TECHNOLOGY Flow pattern changes improve roller cone bit performance

Alan D. Huffstutler
Security DBS Dallas

Improving the flow pattern through and around roller cone bits has increased penetration rate and footage while dropping the cost per foot drilled. These changes to the flow area around the bit help clean the bit and borehole more efficiently.

A typical soft formation sealed-journal-bearing rock bit generates a large amount of formation cuttings. Inadequate removal of cuttings from areas adjacent to the rolling cones restricts the rate of penetration, the life of the seal, and thus the overall bit performance. Improved flow patterns increase penetration rates by limiting the amount of cuttings redrilled and improve bit life by directing abrasive particles clear of the sealing areas.

Two major factors in the overall efficiency of roller cone rock bits are the rate at which formation chips are removed from the borehole and the length of time the bearing seal remains effective.

Historically, rock bit hydraulics have been mainly directed to improving the rate of penetration.¹ These methods typically maximize bit hydraulic horsepower, jet velocity, or jet impact force against system losses to maximize the energy at the bit and minimize drilling costs.² Some test results show that using one or two nozzles rather than three creates conditions that should be more favorable for hole cleaning. Other tests show cleaning should be better with nozzles extended toward the hole bottom.³

Common practice is to maximize hydraulic horsepower by significantly reducing nozzle size and maintaining pressure drop across the bit. Turbulent pressure fluctuations can provide lifting forces sufficient to overcome chip holddown forces and remove rock debris from the hole bottom.4 Many designs have been directed at better bottom hole cleaning and chip removal. Extended jets and asymmetric nozzles have been two approaches for improving bit hydraulics.3 5-8 9 Work has also been done on bottom hole cleaning alone.10 All of these efforts have their merits and are readily acknowledged.

Typically, the nozzles of roller cone rock bits are directed toward the edge of the hole bottom. When three nozzles are run, the return flow exiting from above the cones tends to cross the jet flow, diverting the jet axis up the side of the borehole.11 Extended nozzles provide more hydraulic energy at the hole bottom.

Fully extended nozzle designs, however, have restricted inner flow paths that could promote erosion and failure. The fully extended nozzles can also allow rock formation material to mass around the bearing region, raising their temperature and shortening bit life.¹²

By eliminating the protruding nozzle bosses, increasing nozzle bore size, narrowing the width of the bit arm segments, and providing a convex spherical dome, flow trajectories have been improved. These altered flow trajectories have eliminated "hydraulic dead spots" commonly found around current roller cone rock bit configurations. Nozzles are directed significantly more inward, toward the well bore bottom and away from the edge of the borehole. The flow impacts the bottom of the hole where it is needed most.

Further performance enhancement features include an angled ramp on the shirttail portion of the arm to aid in lifting the cuttings upward, away from the cones and the bearing seals. Changing contours of the bit arms in the nozzle and bearing areas also improves cleaning and prevents cuttings from packing off in the bearing seal area.

Unitary bit body

The use of a unitary bit body enhances fluid return flow around the exterior of the body. The outside diameter of the body is maintained as small as possible while still having a standard American Petroleum Institute bit connection to the drill stem (Fig. 1 [46912 bytes]).

Positioning the nozzles and their associated fluid passageways within the bit body eliminates the need for nozzle housings protruding from the exterior, substantially increasing the available area for return flow between the exterior of the bit and the borehole wall. A convex spherical dome eliminates stagnation of cuttings in the dome area immediately above the cones and promotes movement of cuttings and other debris radially out from the area above the cones toward the annulus.

The cylindrical inner cavity to receive the drilling fluids reduces the fluid swirling action that occurs with conventional designs (Fig. 2 [18632 bytes]). The relatively long and large diameter nozzle bores, which intersect this cavity, provide a more laminar flow, substantially reducing the possibility of erosion of the bores or nozzles.

By positioning the nozzles and their associated bores within the unitary body, the nozzles may be directed inward between adjacent cone assemblies. The nozzles are farther from the borehole and closer to parallel with the projected axis of bit rotation. This inward positioning directs the nozzle trajectory and impact more directly at the bottom of the borehole (Fig. 3 [61500 bytes ]).

Furthermore, it prevents flow trajectories of fluid from above the cones and out from the dome area from redirecting the exiting flow up the side wall of the borehole.

Placing the nozzles and bores within the unitary body eliminates the possibility of weld washouts. The pockets for the attachment of the arms are situated such that welding them to the body does not interfere with any of the nozzles (Fig. 2 [18632 bytes]).

