Abstract: Borehole stability analysis is one of the key scientific issues in the deep drilling engineering. Adoption of a reasonable strength criterion facilitates the prediction accuracy of borehole stability and reduces the chance of borehole breakout. Since the widely-used Hoek-Brown strength criterion neglects the effect of intermediate principal stress, the potential of rock strength cannot be fully realized. To solve the above problems, a newly proposed modified Hoek-Brown strength criterion that overcomes the shortcomings of singularity and non-convexity that exist in most classic strength criteria, is used for the stability analysis of horizontal, vertical, and inclined borehole, respectively. By calculating the minimum mud pressure required for ensuring borehole stability, comparisons with several classic strength criteria such as Mohr-Coulomb, Drucker-Prager, Hoek-Brown, 3D Hoek-Brown, Mogi-Coulomb, and modified Lade strength criterion are conducted, coming to the following conclusions: the predicted results of the adopted modified Hoek-Brown strength criterion are close to those predicted by 3D Hoek-Brown, Mogi-Coulomb, and the modified Lade strength criterion; the relative error between the predicted value and the mud value used in two actual projects are 2.1% and 0.76%, respectively; those results prove the accuracy of the modified Hoek-Brown criterion for borehole stability analysis; the minimum mud pressure increase linearly with increasing borehole depth; meanwhile, it is affected by the inclination angle and the azimuth angle of the borehole; both Mohr-Coulomb and Hoek-Brown criteria are conservative in predicting the minimum mud pressure, increasing the amount of mud that actually used; and the Drucker-Prager criterion over-predicts the rock strength, bringing about an unsafe predictive value of minimum mud pressure; base on the modified Hoek-Brown criterion, the theoretically optimal drilling position can be obtained (i.e. the minimum required mud pressure value), which provides a useful reference for the drilling engineer to optimize the position of borehole.

Key words: borehole stability, modified Hoek-Brown strength criterion, borehole breakout, minimum mud pressure