The difficulty of milling dual casing window with 0139.7 mm is great. The key techniques are analyzed and studied, including the choice of window depth and window azimuth, the calculation of the length of the different gap as well as important nodes, the improvement of milling cones and oblique, the BHA selection, the determination of main widowing parameters, and the approach of walking along the technical casing after windowing. Some conclusions arc obtained as following. The window length should be calculated or estimated before windowing. The window azimuth should be at the smaller gap between the double casings. The whipstock with large bevel angle and high beveled hardness should be taken to ensure the oblique angle. The bottom hole assembly with appropriate stiffness should chosen for windowing or test drilling, to avoid too long window because of too much BHA stiffness, which will reduce too much the devel angle of the whipstock and make the whipstock loss angle buildup capacity, and to ensure the angle holding capacity. The parameters should be timely adjusted when widowing, according to the iron, cement blocks and debris situation carried out with the drilling fluid, as well as the footage, which is the key of efficient, fast and safe windowing for dual casing.%油层套管为(O)139.7mm的双层套管开窗的难度较大,通过选择窗口位置及开窗方位,计算双层套管之间不同间隙窗口的长度,计算重要的节点,改进双层套管开窗铣锥、斜向器,选择开窗下部钻具组合及确定开窗主要参数、开窗后井眼沿技术套管行走的处理方法等关键技术的分析研究.得出了开窗前应计算或预估出窗口长度；开窗方位选在双层套管间隙较小的部位开窗；应用斜面角度大和斜面硬度高的双层套管开窗斜向器,保证斜向器的导斜角度；开窗或试钻选用适当刚度的下部钻具组合,避免钻具刚度太大导致开窗窗口过长使斜向器导斜角降低太多,丧失应有的导斜能力致使开窗开不出去的现象发生,并保证钻具有一定的稳斜作用；开窗时根据钻井液携带出的铁屑、水泥块、岩屑情况以及进尺情况及时调整参数,确保双层套管有效、快速、安全开窗.
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