Bobby Grimes, Dan Scott
Hughes Christensen Co.
Houston
Advances in material and design technologies have enabled softer tungsten carbide insert (TCI) rolling cone bits to lower drilling costs through increases in rates of penetration and total footage drilled.
Soft formation TCI bits of the International Association of Drilling Contractors (IADC) 437 class were introduced more than a decade ago. As the bits evolved and additional sizes were introduced, the applications continued to expand. The early IADC 437 bits replaced steel tooth or harder TCI bits such as IADC 517 bits, thereby giving the operator and contractor opportunities to reduce drilling costs.
The latest version of soft formation bits (IADC 417 and 427 types) mirror the success of the earlier 437 bits, with significant reductions in drilling costs realized throughout the U.S.
BACKGROUND
The original TCI rolling cone rock bits were introduced in 1951, primarily for drilling the extremely hard and abrasive limestone and chert formations of West Texas. The bits featured large quantities of small diameter, durable ball nose inserts with very little projection. The carbide grades for these early inserts were adapted from metal forming applications. Open roller bearings were also widely used at this time.
The introduction of sealed roller bearings in the late 1950s provided longer hours on bottom. This, in turn, allowed TCI bit cutting structures to evolve into designs that were capable of economically drilling medium-hard formations. The superior wear resistance and durability of the carbide inserts provided benefits over the steel tooth bits of the day. These bits used blunt symmetrical insert shapes such as conicals, ogives, and ovoids (Fig. 1). The first chisel-shaped insert was introduced in 1967 and featured a sharp nose radius coupled with a large (90) chisel angle.1 To better balance the wear resistance and breakage resistance of the inserts, special carbide grades were soon developed for the new range of applications.
The introduction of the 0-ring sealed journal bearing in 1969 prompted the development of TCI bits for softer formations because of the increased seal life and load capacity of the new bearing package.2 By the end of the 1970s, the IADC 517 class bit was used in widespread drilling applications. The improvements in steel and carbide manufacturing technologies were key elements in furthering the development of softer TCI bits.
These material improvements combined with improved design technologies led to the successful introduction of a 9 7/8-in. IADC 437 bit in 1979. Other sizes and refinements followed, and the IADC 437 bits became a significant factor-often replacing the harder TCI bits throughout the 1980s.3
With this potential for even softer TCI bits, the first IADC 417 bit was developed, which was then followed by an IADC 427 bit (Fig. 2). Depending on such factors as the formations, rig cost, and operating parameters, these ultrasoft TCI bits now commonly replace harder TCI, steel tooth, and polycrystalline diamond compact (PDC) bits.
DESIGN
The softer bit designs use at least one less row of inserts than the standard 517 and 527 class bits. The inserts may be larger diameter and, invariably, feature increased insert projection from the cone shell. Increased insert projection usually requires deeper cone grooves because rolling cone bits use cone intermesh (the teeth of one cone fit into grooves on the adjacent cones). A decrease in the quantity of the larger inserts results in wider spacing from the crest of one insert to the next. The maximum depth of cut for a row of inserts is determined by the insert projection and spacing. Furthermore, the troublesome phenomenon of bit balling in shale formations is influenced by insert projection and spacing. The longer insert projection and wider insert spacing, combined with the decreased insert row count and total insert count, make the new softer bit designs more aggressive.
Irregular spacing is often used within rows of inserts to decrease the incidence of tracking. Tracking occurs when inserts fall in the same impressions made during previous bit revolutions, rather than produce new intermediate impressions in the borehole bottom. The result of tracking is decreased rate of penetration (ROP) and increased wear of the cutting structure.
Longer insert projections generally result in lower overall insert durability. Improved insert geometries have been developed to provide new levels of durability for high projection inner row inserts. Asymmetric chisel-shaped inserts now significantly outperform previous plain chisel-shaped inserts.4 Two such chisel-shaped inserts and a long projection insert are shown in Fig. 3. The wider chisel insert crests also improve ROP over that of typical 517 class bits.
