Chapter 10: Cement-Quality Logging

Many production logs are aimed at evaluating the competence of a well completion. Knowledge about the completion often is gained indirectly by measuring fluid movements with such logs as radioactive-tracer or temperature logs. Acoustic logging techniques, primarily the cement-bond log and ultrasonic pulse-echo logs, have been used for many years to try to directly measure the quality of the cement between the casing and the formation. Attenuation rate logs have improved on the traditional cement-bond log by applying a compensated measurement that reduces the impact of effects such as tool eccentering. More recently, ultrasonic measurements of the flexure response of the casing have provided a more-thorough interrogation of conditions in the cement/formation annulus. These logs are all active acoustic logs, in that a sound, either audible or ultrasonic in frequency, is generated by the logging tool and then received by one or more acoustic transducers. The primary functions of the cement are to mechanically support the casing and to prevent fluid movement between the various zones in a reservoir and between the reservoir and other zones up or down the hole. Thus, a primary focus of cement-quality logging is aimed at determining whether the cement is of sufficient strength and distribution to prevent fluid communication between zones. A cement-quality log ideally should indicate whether the cement is bonded to the pipe, whether the cement is bonded to the formation, and whether channels are present in the cement. A cement-quality log does not directly measure the capability of the cement to prevent fluid communication; this is inferred from the degree of acoustic coupling of the cement to the pipe and the formation as measured by the logs. For this reason, cement-quality logs are not an absolute measure of the hydraulic integrity of the cement; however, when run and interpreted properly, they are generally reliable indicators of cement placement (McNeely 1973).