Every year, bit designers, like car designers, use newly developed technology to make drill bits run faster, drill deeper and last longer in an effort to give operators more value for their bit dollar. In some cases, the added value is real, while in other cases, it is simply perceived. In the end, it is the operators who must determine which bits actually do perform better. Here are some of the latest technologies being applied to drill bits.

TECHNOLOGY OVERVIEW

Generally, the increased use of computer modeling, for roller cone and PDC bit design and manufacturing, is producing a new generation of bits that delivers breakthroughs in rates of penetration (ROP), increased durability and longer life. These computer models utilize proprietary algorithms to model forces and bit behavior to assure maximum bit performance.

Additionally, computer modeling of the dynamics of interactions between PDC bit cutters and rock allows bits to be custom-designed for specific applications. Computer-aided design tools are being employed to model, with 3D and 4D graphics, the drag, axial and radial forces acting on the bits' cutting surfaces. One manufacturer's cutter/rock interaction model divides the cutting edge into three surfaces: cutting face, chamfer and cylinder surfaces. The computer calculates the cutter's engagement area by meshing each surface into grids, so the cutter orientation's effects on the engagement area can be considered.

Data on advanced cutter wear is one of the results of this modeling. Cutter wear depends on cutting force, relative speed, temperature, cutter material properties and rock properties. Previously, computer models estimated only the wear flat without considering its orientation, as well as the actual diamond thickness, the interface geometry of the diamond layer, and carbide and abrasive resistance. With the newer computer models, cutter wear can be considered three-dimensionally, and all factors neglected by previous models are now easily considered. Bit designers then use this information to devise a bit specifically for a particular job.

Bit balancing. Another development that is becoming more important is bit balancing, Fig. 1. This concept considers the forces acting on the bit to create designs in which no single cone or cutting system is overstressed. This increases cutting efficiency and extends bit service life. Two types of balancing methods are used--force balancing and load balancing.

[FIGURE 1 OMITTED]

Force balancing. Of the three forces acting on a bit--axial force, lateral force and bending moment--it has long been recognized that balancing the lateral force is very important for preventing whirl. In fact, previous concepts of PDC bit force balancing referred only to lateral force balance, due to the belief that once lateral force was balanced, the bit bending moment was balanced also.

However, further study revealed that bit bending moment contributes not only to bit lateral motion or whirl, but also to tilt motion, which significantly affects directional control. Even a perfectly force-balanced bit may exhibit tilt motion, if the axial forces are not balanced. Therefore, balancing the axial forces is equally as important as balancing lateral force.

A PDC bit that is balanced, both in terms of lateral force and bending moment, is a "global force-balanced" bit. Designing such a bit involves adjusting the cutting structure to reduce the imbalance numbers. For example, newer series bits are force-balanced according to a specific set of design criteria that consider the summation of cutter forces to a global, lateral and axial bit imbalance, resulting in a global force-balanced design.

Load balancing. A bit in which the drilling forces acting on each individual cutter are balanced and are evenly distributed across the entire cutting is said to be "load balanced." This technique is meant to prevent cutter wear and excessive point loading that can break or damage cutters.

Roller cone bits are load balanced in two ways--by volume and by force. Volume balancing almost equalizes rock removal among all the cones, while force balancing ensures that all cones are subjected to nearly the same loads, including weight-on-cone, bending moment and force-on-bearing.

For PDC bits, load balancing, which was employed originally on roller cone bits only, is now being used to improve the PDC bit performance. The concept of load balancing is based on the fact that the amount of formation removed by each individual cutter differs and, as a result, the force acting on each cutter also differs. Furthermore, the number of cutters differs from blade to blade. Therefore, the forces acting on each blade differ. To avoid overloading individual cutters and blades, it is necessary to control these load distributions.

Equally distributing the forces minimizes the change in work, or force, among zones of the cutting structure. Thus, designing a "torque- and drag-balanced" PDC bit involves analyzing the distribution of work and forces acting on a cutting structure, with the goal of controlling force distribution over both the blades and cutters. By controlling the force distribution, these bits are able to reduce impact damage and uneven wear while promoting improved ROP.