The stability of the wellbore wall is one of the most crucial issues during drilling oil and gas wells. To avoid wellbore instability, determining a safe mud weight window for minimizing the risk of drilling mud loss/lost circulation and wellbore blowout is very important. If it is calculated accurately, the well drilling will be carried out safely and at a higher speed. In line with achieving these goals, Flac3D software was employed to design a proper mud weight during the drilling operation. In this regard, different zones of a southern Iraq oil field were investigated and the effect of mud weight on wellbore wall stability was examined. The results of modeling indicated that the minimum mud weight in Zone 1 is 5.70 lb/gal, resulting in safe drilling without wellbore wall collapse. The maximum displacements in the X and Y directions in this zone are 0.3621 and 0.8445 mm, respectively. The minimum mud weight in Zone 2 leading to wellbore stability was found to be 6.61 lb/gal, with maximum displacements in X and Y directions limited to 0.2448 and 0.6614 mm, respectively. In Zone 3, the minimum mud weight for safe drilling was found to be 7.43 lb/gal, the highest value compared to other zones, with maximum displacements in the X and Y directions both limited to 0.059 mm. The minimum mud weight for safe drilling was found to be 6.51 lb/gal in Zone 4 and 5.05 lb/gal in Zone 5. The maximum displacements in the X and Y directions are both 0.0848 mm in Zone 4 and 0.291 mm in Zone 5. Thus, the minimum mud weight for maintaining wellbore stability and safe drilling in the whole oil field is 7.43 lb/gal. Considering 7.43 lb/gal as the optimum mud weight in all formations, the drilling mud pressure is consistently smaller than the corresponding pore pressure at all target depths, indicating that the drilling operation is under-balanced in this oil field.