BITS WITH DIAMOND-COATED INSERTS REDUCE GAUGE PROBLEMS

Darren Eckstrom
Smith International Canada Ltd.
Calgary

In highly abrasive formations, failure of the gauge row cutters on tungsten carbide insert bits may occur rapidly, resulting in short bit runs, poor performance, and undergauge hole.

In certain applications, polycrystalline diamond (PCD) enhanced insert bits have longer bit runs and maintain an in-gauge hole which reduces reaming time and wear on downhole equipment. These bits with PCD-coated inserts have reduced drilling costs in several areas of Canada.

PCD has been applied to rock drilling tools for several years because of its high wear resistance. Polycrystalline diamond compact (PDC) bits use polycrystalline diamonds formed in flat wafers applied to the flat surfaces on carbide inserts. The flat PDC cutters drill by shearing the formation.

Smith International Canada Ltd. developed a patented process to apply PCD to curved surfaces, which now allows PCD-enhanced inserts to be used for percussion and rotary cone applications. These diamond-enhanced inserts combine the wear resistance properties of diamond with the durability of tungsten carbide.

Three-cone rolling cutter rock bits have used sintered tungsten carbide as a cutting agent for hard formations for over 30 years.1 Advances in insert geometry and bit design have resulted in increased penetration rates and longer bit life,

Because of the cutting action of three-cone rock bits, the tungsten carbide has to provide both load-bearing strength and wear resistance. Typically, the grade of tungsten carbide with the highest wear resistance also has the lowest impact strength. Thus, the cement tungsten carbide used in three-cone rock bits is a compromise compound with properties that are both wear and impact resistant.

Tungsten carbide insert bits constitute a large portion of rock bits used in Canada, but these have a limited life in many areas because of insert wear and breakage. As a result, there exists a need to go beyond the limits of technology associated with cemented tungsten carbide.

Diamond is the hardest substance known and thus has the highest abrasive wear resistance. PCD has many advantages over tungsten carbide such as a wear ratio of 1,000 to 1 and thermal conductivity five times greater than that of tungsten carbide.

PCD-ENHANCED INSERT

PCD was first produced from carbon by H.T. Hall in 1954. Though applied to various cutting tools, its most widespread use in the oil industry is for polycrystalline diamond compact (PDC) or shear bits. Until recently, PCD wafers could only be bonded to flat tungsten carbide surfaces.

The PCD-enhanced insert was developed as a result of a patented process of bonding PCD onto a curved surface using the Hall Cubic Press. The press uses six anvils to simultaneously apply pressure from all directions while the insert is heated.

The PCDs are formed at a pressure exceeding 600,000 psi and a temperature of 1,400-C. (2,550-F.). For this process, graphite carbon is packed into a pyrophyllite cube. (Pyrophyllite is a natural aluminum silicate mineral.)

As the anvils press against the cube with great force, the cube's volume is reduced by 25%. While the anvils exert pressure on the cube, some of the pyrophyllite is squeezed out to form a nonextruding gasket that allows pressure to rise to 1 million psi.2

The resulting PCD material is a conglomerate of small diamond particles bonded to one another in this sintering process.

This creates a tough body that retains both its shape and its strength. To make an enhanced insert, the PCD is bonded directly to a tungsten carbide substrate.

This kind of insert has shown extremely good wear resistance in various tests. Through continued research, the process was developed to integrate the PCD into the tungsten carbide substrate. Because of the differences in the mechanical properties of PCD and tungsten carbide, transition layers are formed between them to strengthen the bonding.

Each diamond-enhanced insert consists of three layers of PCD, tungsten carbide, and cobalt (Fig. 2). The outer layer contains 90-95% PCD and 5-10% cobalt. The cobalt acts as a catalyst and facilitates diamond-to-diamond bonding.

The cobalt also adds strength by filling the pore space between the diamond crystals. The first composite layer is composed of a 6075% mixture of cobalt and diamond with the remaining 25-30% a mixture of tungsten carbide cemented together with cobalt.

The second composite layer contains the same materials, but the percentage of diamond decreases with a corresponding increase in the percentage of tungsten carbide.

The transition layers reduce residual stress generated during the bonding process and increase the impact strength of the insert.

GAUGE EFFECT

One of the general modes of failure of a three-cone rock bit is cutting structure degradation propagated from the gauge row inward.

As the gauge row wears, the changing load of the bit shifts to inthrusting on the inner bearing (Fig. 1). This shift causes the cone to cock on the journal, and seal bearing failure results as the cocking increases in severity.

This gauge row failure results in poor bit performance and undergauge hole. These problems often require reaming to bottom on the next bit run. Reaming with tungsten carbide bits reduces the effective life of the bit by accentuating the pinching and inthrust loading on the gauge cutters and on the bearings. All of these factors increase drilling costs.

