Abstarct: Geomechanical instability refers to mechanical conditions such as wellbore collapses or failures. Typically it is caused by unknown rock mechanic’s properties and it leads to costs increasing during drilling and completion. X-4 is the exploration well in the north of Iran that experienced variety of wellbore instabilities. The 49% of non productive time (NPT) in this well is incurred as a result of, wellbore instabilities and well control problems. For this well development of a geomechanical model after drilling can be a worthy choice for creation of a process for prediction of drilling hazards and also other items associated with wellbore instability for other proposed wells in that field. For developing a model for this well, in this thesis a one dimensional rock mechanical model is chosen. In that model, the formation elastic properties are calculated baxsed on available petrophysical properties and empirical relations. Because of unavailablity of core lab data, Bradford relation is used as an initial guess for calculation. Then elastic and mechanical properties are optimized baxsed on stress regime, drilling events and caliper log data. The pore and fracture pressure model is constructed baxsed on different available methods. By comparison of results, it is obtained that the Eaton method is the best choice in this case. After that minimum, maximum and vertical stresses are calculated baxsed on fundamental poroelastic theory relations. Then minimum horizontal stress is calibrated by LOT and FIT.finally maximum horizontal stress is calibrated by minimum horizontal stress and area stress regime.. Formation failure is predicted by using Mohr-Coulomb criterion, in which the minimum and maximum stresses are only controlling failure factors at the borehole wall, therefore the intermediate stress is assumed to have no effect. After rock mechanical model (RMM or MEM ) construction, UCS and pore pressure are corrected by normal compaction trend modification. As a result stress regime of region in depth up to around 3500 meter is specified as distension and in deeper interval is specified as compressional and also strain coefficients in the direction of maximum and minimum horizontal stress is calculated 1.9 and 0.4 in order. At the end, the developed model is utilized for prediction of the instability conditions in X-5 well, another appraisal well in that field. Results showed that the developed geomechanical model in the well X-5 successfully predicts the stress regime changes from 2518 meter and from 3900 meter depth in this well obviously. It should be noted that geomechanical model in well X-5 is achieved by using strain coefficients of well X-4. Such accurate prediction in well X-5 undoubtedly confirms the consistency of geomechanical model of well X-4