In recent years, there has been a strong interest in evaluating casing to cement bond with an LWD sonic tool. In 2013, we presented an amplitude-based approach, similar to what is used with wireline tools with its potential and limitations. During this study, extensive modeling was conducted and verified with real data sets to show that the amplitude of the signal propagating inside the tool collar can sometimes become larger than the signal traveling along the casing, which limits the applicability of the first-arrival amplitude for bonding evaluation in thin casings and heavy cements.

From observations made during this investigation, it was noted that when the collar arrival was affecting the casing amplitude signal, the apparent attenuation rate obtained in these conditions anti-correlated with the attenuation measured by a wireline tool. This surprising observation triggered a review of the physics of sonic signal propagation in cased hole and the development of a new formulation of the bond index when an LWD tool is used for the measurement.

This new approach uses the attenuation of the casing and tool collar signal along an array of receivers from an LWD sonic tool, which extends the quantitative cement evaluation to the full range of bond index in all types of casing and cements if the ambient noise level permits.

This new methodology uses the knowledge of the propagation inside the tool collar, which is well characterized for new LWD tools. This different methodology does not require any tool calibration, mud attenuation compensation, or the presence of a free-pipe section for in-situ normalization of the measurements. It is therefore free of user interaction and can be implemented in the downhole tool processor for real-time answers while drilling the cement. This provides a very early knowledge of the casing-to-cement-bond integrity before drilling through the casing shoe, thereby reducing drilling risks. The approach was validated with real examples that showed how the new approach overcomes the limitations of the amplitude-based process formerly used.