Cement Bond Logging - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 213

This same technology can also be applied in cased-holes to evaluate cement
behind casing. An ultrasonic transducer emits a beam of ultrasonic energy in a 300-
600 kHz band. This energy pulse causes the casing to ring or resonate in its thick-
ness dimensions. The vibrations die out quickly or slowly depending on the material
behind the casing. Most of the energy is refl ected back to the transducer where it is
measured; the remainder passes into the casing wall and echoes back and forth until
it is totally attenuated. Figure 12.27 gives a schematic of the transducer and the
compressional wave paths through the completion fl uid, refl ection at the inner cas-
ing wall (fi rst interface), refl ection at the outer casing/cement boundary (second
interface), and refection at the cement/formation boundary (third interface).

Fig. 12.27 Ultrasonic travel paths. Courtesy Schlumberger

By integrating both directional (rotational) and amplitude data of the complete
wave train, 3D views of the state of the materials behind the casing can be constructed
as shown in Fig. 12.28 which shows a portion of the cemented pipe with a channel.

Fig. 12.28 Channel detection using the third echo interface. Courtesy Schlumberger