Abstract

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.

Introduction

Water based drilling fluids have been used for a long time in drilling operations on the Norwegian sector of the North Sea. Most 17 1/2" sections are drilled with water based drilling fluids. For deeper sections the selections of drilling fluid systems varies. In the southern part of the Norwegian continental shelf there exist large regional sand formations that opens for re-injection of drilled cuttings. Furthermore, the shale is generally more reactive. Therefore, use of oil based drilling fluids is common. In the northern part these regional sand formations are absent, and re-injection has been found to be complicated1. Oil based drilling fluids are though still used on some northern fields. Normally, in these fields, the oil wet drilling waste has to be brought onshore for treatment and deposition. Some fields like Heidrun drill all their well sections using water based drilling fluids2,3, even though Heidrun's wells are strongly deviated with well angles in the magnitude of 55–70°.

Unlike when drilling with oil based drilling fluids, water based drilling fluids react with the formation. The potassium chloride (KCl)/polymer based drilling fluid has been the most common water based drilling fluid for the last decade, because of its good inhibitive properties with respect to conserve cuttings. Albeit its good ability to conserve cuttings, the KCl/polymer based drilling fluid has not shown any better performance with respect to wellbore quality than other currently used water based drilling fluids4. In other studies, the KCl has been found to have an adverse effect if used in high concentrations5. The hydrated potassium ion's ability to enter into clay structure creates a slight shrinkage of the clay, followed by micro fractures and production of cavings.

A traditional problem during drilling is to transport cuttings out from the wells. A large emphasis has been put upon optimizing the drilling fluid properties for achieving good hole cleaning. Drill string rotation makes the flow unstable, and in most cases turbulent, for most drilling fluids. The instabilities, or turbulence, increase the shear stress onto the cuttings bed and thereby improves hole cleaning. The best abilities for the fluid to transport cuttings are when the fluid do not consolidate cuttings and it is possible to remove single cuttings particles6,7.

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