The two most commonly used rock failure criteria in wellbore stability analyses are the Mohr-Coulomb criterion and the Drucker-Prager criterion. Neither of these is compatible with 3-dimensional rock strength measurements that have explored the influence of the intermediate principal stress. This paper presents a modified Lade criterion which has the following desirable qualities:

  1. it correctly describes the influence of the intermediate stress on rock strength, and therefore on wellbore stability,

  2. it can be rearranged to give a closed-form solution for critical mud weight, for any wellbore orientation, and

  3. only two rock strength parameters are required, such as cohesion and friction angle.

The modified Lade criterion predicts critical mud weight values that are less conservative than those predicted by the Mohr-Coulomb criterion yet are not as unconservative as those predicted by the Drucker-Prager criterion. It also reveals that the required increase in mud weight when going from low hole angle to high hole angle is not as great as predicted by either the Mohr-Coulomb or Drucker-Prager criteria.

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