ABSTRACT

The usefulness of acoustic information for the analysis of cementation quality has been enhanced by the simultaneous recording of the full-wave display (Variable Density Log) with the first-arrival amplitude (Cement Bond Log). The optimum-spacing requirements of the two logs are somewhat different. For the Cement Bond Log a short spacing is desired in order to show more clearly the attenuation rate of the casing arrival. For the VDL a longer spacing is desired in order to show the formation arrivals in well bonded intervals. To satisfy these differing requirements, a two-receiver system is being used to provide a 3-foot spacing Cement Bond Log (CBL) and a 5-foot-spacing Variable Density Log (VDL) simultaneously. New recording equipment permits the VDL and CBL signals to be recorded directly on the same film. Problems of separate film processing and depth mismatch are thus eliminated. Field examples show that this combination of the long-spacing VDL with the short-spacing CBL can provide a more thorough understanding of cementation quality. Cases of micro annulus and cement channeling, as well as good and poor cement bond, can be recognized. More importantly, rules are proposed which equate CBL-VDL response with hydraulic-seal effectiveness. These have been derived from comparisons of the CBL-VDL response with isolation tests in actual field wells. These comparisons are presented. Field examples from South America illustrate the interpretation techniques.

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