HEAVY WEIGHT ROCK BITS INCREASE PENETRATION RATES IN HARD ROCK

Robert E. Grimes
Hughes Tool Co.
Houston
Floyd C. Felderhoff
Hughes Tool Co.
Midland, Tex.
Harvey Brown Hughes
Tool Co.
Singapore

Faster penetration rates and lower costs per foot result from the use of bits designed specifically for drilling with high weight-on-bit (WOB) in the hard rock formations of the Permian basin.

The dolomite and limestone formations indigenous to this area are more responsive to increased WOB rather than higher rotary speeds (rpm). Historically, the heavier weights required to drill these hard formations severely limited the life expectancy of rolling cone bits.

The development of a heavy weight International Association of Drilling Contractors (IADC) Class 547Y rock bit has been considered successful because of its reliable and economical application at a higher average WOB than other similar class rock bits. The recently introduced design has proven its ability to accommodate the increased loads required to overcome the relatively high compressive strengths of the hard rocks in the West Texas/southeastern New Mexico area. Furthermore, the new bit can maintain and often increase rates of penetration (ROP) over Ion-er sections of hole. This results in lower cost per foot for drilling in this area.

In these hard formations, it was recognized that an increase in the life of the cutting structure could lower the cost per foot drilling and that higher WOB maximizes penetration rates.

A program was initiated to develop a tungsten carbide insert (TCI) bit that could accommodate weights exceeding 60,000 lb (267 kN) while drilling up to 50% faster than standard rolling cone bits in hard rock applications. To increase bit longevity, the design program focus was three-fold: an increased cutting structure durability, an effective high load/low rpm bearing, and a strengthened bit body.

Historically, long operating hours and ever-increasing weights have been standard operating parameters for rolling cutter rock bits used to drill the limestone and dolomite formations of West Texas and southeastern New Mexico. These parameters, coupled with the corrosive downhole environment and rough running conditions common to this drilling area, can severely limit the useful life of rock bits.

Numerous developments in rock bit technology have arisen from challenges in this hard rock environment. For example, tungsten carbide insert (TCI) bits were first introduced in 1951 and have subsequently undergone extensive development. 1 2 Similarly, the development of journal bearing rock bits in 1969 was considered a milestone in hard rock drilling because of the ability of journal bearings to withstand high unit loading without risk of rapid bearing failure. 3

Formations in the Permian basin mainly consist of limestones, dolomites, sands, and shales. This project was specifically aimed at the limestone and dolomite formations, which possess relatively high compressive strengths.

A typical well program in the West Texas/southeastern New Mexico area starts with a 17-1/2-in. (445 mm) surface hole drilled to approximately 500-600 ft (150-180 m), with an 11-in. to 12-1/4-in. intermediate hole drilled from 2,500 ft to 4,000 ft. A 7-7/8-in. TCI bit is used to drill out the 8-5/8-in. intermediate casing. Two to four more TCI bits are then used to drill to total depth at around 8,000-12,000 ft (2,440-3,650 m), depending on the geology. Bit types used in the 7-7/8-in. section of hole are usually in the IADC Class 547 to 627 range.

HIGH WOB

When relatively heavy weights, i.e., 6,500 lb/in. of bit diameter (1,140 N/mm), are run on rock bits in combination with long service hours, the propensity of fatigue crack development in the steel cone shells and rock bit bodies increases greatly. Excessive weights also lead to broken inserts, predominantly on the inner and middle rows of inserts, as opposed to broken teeth in the outer heel rows, which normally indicates excessive rpm.4 Coring, or breakdown in the cone nose areas, and weld washouts have also been observed to increase with corresponding increases in WOB.

The potential problems inherent with high bit weights are further aggravated by the natural brine mud systems used routinely in these wells. This type of mud characteristically has high concentrations of chlorides; thus, general corrosion pitting, and in particular stress corrosion cracking, may significantly reduce the strength and fatigue life of all steel rock bit components. Moreover, because of the nature of the formations and operations, it is common for bits to "run rough" while in service in these areas, which can lead to relatively high impact loading. This tendency further increases the likelihood of catastrophic failure of various rock bit components.