APPLICATIONS

Bits with PCD-enhanced inserts on gauge rows have been tested and run successfully in Canada. These bits were run in areas and at depths where typical offset wells have shown a trend for the bits to be pulled undergauge with low rotating hours. Geographically, these areas are located west of the 5th and 6th meridian in Alberta and northeast British Columbia.

In contrast to tungsten carbide insert bit performance, less than 20% of all diamondenhanced insert bits run were graded out of gauge. Those PCD insert bits graded undergauge were less than 1/16 in. undergauge.

Drilling records of offset wells in the Rocky Mountain House area show that drilling the intervals of the Viking, Mannville, and Glauconite formations, at a depth of 9,500-11,200 ft, requires two to four International Association of Drilling Contractors (IADC) code 5-3-7 or 6-1-7 bits. The average dull bit conditions were 6-8 for inserts on all rows, 4-6 for the bearings, and 1/4 in. out of gauge.

A 121/4-in. diamond-enhanced insert test bit, IADC code 5-4-7, drilled the entire section in 125 hr. The bit was pulled because of bottom hole assembly (BHA) problems, not bit failure. It was graded 5 for the inner teeth, 1 FC (flat crested wear) for the outer teeth, and in gauge (Fig. 3).

Prior to 1990, all diamondenhanced inserts were pressed into rock bits with conical inserts. In late 1990, diamond-enhanced inserts were pressed into the gauge rows of 83/4-in. IADC code 51-7X bits (X denotes chisel shaped inserts). Three of these bits were tested in the Peace River Arch in northwest Alberta. Efforts were made to obtain representative full life field tests under typical drilling conditions in the area.

A typical well profile for the area consists of drilling a 131/4-in. hole to 1,250 ft, setting 91/8-in. casing, and then drilling out with an IADC code 5-1-7X 83/4-in. bit. The offset wells generally took three bits to reach 6,000 ft: two IADC code 5-1-7X bits and one IADC code 5-2-7X bit.

The first test bit drilled the interval to 4,740 ft in 113 hr with a cost savings of over $1.00/ft. The second test bit cannot be included in the analysis because of operational changes.

The economic analysis of the third bit is not yet complete because it is still scheduled for a third rerun. Its first run drilled 2,130 ft in 50 hr, and the second run drilled 2,750 ft in 41 hr. This is much better than the bits used on the offset wells.

HORIZONTAL WELLS

In 1989, an operator planned to drill a horizontal well in northwestern Alberta, but the company had concerns about short bit life and undergauge hole similar to the problems it encountered in the horizontal section on a previous well in the area.

The geographic formation is the Cadomin characterized by conglomerate, usually hard. The matrix is generally fine-to-coarse-grained sand, and the cement is silica. Chert and quartzite are the predominant clast lithologies, but sandstone is present in some areas. 4

The extreme abrasiveness of this formation lithology makes reaming an undergauge hole extremely difficult.

By using PCD-enhanced gauge inserts on 6-in. IADC code 6-2-7 and 6-3-7 rock bits, the operator drilled two horizontal wells in the area without experiencing any undergauge problems on the horizontal extensions. The PCD-enhanced gauge row inserts showed no measurable abrasive wear, but the tungsten carbide inner row inserts were worn flat to a 7-8 condition (Fig. 4).

Given the wear condition of the inner rows, a rock bit with standard carbide on the gauge would have rounded off and gone undergauge in the horizontal section. The PCD-enhanced gauge inserts allowed the bit to drill its full life with full gauge.

Based on these case histories and other test runs, the PCD-enhanced gauge inserts have proven to be technically sound. Maintaining the gauge cutting structure plays a critical role in rate of penetration and bit life. A sharp gauge insert reduces inthrust loading on the bearing, preventing premature bearing seal problems.

These examples represent a small percentage of the total number of PCD-enhanced insert bits run in Canada. Much of the test data cannot yet be published because of customer confidentiality.

ACKNOWLEDGMENT

The author would like to thank Mickey Sutherland, manager for Anderson Exploration; Steve Hall, drilling engineer for Canadian Hunter Exploration; and the Mega Diamond Division of Smith International for support on this article.

REFERENCES

Salesky, W.J., and Payne, B.R., "Preliminary field tests of diamond-enhanced inserts for threecone rock bits,"paper 16115, presented at the SPE/IADC conference, New Orleans, mar. 15-18. 1987.
Nassau. K., and Nassau, J.. "The history and present status of synthetic diamond," Lapidary Journal Inc., Vol. 32, No. 1, April 1978.
Drilling records, Division Canamera Equities.
Glass' D.J., Lexicon of Canada Statigraphy, Canadian Society of Petroleum Geologists, Vol. 4, 1990.