Investigation of Wellbore stability in a Horizontal well drilled in interbedded Sandstone Reservoir of Zubair Field

Drilling a well in safe and cost-effective manner is the main target of drilling engineers. Thus, wellbore stability, especially in highly deviated and horizontal wells, is the major obstacle in well design and, consequently, investing in many oil and gas fields around the world. These impediments are varied, such as lack of hole cleaning due to accumulation of splintered, blocky or tabular fragments, hydraulic hammers, tight holes, pipe sticking, lost circulation, poor cementing, sand production, reservoir compaction, surface subsidence and many others. All these problems may cause a significant increase in nonproductive time of drilling or production. The purpose of this thesis is to determine the safe mud window for a proposed horizontal well in one of the most stressful fields in the southern part of Iraqi oil fields. The study is based on real data of eleven wells, here a parameters were implemented depending on the data of Triaxial and Brazilian tests of four wells that have been drilled in Zubair field. On the other hand, a dynamic model was employed in order to determine the rock mechanical strength parameters along the entire wellbore. After running the dynamic and static models, a matching has been carried out to define the uncertainty. Further, one of the most important considerations that was taken into account is the selection of an appropriate mechanical failure criterion. Formation pressures play a major role in any geomechanics study. Accordingly, direct method was used to determine the pore pressure in the permeable formations, while indirect techniques were exploited to define the pore pressure in impermeable lithologies. Subsequently, the geomechanical model was built by Schlumberger’s Techlog to determine the mud window for a horizontal well. Finally, due to the uncertainty in some results of log derived parameters, probabilistic analysis was performed by using Monte Carlo method to create a more realistic study.