(a) Cement bond logs (CBL)
The cement bond logging tools have become the standard method of evaluating cement jobs since they not only detect the top of cement, but also indicate how good the cement bond is. The CBL tool is basically a sonic tool which is run on wireline. The distance between transmitter and receiver is about 3 ft (Figure 26). The logging tool must be centralised in the hole to give accurate results. Both the time taken for the signal to reach the receiver, and the amplitude of the returning signal, give an indication of the cement bond. Since the speed of sound is greater in casing than in the formation or mud the first signals which are received at the receiver are those which travelled through the casing (Figure 27). If the amplitude (E1) is large (strong signal) this indicates that the pipe is free (poor bond). When cement is firmly bonded to the casing and the formation the signal is attenuated, and is characteristic of the formation behind the casing.

CBL tool

receiver signal

(b) the Variable Density Log (VDL)
The CBL log usually gives an amplitude curve and provides an indication of the
quality of the bond between the casing and cement. A VDL (variable density log),provides the wavetrain of the received signal (Figure 28), and can indicate the
quality of the cement bond between the casing and cement, and the cement and
the formation. The signals which pass directly through the casing show up as
parallel, straight lines to the left of the VDL plot. A good bond between the casing
and cement and cement and formation is shown by wavy lines to the right of the
VDL plot. The wavy lines correspond to those signals which have passed into and
through the formation before passing back through the cement sheath and casing
to the receiver. If the bonding is poor the signals will not reach the formation and
parallel lines will be recorded all across the VDL plot.

The interpretation of CBL logs is still controversial. There is no standard API scale
to measure the effectiveness of the cement bond. There are many factors which can
lead to false interpretation:

1. During the setting process the velocity and amplitude of the signals varies significantly. It is recommended that the CBL log is not run until 24 – 36 hours after the cement job to give realistic results.
2. Cement composition affects signal transmission.
3. The thickness of the cement sheath will cause changes in the attenuation of the signal.
4. The CBL will react to the presence of a microannulus (a small gap between casing and cement). The microannulus usually heals with time and is not a critical factor. Some operators recommend running the CBL under pressure to eliminate the microannulus effect

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