Experiments were conducted to study how sonic and ultrasonic cementation logs are affected by microannuli. By creating both gas- and liquid-filled microannuli of known sizes at the casing-to-cement interface, measurements could be recorded and analyzed.
Depending on the nature of the fluid and the size of the microannulus, results showed that all cementation logs were sensitive to microannuli. Gas at the casing-to-cement interface had a very strong effect on the ultrasonic measurement but had minimal effect on traditional sonic Cement Bond Log (CBL) measurement if the microannulus was in the range of one m. CBL measurement was very sensitive to water-filled microannuli. Ultrasonic measurements were capable of evaluating the quality of the cement behind a water-filled microannuli as large as 100 m. Liquid-filled microannuli could be identified through the large coupling attenuation measured with multi-spacing sonic tools.
Analysis of these experiments brings new insights to cementation-log interpretation.
Interpretation of cementation logs is without any doubt subject to controversy. For over 30 years, CBL measurements have been used to evaluate cement jobs. More recently, special tools have been developed to overcome some of the limitations of the traditional sonic measurements. Use of ultrasonics, has made it possible to improve cement evaluation, mainly through a spatial resolution which allows a much clearer identification of incomplete mud removal. However, using higher frequency waves renders the signal more sensitive to the local events, such as poor pipe-surface condition.
In many cases, field experience and theories have helped people to better understand the response of the tools to specific situations. When a small gap exists between the casing and the cement, the response of acoustic and ultrasonic tools is affected. There is a microannulus effect which renders cement job evaluation even more difficult. Several articles have been published to explain how a microannulus can be created or induced and how this affects cementation logs. Temperature or pressure changes are the most common causes of microannuli (see Appendix 1 for usual formulae).