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^
