Methods to evaluate conventional cements with logging data may not yield valid results in foamed cements. Logging tools commonly used in cement evaluation are the traditional cement bond log (CBL) tools and the modern ultrasonic scanning tools. CBL tools provide waveform images and acoustic amplitudes that together help describe cement-to-pipe and cement-to-formation bonding. Ultrasonic scanning tools contribute circumferential images and detailed information regarding the cement-to-pipe bond. The major problem in evaluating foamed cements is that their impedance values can be below that of annular fluids, such as mud or water. Thus, a standard interpretation of ultrasonic images and data in foamed cements may provide an incorrect diagnosis of the cement bond and could lead to unnecessary remedial cementing activities. A new interpretation method expands and improves on previously published methods to effectively evaluate foamed cements with the common cement-evaluation tools. This method uses ultrasonic scanning data to provide detailed information regarding the cement-to-pipe bond and allows foamed cement to be distinguished from mud or spacer fluid behind casing. Cement-slurry weight and composition do not affect this technique. The technique does not require additional logging tools or passes and thus does not introduce additional logging or rig expenses. Examples are presented showing that the new method is valid and is effective in both time and cost. These examples will illustrate several new ultrasonic cement-bond curves that were developed as a result of the new method and that, when used in conjunction with CBL amplitude data, improve foamed-cement evaluation. In the examples, the interpretation is focused to answer the basic question, "Should remedial cementing be performed, or should the well be perforated for production?"

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This paper will demonstrate cased-hole applications of a new ultrasonic scanning tool. Field logs are presented from environments containing various wellbore, casing, and cement slurry types. Cement evaluation and casing inspection examples will include logs from wells with heavy completion fluids, lightweight cements, heavyweight casing, and hastelloy pipe (used in highly corrosive environments). Very high horizontal and vertical sampling rates permit simultaneous cement evaluation and casing inspection. The data are corrected for direction and tool rotation using a navigation package. Data such as acoustic impedance, cement compressive strength, casing thickness, casing outer diameter and/or radius, casing ovality, tool eccentricity, and 40 to 100 calipers may all be recorded in real time in conventional or imaging formats. Improved processing software provide two- and three-dimensional images. It was observed that differing wellbore and casing conditions require different logging parameters (such as scan rate and sample density). Thus, the mechanical and electrical configuration of the tools are flexible to maximize signal response in all possible downhole conditions. Multiple scanning heads with selectable transducer characteristics are available. Downhole processing also reduces telemetry demands and allows other tools to be combined with the new scanning device to provide additional real-time data without reducing logging speed or delaying analysis processing until after logging. Introduction Downhole ultrasonic scanners have been used primarily for imaging wellbores. A new scanner that furnishes improved images and that offers remarkable casing inspection and cement evaluation capabilities has been developed and field proven. The new device operates in two modes: image mode and cased-hole mode. In image mode, the scanner evaluates only the "inner" surface of the target (the formation bounding the wellbore or the inner wall of the casing). High vertical and azimuthal sampling provides high-resolution, two and three-dimensional images. These images are useful in locating fractures, identifying borehole breakout, delineating bed boundaries, studying formation textural features along the borehole wall, and evaluating casing integrity by revealing distortion, wear, holes, parting, and other anomalies on the inner wall of the casing. When cement evaluation or a more complete casing assessment is needed, the ultrasonic scanner operates in cased-hole mode. The full circumferential maps of casing thickness and acoustic impedance generated from the measurements made in this mode can be used to reveal thinned casing and to clearly distinguish between cement and fluids in the annular space behind casing. In cased-hole mode, a significant part of the acoustic waveform is processed. Thus, high telemetry data rates, intense processing capabilities, and selective transducer frequencies are required. Tool Operation The new scanner, designated as the Visualization version of Circumferential Acoustic Scanning Tool (CAST-V), uses two ultrasonic transducers: a primary transducer and a secondary transducer. The primary transducer is mounted in a rotating scanner head and is in direct contact with the wellbore fluid. The scanner head rotates continuously about the tool axis, transmitting ultrasonic signals and receiving reflections from the casing or formation. P. 79^

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Complex, or non-standard, cement slurries create special, difficult cement evaluation problems which are magnified when high density drilling fluids are used. These complex slurries include lightweight cements (water-extended, ceramic bead, and nitrified cements), gas-channeled cements, multistage density cements, as well as intervals of cement that are contaminated with drilling fluid. Current cement evaluation methods, via conventional logging techniques, are based on signal amplitude, acoustic impedance, and compressive strength relationships. These properties are derived from the materials behind the pipe and interpretation methods rely on how these properties differ in fluids versus standard cement. This desired contrast is not always evident in complex cements; however, statistical variance techniques have been developed that permit accurate interpretations of zone isolation in low-density cements. This new approach to determining zone isolation in lightweight and complex cements with ultrasonic cement evaluation tools is called Statistical Variation Plot (SVP) and is based on the following two factors:Distinguishing solid materials from fluids in the casing-borehole annul usAssuming that solid material cannot be displaced during the remedial cementing operation whereas fluids may be displaced1. It will be shown that this technique is superior to conventional evaluation techniques when the acoustic properties of the cement are similar to the acoustic properties of the annular fluid.