Summary

The CET (*) tool was initially designed as a Cement Evaluation Tool, based on the ultrasonic resonance of the casing. This resonance, induced and sensed in a pulse-echo mode, depends on the thickness of the pulse-echo mode, depends on the thickness of the casing, on the medium behind the casing, and is also sensitive to casing wear and corrosion. In addition to the exploitation of the amplitude of the echoes and of its eight high resolution (0.1 mm) calipers, the tool was modified to give eight local steel thickness measurements, 45 degrees apart. Thickness is obtained with a resolution of 0.1 mm (0.004 in.) through the determination of the resonant frequencies of the pipe wall.

The technique has been widely field-tested, in Europe and in North America, and log examples show the ability of this new tool to detect changes of casing weight, distortion, perforation damage, scaling, wear from the drill pipe, corroded zones, acid attack …

This new tool performs the same cement measurements as the CET tool, and has the same size. By providing cement and corrosion evaluation in a single run, it is a unique means of casing investigation.

Introduction

The Cement Evaluation Tool contains eight high frequency ultrasonic transducers examining eight different azimuths of the casing with a very fine vertical resolution. The basic idea is to make the casing resonate in its thickness mode. The transducers act as transmitters and receivers, each transducer emitting a short pulse of acoustic energy and then receiving the echo from the casing. A 9th transducer in the tool is used as a reference to measure the properties of the mud. For each waveform received by each transducer, several measurements are made (figure 1):

  1. Transit time (DT)

    This measurement is the interval of time that separates the firing of the transducer from the reception of the echo. This delay is proportional to the distance from the transducer to the casing. It is used to compute the caliper outputs of the tool.

  2. Peak value (W1).

    The first part of the pulse is the reflection from the inside surface of the casing. Its amplitude (W1) is a characteristic of the sensitivity of the transducer, of the attenuation of the fluid inside the pipe, and of the roughness of the inside surface of the pipe. It does not depend on casing thickness or cementation.

  3. Resonance amplitude (W2 and W3).

    The amplitude of the resonance that follows the peak value depends on the materials that are behind the casing, which can be cement, mud or gas. Through the measurement of resonance amplitude, it is possible to evaluate the acoustic properties (acoustic impedance) of these materials. To achieve that, the received waveform is rectified and integrated within two gates (W2 and W3) that cover part of the resonance decay.

  4. Resonance frequencies.

    The normal resonance modes of the casing wall have frequencies which are approximately proportional to the inverse of its thickness. They also depend on the speed of sound in steel which is almost a constant. They were not measured by the CET tool until now; the tool has been modified to determine these frequencies and to derive information on casing thickness.

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