Ultra Sonic Image Logs (USI, RBT)
In addition, precise acoustic measurements of the internal dimensions of the casing and of its thickness provide a map-like presentation of casing condition including internal and external damage or deformation.
Rotating head ultrasonic (acoustic) imaging tools are the current state of the art for cement and casing integrity mapping. The rotating head gives greater circumferential resolution than the segmented CET and CMT class of tools. The casing inspection capability cannot be accomplished by other cement evaluation tools.
The sonde includes a rotating transducer subassembly available in different sizes to log all normal casing sizes. The direction of rotation of the subassembly controls the orientation of the transducer – counterclockwise for the standard measurement mode (transducer facing the casing or the borehole wall), and clockwise to turn the transducer 180 degrees within its subassembly (transducer facing a reflection plate within the tool) to measure downhole fluid properties. The fluid properties are used to correct the basic measurements for environmental conditions.
of the reflected ultrasonic waveforms provides information about
the acoustic impedance of the material immediately behind the
casing. A cement map presents a visual indicator of cement quality.
Impedance is measured in units of megaRayls.
Like the CET, the USI tool analyzes the decay of the thickness-mode resonance signal contained in the reflected acoustic pulse, but the analysis is performed in a different manner. The CET tool has eight fixed transducers in a helical array, 45 degrees apart azimuthally each seeing only a small segment of the casing. The USI tool has a single rotating transducer that looks all around the casing.
As the acoustic impedance of the casing material and of the borehole fluid are essentially constant, the signal inside the casing decays at a rate that is dependent on the acoustic impedance of the material outside the casing.
In contrast to CET processing, which uses traditional energy windows, USI processing derives acoustic impedance directly from the fundamental resonance to measure the following:
1. The acoustic impedance of the cement or whatever material is between the casing and the formation.
2. Casing thickness from the natural resonant frequency of the casing, which is approximately inversely proportional to the wall thickness.
3. Internal casing radius. The time between the firing and the major peak of the echo is measured by locating the waveform peaks. Time is converted to a measurement of the internal radius using the fluid properties measurement to compute the velocity of sound in mud, taking into account the transducer’s own dimensions.
4. Casing inspection. The inside and outside diameters are determined from the transit time and casing thickness measurements. The maximum amplitude of the waveform provides a qualitative measure of the internal surface rugosity of the casing.
Several presentations are available to address specific applications. Negative conditions are indicated by the color red. For example, red curves represent outputs for tool eccentering, minimum amplitude, maximum internal radius, minimum thickness, gas index, and so on. Increasing intensity of red in the images represents increasingly negative conditions such as low amplitude, metal loss, and the presence of gas in the cement map. The gas may be intentional, as in foam cement, or unintentional from gas invasion as the cement cures.
The following log presentations are available from USI recordings:
1. Fluid properties presentation, including fluid acoustic velocity, acoustic impedance of fluid, and thickness of reference calibrator plate.
2. Cement Presentation, including cement properties curves, cement map, and casing dimensions, plus synthetic bond index and minimum, maximum and average values of acoustic impedance. Two cement images are generated, one with and one without impedance thresholds.
3. Corrosion Presentation with casing profile, casing reflectivity, casing Internal radii, thickness image, Internal and external radii, average and maximum thickness,
4. Composite Presentation, with cement, corrosion measurements, and processing flags. Two acoustic impedance images are presented: one on a linear scale and one with thresholds corresponding to the acoustic impedance of gas and mud.
These thresholds can be varied for conditions such as light cement (where lower acoustic impedance indicates lower fluid cutoff) and heavy mud (with a higher fluid threshold cutoff). Check the colour scale on each log.
Internal radius images:
Alternate images that plot internal radius and thickness versus API specifications of the casing are available.
The acoustic impedance of the mud must be accurately known to within 10 percent in order to obtain a 0.5-MRayl accuracy in cement. The acoustic impedance of the mud is provided by the downhole fluid properties measurement, which is normally acquired while tripping into the well.
A microannulus affects the apparent cement acoustic impedance. Laboratory experiments show that a 100-micron (0.004 inch) microannulus results in a 50 percent loss in apparent impedance. Even the smallest liquid-filled microannulus causes the loss of shear coupling into the cement and a drop of approximately 20 percent in impedance. Whenever the presence of a microannulus is suspected, the USI tool should be run under pressure to obtain an improved acoustic impedance measurement.
A dry microannulus is called micro-debonding and gives a patchy looking cement image.
The USI tool can resolve the impedance of the material filling a channel down to 1.2 inches, which is therefore the minimum quantifiable channel size. The angular resolution improves for larger diameter casing, from 30 degrees in 4.5-in. casing to 10 degrees in 13 3/8-in. casing. However, interpretation is required since channels are not always surrounded by high-impedance cement nor are they always filled with low impedance material.
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