Abstarct: The exploitation of oil resources is associated with many challenges and costs that most of them are related to drilling operations. Wellbore drilling is one of the main processes of exploration, evaluation, and development of oil and gas reservoirs and is always presented as one of the main challenges in the upstream industries. One of the main concerns in the drilling industry is the stability of wells and determining the optimal weight of drilling mud. The purpose of this research is to evaluate the stability of the studied wells in the Lali field and to provide the basis for selecting the best drilling direction and reducing the problems and costs associated with it. In order to determine the safe mud window and to provide the basis for selecting the best drilling direction, the status of factors such as stress conditions, tectonic factors, and geomechanical parameters and physical characteristics of the reservoir rock should be evaluated. One-dimensional models show the parameters’ changes in the direction of the wellbore drilling properly but are not able to show the anisotropy of horizontal changes in the field. Therefore, in this research, three-dimensional geomechanical modeling has been used to identify and investigate the general condition of the field in different directions. In this thesis, the relevant studies on geomechanical considerations of well stability evaluation have been performed. Then, the data are collected in the Lali field, and baxsed on them, the one-dimensional geomechanical model of the intended wells is constructed. Then, a geomechanical model has been developed for the Asmari reservoir of the Lali field to evaluate the anisotropy of effective stress distribution and geomechanical properties of the region. Construction of a three-dimensional structural model, griding, magnification of data scale, and finally, expansion of obtained parameters from one-dimensional geomechanical models in a part of the Lali field reservoir is one of the steps that have been done to build the three-dimensional geomechanical model. by using Columbus's Mohr criterion as a failure criterion that is more adjustment with the events of the study field, the stability status of wells and drilling risk in the reservoir has been evaluated. Finally, the optimal direction for wellbore drilling in the Asmari reservoir of Lali field is determined. The cause of instabilities and failures in the wells in this research has been distinguished from inappropriate mud weight selection. According to the fluid pressure used for drilling many wells in the Asmari reservoir of the Lali field is low and close to the critical mud pressure, with small changes, this pressure has got out of the safe mud window and the wellbore wall position closes to rupture. According to the geomechanical model, the safe window width of the drilling mud is reduced by moving towards the center of the Asmari reservoir. This trend was observed in other areas of the Asmari reservoir. Therefore, in these areas, the weight of the mud must be carefully selected. Zones 7 and 6 of Asmari, with an average of 95.22 and 92.32 MPa, respectively, have the lowest safe mud window width in the tank. According to the stress conditions in Lali field, which was obtained as reverse fault along strike slip, the most stable drilling direction to minimize instability is the direction of maximum horizontal stress (N040E).