Fluid flow enhancing arms

The attached arms which contain the bearings and support the rolling cutting elements are narrowed to increase the return area between the exterior of the bit and the wall of the borehole. By maintaining an equal radial distance from the center of the bearing 360^o around the bearing on the arm, the width of the arm becomes equal to twice this radial distance. Because the width of the arm does not become greater at any point, valuable return area is increased.

Maintaining this equal distance around the entire circumference of the arm bearing also relieves the throat area on the arm. Thus, the portion of the cone backface which tends to have sticky abrasive formation particles adhere to it, that extends beyond this distance, does not abrade the portion of the arm bearing in the throat area creating shale burn. This equal distance also eliminates shirttail ears, which can cause wedging of cuttings and other debris into the cone backface and the sealing area on the leading side of the arm.

The enhanced contours from the slim arm design and the throat relief allow the fluid trajectories to flow more smoothly around the sealing areas and between the body and the borehole. This smoother flow improves hole and bit cleaning.

The angled ramp on the shirttail also aids in the evacuation of cuttings (Fig. 4 [44450 bytes]). This ramp gives added lift to cuttings exiting from below the cones near the borehole wall and helps direct the fluid return flow up in this turbulent area. To increase the return area and help create smooth flow trajectories past the bit, the arms are contoured inward.

Enhanced return area

The combination of arm width, body diameter, and the elimination of exterior nozzle bosses increased the return area between the exterior of the bit and the borehole wall by slightly more than 20% on the experimental 77/8-in. test bit. The increased return area is approximately 17% for both the 11-in. and the 121/4-in. experimental bits.

Obviously, the bit size, body outer diameter, and arm width all contribute to the amount of increased area. A typical cross-sectional area comparison is shown in Fig. 5 [59460 bytes].

Visual flow tests

Flow visualization experiments in a Plexiglas cylinder were conducted on 77/8-in. and 81/2-in. Type 447 and 437 (International Association of Drilling Contractors bit classification) roller cone bits with the enhancement features. Bottom hole patterns were machined in the Plexiglas to simulate bottom hole flow conditions. Water was circulated at 100 and 150 gpm through the 77/8-in. bit and 150 and 250 gpm through the 81/2-in. bit.

Small plastic particles were introduced into the fluid flow for the visualization tests. As these particles were carried in the flow around the cones, arms, and bodies, high-speed photography recorded their path.

The 77/8-in. enhanced return area bit was compared to a conventional 77/8-in. IADC 447 class roller cone bit using identical cutter cones and flow rates. The visual tests revealed obvious differences in flow trajectories of the fluid around the throat areas of the arms, the central dome area, and above the nozzle exits adjacent to the body and cylinder (borehole) diameter (Fig. 6 [73358 bytes]).

Bead movement around the conventional bit occasionally stagnated in the dome area, and swirling occurred in and around the protruding nozzle bosses. Some of the beads fell toward the hole bottom until they were swept back toward the bottom by the jet stream of the nozzle. Other beads that flowed up past the nozzle bosses swirled in the area immediately above the nozzle bore.

Conversely, the beads flowed smoothly along the convex dome of the experimental enhanced return area bit, flowing out toward the body outer diameter. Once at the area between the exterior of the bit and the wall of the cylinder, the beads would normally flow up in the channel created by two arms and body outer diameter.

It was generally agreed that even though the velocities in the enhanced return area bit were obviously somewhat slower in the areas adjacent to and above the nozzle exits, the flow was more laminar without the eddies that created the swirling and redirected motion of the beads around the conventional bit.

Field tests

The field tests were conducted where direct offset data were available for reasonable comparison of similar IADC class bits at similar drilling parameters and depths. The data are presented in terms of average percent improvement of the test bits compared to average offset data.

The data were compiled from ten experimental 77/8-in. IADC 447 test runs. The test bits averaged 9.6% more footage, a 16.5% penetration rate increase, and a 14.2% reduction in cost per foot as compared to the average offsets. One bit drilled 5,150 ft in 159.5 hr; the dull bit grade revealed all seals effective.

Five 11-in. IADC 537 test bits were run. Averages for these test bits showed an increase of 11% in the penetration rate, 3.8% more footage, and 9.8% lower cost per foot, compared to the averaged offsets. Most of the 11-in. test bits were run on multiple runs, some as many as six times. Three bits accumulated more than 200 total hr each.

