Microannulus Identification and Quantitative Evaluation in Cased Wells

Downhole temperature and pressure changes in cased wells often lead to microannulus between casing and cement sheath. Conventional cement bond log/variable density log (CBL/VDL) logs make it difficult to distinguish microannulus from bad bonding conditions. The flexural Lamb wave from the casedhole pitch-catch measurement is sensitive to microannulus in the fast cement condition, which allows for identifying the microannulus. We present such an identification method, which, when integrated with the CBL casing wave amplitude and flexural Lamb wave attenuation, allows us to quantitatively invert the microannulus width. The casedhole flexural Lamb and CBL casing wave characteristics are modeled with varying microannulus widths. The results show that, for a cased borehole with fast cement, the attenuation of the flexural Lamb wave increases if wet microannulus occurs. The attenuation decreases gradually with the increasing microannulus width. The attenuation difference between wet microannulus and good bonding conditions will decrease gradually with the increase of frequency in the 100-250 kHz frequency band. On the other hand, in CBL/VDL logging, the amplitude of the casing wave is low, with good bonding conditions. When microannulus occurs between the casing and cement sheath, the amplitude of the casing wave increases significantly, close to 40% of the free casing value. Therefore, by integrating the attenuation of the flexural Lamb wave and the amplitude of the CBL/VDL casing wave, the microannulus can be identified, and its width can be estimated. Applying this method in the Tarim Oil Field of west China, a large number of microannulus sections have been identified in the deep or ultradeep cased wells. An application example for an ultradeep cased well illustrates that the good bonding section shows low flexural wave attenuation and low VDL casing wave amplitude, and the microannulus section shows high flexural wave attenuation and high VDL casing wave amplitude. Two logging runs with different center frequencies show that the attenuation difference of the well-cemented and microannulus sections decreases with the increase of center frequency. The field data and theoretical modeling results are consistent with each other. Using the frequency dependency of the attenuation data, the microannulus width is quantitatively calculated. This downhole microannulus identification and estimation method can replace the traditional well pressurizing method, and the results can help field engineers in their decision-making process.