a clear trend of impedance evolution that correlates well with the direct impedance measurement of cement media.
With the advent of the API RP 65 – Part 2 (post-Macondo), the concept of well integrity evaluation has been given an entirely new meaning and importance. Watching acoustic logs in isolation without sufficient correlation with the formation evaluation logs and cement placement simulation yields a lot of unanswered questions. It was at this stage that Schlumberger (SLB) was approached by Saudi Aramco's drilling optimization team to evaluate the zonal isolation between the Formations Shale A and Sandstone B in Well-A, which was a well from the unconventional resources group. The objective was to assess the zonal isolation with a high level of assurance and provide feedback for obtaining better results in subsequent wells. The paper showcases the technique applied to assess the well integrity through an integrated holistic approach for the first time in the Middle East that looks all data available rather than being confined to looking up the acoustic logs in isolation that many times provides only part of the answer.
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