High bit weights also result in high radial and axial thrust loading of the rock bit bearings. A common dulling mode of sealed roller bearing bits is spalled roller races. Essentially, spalling is a fatigue mechanism, the incidence of which increases with both hours and weight. Journal bearings provide the advantage of distributing the bearing loads over a finite area of the journal surfaces, in contrast to the discrete lines of contact inherent with roller bearings.

Thus, journal bearings provide lower unit loads and essentially no problems with fatigue at the bearing surfaces. The failure mechanism for overloaded journal bearings is galling, which is an adhesive wear phenomenon. As long as a clean grease environment is maintained, the present range of rock bit operating parameters in the subject drilling area rarely results in galled journal surfaces.

Heat generation within sealed rock bit journal bearings is proportional to WOB. Excessive heat will adversely affect the performance and life of the primary elastomeric bearing seals because the physical properties of elastomers degrade rapidly with increasing temperature. 5 Thus, the load capacity of sealed rock bit journal bearings may actually be determined by the ability of the primary bearing seals to survive the thermal environment produced by the high load conditions.

Failure of the bearing seals terminates the useful life of a bit, and if the bit is not pulled, seal degradation will eventually cause lost cones.

BIT DEVELOPMENT

Many of the wells drilled in this area penetrate the Permian dolomite and limestone, the Pennsylvanian limestones, and the Mississippian dolomite and limestone formations, all of which have compressive strengths in the 20,000-30,000 psi (140-210 MPa) range. These formations respond well to a chipping and crushing type of drilling action, rather than the sliding and scraping type of action used by softer formation bits. Woods determined that in these relatively hard West Texas formations, drilling rate was directly proportional to weight.6 These observations were later confirmed by Tsai, et al., who reported that drilling rate of TCI bits in hard West Texas formations is affected more by weight than rotary speed and that an increase in penetration rate was realized with even moderate increases in weight.

If hole deviation is not a problem, the appropriate WOB is usually determined by previous experience with similar bits run in the same application. By avoiding premature cutting structure breakdown and bearing failures, an acceptable WOB ostensibly would produce acceptable penetration rates and long bit life. Upon reviewing many bit runs, it was determined that a significant increase in WOB should produce much higher drilling rates, thereby significantly reducing the cost per foot.

With the rock bits available at that time, it was believed this approach would compromise bit reliability and result in unacceptably short runs. It is noteworthy that drillers in the Permian basin were already headed in this direction. A comprehensive study of data from bits run from January 1985 through March 1992 indicates the WOB values for IADC Class 547Y bits have increased an average of 1,300 lb annually since 1985 (Table 1).

It was concluded that reduced drilling costs in these hard rock applications could be realized with the development of new rock bit designs capable of operating reliably under heavy weights. Such heavy weight bits must have extremely durable cutting structures, bearings capable of surviving under high loads for long periods of time, and strong bit bodies. A goal was established to accommodate bit weights up to 8,230 psi of bit diameter (i.e., 65,000 lb on a 7-7/8-in. bit). However, compromising overall bit reliability would negate the advantages associated with increasing the penetration rate by drilling with high weight.

CUTTING STRUCTURE

Critical to the success of a heavy weight bit was the design of the cutting structure. In general, an optimum bit design depends on all the components being properly sized to fit within the given hole size. For instance, enlarging the diameter of the journal bearing to increase the load capacity of the bit may result in the steel cone shell becoming too thin, thereby leading to cone breakage. Conversely, a considerable increase in cone shell thickness may not leave enough room in which to fit a journal bearing large enough to accommodate high bit weights.

Every component in a rock bit design competes with at lease one other component for space. Therefore, the optimum bit design from a structural integrity standpoint strikes the proper balance between all its components. Fig. 1 illustrates the proportions in a conventional rock bit design of IADC Class 547Y.

For the new, heavy weight 547Y bit developed for West Texas applications a moderately high density of insert rows, and corresponding insert counts within the respective rows, was deemed desirable. This arrangement provides both durability and a smooth running bit. Relatively low cone offset, or skew, was used to improve the cutting structure durability and to minimize wear of the tungsten carbide inserts from sliding and scraping (Fig. 2).

