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Comparative analysis of the predictability of established rock failure criteria around boreholes

Kopasakis Stylianos

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Summary

The purpose of this diploma thesis is the calculation of the stresses around a vertical borehole and the calculation of the necessary mud unit weight for the avoidance of the rock failure. For the estimation of the mud unit weight three types of failure criteria have been used: the Mohr-Coulomb, the Modified-Lade and the Mogi-Coulomb. Additionally, the analytical Kirsch solutions have been considered for the calculation of the in situ stresses around the circular borehole. These solutions have been programmed in MATLAB in order to determine the range of the unit weight of the mud for the prevention of failure at the walls of the borehole. For the validation of this methodology, the results of the algorithm were compared with four borehole case studies in order to identify the most appropriate failure criterion. More precisely, the Kirsch solution is used to calculate the stress field around the perimeter of the borehole. The boundary conditions were found in the data given in the case studies. Subsequently, the calculated stress field was converted into the effective principal stresses σ1΄, σ2΄ ,σ3΄.Finally, these principal stresses were used with the aforementioned failure criteria to estimate if the borehole will fail. At the end of this diploma thesis the results of the algorithm were compared with the results of the literature in order to identify the failure criterion which give the most appropriate range of the mud weight unit for better prevention of failure at the borehole walls. Additionally suggestion for further studying of the problem are given.

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