Ultra Sonic Image Logs (USI, RBT)
The USI Ultra Sonic Imager and UBI Ultrasonic Borehole Imager tools are acoustic borehole imaging logs. The USI is a cased hole cement bond / cement mapping tool with output similar to the CET, but with more complete coverage of the borehole. The UBI is an openhole tool for formation imaging. They are physically similar to each other but use different transducers. This tool type is sometimes referred to as a rotating-head bond tool (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.

Analysis 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.







INTERPRETATION RULE 1: Low Amplitude = Good Cement
INTERPRETATION RULE 2: High Attenuation = Good Cement
INTERPRETATION RULE 3: High Bond Index = Good Cement
INTERPRETATION RULE 4: High Compressive Strength = Good Cement
INTERPRETATION RULE 5: High Acoustic Impedance = Good Cement

RAYL: A unit of specific acoustic impedance in the centimeter-gram-second system of units, = 10 pascal-seconds per meter. A specific acoustic impedance has a magnitude of 1 rayl when a sound pressure of 1 microbar produces a linear velocity of 1 centimeter per second. The unit is named for John William Strutt, third Lord Rayleigh (1842-1919).

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.

5. Impedance Images:
        Linear color scale
        White < 0.5 MRayl = no cement.
        Colors from yellow to brown represent steps of 0.5 MRayl.
        Black < 8 MRayl - good cement.

With thresholds
        Red < 0.3 MRayl = gas intrusion
                      from formation or gas breakout

in foam cement.
        Blue < 2.6 MRayl = fluid – water
                      or mud in cement.
        Yellow < 3 MRayl = foam cement
                      or very contaminated neat

Colors from yellow to brown represent steps of 0.5 MRayl and indicate solids, eg. Cement or cement extenders. Black < 8 MRayl = high quality neat cement.

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.

6. Amplitude images:
The amplitude image, derived from the amplitude of the main echo of each waveform, represents the reflectivity of the internal surface of the casing. The image is normalized with respect to the maximum value at a given depth, and all points are presented in terms of attenuation from the maximum amplitude at that depth. Normalized minimum amplitude curves are plotted.

Linear color scale
        Black = low signal (-6 dB) = good bond
        Colors from dark red to white represent steps of 0.5 dB
        White = high signal (0 dB) = no bond

7. Diagnostic images:
Processing flags – diagnostics for the field engineer.

8. Internal radius images:
The internal radius image shows the variations around IRAV, the average
radius at each depth. Two color scales are used – blue to white for
internal radii less than IRAV and white to red for internal radii greater
than IRAV. Each color step represents 0.008 in.

9. Thickness images:
The thickness image shows the variations around THAV, the average
value of the thickness at each depth. Two color scales are used – red
to white for thicknesses less than THAV and white to blue for thicknesses
greater than THAV.

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.

The following examples are courtesy of Schlumberger.

Typical USI composite presentation with cement images, synthetic bond index, and casing image.

Combination of USI and CBL-VDL gives more complete picture – high resolution from USI and formation arrivals on VDL.

Good cement – low amplitude on CBL, high Acoustic Impedance on USI, no serious channels on image.

Poor cement – high amplitude on CBL, low Acoustic Impedance (blue colour) on image indicates large void space filled with liquid (mud).

USI-VDL and USI before and after squeeze.

CBL and USI in foam cement.

Microannulus – high amplitude on CBL and VDL, medium to high acoustic impedance on USI.

Micro-debonding – similar to microannulus but patchy instead of continuous.

Samples of cement jobs with problems, and possible cures.

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