Imaging of annulus-formation interface using reverse time migration based on ultrasonic pitch-catch measurements: Case studies from an experimental well and a field well.

Cased-hole logging using ultrasonic pitch-catch modality is a proven technique for cement bond evaluation in cased holes. However, complex measuring environments such as tool and casing eccentering makes it difficult to precisely separate and pick the third interface echoes from full ultrasonic Lamb waves, leading to ambiguous identification of annulus-formation interface. To overcome this problem, we propose an improved cased-hole reverse time migration approach for ultrasonic pitch-catch measurements to image the annulus-formation interface. The missing ultrasonic Lamb waveforms between far and near receivers due to insufficient spatial sampling are reconstructed based on the optimized theoretical phase velocity of zero-order anti-symmetric Lamb mode waves. Additionally, we apply the hybrid illumination imaging condition to mitigate the imaging noises around the true reflectors and sources. Data examples from a physical experimental well and a field well demonstrate that the proposed approach is an effective method for characterizing the annulus-formation interface without requiring precise knowledge of the velocity distribution in the region behind the casing. Furthermore, test on an experimental well has indicated that the method has the potential to detect the absence of cement in the annulus medium.