Identification Method of Microannulus Based on IBC and CBL/VDL Logging

To accurately study the influence of microannulus thickness on the evaluation of cementing quality by different logging instruments, this study analyzed the response characteristics of CBL/VDL logging casing waves, IBC-induced resonance waves, and flexural waves to the presence of microannulus. The study focused on how the microannulus between the casing and cement sheath as well as the density of the cement slurry, impacts these three wave modes. The findings reveal that the casing wave in CBL/VDL logging is consistently sensitive to the presence of microannulus, with amplitude increasing as the microannulus thickness (fluid-filled) grows. A microannulus as thin as 0.01 mm can cause the relative amplitude of the casing wave to reach 30%–50%, leading to an assessment of poor cement bonding. The resonance wave amplitude, however, is not significantly affected by microannulus thinner than 0.01 mm, and the attenuation of the flexural wave only slightly decreases when low-impedance cement is cemented behind casing. In contrast, when high-impedance cement (generally conventional cement and heavy cement) is coupled, the presence of microannulus markedly increases attenuation, causing the flexural wave to reach a high attenuation value. In gas-liquid-solid phase identification, the medium behind the casing is likely to be identified as solid cement, making the combination of CBL/VDL and IBC logging results effective for identifying microannulus intervals. Field oil test results from multiple wells show that wells with high CBL casing wave amplitudes and high flexural wave attenuation values consistently demonstrate better cement sealing capabilities, confirming that these logging responses are characteristic of microannulus. This method of microannulus identification, based on IBC and CBL/VDL logging avoids the traditional wellbore pressure logging method and obtains a good application effect, and it is worthy of promotion and application.