Abstract
Reliable assessment of cement-casing bonding is essential for secure and long-term well integrity, particularly in plug and abandonment (P&A) operations, where microannulus formation can create leakage pathways. Acoustic logging is widely used in the field for this purpose but their interpretation can depend strongly on the signal processing algorithm used. This study investigates the capability of two ultrasonic techniques: pulse-echo (PE) and pitch-catch (PC), as well as their analysis algorithms, to detect and characterize microannuli under controlled laboratory conditions designed to replicate casing-cement interfaces.
Cement–casing samples were prepared with and without intentionally induced microannuli. A reference sample was cured at ambient pressure, while a second sample was cured under internal pressures of 5 bar to generate microannuli upon pressure release. Acoustic logging was performed using a rotating 5 MHz transducer system. PE data were analyzed using three industry-standard algorithms, together with a laboratory-optimized method. PC measurements were processed following a published laboratory-tailored attenuation analysis procedure.
The reference sample exhibited more homogeneous thickness and impedance signals across all PE algorithms. PC attenuation measurements showed low and uniform energy loss along the casing wall, with attenuation values corresponding those associated with well-bonded cement-casing. Leakage testing confirmed the absence of hydraulic pathways in this sample. In contrast, the sample cured under 5 bar internal pressure displayed increased heterogeneity in both PE and PC responses. PE-derived impedance values varied significantly depending on the interpretation algorithm highlighting algorithm-dependent uncertainty. PC measurements showed systematically increased attenuation, consistent with energy leakage from the casing to an unbonded water-filled annular region. Leakage experiments corroborated these findings, with measurable gas flow occurring at differential pressures as low as 2 bar.
These results demonstrate that acoustic logging can detect microannuli at laboratory scale, but interpretation strongly depends on the processing algorithm. The combined acoustic and leakage rate findings emphasize the need for refined calibration to ensure reliable cement-casing bond evaluation in P&A applications.