Abstract: Restricted by the existent casing program,formation and the requirement to avoid acquifer,when sidetracking horizontal wells into Ordovician reservoir in Tahe Oilfield,the kickoff point must be moved up to upper Carboniferous mudstone formation.The two formations belong to different pressure systems,so how to effectively seal the Carboniferous unstable formation is the key for developing the Ordovician remaining oil efficiently.Using expandable casing as intermediate casing to seal complex formations during drilling,provides a new way to solve the problems encountered in sidetracking horizontal wells in Tahe Oilfield by optimizing the casing program and follow-up drilling process.According to the difficulties encountered in deep sidetracking from φ177.8 mm casing in Tahe Oilfield,through dissecting the key technologies of expandable casing,such as expandable pipes and expansion thread,the operation process and procedure of running expandable casing in deep wells were improved,finding out a set of drilling technology with expandable casing.The technology has been tested in 3 wells in Tahe Oilfield,in which the window were all at the Carboniferous layer below 5 100 m,the inclined segments after sidetracking sealed the complex formation successfully,realizing the first sealing with domestic expandable casing in open hole,providing a new technical means to seal complex formations during drilling.