Selective cementing technology has been used to exploit the carbonate reservoir, which has satisifed the needs of multi-stage reform and later stimulation of reservoir. External Casing Packer (ECP), which is the key tool to selective cementing technology, is discussed in this paper to explore its main failure causes during the selective cementing operation. Analysis of the bottom hole tempera-ture, running-in inlfuencing factors, sealing position diameter and pipe string structure shows that rubber tear strength reduced by higher bottom temperature, unsmooth running-in inlfuenced by oversized hole curvature, intensiifed friction during running-in induced by incompetent drifting measure, loading capacity reduced by oversized sealing position diameter and unreasonable pipe string structures, and non-effective protection to the ECP are the main causes of the ECP failure. According to the failure causes above, corresponding technical measures have been established:the actual maximum operating temperature is rated operating temperature multiplying by a safety factor; using the effective technical measures to control the borehole curvature; simulating drifting with the watermelon mill; choosing the rational borehole diameter of the sealing up position and connecting short casing iftted with centralizer at both ends in the ECP. These measures are signiifcant for reducing the possibility of ECP failure and improving the safety reliability of selective cement-ing technology.%采用选择性固井工艺开发碳酸盐岩油气藏，能满足储层多段改造和油田后期增产开发需求。管外封隔器（ECP）是选择性固井应用中的关键工具，针对选择性固井实践中ECP失效问题进行了研究，对井底温度、下入过程影响因素、涨封位置井径及管串结构等方面进行了分析，结果表明导致ECP失效的主要原因分别是井底温度过高导致橡胶撕裂强度降低，井眼曲率过大影响工具顺利下入，通井措施不到位加剧下入过程中摩阻，涨封位置井径过大降低承压能力以及管串结构不合理对ECP起不到有效的保护作用等。针对上述失效原因，制定了对应的技术措施：实际最高工作温度设定为额定工作温度乘以安全系数，采用有效的技术措施控制井眼曲率，利用西瓜铣进行模拟通井，合理选择涨封位置井径以及在ECP两端连接装有扶正器的短套管等。这些措施对减少ECP施工失效、提高选择性固井施工成功率具有指导意义。
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