The Zubair Formation is the most prolific reservoir in Southern Iraq. Decline of reservoir production is a common features of many mature petroleum fields. Drilling a long horizontal well is one of the many technologies commonly used to enhance the production in these fields. The main challenges that were encountered while drilling such horizontal wells in Zubair Formation is to maintain wellbore stability especially across the weaker shale formations overlaying the reservoir section (pay zone). The first horizontal well was lost mainly due to severe instability problems. To successfully achieve the above objective, a comprehensive geomechanics study was carried out to understand the causes of the wellbore failure and to optimize the most appropriate drilling strategy.
In this paper an advanced geomechanical model was conducted by constructing a calibrated post-drill 1D mechanical earth model (1D-MEM) using field and laboratory data for the Zubair Formation. From the offset wells data, open hole wireline logging measurements were used to estimate the in-situ principal stresses magnitude and its orientations, pore pressure, and rock mechanical properties. The generated model was then applied with Mogi-Coulomb failure criteria to analyze the wellbore stability problems for wells with horizontal well profiles and to determine optimum mud weight and safe wellbore trajectory for future successful drilling operation.
The analysis identified that the mud weight, that was used previously, while drilling the overlying shale formation in the deviation build-up section 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. It is therefore recommended to increase the mud weight as required based on the trajectory of the planned well. The design of the future horizontal wells through the problematic Zubair shale formation can greatly improve based on this study.
The Zubair Formation is comprised mainly of alternating shale and sandstone, with minor streaks of limestone and siltstone. It is recorded as oil-bearing in 30 structures, which contain about 30% of Iraq's hydrocarbon reserves (Jassim and Goff, 2006). This multilayered reservoir has been subdivided based on its sand/shale ratio into five members: upper shale, upper sand, middle shale, lower sand, and lower shale. The Zubair Formation has shown to have significant geomechanical problems for several wells in these fields, based on the issues experienced during drilling stages such as lost circulation, wellbore collapse, shale caving, stuck logging tools, stuck pipe, and loss of some borehole sections. It seems that these issues manifest at the interfaces of the weak and non-depleted shale and depleted sandstone sections.