An integrated wellbore stability study to mitigate expensive wellbore instability problems while drilling into Zubair shale/sand sequence, southern Iraq

”The Zubair Formation is the most prolific reservoir in Iraq, which is comprised of sandstones interbedded with shale sequences. Due to the weak nature of the shale sequence, the instability of a wellbore is one of the most critical challenges that continuously appears during drilling across this formation. Historically, over 90% of wellbore problems in the Zubair Formation were due to wellbore instability. Problems associated with wellbore instability, such as tight hole, shale caving, stuck logging tools along with subsequent fishing, stuck pipe, and sidetracking result in increasing the non-productive time. This non-productive time has cost an enormous amount of money. The main objective of this research is to reduce the drilling time and cost for 8 ½” phase of wells in Zubair Formation by minimizing wellbore stability problems. This will be achieved by different laboratory tests on core samples from the targeted formation to obtain the rock mechanical properties and by applying a geomechanical model based on offset well data coupled with suitable rock failure criteria to obtain a safe mud weight and an appropriate well trajectory. Furthermore, this project presents some of the primary laboratory and wellsite testing techniques that are often used by mud engineers to characterize and remediate drilling fluids and shale interactions to improve the selection of chemical additives for clay inhibit. The present research work can be applied as a cost-effective tool to assess and address existing wellbore instability problems and to guide future neighboring wells for better drilling efficiency by reducing the non-productive time and well costs”--Abstract, page iv