In this research, the stability analysis of vertical and horizontal wellbores is carried out using FLAC3D numerical code by considering three varied stress regimes. The Normalized Yielded Zone Area (NYZA, i.e. the ratio of surrounding yielded cross-sectional area to initial area of wellbore) is determined for different mud pressures as well as diverse orientations of wellbore. By means of MATLAB software the best curve is fitted to the recorded points and then the optimized mud pressure is calculated using these plots. The results show that, the optimized orientation of wellbore is closely pertinent to the in-situ stress regime. Finally, the resulted mud pressure was compared with the mud pressure obtained from analytical solution by Mogi-Coulomb criterion. The comparison between numerical and analytical solution proved that the NYZA method may underestimate the drilling mud pressure and should be used cautiously.

Keywords: Finite difference, NYZA, Optimum mud pressure, Stress regime, Mogi-Coulomb criterion.