Cement Sheath Failure Mechanisms: Numerical Estimates to Design for Long-Term Well Integrity

Abstract

Well cement should provide structural support and zonal isolation through the entire service lifetime of a well. However, even if a good primary cement job is achieved, variations in temperature and pressure over the lifecycle of the well are likely to induce different failure mechanisms that threaten the set cement sealing integrity. This work presents an extensive finite element study with the aim to assess the relevance of casing-cement-formation material properties, geometric parameters and characteristic well-loading events, in contributing to the occurrence of cement sheath failure mechanisms. Special focus is given to thermal-related loading events. There is a discussion of main assumptions to describe and estimate the different cement sheath types of failure. A large number of test cases are defined on the basis of an automated sensitivity screening of random input properties of the wellbore components, in a conventional production casing section. The influence of casing stand-off positions and initial defects in the cement sheath are also evaluated. At the end of this paper, the approach is narrowed-down to be applied to a specific well case, in which the impact of transient well heating/cooling events on cement sheath integrity is assessed. Based on the results, guidelines for the assessment of long-term cement sheath integrity and selection of cement systems mechanical properties are given to mitigate the risk of losing zonal isolation.