When a well is drilled, the load carried by the removed rock is then taken by the adjacent rock to re-establish equilibrium. As a result, a stress concentration is produced around the well, and therefore the in situ stresses are modified. If the stress somewhere does not satisfy the failure criterion for the rock, the rock fails and lead to instability. The wellbore instability results in substantial economic loss. Therefore, it is necessary to study factors affecting wellbore stability. In this research, the effect of four factors including: in situ stress, pore pressure, inclination angle of well and azimuth angle of well from the major horizontal stress on the wellbore stability were analyzed, using numerical modelling. For this purpose, FLA C3D software was used. As wellbore instability criterion, the Normalized Yielded Zone Area (NYZA), which is the cross-sectional area of the yielded rock around the well divided by 'the area of the original borehole was used. The results obtained show that for vertical well position, the best situation is achieved when the horizontal in situ stress is isotropic. Also, it is concluded that instability increased with increasing of well's inclination angle to vertical direction and increasing of azimuth angle in relation to major horizontal stress.
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