This paper is one of two describing changes to the IADC Classification and Dull Grading Systems for fixed cutter bits. Dull grading system revisions, described herein, were implemented to improve utilization and effectiveness of the dull grading system. Classification system changes were required as a result of improvements in bit technology and applications, and are detailed in the companion paper SPE 23940.

SPE Members Abstract One concept to avoid downhole vibrations with PDC bits requires a net side force that rotates with the bit and pushes a low friction gage pad against the borehole (anti-whirl bit). When drilling with an angle build assembly or in an angled hole, a non-rotating side load is applied to the bit. When drilling with a steerable assembly in the rotate mode, the bit axis is tilted relative to the profile of the bottom of the hole. This axis tilt affects the bit cutting forces and may consequently affect bit stability. This paper investigates these non-axisymetric loads for their effects on the stability characteristics of anti-whirl designs as they compare with standard PDC bits. A second hypothesis holds that standard PDC bits occasionally whirl on directional bottomhole assemblies, which leads to overgage holes and ledging. The build rates, hole diameter, and ledging characteristics of anti-whirl bits are compared to standard PDC bits. Comparisons were established in lab tests using standard and anti-whirl PDC bits both in an anisotropic formation (Green River shale) and in an isotropic formation (Carthage limestone) with a nonaxisymetric loading. Further, a comparison is made between two directional wells drilled at a field research facility using the same surface hole. One well was drilled with a standard PDC bit; the other with an anti-whirl PDC bit. The two wells are compared for surface vibration level, rate of penetration, build rate, dull condition of the bits, and hole diameter and ledging from calipers of the wellbore. Introduction Suppression of the bit whirl phenomena has dramatically changed the design and operation of PDC drill bits. One of the methods used to control whirl involves designing the cutter locations to generate a resultant imbalance force that is directed toward a low friction gage pad. This design technique stabilizes the bit against the hole wall. In a recent study, three non-axisymetric load cases that may affect stability were examined. When drilling with directional assemblies, or in high angle wells, an external side load is applied to the bit. In anisotropic rock, the cutting forces change as the bit rotates, thereby changing the imbalance force vector of the bit with time. Lastly, when drilling with steerable assemblies, the bit axis is tilted relative to the direction of bit motion. This modifies the cutting forces that are generated, thus modifying the resultant imbalance force. Since bit stability was uncertain in the above situations, early field tests of anti-whirl designs were restricted to straight, low-angle holes, drilled with conventional rotary assemblies. In order to eliminate these restrictions and understand the effects of loading as previously described, lab and field comparisons were undertaken on conventional and anti-whirl PDC bits. Lab tests were conducted in both anisotropic and isotropic rocks with non-axisymetric loading. The resulting loads and displacements were then recorded. P. 727^

Highly plastic and over-pressured formations are troublesome for both roller cone and PDC bits. Thus far, attempts to increase penetration rates in these formations have centered around re-designing the bit or modifying the cutting structure. These efforts have produced only moderate improvements. This paper presents both laboratory and field data to illustrate the benefits of applying a mirror polished surface to the face of PDC cutters in drilling stressed formations. These cutters are similar to traditional PDC cutters, with the exception of the reflective mirror finish, applied to the diamond table surfaces prior to their installation in the bit. Results of tests conducted in a single point cutter apparatus and a full-scale drilling simulator will be presented and discussed. Field results will be presented that demonstrate the effectiveness of polished cutters, in both water and oil-based muds. Increases in penetration rates of 300400% have been observed in the Wilcox formation and other highly pressured shales. Typically, the beneficial effects of polished cutters have been realized at depths greater than 7000 ft, and with mud weights exceeding 12 ppg. Introduction Historically, drill bits employing polycrystalline diamond to medium-hard formations, with low abrasivity. Nearly 70% of the formations drilled with PDC bits are shales, but successful applications in carbonates, evaporites, mud and siltstones, and sandstones are also common. Extensive studies performed in laboratory wellbore simulators have also provided important data for the proper geological applications and drilling practices that optimize drilling with PDC bits. Even though a PDC bit is deemed appropriate to the targeted formation, efficient, and thus economic, drilling performance depends on an optimum combination of bit design, operational practices, mud chemistry and hydraulics. Formations that can be successfully drilled increase steadily as knowledge is advanced in all these areas. Paradoxically, PDC bits occasionally perform unsuccessfully in formations they would seem to be ideally suited to drill. The U.S. Gulf Coast is one region where this is known to occur, even though the formations are not exceptionally strong or abrasive. There, the typical problem is the severe drop in the rate of penetration (ROP) experienced when drilling deep shale, especially with heavily weighted muds. These types of rock should be well suited to the shear cutting action of PDC bits, but surprisingly, PDC, as well as roller cone bits, have proven to be less than effective in drilling these formations. Increasing weight-on-bit early in the drilling cycle to reach an acceptable ROP is often unsuccessful, resulting in the bit being pulled prematurely. Examination of the bit often reveals no sign of balling or cutter damage, despite the application of heavier weight. P. 277