Water based drilling fluids have been used in many North Seadrilling operations during the last decades. Traditionally, ahigh focus has been directed towards creating inhibitivedrilling fluids with high low shear rate viscosity for optimisingcuttings transport and intact cuttings on the shaker. It has,however, been recognized that there is no direct connectionbetween cuttings quality and hole stability. In the current fieldit was also found that a low gel/viscosity drilling fluid iseffective in transporting cuttings out of the wellbore.Following this, the content of high molecular weight polymershave been reduced and exchanged with low molecular weightpolymers to enhance hole cleaning throughout the last years.Fewer cuttings are entrapped in beds down hole and aretransported up to the surface. The current paper demonstrateshow this low viscous/low gel strength drilling fluidcontributed to improved drilling performance, where animprovement in tripping time, casing running speeds and totaldrilling time was observed.In addition to changes in the viscosity a wide range inNaCl and KCl salt concentrations and mixtures have beentried and field tested. The project is currently using only amoderate concentration of KCl, which has improved holestability. The frequency of unstable hole occurrences in aparticular troublesome formation are today greatly reduced byusing lower KCl salt concentration and improved well design.The current paper explains in detail how the drilling fluiddesign optimizes the chemical performance and the holecleaning performance during the drilling operation. Thedrilling performance for this particular field improvedsignificantly during the last three years where the total drillingtime was reduced with roughly 50% compared to earlierpractice. The reasons for the improvements are mainly thechanges in well design combined with the use of an effectiveWBM system.
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