Successfully replacing oil-based drilling fluids with water-based drilling fluids: Case study demonstrates value of extensive planning and execution in an extended-reach well

An extended-reach well was planned and successfully drilled with a water-based drilling fluid in an area where wells historically have been drilled with mineral oil- and ester-based drilling fluids-the North Sea central Graben. The high cost of cuttings drilled with inverts that must be shipped to shore for processing and disposal coupled with the potential shutdown of drilling operations in times of bad weather (when cuttings could not be transferred from the rig to supply vessels) led the operator to revisit the use of water-based drilling fluids, especially in the large-diameter hole section in which large amounts of cuttings are generated. However, many water-based systems have been used in the central Graben with little success. The need to develop a stable, shale-inhibiting, water-based mud (WBM) system capable of performing well in extended-reach drilling (ERD) applications was identified. Extensive prewell modeling of wellbore stability in problematic shale zones and of hole cleaning in the large-diameter interval contributed to the success of this challenging project. This article discusses the planning, preparation, and successful drilling of the intermediate interval with a water-based drilling fluid.