The Zubair Formation is the most prolific reservoir in Iraq, which is comprised of sandstones interbedded with shale sequences. Drilling deviated boreholes in this formation has always been a challenge due to the weak nature of the shale sequence. Historically, over 90% of wellbore problems in the Zubair Formation are due to wellbore instability. These wellbore instability problems, such as hole collapse, tight hole, stuck pipe, and sidetracks results in increasing the nonproductive time (NPT) and well costs. The present study aims to construct a calibrated post-drill 1-D mechanical earth model (MEM) that can be used to conduct a comprehensive geomechanical analysis of the Zubair Formation.

A practical methodology was detailed for (1) constructing the 1-D MEM by using data from several open hole logs and (2) calibrating such a model using all the available data (e.g., observations of wellbore failures, measurements of well testing, laboratory test results of rock mechanical properties, etc.). Three failure criteria (i.e., Mohr-Coulomb, Mogi-Coulomb, and Modified Lade) were used to perform a root cause analysis of borehole breakouts and to address the minimum required mud weight to minimize wellbore instability-related issues for different well trajectories.

The wellbore failure analysis showed that the mud weight, that was used previously, is insufficient to support the rock on the wellbore wall, and it is not appropriately adjusted according to the variation in the wellbore azimuth and inclination. In addition, the heterogeneity of the Zubair Formation and the inappropriate drilling practice have magnified the severity of the drilling problems. The predictions resulting from this study can be used as cost-effective tools when planning for future neighboring deviated wells to create better drilling efficiency by reducing the nonproductive time and well costs.

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