How to manage wellbore stability in the Vicking Graben tertiary shales by using mud systems environmentally friendly?

Abstract

Abstract This paper deals with the drilling of the 17"1/2 section of the Dunbar field (Viking Graben - North Sea). In the past, this formation was systematically drilled with a conventional WBM in underbalanced conditions (mud weight less than formation pore pressure estimated at 1.4SG). With such a mud system, the well was highly unstable particularly in the Eocene shales (very large amounts of cavings were produced, calliper was very overgauged) and the final trip was particularly tight. Following three remedial side tracks experienced over the last two years, an extensive study has been carried out. The main recommendations were to drill the Eocene in overbalanced conditions (MW>1.4SG) with a Silicate Mud. The results obtained on two wells slightly inclined (20° and 35° inclination at the 13"3/8 casing shoe) were very encouraging. By contrast to previous underbalanced experience, the hole was perfectly in gauge and trips much easier (regular wiper trips to be performed). However, even in the case of a perfectly stable rock and to ensure a good hole cleaning, the wells had to be circulated at a high rate and a sufficiently thick rheology. These results have then been confirmed on a third well (64° at the 13"3/8 casing shoe) with a maximum mud weight equal to 1.45SG. However, this MW was not sufficient for 72° inclination (the well was too unstable and had to be abandoned).