METAL SEALED CONE BITS REDUCE COSTS IN ABRASIVE DRILLING
Rodrigo Cortes, G. Hocol
SA Neiva
Colombia
Carlos Valbuena, Victor Cuellar
Hughes Christensen Co.
Maracaibo, Venezuela
Metal sealed rolling cone bits have cut drilling costs by increasing the footage drilled per bit and by increasing the penetration rate in several wells in South America.
The metal seals double the bearing life compared to conventional elastomer sealed bits, thereby allowing the bit to stay on bottom longer.
In Colombia, an operator required that only one bit be used to drill an entire section of hard, abrasive sandstone. Each of the four previous offset wells used numerous bits in this section; 10 elastomer sealed bits were used in one of these wells. Furthermore, the bit had to operate under unusually high speeds to avoid hole washouts. A single metal sealed tungsten carbide insert (TCI) bit drilled the abrasive interval with no problems.
In Venezuela, metal sealed bits were used to lower drilling costs in both relatively moderate and aggressive drilling conditions.
METAL SEAL DESIGN
Lowering drilling costs by extending bit life has pushed the development of rock bit technology during recent years. Advances in bit design, such as 0-ring sealed journal bearings, have extended bit life appreciably. Yet because drilling parameters have become more aggressive and downhole conditions more hostile, elastomer bearing seals are pushed to their limits in high-speed drilling and in drilling deep, hot holes.
Metal sealed bits, developed in the late 1980s, generally have longer bit lives under such harsh conditions.
Metal seal technology and its use in rock bits has been well-documented in the drilling literature.2-5 Fig. 1 shows the small cross-section, compensating metal-to-metal face seal. The seal consists of two hard metal alloy seal rings supported by two elastomer 0-rings (energizers). Dynamic sealing occurs at the metal ring faces which are in lubricated contact. The energizers are static seals and force the opposing metal ring faces into sealing contact.
Self-generated frictional heat is one of the primary causes of premature bearing failures in conventional 0-ring sealed bits. With metal sealed bits, the low coefficient of friction between the polished sealing surfaces reduces heat, enabling the seal to maintain integrity at higher rotational speeds for longer operating periods. Frictional drag between the polished and lubricated faces of the metal seal is 70-80% less than that of a conventional 0-ring seal.
Metal face seal technology has therefore expanded the operating envelope of rolling cone drill bits. Initially, the metal sealed bits were used exclusively in harsh drilling environments where faster penetration rates, and hence lower cost per foot, generally hinged on higher rotational speeds.
Metal sealed bits have also been used recently in moderate drilling conditions.
Rolling cone bits with metal face seals have reduced drilling costs because these bits can drill for longer periods at higher speeds than conventional bits.
BALCON FIELD
The first well in the Balcon field in the Neiva basin of south central Colombia was spudded in July 1988.
The most challenging aspect of Balcon field development drilling was in the 12 1/4 in. interval, which typically runs from about 1,400 ft to 7,400 ft. In the upper range of the 12 1/4 in. section to about 3,966 ft, a typical Balcon well encounters the Chicoral group, a conglomerate that consists of interbedded layers of chert, claystones, sandstone, and mudstones.6 The Guaduala group (to about 6,622 ft) is characterized by claystone and a sandstone-claystone sequence. The operator wanted to drill this group with only one bit.
Down to about 7,404 ft, the 12 1/4 in. section encounters the Monserrate formation, which is predominantly a sandstone with some interbeds of siltstone, chert, and minor shale.
In the four previous offset wells, these highly abrasive formations caused numerous bits to be out-of-gauge when pulled out of the hole. The out-of-gauge drilling can decrease penetration rates and cause undergauge bottom hole assembly components. An out-of-gauge hole frequently requires the next bit to ream to bottom.
To minimize the gauge problem, a metal sealed bit was used in the abrasive Chicoral and Monserrate formations. The metal sealed bit's gauge protection included ovoid heel and gauge row inserts. The ovoid shape maximized the durability of the heel row. The heel row inserts were fabricated from a proprietary carbide grade that has about 20% better abrasion resistance than conventional carbide grades. The bits also were designed with about 58% more carbide on gauge for additional abrasion protection.
Of the formations encountered, the Chicoral group was the most challenging and expensive to drill. Thirteen drillstring washouts occurred during drilling of this section, which averaged slightly more than 1,836 ft long.
Three washouts occurred in well Balcon 1 (1,406-2,636 ft), two in well Balcon 2 (1,950-3,300 ft), five in well Balcon 3 (2,360-3,710 ft), and three in well Balcon 4 (1,710-3,910 ft).
The Balcon 5 well was spudded in August 1991. A single 12 1/4 in. metal sealed rolling cone bit (IADC 517 class) was used to drill the Chicoral group (Fig. 2).
The bit was run on a positive displacement motor in the 12 1/4 in. section through the Chicoral group (1,610-2,825 ft) to reduce the extremely high drillstring vibrations that plagued the previous four wells.
Bottom hole assembly (BHA) twist offs and the subsequent washouts had been prevalent during drilling of this conglomerate formation in the offsets. The use of a motor minimized BHA damage by decreasing the vibrations per bit rotation.
Using the motor, however, meant that the bit would have to rotate between 387 and 400 rpm and also be able to resist the abrasive formation.
The bit ran at about 400 rpm.
The formation was drilled with no drillstring washouts.
In the 12 1/4 in. section of Balcon 5, the number of bits used was significantly less than that for the four offset wells.
Forty elastomer sealed bits were used in the offsets, but only four metal sealed bits (IADC 517, 417, and 537 classes) drilled the entire 12 1/4 in. section of Balcon 5.
