Cased-hole reverse time migration imaging using ultrasonic pitch-catch measurement: Theory and synthetic case studies

Cement-bond evaluation is of great significance in oil/gas development, geothermal production, and CO2 storage. Advanced ultrasonic pitch-catch measurement exhibits poor performance in determining a cement-formation interface due to weak reflections from the cement-formation interface (third interface echo) in nonaxisymmetric complex ultrasonic environments. To deal with this issue, we use a reverse time migration (RTM) approach for ultrasonic pitch-catch measurements to image the cased-hole structure, especially for the cement-formation interface. To further enhance cased-hole RTM feasibility, a phase-shift interpolation technique is applied to reconstruct ultrasonic array waveforms from limited receivers in the pitch-catch measurement. Synthetic examples demonstrate that RTM is capable of imaging the ce- ment-formation interface with a high resolution under various ec- centerings of casings and tools. This study illustrates that small errors introduced by phase-shift interpolations do not greatly de- grade the imaging result of RTM. Furthermore, tests for inclined cement-formation interfaces demonstrate that the RTM is able to recover the true position and geometry of the cement-formation interface when a smoothed velocity model is available. In addi- tion, although RTM is sensitive to the velocity perturbation, it is possible to image the cased-hole structure by using a multistep strategy assuming that the position and thickness of the steel cas- ing are unknown.