Abstract

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.

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