ABSTRACT:

Different from the prevalent methods that treat rock formation as isotropic elastic, poroelastic, and/or elastoplastic materials, this paper presents a set of semi-analytical solutions for the wellbore stability problem by adopting the widely used anisotropic critical state plasticity model originally proposed by Dafalias, 1987. This model is capable of taking mechanical anisotropy into account and therefore the wellbore stability analysis presented in this paper is more realistic. By carefully choosing an independent auxiliary variable, the plastic zone solution is reduced to solving a system of seven partial differential equations with the three stress components, specific volume, and three anisotropic hardening parameters being the basic unknowns. Parametric studies have been conducted to explore the influence of overconsolidation ratio and inherent anisotropy. The results show that both these two plasticity features exert pronounced effects on the response of the drilled wellbore. In addition, the feature of anisotropy has also been discussed. It is found that there is a distinct difference between the results with and without considering the anisotropy in the wellbore stability problem.

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