Safe Mud Weight Window Determination: A Case Study from Southern Iraq


The Lower Cretaceous Zubair formation is comprised of sandstones intercalated with shale sequences. The main challenges that were encountered while drilling into this formation included severe wellbore instability-related issues across the weaker formations overlaying the reservoir section (pay zone). These issues have a significant impact on well costs and timeline. In this paper, a comprehensive geomechanical study was carried out to understand the causes of the wellbore failure and to improve drilling design and drilling performance on further development wells in the field. Failure criteria known as Mogi-Coulomb was used to determine an operating mud weight window required for safe drilling. The accuracy of the geomechanical model was examined by comparing predicted wellbore instability with the actual wellbore failure shown on caliper logs at a given mud weight. The analysis identified the cause of wellbore instability, as being inadequate mud weight while drilling the overlying shale formation. In addition, this case demonstrated that the safe operating mud weight window was found in the range of 1.35-1.55 g/cc, while 1.21 g/cc had been applied in practice. The results of this study can be used as a reference for optimal mud weight selection to mitigate wellbore-stability problems in nearby wells in this field.

Meeting Name

53rd U.S. Rock Mechanics/Geomechanics Symposium (2019: Jun. 23-26, Brooklyn, NY)


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Boreholes; Geomechanics; Infill drilling; Rock mechanics; Shale; Stability, Development wells; Drilling performance; Geomechanical model; Geomechanical studies; Mud weight windows; Wellbore instability; Wellbore stability; Zubair formations, Oil field equipment

Geographic Coverage


Document Type

Article - Conference proceedings

Document Version


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© 2019 American Rock Mechanics Association (ARMA), All rights reserved.

Publication Date

01 Jun 2019