Abstract：Evaluation of the thickness of microannulus in cased borehole is very important in petroleum engineering.The dependence of amplitude of first break for acoustic logging in cased borehole on the thickness of microannulus is studied in this paper with Finite Element Method(FEM).The numerical resuits show when the thickness of microannulus is less than 1 mm,the variation in the thickness of microannulus has a minor effect on the amplitude of acoustic first break;but when the thickness of microannulus is larger than 1 mm, the coupling of casing with cement becomes weaker, the acoustic energy is difficult to enter the beds ,leading the amplitude of casing wave to increase.The paper considered that the 1 mm thickness of microannulus can be taken as a critical value having influnence of microannulus on well-logging aimed at well-cementing quality.
