ANTIWHIRL PDC BIT DESIGNS REDUCE VIBRATIONS

Polycrystalline diamond compact (PDC) bits designed to eliminate whirling can improve penetration rates and create less vibration in downhole assemblies.

They also can improve core recovery.

The use of antiwhirl bits in directional drilling is promising and has prompted development of a number of field guidelines.

These bits control whirl by the cutter locations which generate a resultant imbalance force that is directed toward a low friction gauge pad. The bit is then stabilized against the borehole wall.

Numerous laboratory and field tests have proven the tendency for conventional PDC bits to whirl under certain drilling conditions. The vibrations generated by the bit during whirling can contribute to cutter failure and damage tubulars and downhole electronics inside measurement-while-drilling tools.

When a bit whirls, it walks around the well bore backwards, often increasing hole size and reducing penetration rates because of inefficient drilling. Whirling is also thought to shorten bit life.

The design rules and testing of antiwhirl PDC drilling and coring bits were covered in papers presented at the 1992 SPE Annual Technical Conference and Exhibition, Washington, D.C., Oct. 4-7.

DESIGN AND TESTING

Specially designed antiwhirl polycrystalline diamond compact (PDC) bits have a reduced propensity to vibrate and whirl backwards, thereby improving penetration rates and bit performance, according to C.H. Cooley and P.E. Pastusek of Hughes Christensen and L.A. Sinor of Amoco Production Co. in the paper "The design and testing of anti-whirl bits."

The positioning of the individual cutting elements to create a net force imbalance eliminates the detrimental backwards whirl of PDC drag bits.

The imbalance force pushes the bit against the side of the borehole, which in turn creates a stable rotating condition that resists backwards whirling.

For a bit to drill without whirl, the cutter placement must produce a cutting force vector of proper magnitude and direction. Each antiwhirl bit design has a sector on the bit where cutting elements may not be placed (the cutter devoid area) without decreasing bit stability.

Experience has shown that antiwhirl bits will whirl at low depths of cut and light bit weights.

These bits are more accurately called whirl-resistant bits because they operate smoothly over a wider range of conditions than conventional bits.

Downhole pressure may alter the penetration rate at which both antiwhirl and standard bits make the transition to smooth drilling because more weight is required downhole due to the pressure-related increases in rock strength. For example, in laboratory testing, anti-whirl bits made the transition to smooth drilling at about 12-24 ft/hr, whereas conventional PDC bits made the transition at about 96 ft/hr while drilling under the same conditions.

There appears to be no trend relating bit size to stability: one 17/2 in. antiwhirl bit has been among the smoothest tested.

At Amoco's Catoosa, Okla., test well where conditions can be easily controlled, antiwhirl and conventional PDC bits were run in hard limestone formations. A standard PDC bit stopped drilling at 400 ft because of broken cutters. In contrast, an antiwhirl bit drilled with no problems to 1,500 ft through hard limestones with compressive strengths in excess of 60,000 psi.

Field test results of the antiwhirl bits have been harder to analyze because of the variability in formations drilled, variability of drilling parameters (weight on bit, rotary speed, etc.), and the diversity in drilling rigs and equipment.

The majority of the 100 or so antiwhirl PDC bit runs to date in the field have been successful, with most of the operators rating the bit performance good.

CORE BITS

Coring operations can become more economical with antiwhirl core bits which give improved coring rates, better recovery, good quality cores, and longer bit life.

Core bits are susceptible to downhole vibrations similar to those that affect full bore bits.

The elimination of bit whirl reduces typical core jamming problems.

Additionally, the cores are generally smoother and closer to gauge diameter than those drilled with standard PDC core bits.

Coring results with 43/4-91/s in. antiwhirl PDC bits have been successful. The antiwhirl technology has also allowed recovery of wire line retrievable cores with conventional rigs and drillstrings instead of mining rigs and slimhole technology, according to L.A. Sinor, T.M. Warren, S.M. Behr, and M.R. Wells of Amoco Production Co. and consultant J.R. Powers in "Development of an anti-whirl core bit."

Continuous coring in larger holes has historically suffered from low retrieval rates basically because it is difficult for a bit to cut a smooth core small enough to be retrieved through a conventional size drillstring. Catching the core or jamming the core barrel is also a problem, along with poor bit performance. The rate of penetration is usually very slow, such that the core becomes too expensive to retrieve. Another problem is that PDC bit life may be too short, requiring frequent trips.

The antiwhirl technology eliminates bit vibration to allow a better quality core. Also, technology borrowed from the mining industry improves bit performance by increasing conventional rotary speeds seven fold, to speeds of 700-900 rpm. The higher speeds improve the rate of penetration in firm formations.

The combination of these technologies has allowed recovery of cores in a number of operations where conventional coring techniques previously failed.

NONAXISYMMETRIC LOADS

A series of laboratory and field tests suggests that antiwhirl bits are stable over a wide range of side load conditions, according to P.E. Pastusek, C.H. Cooley, and M. Anderson of Hughes Christensen and L.A. Sinor of Amoco Production Co., in "Directional and stability characteristics of anti-whirl bits with non-axisymmetric loading. "

Compared to conventional polycrystalline diamond compact (PDC) designs, antiwhirl bits tend to drill more in the direction where they are pointed and are less influenced by where they are pushed.

Three nonaxisymmetric load cases may affect bit stability:

An external side load is applied to the bit when directional assemblies are used or when high angle wells are drilled.
In anisotropic rock, the cutting forces change as the bit rotates, thereby changing the imbalance force vector of the bit with time.
With steerable assemblies, the bit axis is tilted relative to the direction of the bit motion, modifying the resultant imbalance force.

Antiwhirl bit designs evaluated for stability were found to be less sensitive to anisotropic formations and external side loads than conventional bits.

The axis tilt encountered in drilling with steerable systems in both rotary and sliding modes does not affect stability of the antiwhirl designs tested.

The build rate of antiwhirl bits on steerable systems is lower than that for conventional PDC bits.

The lower rate is attributed to the smooth gauge pad design on the antiwhirl bits.

Hole spiraling with steerable systems appears to be related to bit whirl, with the wave length controlled by the distances between the bit, stabilizer, and motor bend.

The drilling of two test wells on steerable and angle build motor assemblies effectively compared the response of conventional and antiwhirl PDC bits.

These tests helped develop a few practical field guidelines:

High torque/low speed motors should be used. Because harder formations can be drilled with antiwhirl bits, a high torque motor permits use of the higher bit weights required for bit stability.
To begin drilling, the bit should touch bottom with half the normal drilling flow rate, and then the weight and flow rate should be brought up simultaneously. This procedure should reduce the possibility of off-bottom whirl continuing once drilling resumes.
In steering mode, the drill pipe speed should remain slow (20-40 rpm). Higher speeds lead to instability.
The well plan and bottom hole assembly should be adjusted for the differences in build rate; this depends on the previous bit used and whether it drilled an over-gauge (whirled) hole.