Determination of breakout failure zone around the borehole using the Mohr-Coulomb and Hoek-Brown failure criteria

By drilling borehole in the ground, the distribution of stress around it changes and stress concentration is created. If the shear stresses induced by in- situ stresses is more than rock strength, it causes a kind of failure around the borehole, which is called breakout. It has been observed that breakout failure zones are initiated and propagated in the direction of minimum in- situ stress. In this paper, by assumption of elasticit behavior of rock mass, the analytical 2D analysis of breakout failure using the Mohr-Coulomb and Hoek-Brown failure criteria is addressed and the failure zone is obtained by using these two criteria. According to the results of the mathematical model, the effective parameters in the depth and width of the breakout occurring around the borehole are depended on the mechanical properties of the materials in the medium as well as the amount and ratio of in- situ stresses. If the ratio of stresses is one, breakout failure will not occur. Also, with increasing the rock quality, the breakout depth decreases, and with decreasing rock strength and increasing the amount and ratio of stresses, the breakout area becomes larger.