Long projection conical inserts have been used by the industry for more than a decade. The long projection produces a high ROP, and the conical shape provides durability.
Sealed journal bearing technology has also advanced during the last few years. Improved seal gland geometries and elastomer seal materials provide longer service life under more severe operating conditions. Furthermore, higher capacity metallurgical bearing couples have been combined with new designs which maximize the size of the critical bearing components. Advanced bearing lubricants have also been developed to increase load capacity at elevated temperatures. The results of these developments are new bearing packages with greater tolerance to weight on bit (WOB), rotational speed, longer hours, and service temperatures.
METALLURGY
The ultrasoft-formation TCI bits employ state-of-the-art materials technology. The hot isostatic press (HIP) or sinter-HIP process is used to ensure production of carbide inserts of consistently high integrity. The tungsten carbide inner row inserts are made from one of several proprietary grades of cemented carbide which were developed specifically for large projection inserts. The new carbide grades resulted from a research and development program specifically for the rock bit industry. Aggressive insert geometries, coupled with the new wear-resistant carbide grades, allow higher levels of performance in soft formation drilling.
The proprietary steel used in the rolling cones was also developed specifically for this application. The steel is a very clean, low sulfur, inclusion-shape-controlled alloy.
The steel has a high toughness and impact strength and very predictable core hardness. These properties are important for cutting structures with larger diameter, longer projection inserts. The deep cone grooves associated with high projection inserts result in thinner cone shell sections, so high values of toughness are required to resist cone cracking or breaking. High strength is also required to adequately grip the long inserts because the higher bending moments produced by the longer projections may result in rotated or lost inserts during service.
SIMULATOR TESTING
A high-pressure, bottom hole simulator test facility is required to accurately study the relative drilling performance of different bit types under controlled laboratory conditions.6 Drilling tests were performed at 2,000 psi bottom hole pressure in Mancos shale with a mud flow rate of 280 gpm and 2.8 hydraulic horsepower/sq in. Fig. 4 shows the drilling response of new 417, 427, 437, 517, and 527 bits rotating at 120 rpm.
These data clearly show the increased ROP characteristic of the softer bit types in this medium-hard, balling-prone shale formation. The ability of the softer bit types to drill at higher ROPs with lower WOB can result in improved bit life or total footage drilled because less energy is put into the bit to accomplish the same amount of work.
The softer bit must, of course, possess adequate cutting structure durability to survive any harder formations which are encountered during a run.
MARKET
A review of the soft TCI bit market (all bits IADC class 527 and softer) was made from a rock bit data base for the period 1986-1991 in North America. This analysis indicated an increased use of the IADC 437/447 bits, which continue to replace the firmer 517/527 class bits.
For example, in the 12 1/4-in. size, nearly half of the soft formation tungsten carbide insert (TCI) bits are now softer than IADC 517, as compared to less than one quarter of the bits in 1986. This represents a significant shift in applications and drilling economics.
The ultrasoft IADC 417/427 bits were introduced in the late 1980s in North American markets. The growth in applications of these ultrasoft TCI bits follows a pattern similar to that of the earlier IADC 437/447 bits, albeit at lower overall levels.
The 417/427 class bits have 6% of the market; nevertheless, this is a sizable quantity of bits because of the relatively large total market considered.
In 1991, the 16-in. and 17 1/2-in. IADC 415 bits were introduced, largely for international offshore drilling applications. These bits have become well established in the North Sea and Southeast Asia, where their versatility has allowed them to replace PDC, steel tooth, and firmer TCI bits.5 7
CASE STUDIES
A review of several case studies from various applications across North America demonstrates the performance advantages and potential cost savings realized with the soft formation TCI bits.
Table 1 compares the use of 437 bits and 517 bits in the pressurized shale drilling of South Louisiana.7 In the 6 1/2-in. hole section, the 437 bits cut costs nearly $18/ft through increased footage (+ 75%), hours (+ 39%), and ROP (+26%). In the 8 1/2-in. hole section, the 437 bits saved approximately $20/ft; footage (+61%), hours (+20%), and ROP (+34%) all increased.
