Water based drilling fluids have been used in many North Sea drilling operations during the last decades. Traditionally, a high focus has been directed towards creating inhibitive drilling fluids with high low shear rate viscosity for optimising cuttings transport and intact cuttings on the shaker. It has, however, been recognized that there is no direct connection between cuttings quality and hole stability. In the current field it was also found that a low gel/viscosity drilling fluid is effective in transporting cuttings out of the wellbore. Following this, the content of high molecular weight polymers have been reduced and exchanged with low molecular weight polymers to enhance hole cleaning throughout the last years. Fewer cuttings are entrapped in beds down hole and are transported up to the surface. The current paper demonstrates how this low viscous/low gel strength drilling fluid contributed to improved drilling performance, where an improvement in tripping time, casing running speeds and total drilling time was observed.
In addition to changes in the viscosity a wide range in NaCl and KCl salt concentrations and mixtures have been tried and field tested. The project is currently using only a moderate concentration of KCl, which has improved hole stability. The frequency of unstable hole occurrences in a particular troublesome formation are today greatly reduced by using lower KCl salt concentration and improved well design.
The current paper explains in detail how the drilling fluid design optimizes the chemical performance and the hole cleaning performance during the drilling operation. The drilling performance for this particular field improved significantly during the last three years where the total drilling time was reduced with roughly 50% compared to earlier practice. The reasons for the improvements are mainly the changes in well design combined with the use of an effective WBM system.