Application of Enhanced Ultrasonic Measurements for Cement and Casing Evaluation

Abstract

Abstract The hydraulic isolation of the wellbore casing and cement is critical for the completion of production and injection wells. Zonal isolation prevents the production of fluids from non-completion intervals, contamination of ground water by fluids in the wellbore, and allows conformance control of injected fluids. Current acoustic evaluation techniques may be limited by the acoustic properties of the material behind casing and by the inability to see beyond the cemented region near the casing. A new ultrasonic imaging tool has been developed to address these limitations. The new imager tool combines the classical pulse-echo technique with a new ultrasonic technique that provides temporally compact echoes arising from propagation along the casing and also reflections at the cement-formation interface. Processing these signals yields unprecedented characterization of the cased-hole environment in terms of the nature and acoustic velocity of the material immediately behind casing, the position of the casing within the borehole, and the geometrical shape of the borehole.5 In order to provide answers to the casing/cement evaluation questions, a field study was performed to evaluate the results provided by both sonic and this new ultrasonic tool in the different cement materials, drilling fluids, and casing sizes. Field examples are presented to illustrate the actual response of the new ultrasonic tool to these various completion environments including wells cemented with conventional and lightweight cement. The results demonstrate enhanced cement evaluation for all cement types and a significant reduction in the uncertainty in making a squeeze or no-squeeze decision. The new cement evaluation tool implements both the traditional pulse-echo technique and the new flexural wave concept. The flexural mode enables deep imaging of the cement sheath up to the cement-formation interface. In addition, the measurement of the borehole geometrical shape makes it possible to evaluation double casing string conditions for potential damage.