In the Oseberg field in the Norwegian sector of the North Sea, extended reach drilling (ERD)has been employed to increase oil recovery. Wells up to 9,327 m total depth (TD) have been drilled. Hole stability problems were experienced in the Upper Jurassic Draupne Formation (Kimmeridgian shale) in a number of very highly inclined wells; these problems had not been experienced at lower inclinations. A rock mechanical study was carried out to explain and overcome the stability problems.

A series of thick-walled hollow cylinder (HC) tests were carried out on core material and analogous outcrop samples in order to identify the mechanisms of the hole problems, together with triaxial tests to obtain modelling parameters. Hole deformations were measured during external pressurization of the cylinders, and after testing the samples were sectioned and exiunined for damage. A very strong influence was found of the angle between the HC axis and the bedding plane of the shale. Specifically, if the HC axis lay more than approximately 100 away from the bedding plane, the hole was relatively stable; if the HC axis was parallel to bedding, the hole was highly, and catastrophically, unstable, forming very large regions of cavings. An anisotropic elastic model for the stresses around the hole did not predict such a sudden change in behaviour.

It was finally concluded that the borehole instability related to strength anisotropy associated with the pronounced bedding of the organic-rich Draupne shale. Spallings from the high side and the low side of the hole occurred at a number of different mud densities when the inclination was sufficientiy high. The decision was made to modify the wellpath so that the "angle of attack" between the well trajectory and the bedding planes in the Draupne Formation always exceeded 200. The world record well C-26 (9,327 m; 7,853 m horizontal reach) was sidetracked due to stuck pipe after an unsuccessful high-angle Draupne penetration. The sidetrack was stable at practically the same mud density, but with a lower inclination.

The reported type of hole instability is widely recognized in hard-rock applications of rock mechanics (mining, tunnelling), but is relatively new to petroleum-related rock mechanics.

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