A New Theory on Cutter Layout for Improving PDC-Bit Performance in Hard-and Transit-Formation Drilling

It is well-known that cutter layout on a polycrystalline-diamond-compact (PDC) bit plays a key role in bit stability and drilling efficiency. In today's PDC bit, cutters are laid out according to two general principles: single set or track set. A single-set cutter layout is defined as having no cutters at the same radial or axial position after cutters are rotationally projected into a radial plane passing through the bit axis (this is referred to as bit profile in the industry). On the other hand, a track-set layout is defined as having at least two cutters at the same radial and axial positions on a bit profile but on different blades. In most track-set designs, cutters on primary blades are single set, and cutters on minor blades are redundant to those on primary blades. An extreme version of track set is called track loc, in which there is only one primary blade and cutters on all other blades are redundant to those on the primary blade (Weaver and Clayton 1993). Figs. 1a through 1c depict single set, track set, and track loc, respectively, demonstrating that the major difference is the engagement area or width of indentation of cutters while drilling. The single-set cutter has the smallest indentation width, and the track-loc cutter has the largest indentation width. According to laboratory and field tests, a PDC bit with single-set cutters drills much more efficiently than a PDC bit with track-set cutters (Besson et al. 2000). On the other hand, a PDC bit with track-loc cutters is extremely stable but extremely inefficient. Bit stability may be enhanced by laying out the cutters so that the bit lateral forces are balanced or minimized. This is called a "force-balanced" PDC bit (Glowka 1989; Behr et al. 1993; Clayton et al. 2005). In some applications, cutters are laid out so that a large lateral imbalance force [as high as 15 to 20% of weight on bit (WOB)] is created and directed toward a so-called low-friction gauge pad, which is on one side of the bit. The bit is called an "anti-whirl" PDC bit (Brett et al. 1989; Warren et al. 1990). Successful field applications of this technology have been reported (Warren et al. 1990). In this paper, it is found that the force-balanced condition of a PDC bit with single-set cutters may be violated during transit-formation drilling. A new cutter layout procedure is proposed, which not only ensures the force-balanced condition during transit drilling but also significantly improves drilling efficiency. The concepts developed in the paper are first verified by laboratory tests of a core PDC head and then a full PDC bit. Two case studies of the new bits in drilling through hard and transit formations are detailed in the paper.