Ultimate Strength Reliability Analysis Applied to the Design of Cement Sheaths in Oil Wells

Abstract

Abstract In offshore wells, cement sheaths are responsible for isolating the casing from the formation, protecting the latter from corrosion, and preventing fluid migrations that can cause severe blowouts in extreme situations. The most usual methodology applied in the structural evaluation of cement sheaths is deterministic, where characteristic values are employed to model material properties and loads. At the end of the analysis, stresses (or any other derived parameter) are compared to allowable limits. Although this design methodology is relatively simple, it doesn't consider uncertainties regarding material properties and loading conditions. Moreover, no information is provided concerning how far the structure is from failure. These issues can only be addressed using a statistical approach. In this way, this work aims to propose a reliability-based methodology to be applied to the design of cement sheaths. Using the FORM method and a structural analysis computer program (TENCIM-1D – Toledo Filho et al. 2020), failure probabilities can be easily accessed with low computational costs. These values can then be compared to target failure probabilities specified in offshore standards. In addition, the FORM method can also improve the design quality, as one of its subproducts – the derived importance factors – may help to identify the properties and parameters that most contribute to the obtained failure probabilities. Therefore, these properties and parameters are the ones that need to be better described statistically, either through experiments to survey physical properties or through field measurements. The obtained results indicate the feasibility of the proposed methodology and the need to develop studies to determine the probability distributions, mean values and standard deviations of some physical properties and loads.