The two most commonly used rock-failure criteria in wellbore-stability analyses are the Mohr-Coulomb criterion and the Drucker-Prager criterion. The literature gives many examples that demonstrate the application of these two criteria to wellbore-stability computations. These two failure criteria represent two extreme treatments of the intermediate principal stress, σ_2. The Mohr-Coulomb criterion assumes that σ_2 has zero influence on rock strength. Conversely, the Drucker-Prager criterion (sometimes referred to as a linear extended von Mises criterion) gives as much weight to σ_2 as it does to the major and minor principal stresses, σ_1 and σ_3, respectively. If the criteria are both fit to the same set of triax-ial-compression-strength data (σ_1 > σ_2=σ_3), the Drucker-Prager criterion predicts greater rock strength than the Mohr-Coulomb criterion for all stress states other than triaxial compression.
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