The metal sealed bits averaged nearly three times the footage drilled and had twice the operating life compared to the elastomer bits used in the offset wells (Table 1).
Table 2 shows the cost savings achieved with the metal sealed bit on the Balcon 5.
The calculations for Balcon 5 assume an operating cost of $7,500/day and a trip rate of 1,000 ft/hr. Note that the cost per foot dropped $13-70/ft, even though the initial cost of the metal sealed bit was about $5,550 more.
Bearing reliability was the primary reason for the improvement in bit performance. The average number of revolutions for the metal sealed bits was substantially greater than that for the elastomer bits. The seal effective rate was also improved (Fig. 3).
The doubling in bearing life expectancy increased the hours drilled with the bit. The use of metal sealed bits on Balcon 5 helped reduce drilling costs by 34% because of the fewer number of bits required to drill the 12 1/4 in. section.
From surface to total depth, bit costs in Balcon 5 were reduced $125,000. A major contributor to the total well savings of $530,323 compared to the best available offset, Balcon 2, was the 8 day reduction in actual rotating time on Balcon 5.
For the five Balcon wells, the total rotating times were 956 hr for Balcon 1, 644 hr for Balcon 2, 884 hr for Balcon 3, 1,058 hr for Balcon 4, and 460 hr for Balcon 5.
It should be noted that improvements in the drilling fluid program and rig maintenance also contributed to the overall cost savings realized on Baleen 5. In the four offset wells, the 6 in. interval in the Cabellos formation, a section that runs about 9,812-10,433 ft, was drilled in an overbalanced condition that severely hampered penetration rates. In Balcon 5, the mud weight was lowered to 8.9 ppg, which improved penetration rate and significantly reduced formation damage.
LANGUNILLAS FIELD
Metal sealed bits reduced drilling costs in both aggressive and normal drilling conditions in the 8 1/2 in. interval of wells drilled in the Langunillas field in the northern portion of Lake Maracaibo, Venezuela.
Wells drilled in the Langunillas field penetrate Miocene and Eocene formations. The Miocene is characterized by a consistent sequence of clays interbedded with laminar sandstones with poor consolidation. The Miocene section has a series of massive sandstones interbedded with hard and abrasive shales.
Well diameters for this region are typically 17 1/2 in., 12 1/4 in., and 8 1/2 in. The 8 1/2 in. interval has the largest application of metal sealed bits.
The 8 1/2 in. metal sealed bits (IADC 517 class) used in the Langunillas field significantly reduced cost per foot in both high and relatively low speed applications (Table 3).
Based on a rig rate of $300/hr, the metal sealed bit at 125 rpm realized a 15% cost savings compared to the elastomer sealed bits used in the best offset wells. Although the penetration rate was only slightly higher, the cost reduction was a result of the 50% increase in footage and operating hours.
Table 4 shows the reduction in costs for higher rotational speeds (the bit speed was increased 110%). Assuming an operating cost of $1,350/hr, the cost per foot was reduced about $30, although the average cost savings per bit run were $18,512 compared to elastomer bits run at similar speeds in the field.
Both footage and penetration rates were 35-40% greater at the higher speeds.
The metal sealed rolling cone bit is cost effective if rig rates are low or if the targeted formation does not respond to increased bit speeds.
With normal rotary speeds, the metal sealed bit reduces drilling costs by increasing footage and operating hours.
If the rig rates are high or if the drilled formations respond to higher bit speeds, the metal sealed bits can help lower drilling costs by increasing the footage and penetration rates.
The 8 1/2 in. IADC 517 class bit realized a 546% return on investment when run at lower speeds and a 210% return when the speed was increased.
ACKNOWLEDGMENT
The authors thank Hughes Christensen Co. and Hocol SA for permission to publish this article and the petroleum engineering, drilling engineering, and drilling operations departments of Lagoven SA in Tia Juana, Venezuela, for their cooperation and assistance.
REFERENCES
- Shepherd, W.L., and Klingensmith, D.L., "Improvements in Rock Bit Performance," presented at the ASME Energy-Sources Technology Conference and Exhibition, New Orleans, February 1990.
- Burr, B.H., "Earth Boring Bit with Pressure Compensating Rigid Face Seal," U.S. Patent No. 4,516,641, 1985.
- Kelly, J.L. Jr., and Ledgerwood, W.L. III, "Performance Evaluation of a New Rock Bit Bearing Seat," presented at SPE/IADC Drilling Conference, New Orleans, Mar. 15-18, 1988.
- Schmidt, S.R., Wittry, A.E., Burr, B.H., and Kelly, J.L. Jr., "Metal Bearing Seal Technology Improves Drilling Efficiency of Rolling Cutter Bits in High-rpm North Sea Applications," presented at the SPE/IADC Drilling Conference, Amsterdam, Mar. 11-14, 1991.
- Burr, B.H., and Jarding, J.J., "The Metal Seal-State of the Art Rock Bit Technology for Geothermal Drilling Applications," Geothermal Resources Council Transactions, Vol. 14, Part 1, Honolulu, 1990.
- Colombian Society of Petroleum Geologists and Geophysicists, "Geological Field Trips-Colombia 1959-1978," Ediciones Geotec Ltda., 1979, pp. 262-268.
- Scott, D.E., Zahradnik, A.F., and Schmidt, S.R., "Enhanced Gage Improves Rolling Cone Bit Performance in Abrasive North Sea Sands," presented at the Offshore Technology Conference, Houston, May 6-9, 1991.
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