An integrated one-dimensional geomechanical model to identify the optimal mud weight and the well trajectory for the Zubair formation.

The Zubair formation is a significant producing reservoir in southern Iraqi oil fields. During the drilling of this formation, many wellbore instability issues were encountered, including shale caving, tight hole, lost circulation, and pipe sticking. These challenges significantly lead to an increase in non-productive time (NPT). The drilling data analysis indicated that the wellbore shear failure was the principal cause of these issues. The main objective of this study is to build a one-dimensional geomechanical model using core and well logging data from offset wells. Thus, the safe mud weight necessary to prevent the initiation of the shear and tensile failures against the Zubair formation was determined by employing the Mogi-Coulomb failure criterion. The results revealed that the horizontal and strongly deviated wells are less secure and stable than vertical and slightly deviated wells (less than 30°). This is based on a sensitivity analysis of the wellbore at a specific depth. The recommended mud weight for drilling wells having angles of inclination that vary from 0 to 30° is 11.9 to 12.3 ppg. A 140° NW-SE azimuth is the optimum azimuth to drill the deviated and horizontal wells which is analogous to the orientation of the minimal horizontal stress. The research's findings can be applied to planning upcoming wells in the study region's vicinity. [ABSTRACT FROM AUTHOR]