Evaluation of Breakout by Mogi-Coulomb Failure Criterion as a Method for Estimating in situ Stresses

Determining the directions and the values of in-situ stresses is of great importance as the most fundamental parameter needed to evaluate the borehole wall's stability. One of the methods for determining the directions and values of in-situ stresses is the borehole breakout phenomenon, which is developed by an increase in shear stress due to the concentration of compressive stress around the borehole and leads to the failure and collapse of the drilled borehole wall. In the present article, the width and depth of the failure were obtained by combining the stress relations around the borehole based on the theory of elasticity and through the Mogi-Coulomb failure criterion. Then, the minimum fluid pressure inside the borehole was evaluated to prevent damage to the borehole wall, and the effect of physical properties of materials, in situ stress ratio, and fluid pressure on the breakout dimensions was investigated. In the second section of this article, 215 breakout analyzes for different mechanical properties and in situ stresses were carried out for the selected sandstone. The objective of the present study is to evaluate the relationship between the width and depth of failure. The correlation coefficient between the width and depth of failure was equal to R=0.74 using simple linear regression. Moreover, the correlation coefficient's value was calculated to be R=0.82 based on the gene expression programming (GEP) method. It means that there is a moderate to strong correlation between these two parameters, and if the values of in situ stresses σ_h, σ_H to be obtained based on the breakout geometry, only the value of one of these stresses can be obtained due to the high correlation between width and depth of failure.