Abstract: During the well completion testing to a certain extra-deep complicated well with 8 024 m completion depth, leakage takes place with the secondary drilling case at the point of 2 964.6 m.Based on relevant investigation and theoretic calculation, the reasons for the said leakage are indentified as follows.Due to poor cementation quality of the casing, there exist cement-free sections, and this causes low sitting/hanging tonnage of the casing string, and thus leads to the casing in the cement-free section bends, and gets unstable.And because of rather long third drilling period, slow drilling footage and over frequent drill up-and-down operations, the drilling tool constantly abrades the inner surface of the casing, and once the casing is worn to be too thin to bear the load in the well, leakage failure eventually takes place.