Numerical Study on Wellbore Stability Analysis Considering Volumetric Failure

Michel N. Zahn, Lúcia Carvalho Coelho, Luiz Landau, José Luis Drummond Alves


When a wellbore is drilled, the equilibrium in-situ stress is changed. In order to support the stress relief induced by the drilling and to prevent hydrocarbon influx into the cavity, the borehole is filled with a fluid. These operations create new stresses configurations. A main point in wellbore projects is the definition of the drilling fluid density to keep the wellbore stable. The upper bound to the fluid density is the collapse stress that is the limit to shearing. The lower bound is the fracture stress that limits the tensile failure. The fluid densities between these limits is named safe mud weight window. Conventional wellbore stability analysis usually considers the effects of shear or tensile failure. Recent physical models pointed the possibility of compactive volumetric failure around boreholes. Other numerical studies showed that this type of failure may be attained by stress level compatible with reservoirs in under production in high porosity rocks. This paper applies numerical modeling by finite element method with ANSYS® software to study stability of oil wells. It simulates the compactive behavior of rocks using a cap plasticity model. The cap model data was collected from literature. Parametric studies were conducted in horizontal wellbores to evaluate the conditions of volumetric failure to occur. The study pointed the tangential stress concentration as a critical condition for volumetric failure around boreholes.

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