Deep wells drilled into the Jurassic formations of North Kuwait pose a drilling challenge due to the existence of low pressure reservoirs separated by an extremely high pressure salt zone. Currently, the seven string casing design in these wells leaves no further option for zonal isolation. As a result, a wide range of pore pressures are encountered while drilling these formations. Depleted zones/reservoirs in these wells suffered multiple instances of differentially stuck pipe and side tracks. The end result has been a significant loss of expensive rig days. These sections are drilled using oil based mud (OBM) with a high oil/water ratio and mud densities ranging from 10 to 17 ppg.
To prevent a reoccurrence of the differential sticking problems, extensive laboratory work was undertaken to determine how to negate the effects of high differential sticking force while drilling depleted zones. Laboratory tests and fracture modelling based on historical data were carried out to develop a suitable fluid formulation to address these challenges. This work indicated that a novel combination of glycolic polymer lubricant and sized marble bridging agent provided the optimum combination of lubricity and filtration control to minimize differential sticking tendency.
A fluid formulation based on the laboratory results was utilized to drill subsequent wells in this field. The depleted intervals were drilled using a similar OBM treated with a combination of unique lubricant and sized marble bridging agent to reduce differential pressure transmission across the depleted zones. The intervals were drilled and logged without the occurrence of any problems of differential sticking. The troublesome sections were completed incident free resulting in huge savings in cost and rig time.
This paper describes the historical challenges in this field and present case studies demonstrating how the solution was successfully implemented in the field.