In recent years drilling for hydrocarbons have become a much more challenging task as nice, blanket-type reservoirs are now part of history and appear mostly in textbooks and old reports. The current economic and political world environment demands fewer, more "difficult" wells to be drilled and produced. Wellbore stability problems account for huge over-costs worldwide.

Tectonically active areas, thin interlayered horizons, unconsolidated rocks; deep and hot formations are part of the geological environments where hydrocarbons are currently produced from. The degree of planning and problem anticipation at both drilling and production stages will determine the difference between a productive reservoir and a technical/economical failure.

In the literature, some specific cases have been analyzed but the lack of a more general methodology for wellbore design is a problem that needs to be addressed. The vast majority of models utilize two-dimensional (2D) descriptions of the problem in order to determine the best well trajectory from a geomechanical point of view. In this paper, several situations are presented, where different stress field patterns were assumed for different rock types. The generality of this study provides a tool for well design under different stress environments. The maximum principal stress was allowed to follow different directions in different planes (horizontal, vertical and inclined) depending upon the case being analyzed.

The model represents wellbores drilled in shallow, medium-deep, and deep basins. The effect of both differential stress and loads magnitude were evaluated throughout this work.

The model developed here was based on the finite element method, for both linear-elastic and elasto-plastic materials. The description of the problem is a fully three-dimensional (3D) representation of the wellbore and the applied stresses. This paper has two parts: a theoretical approach, which provides the basis for the solution setup, and a set of numerical experiments with their correspondent results. Since several real-life situations were evaluated, the potential applicability of the results obtained here is apparent; and the results of these simulations could be used as a cross check for calculations made for more particular situations in the field. The aim of this work is to provide a more global vision of the effects of stress variations upon the optimum design of oil and gas wells.

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