Integrated 3D Mechanical Earth Modelling to Intensively Investigate the Wellbore Instability of Zubair Oil Field, Southern Iraq

Wellbore instability in the Zubair oil field is the main problem in drilling operations. This instability of the
wellbore causes several problems including poor hole cleaning, tight hole, stuck pipe, lost circulation, bad
cementing, and well kick or blowout that lead to an increase in the nonproductive time. This article aims to set
up an appropriate drilling plan that mitigates these instability issues for further well drilling. Field data from six
wells (logs, drilling and geological reports as well as offset well tests) were used to build a one-dimensional
mechanical earth modeling for each well. The constructed model of selected wells was combined to construct the
three-dimensional mechanical earth modeling, which can enable the distribution of all estimated geomechanical
parameters along the field of interest. The results revealed that the strip slip and normal faults are the common
fault regimes in Zubair oil field located in carbonate rocks and clastic rocks, respectively. The Mogi-Coulomb
failure criterion is conservative in determining the minimum and maximum mud weight, and it agrees with the
determination of real failure from the image/and caliper logs. The best direction to drill the deviated and
horizontal wells was towards the minimum horizontal stress with 140o azimuth from the north. The recommended
ranges of mud weight along the sections of 12.25″ and 8.5″ of the highly deviated and horizontal wells were
(between 1.46 and 1.58 gm/cm3) without any expected wellbore instability problems. Based on the outcomes of
3D model, it is expected that the wellbore instability issues are most likely to occur in the domes of; Shauiba and
Hamma and in formations; Tanuma, Khasib and Nahr-Umr. In contrast the less wellbore instability problems are
expected to expose in Rafidya dome. The study presents an appropriate mud window that can be used to design
to minimize the wellbore stability problems when new wells will be drilled in Zubair oil field.
Keywords: Dome; Wellbore stability; Zubair Oil field; Rock

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