Wellbore Stability Analysis In Porous CarbonateRocks Using Cap Models

It is well established that hydrocarbon wellbores fail under tension mode or shear compressive mode. Although there are sophisticated constitutive models that may represent the behavior of the rock, the lack of data available lead to the use of simple models such as elastic models with a failure criteria, and the DruckerPrager or Mohr-Coulomb elastic-plastic models with associated or nonassociated flow rules. Recently, many studies have been published on the behavior of porous rocks. Mechanisms of pure compression failure (no shear) have been described. Cap plasticity models may represent the material pure compression failure. An experimental program and numerical simulations on porous carbonate rock from Campos Basin, offshore Brazil, carried out investigation on this phenomenon. The experimental program consisted on defining the hydrostatic strength of the rock through laboratory tests for stress paths that varied from hydrostatic to uniaxial. All tests followed paths below the shear line. Data from these tests was fitted to a cap plasticity model. Parameters obtained were used on stability analysis of a horizontal borehole drilled in the same field. A comparative study of the Mohr-Coulomb shear model and the cap model was carried out. The Mohr-Coulomb shear model pointed to stability of the borehole in the compressive mode, while the cap model indicated damage to the rock in the vicinity of the well.