Effect of Mechanical and Geometrical Properties of Cement on Wellbore Stability Using 3-D Analysis

Abstract

Especially in the long term, the role of the mechanical and geometrical properties of cement is very important to prevent cement failure and wellbore instability. Cement failure is the result of developed plastic strain. In this study, based on the Drucker-Prager criterion and using 3-D finite element analysis, the role of the mechanical and geometrical properties of cement on its stability are scrutinized. In addition, because cement must be designed based on the properties that are the least horizontal stress sensitive, to prevent cement failure and wellbore instability in the long term, stress sensitivity analysis is done. According to the results, when cement is in the plastic mode, improving the mechanical properties of the cement (either Young’s modulus or Poisson’s ratio or uniaxial compressive strength) could be helpful to prevent plastic strain development in cement. However, in such conditions, an increase of the cement’s thickness might be helpful. When the casing is not located in the center of the well, where the ratio of the cement thickness to the casing thickness is less than one, plastic strain will develop in the cement if the cement’s mechanical properties are weak. The results show that by increasing Poisson’s ratio of the cement, developed plastic strain in the cement decreases logarithmically. Moreover, by increasing either cohesion or the internal friction angle of the cement, the developed plastic strain in the cement decreases exponentially. In addition, among the cement’s mechanical properties, Poisson’s ratio and elastic modulus are the least horizontal stress sensitive ones.

doi:10.14456/WJST.2015.59