Much of the design program was directed to the heel rows, which are responsible for cutting the largest kerf of hole bottom and maintaining hole gauge. Heel row breakdown is a common dulling mode in hard formations. Furthermore, sliding and scraping of the heel rows should be minimized by using a relatively low cone offset to reduce insert wear and improve gauge holding ability. Additionally, the nose area of the cones must be properly designed to cut the center of the hole without coring.

The grades of tungsten carbide incorporated in the inner row and heel row inserts must provide the proper balance between wear resistance and toughness. The insert geometry also comes into play (projection, nose radius, etc.) because a durable insert shape allows the use of a harder and more wear resistant carbide grade. Wear resistance (or hardness) and the toughness of tungsten carbide are, for the most part, inversely proportional. For West Texas applications, ample toughness is required to penetrate hard rock under heavy bit weights; however, sufficient wear resistance is demanded to minimize compact wear while achieving the high footage and long service hours desired.

Insert retention is a priority in heavy weight bits because the occurrence of lost and rotated inserts increases as bit weights increase. Accordingly, uniform and consistent cone steel core hardness is required. Relatively high hardness levels are required to provide the high strength demanded for insert retention. The hardness should not be high enough to compromise the toughness of the cone shell, resulting in cracked or broken cones.

For each row of inserts, emphasis was placed on attaining proper balance between insert hole depth, the number of inserts within each row, the minimum section between adjacent insert holes, and the minimum section between the insert hole bottoms and bearings. Note that while deepening the insert holes improves retention, it also decreases the section between the insert hole bottom and the cone bearing surface. Similarly, increasing the quantity of inserts within a given row will increase the durability of the row but will correspondingly reduce the section between adjacent insert hole bottoms, which may lead to cone shell fatigue.

The West Texas heavy weight bits were designed with relatively deep insert holes to prevent insert loss caused by the cone shell erosion experienced during long bit runs. Moderate sections between adjacent compact hole bottoms were used to improve resistance to fatigue cracking. Breakage resistance of the steel cone shells was increased by the relatively thick cone shells.

BEARING DESIGN

The load capacity for a journal bearing is proportional to the journal diameter and length. Thus, the conventional wisdom would be to increase the journal diameter to provide a high-capacity bearing. This approach, however, poses a problem.

A large journal diameter precludes the optimization of the other important parameters, primarily the cone shell thickness, insert hole depths, seal coverage by the shirttail, etc. On the new design, a proper balance was attained by slightly decreasing the journal diameter. This provided the room to strengthen the cutting structure elements and better protect the bearing seal.

All of the head bearing load surfaces were first carburized, which produces a hard, strong case. A special boronizing treatment over the carburized case resulted in an extremely hard case approximately 0.004-in. thick. Cone bearing surfaces were also carburized, and the cone journal surface was inlaid with copper, which provides solid lubricant and conducts bearing-generated heat away from the bearing interfaces. The boronized vs. copper inlay bearing couple provides a high load, moderate rpm bearing, which displays very low wear rates while in service.

As shown in Fig. 3, ring-lock cone retention was used as opposed to the more conventional ball-lock cone retention method. The ring-lock is a more compact design than the ball-lock, which allows an additional 0.25 in. in journal length, thicker cones, and more space for insert hole bottoms. Thus, the high load capacity required for this heavy-weight drilling application was achieved by a combination of the special bearing metallurgy and a properly sized journal bearing with ring-lock retention.

Leg breakage resistance was increased by thickening the rock bit legs. Leg thickness is partially determined by the location of the bearing. For example, pushing the bearing deeper into the cone increases leg thickness and improves seal protection, but also decreases the cone shell thickness.

Throughout the design and manufacturing processes, stress raisers were eliminated in critical areas of the rock bit legs (e.g., the drilled hole connecting the lubrication system to the bearing was centrally located and properly sized so as to not weaken the leg). The ring-lock cone retention system results in a stronger leg because it eliminates the ball plug hole and ball plug weld in a conventional ball-lock cone retention system.

