Production drilling on the Gullfaks platforms was planned using oil based mud for the whole well path. However, in light of the growing environmental concerns about the discharges of oily cuttings, potential health hazards and in some situations reduced safety margins connected to the use of oil based drilling fluids, it was decided in 1988 to drill as much as possible of the hole volumes in the remaining wells on the Gullfaks field with water based mud. It was also a goal to use water based drilling fluids consisting of readily biodegradable and non toxic chemicals.

This paper discusses the reduction in discharged oil on drill cuttings from the three Gullfaks platforms. This reduction is approximately 95 % per well drilled with water based mud compared to the discharge from the first nine wells on Gullfaks A drilled with oil based mud. The time/depth curves for the drilling operation, from drilling out of the conductor shoe to start drilling the reservoir section on Gullfaks A, are also presented and analyzed, showing an increased drilling rate of 38 % with water based mud compared to oil based mud. The water based mud system used is described.

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