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Historically, the Upper Jurassic Brae conglomerates intrinsic to North Sea Block 16/17 have been drilled with IADC 517 to 617 type roller cone bits. Unfortunately, excessive wear to the cutting structure and gauge area of these bits dramatically limited their useful life and made them uneconomical. Runs shorter than 100 ft with penetration rates as low as 4 ft/hr was not uncommon with these bits. This was true even for roller cone rock bits incorporating enhanced gauge protection.

The decision to employ fixed cutter bits in Block 16/17 was initially frustrated with the inability of existing designs to effectively drill the conglomerates. However, a new fixed cutter design concept utilizing impregnated grit-sized diamonds has greatly improved drilling performance with substantially lower cost per foot.

Field tests have shown average penetration rates of more than 1 5 ft/hr with bit life averaging 400 ft. Penetration rates have remained steady throughout the target interval. Further, no significant gauge wear has been observed, despite the fact the bits were run on turbines.

This paper describes the development of the IADC M841 class fixed cutter bit and its application in these formations. The cost savings achieved with these bits and the future potential of the M841 impregnated diamond drill bit will be analyzed. The laboratory testing leading up to the development of the impregnated diamond bit will be detailed.


Block 1 6/1 7, also known as the T-Block, is located about 120 miles from Aberdeen in the Central North Sea (Fig. 1). Phillips Petroleum made the first discovery in the region in 1976 and since then four distinct fields have been identified: Tiffany, Toni, Thelma, and South East Thelma (Fig. 2). Since assuming operatorship of the block in 1986, Agip UK has thus far decided to develop the Tiffany and Toni fields. To date, 20 exploration and appraisal wells have been drilled on the block. Four development wells have been pre-drilled for the Tiffany platform, which became operational in the summer of 1993, and five wells have been drilled through a subsea template installed for the Toni development.

Historically, the Brae conglomerates of the Upper Jurassic sequence of the Central North Sea have proved extremely difficult to drill. The unique combination of a hard and abrasive sandstone matrix with a pebble and occasional breccia conglomerate found in Block 16/17 has made it a particularly difficult area to drill. The formations encountered there have proved impenetrable for both insert roller cone or standard fixed cutter bits.

Since 1 988, Agip UK has been involved in an extensive program to develop a fixed cutter bit capable of drilling the difficult Brae conglomerates of Block 16/17. What resulted was a partnership with a bit manufacturer, which led to the successful development of the IADC M841 type impregnated diamond fixed cutter drill bit,

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