Final assembly of rock bits is accomplished by precisely welding together three bit thirds, with each bit third comprised of a head section (or leg) and a cone. Rock bits that are run under heavy loads are more likely to experience assembly seam weld washouts during service. Normally, this type of failure is associated with fatigue cracks that develop within the bit body while drilling. The hostile operating environment of the West Texas area increases the importance of maintaining the proper weld groove geometry and the quality of the weld application to avoid the premature termination of a bit run because of a weld washout.

FIELD RESULTS

In 1985, the average WOB for 7-7/8-in. IADC Class 547Y bits in this area was 37,700 lb. With steady increases each year, the average WOB through March 1992 has risen to 48,400 lb. The average rotary speed has remained essentially constant at approximately 69 rpm over this period (Table 1).

The first order of the new heavy weight IADC Class 547Y bit was introduced to the West Texas counties of Martin and Upton in 1988. Average operating parameters for the new bit were 61,000 lb and 72 rpm in 1988. This represents a 39% increase in WOB. The heavy weight 547Y drilled successfully in Martin County with a 15% lower cost per foot, a result of the 18% increase in drilling rate over the next best IADC Class 547Y bit (Table 2). The operator saved an average of $3,400 per bit run.

The early results were even more impressive in Upton County. The average WOB increased only 10%, from 52,000 lb to 57,000 lb; however, the penetration rate increased 14%, from 36.1 to 41.1 ft/hr. Furthermore, it drilled an average of 59% more hole, making 3,236 ft of hole as compared to the 2,030 ft achieved by the next most cost-effective bit. Once again, the 26% lower cost per foot resulted in an average savings of $5,500 on each bit run.

Since its introduction in 1988, the heavy weight 547Y has also been applied successfully in essentially all areas of West Texas and southeastern New Mexico. Most of the runs were concentrated in Eddy and Lea Counties, New Mexico. In these wells the bit drilled at consistently higher WOB, with higher overall penetration rates and footage drilled, and with corresponding lower average cost per foot figures.

It should be noted that not all drillers in the region are willing to run the levels of WOB under which the subject bit was designed to operate. Although the bit has been shown to maintain its integrity and perform well under the heavier WOB, the economic conditions required to run the heavier weights may not be present. Other factors may come into play, such as the overall condition of the drillstring and the total number of drill collars available at the rig site.

In some circumstances it may be necessary to run a softer IADC class TCI bit at lower operating parameters. While similar results may be obtained in this manner, the overall reliability in drilling these long hole sections will decrease because of the reduced durability of the softer cutting structure.

ACKNOWLEDGMENT

The authors wish to thank the management of Hughes Tool Co. for permission to publish this article.

REFERENCES

1. Newman, E.F., "Design and Application of Softer Formation Tungsten Carbide Rock Bits," SPE/IADC paper No. 11386, presented at the SPE/IADC Drilling Conference, New Orleans, Feb. 20-23, 1983.

2. Shepherd, W.L., and Klingensmith, D.L., "Improvements in Rock Bit Performance," presented at the ASME-ETCE conference, New Orleans, Jan. 14-18, 1990.

3. Deane, J.D., Doiron, H.H., and Tompkins, L.B.,"The Economics of Running Journal Bearing Tooth Bits at High Rotary Speed," presented at the SPE/IADC Drilling Technology, Conference, Houston, Mar. 19-21, 1984.

4. Hampton, S.D., Garris, S., and Winters, W.J., "Application of the 1987 Roller Bit Dull Grading System," presented at the SPE/IADC Drilling Conference, New Orleans, Mar. 15-18, 1987.

5. Kelly, J.L. Jr., and Ledgerwood, L.W. III, "Performance Evaluation of a New Rock Bit Bearing Seal," SPE/IADC paper No. 17186, presented at the SPE/IADC Drilling Conference, Dallas, Feb. 28-Mar. 2, 1988,

6. Woods, H.B., "Parameters of Drilling Engineering Systems," presented at AAODC Seventh Rotary Drilling Conference, Houston, Feb. 27, 1969.

7. Tsai, C.R., and Robinson, L.H., "Improve Drilling Efficiency with Two Nozzles and More Weight-On-Bit," presented at the SPE/IADC Drilling Conference, New Orleans, Feb. 20-23, 1983.