Problems encountered in drilling Tertiary clays and shales in the southern part of the Norwegian Continental Shelf and the mud system most frequently used in this area, are reviewed in this paper.
Severe viscosity problems frequently arise while drilling the top 10,000 feet of hole, which predominantly consists of clays and shales, and are the direct cause of several problems like swabbing formation; blocking of flowlines and shaker screens by very soft sticky clay; sticking of drill pipe; and bit and collar balling.
The viscosity problems are the result of lack of solids removal equipment, the dispersing type drilling fluid used which makes the solid separation more difficult, and the facts that the lower 5,000 feet of formation is overpressured, behaves plastically, and has a high montmorillonite content.
A salt solution base mud is discussed as an alternative to the commonly used lignosulfonate system. Hydration tests of formation samples in various salt solutions indicate that low density solid content in the drilling fluid might be controlled more economically by the use of a salt water base mud and centrifuges and fine screen shakers. To date mud properties have partially been controlled in the unsatisfactory manner of discarding old mud and making up new mud.
It is concluded that a saving of 25% on the cost of barite alone seems possible with a salt solution base mud. possible with a salt solution base mud. More important however, are the potentials for reducing rig time and potentials for reducing rig time and consequently cost.
This study presents mud programs and drilling problems pertaining to the Tertiary clays and shales in the southern part of the Norwegian Continental Shelf (see Fig. I).