The effect of varying shale concentrations on wellbore instability in drilling operations for Iraqi oil fields

Drilling and completion in shale formations create problems in terms of wellbore instability in certain situations globally, which causes substantial additional expenditure for the drilling industry by increasing Non-Productive Time (NPT). It is also important to take into consideration the role of shale concentration in contamination by drilling fluids that could result in wellbore instability problems. This paper studies the effect of variations in shale concentrations in cases with water based muds (WBM) to determine the rheological properties of such muds. The shale samples employed in this study were drawn from the Rashdia region of the East Baghdad oilfield. The base drilling mud has a density of 8.86 lb/gal, and its specific rheological properties were measured using a viscometer. Different shale concentrations were then added to the base mud (0.5%, 2%, 5%, 8% and 10%), and the rheological properties for the resulting new muds measured and their plastic viscosity determined. The increases in the respective rheological values were 11.8%, 21.2%, 25.6%, 33.0%, and 54.7% for the specified shale contamination levels in this mud system. This confirms that the proper design of drilling fluids must be considered when drilling shale formations to minimise or mitigate potential instability problems.