Abstract: The total mumbers of casing failure wells of China were over ten thousands, and the great part of casing failure wells centralize in trorblesome formations such as saline rock, shale, gypsum rock, gypsum-salt rock and so on. This paper analyzes in detail the irrational rules if the casing desgin at formations in our coutry. We have researched for five years. At this base, we presents the scheme design of double-layer casing, which is a special scheme design of local double-layer casing. Its optimum, economic evaluation and construction technique were discussed. The result, the local double-layer casing, is the best consideration to solve casing failure problems in high pressure formation .　

Key words: Casing string design, High pressure reservoir, Formation pressure, Casing collapse, Double-layer combined casing, Economic evaluation