Study on Mohr-Coulomb-based three-dimensional strength criteria and its application in the stability analysis of vertical borehole

摘要

In the present study, the six extensions for the Mohr-Coulomb criterion into three-dimensional stress state are discussed in detail. The prediction of strength from these criteria are compared with the experimental data of true-triaxial tests from 16 rocks. These extended criteria are Mogi-Coulomb, Modified Mohr-Coulomb A and B, and Mohr-Coulomb criterion with three different deviatoric functions (elliptical, hyperbolic, and spatial mobilized plane functions). The strength parameters of these criteria are calculated using the triaxial compression points from the true-triaxial tests data. Such consideration is taken because in general triaxial compression tests, data are only available for the majority of rocks. It is observed from the comparison that on average Mohr-Coulomb criterion with elliptical smoothening (MC-ED) performed better than other criteria (considered in the study). Hence, MC-ED is considered for vertical borehole stability analysis. MC-ED criterion also satisfies the Drucker stability postulate. To demonstrate the application of MC-ED criterion in the borehole stability analysis, four cases are considered. Results from the MC-ED criterion are compared with the Mohr-Coulomb criterion. The Mohr-Coulomb analysis calculates the higher minimum overbalance pressure required as compared to the MC-ED and lower fracture pressure. It is observed that MC-ED results are relatively closer to the actual values used in the case study considered in this paper.