To ensure well's production cycle as long as possible, wellbore integrity becomes very significant and is attracting more and more attention. Among numerous researches about the influence factors of wellbore integrity, casing impairment is a common phenomenon. Usually, casing impairment leads to loss of pressure integrity, pinching of production tubing, or an inability to lower workover tools, etc. Downhole cemented casing bears various loads during its working life. Among these loads, inner pressure variation and wellbore temperature's effect are all challenges to cemented casing's good performance. Moreover, the outer in-situ stress is also a non-negligible factor especially when the distribution of the outer in-situ stress is complicated. So, how to avoid casing impairment and how to select appropriate casing type become a significative work which will save cost and is beneficial to the wellbore's longterm integrity.

In this paper a coupling mathematical analysis model of downhole cemented casing-cement sheath-formation rock has been built. Through elastic mechanics method, calculating programme programmed by computer language and graphical output, this paper presents a detailed analysis about the cemented casing stress field under arbitrary in-situ stress (especially non-uniform in-situ stress) field considering pressure variation coupling with temperature effect. It shows that whether the in- situ stress field is uniform or non-uniform, the tangential stress is so much larger than the radial stress. When the in-situ stress's values are relatively large, proper inner pressure increase can compensate for the casing's enduring performance. The research findings can be used to forecast the mechanical failure type of cemented casing. Furthermore, it will be a useful guideline for casing type selecting.

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