In Lafayette Parish, La., bit records from nine wells in a close area were studied. The four recent wells drilled with 12 1/4-in. metal face sealed 437 bits showed advantages in footage (+ 87%), hours (+16%), and ROP (+62%) over the elastomer sealed 517 bits used on five prior offsets. Even with the price premium for the metal face sealed bits, the wells were drilled at $19/ft less than the offsets.
Table 2 shows the potential gains in ROP for the most recent 8 1/2-in. IADC 417 bits with metal face seals compared to other conventional TCI bits in Terrebonne Parish, La. The ultrasoft 417 bit saved more than $20/ft compared to the 517 bit, primarily because of the higher penetration rates and increased footage drilled. Note how the field data rank the ROP of the 417, 437, and 517 bits essentially the same as the laboratory data shown in Fig. 4.
In Irion County in West Texas, the ultrasoft conical 427 TCI bit cut costs of more than 30% compared to the firmer TCI bits used on the same rig on offset wells. Nearly a mile of sands, shales, and limestones was penetrated with one bit, drilling out from under surface casing (Table 3). The greater than 50% increase in ROP was achieved with the same moderate operating parameters used on previous wells with 527 and 537 bits.
The 417 bit has cut costs in traditional steel tooth applications as well. Table 4 summarizes the performance of bits in the 12 1/4-in. hole sections on two wells drilled by the same rig in Dimmit County, Tex. The 12 1/4-in. interval drilled with the 417 bit cost 11% less than the offset well drilled with numerous 116 class bits. In Campbell County, Wyo., the 7 7/8-in. conical IADC 427 bit was able to replace several steel tooth bits, thereby saving trips.
The ultrasoft bits have also found applications in Alberta (Table 5). A single 7 7/8-in. 427 bit replaced three to four bits on nearby offset wells. This bit run doubled the overall ROP through this interval and eliminated two to three bits and related trips. The cost per foot was $5.23 compared to the $11.40/ft average for the offsets. The 8 3/4-in. 437 bit also outdrilled two to three 517 bits in Alberta. The overall interval ROP was nearly doubled, and one to two TCI bits and related trips were eliminated. Cost per foot dropped 52%.
These examples from North American drilling operations demonstrate the continued improvements and advancements in tungsten carbide insert bits.
REFERENCES
- McElya, F.H., "Specialty Shaped Inserts for Compact Rock Bits," U.S. Patent 3442342, 1969.
- Galle, E.M., "Seal Means for Drill Bit Bearings," U.S. Patent 3397928, 1968.
- Newman, E.F., "Design and Application of Softer Formation Tungsten Carbide Rock Bits," IADC/SPE Paper 11386, presented at the 1983 IADC/SPE Drilling Conference, Dallas, Feb. 20-23, 1983.
- Bozarth, R.O., "Rock Bit with Specially Shaped Inserts," U.S. Patent 4108260, 1978.
- Scott, D.E., and Zahradnik, A.F., "New Generation of Soft Formation TCI Bits Reduces Drilling Costs in High-Cost Environments," Paper 7000, presented at the 1992 Offshore Technology Conference, Houston, May 4-7.
- Ledgerwood, L.W., and Kelly, J.L. Jr., "High Pressure Facility Re-Creates Downhole Conditions in Testing of Full Size Drill Bits," Paper 91-PET-1, presented at the American Society of Mechanical Engineers Energy-Sources Technology Conference and Exhibition, Houston, Jan. 20-23, 1991.
- Patrick, B., Bond, D., Zahradnik, A., Mackie, C., and Brown, L., "Application and Development of Very Long Insert Rolling Cone Bits to Drill Interbedded Lithologies," Paper 24585, for presentation at the 1992 SPE Annual Technical Conference and Exhibition, Washington D.C., Oct. 4-7.
- Owen, B., "Soft-Structure Drill Bits Conserve Money in Southern Louisiana's Sticky Shales," Gulf Coast Oil World, July/August 1991, pp. 28.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
