Novel emulsifiers in reversible invert emulsion drilling fluids offer a unique opportunity for operators to control the wettability properties of the mud. Reversibility of the invert emulsion will provide economic advantages in drilling and completion and increased productivity. Optimal drilling performance and formation damage control are usually achievable by oil wetting the drill solids, formation and drillstring. On the other hand, optimal completion of a well - filtercake removal and cement bonding -- and disposal of drilled cuttings are best achieved by maintaining all surfaces water-wet. Filtercake removal is especially critical in the completion of horizontal openhole wells.

The reversible invert emulsion drilling fluids can be readily and reversibly converted from a water-in-oil emulsion to an oil-in-water emulsion and back to a water-in-oil emulsion using an acid-base chemical switch. This drilling fluid system is simple and easy to run without any drastic changes in current invert-emulsion technology. A novel surfactant package forms an invert emulsion in the presence of lime and a direct emulsion in the presence of acid. Reversible emulsion technology uses new emulsifiers that allow the drilling fluid to cycle between oil-wetting and water-wetting as frequently as one desires. Protonation and deprotonation of the surfactant is used to control the emulsion phase without drastic and uncontrollable viscosity swings.

The reversible invert emulsion drilling fluid has been successfully run in a North Sea well. The paper presents field data illustrating the economic benefits of this system and discussion on how the ability to change wettability can influence drilling and completion performance.

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