Cement evaluation plays an important role in determining well integrity, to ensure zonal isolation in a wellbore. Cement evaluation has recently evolved to include Logging-While-Drilling (LWD) sonic tools due to the multiple benefits of LWD logging compared with conventional methods, including tool conveyance, reduced tool eccentering effects, time lapse evaluation, etc. On the other hand, there is an LWD-specific challenge which is the contamination of the casing mode by the drill collar mode. The recent development of the algorithm to use the attenuation through receiver array of both the tool and casing arrivals, gave an initial insight into how to overcome the problem.

We investigated the applicability of a hybrid method using both the signal amplitude and attenuation which was introduced in 2014, and have now further enhanced the processing method. A new processing workflow has been developed to improve the measurement. In the new workflow, the waveform envelope is initially calculated by the Hilbert transform, and the signal amplitude is then evaluated in a time window, in which both casing and collar modes are present. Next, the apparent attenuation rate of the signal across the receiver array is considered. The applicability of the method is validated by an intensive numerical modeling study using finite difference methods (FDM) with detailed tool structures in various casing sizes, mud properties, and cement types. We also conducted a sensitivity study of the apparent signal attenuation to the bonding over the receiver array and the bonding over the transmitter-receiver (TR) spacing, respectively, to identify in which zone of bonding the hybrid method is more sensitive.

Field data examples of quantitative cement evaluation are presented for different LWD tool sizes and casing sizes together with wireline reference to demonstrate the validity of the new method and the suggested quality control processes. All of the results finally prove that the hybrid method using both amplitude and attenuation is the right way for full-range cement bond evaluation with LWD sonic tools.