Wellbore stability analysis and selecting safe mud weight window for Mishrif reservoir in Southern Iraq

Borehole instability problems lead to increased non-productive time, essentially during the drilling of shale formations. Past drilling experiences exhibited several problems: mud losses, tight hole, caving, mechanical stuck pipe, and extensive hole collapse. The study aims to construct a mechanical earth model (MEM) to analyze the wellbore stability. Thus, MEM was employed to define a safe mud weight window and improve the drilling performance on development wells. The specific analysis of wellbore instability showed that the primary cause of those instability problems was applying inadequate mud weight to drill the Tanuma shale formation. In addition, the minimum required drilling density to drill through the Tanuma formation is a strong function with a wellbore inclination. Thus, for the Tanuma formation, the model indicates that wellbore inclinations above 50° should be avoided. NW–SE azimuth provides less risk in terms of hole collapse in the Tanuma formation. Also, the sensitivity points showed that the appropriate mud weight for horizontal wells in Mishrif formation was 12.5 ppg.