This paper discusses well integrity evaluation using LWD sonic monopole data. The first part of the paper explains the methodology which enables full range cement bond index determination i.e. 0 to 1; a hybrid method which uses both amplitude and attenuation through a receiver array (Pistre et all, 2014). The methodology was enhanced at each processing step to be applicable in wide range of conditions including quality control of the processing results. The later part of the paper shows a real world example that demonstrates how the data was used as input to decisions regarding well integrity. With the example case study, the benefits of LWD cement evaluation were confirmed.
The basic principle of sonic cement evaluation with Wireline is based on the acoustic signal of the casing extensional mode traveling through the casing. When no cement is present the casing is free to ring and the signal which reaches the receivers is large. Contrarily, when cement is present behind the casing, the casing extensional mode is attenuated and the received signal is much smaller.
In LWD sonic cement evaluation, drill collar acoustic propagation is one of the biggest concerns since it becomes mixed with the casing mode. The presence of the tool collar mode limits the bond index range which can be measured using an amplitude only approach (Blyth et. al, 2013).
In order to determine the cement bond index in high bonding conditions, an attenuation based approach is utilized. By looking at the change in amplitude across an array of receivers, the apparent attenuation of the casing mode can be determined. That rate of attenuation is then converted into a bond index value based on a model which takes both casing and collar modes into account.