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

Abbas Dakhiel

doi:10.46717/igj.54.2e.4ms-2021-11-20

Abstract

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.

Highlights

Zubair oil field is one of the largest fields in Iraq (Al-Jaberi and A-Jafar, 2020)
The Zubair field has anticline structure with four domes; Al-Hammar, Shuaiba, Rafidya and Safwan, (Program, 2015). It is primarily consisted of four reservoirs; Upper Cretaceous Mishrif Limestone, Lower Cretaceous Upper Shale Member, Lower Cretaceous Upper Sandstone (3rd Pay) and Lower Cretaceous Lower Sandstone (4th Pay), (Fig. 1)
It is worth noting that the field is composed of a fourth dome (Safwan dome), but it was not included in the study due to the lack of basic data for the construction of the mechanical earth model

Summary

Introduction

Zubair oil field is one of the largest fields in Iraq (Al-Jaberi and A-Jafar, 2020) It was discovered in 1949, and produced for the first time in 1951, it is located in southern Iraq. The Zubair field has anticline structure with four domes; Al-Hammar, Shuaiba , Rafidya and Safwan, (Program, 2015). It is primarily consisted of four reservoirs; Upper Cretaceous Mishrif Limestone, Lower Cretaceous Upper Shale Member, Lower Cretaceous Upper Sandstone (3rd Pay) and Lower Cretaceous Lower Sandstone (4th Pay), (Fig. 1). The Lower Cretaceous Zubair reservoir is a main producing horizon in the Zubair oil Field, southern Iraq,(Al-Jafar and Al-Jaberi, 2019).The well design in the Zubair oil field is generally consisted of four sections; 23", 17.5", 12.25": and 8.5". Sections 12.25" and 8.5" are among the most

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