fax 01-972-952-9435. AbstractMaersk Oil and Gas AS are typically drilling the carbonate reservoirs in the Danish Sector of the North Sea in 8½" horizontal hole sections, with section lengths regularly over 15,000 ft. The two lithological components of the formation show a large variation in geo-mechanical properties: a soft, plastic chalk matrix and large, hard, brittle, chert nodules. Impact loading on cutting structures and penetration difficulties in the hard, microcrystalline chert are generally the limiting factors on bit longevity and performance. These long horizontal sections highlighted the opportunity for optimising the durability of single bit runs. In June 2001 Maersk Oil implemented a bit development priority that was to produce significant, beneficial results.