Sonic and ultrasonic measurement applications for cased oil wells

Elastic waves have numerous oilfield applications, most pertinently for well integrity characterisation of cased wells. The measurements are used to determine the quality of the cement placed between the casing and formation rock or between multiple casing or liner strings. In addition to supporting the casing and protecting it from corrosive fluids, the cement is used to prevent hydraulic communication between formation layers that were isolated from each other before the well was drilled. This paper focuses on low-frequency sonic measurements and high-frequency ultrasonic measurements that are either currently used or in development for cement evaluation. The domain has seen tangible progress, with advances in the understanding of the physics related to the measurements and the development of effective modelling, signal processing, inversion and interpretation methodologies. The measurements leverage the physics associated with the guided waves excited in the casing string, which propagate axially along the direction of the borehole. Signal processing and interpretation rely on phase or dispersion information, as well as time-of-flight and amplitude variations across receivers, to provide a diagnosis of cement quality and an estimation of the material properties of interest.