Wellbore instability is one of the major problems that engineers meet during drilling and cause nonproductive time, which consequently leads to over cost the well. The causes of wellbore instability are often classified into either mechanical (for example, failure of the rock around the hole because of high stresses, low rock strength, or inappropriate drilling practice) or chemical effects which arise from damaging interaction between the rock, generally shale, and the drilling fluid. This work deals with generating an integrated wellbore stability analysis for Mishrif formation in Southern Iraq. Many wells in this area showed that the additional cost of the well is due to instability problems. Drilling these wells without finding a solution for loss problems is considered a significant loss in time and cost. The obtained results revealed that the use of inappropriate mud weight is a major cause of the wellbore instability. The optimal solution to this problem is to follow the best path of the well and the use of safest window for mud weight. The optimum mud weight which can be successfully applied in Mishrif Formation ranges from 8.8 to 9 ppg and the maximum angle should be 40 degrees and the best scale will be 40-60 degrees northwest-southeast. The most stable direction to drilling is the NW-SE direction which is the direction parallel to Shmin.
Wellbore instability is a function of imbalance in the required wellbore pressure applied and the fluid pressure in the formation, in addition to chemical interactions between the formation and the drilling or completion fluids, and interactions between these fluids and native formation fluid . Deviation and azimuth of the well also influence the wellbore stability as the stress distribution around the wellbore is dependent on the orientation of the wellbore, with respect to the in-situ stresses and the hoop stresses introduced during drilling the wellbore . To avoid wellbore instability problems in drilling, a proper well design needs to be developed for the formations to be drilled and completed for production, which requires understanding the in-situ stress state, pore pressure, and geomechanical properties of the reservoir formation .