Results for three experimental 121/4-in. IADC 517 bits showed a 19.7% increase in penetration rate and a 7.6% decrease in cost per foot compared to the offsets for the footage drilled. The test bits also showed a 54.9% increase in footage, resulting in a 41.6% decrease in the cost per foot compared to the overall offset averages for the North Sea field where these two bits were tested.

The 121/4-in. bits tested in the North Sea experienced flow rates in the range of 850-950 gpm; the dull bit conditions revealed no signs of fluid erosion in the fluid cavity or nozzle bores of the body.

Results

The enhanced return area features of the test bits improved overall bit performance, including increases in footage drilled and rate of penetration. The cost per foot decreased for these bits.

Initial laboratory visual flow tests and field test results indicate that the enhanced return areas around the cones and bearings reduced formation packing and abrasion adjacent to the bearing sealing surfaces.

The enhanced areas between the bit body and the borehole improved flow trajectories, which improved borehole and bit cleaning. Moving the nozzles inward, away from the corner of the borehole bottom to the position between the cones, appeared effective in cleaning the borehole and the cones without detrimentally affecting the cones.

The cylindrical fluid cavity and location of the nozzles within the unitary body virtually eliminated any fluid erosion, even at flow rates as high as 950 gpm. Field tests showed the bits met the demands of the drilling environments. Some of the test bits experienced 70,000 lb weight on bit, and others had runs totaling more than 200 hr, proving the strength and integrity of the overall bit design.

Cleaning the hole bottom with various nozzle configurations concentrating on impact force, jet velocity, and hydraulic horsepower is an effective method of increasing the rate of penetration. Removing the nozzle flow off the side of borehole, cleaning the cutting elements of the cones, and providing improved flow trajectories around and above the cones and bit body also improves bit performance. These changes help prevent bit balling and keep the sealing areas free of abrasive debris.

Acknowledgment

The author thanks many individuals for contributions and assistance in the preparation of this article, with special thanks given to Ken Bramlett and Mike Huskey of Security DBS.

References

1. Slaughter Jr., R.H., "Development, Laboratory, and Field Test Results of a New Hydraulic Design for Roller Cone Rock Bits," Society of Petroleum Engineers paper 14220, presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, Sept. 22-25, 1985.

2. Wells, M.R., and Pessier, R.C., "The Effects of Asymmetric Nozzle Sizing on the Performance of Roller Cone Bits," paper 25738, presented at the Society of Petroleum Engineers/International Association of Drilling Engineers Annual Drilling Conference, Amsterdam, Feb. 23-25, 1993.

3. Sutko, A.A., and Myers, G.M., "The Effect of Nozzle Size, Number and Extension on the Pressure Distribution Under a Tricone Bit," SPE paper 3109, presented at the SPE fall meeting, Houston, Oct. 4-7, 1990.

4. Wells, M.R., and Pessier, R.C., "Asymmetric Nozzle Sizing Increases ROP," Drilling Contractor, September 1993, pp. 50-51.

5. van Lingen, N.H., "Bottom Scavenging-A Major Factor Governing Penetration Rates at Depth," Journal of Petroleum Technology, February 1962, pp. 187-96.

6. Feenstra, R., and van Leeuwan, J.J.M., "Full-Scale Experiments on Jets in Impermeable Rock Drilling," Journal of Petroleum Technology, March 1964, pp. 326-29.

7. Pratt, C.A., "Increased Penetration Rates Achieved with New Extended Nozzle Bits," Journal of Petroleum Technology, August 1978, pp. 1191-98.

8. Baker, W., "Extended Nozzle Two-Cone Bits Require Precise nozzle Sizing for Optimum Performance," SPE paper 8379, presented at the SPE Annual Conference and Exhibition, Las Vegas, Sept. 23-26, 1979.

9. Warren, T.M., and Winters, W.J., "The Effect of Nozzle Diameter on Jet Impact for a Tricone Bit," Journal of Petroleum Technology, February 1984, pp. 9-18.

10. King, I., Wells, M.R., Pessier, R.C., and Besson, A., "A Methodology Using Laboratory Experiments and Numerical Modeling to Optimize Roller Cone Bit Hydraulics," SPE paper 28315, presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Sept. 25-28, 1994.

11. Wells, M.R., "Dynamics of Rock-Chip Removal by Turbulent Jetting," SPE Drilling Engineering, June 1989, pp. 144-52.

12. Manual from the Security DBS Rockbit/Drilling Tool School.

Based on a presentation at Energy Week Conference & Exhibition in Houston, Jan. 29-Feb. 2.

The Author