Selecting the Proper Rotary Rock Bit

Abstract

Introduction On Aug. 10, 1909, Howard R. Hughes patented the first rotary rock bit, which he called a "boring drill." The drill resembled two pipe reamers set on journal bearings attached to a drillpipe sub. This invention enabled rock formations to be drilled and was the starting point for 62 years of rock-bit development. This development history is traced in Fig. 1. It is not our purpose here to discuss these changes, nor to detail the basic designs of rock bits. The major bit manufacturers provide, for the asking, very good manuals on this subject. It is our purpose to provide the engineer who plans casing and drilling programs with the plans casing and drilling programs with the information he needs to take advantage of the most modern bit technology. Casing programs that force the use of obsolete bit types, and drilling programs that do not run the new bits properly are programs that do not run the new bits properly are more costly than they need be. Optimization of drilling is coming into its own after 10 years of intensive development. Often, however, the optimization procedures are handicapped by preset casing programs that force the use of off-size or obsolete bits. How good is modern bit technology? If you are drilling a "standard" hole size, the answer is "great." We are drilling footages and hours on single bits that would have been unbelievable just 2 years ago. Many of the new design features, which were experimental in the early 1960's, have been made generally available in the last 4 years. As shown in Fig. 1, it is estimated that improvements in rock bit development have been 30 percent greater in the last 4 years than in the 50 previous years. In fact, some optimum drilling programs planned just last year for ultradeep wells have become obsolete before the wells have been completed because of the introduction of new types of bits that were not generally available when the wells were originally planned. planned. Roller-Rock-Bit Classification Complete rock-bit nomenclature can be found in most bit catalogs and, therefore, will not be introduced here. However, several terms should be described for the following discussion. Rock bits are usually classified according to tooth material and height, cone offset, and journal angle. The teeth may be hard-faced, case-hardened, or inserts of tungsten carbide. The cone offset, sometimes called "skew," is a measure of how far the center line of a cone misses the center of the bit. Journal angle is the angle the center line of the bearing journal makes with the bottom of the hole. A rock bit may be built with any one of four different types of bearing assemblies: nonsealed roller, sealed roller, sealed journal, and "super" journal, with the price increasing in the order listed. A rock-bit classification guide based on these features for the four major bit manufacturers is given in Table 1. Bits on the same fine in the chart are designed to withstand roughly the same operating conditions. JPT P. 1359