DOME-SHAPED POC CUTTERS DRILL HARDER ROCK EFFECTIVELY

D.P. Moran
Smith International Inc.
Houston

Rock mechanics and sonic travel time log data indicate that bits with convex-shaped polycrystalline diamond compact (PDC) cutters can drill harder rock formations than comparable bits with flat PDC cutters.

The Dome-shaped cutters have drilled carbonate formations with sonic travel times as small as 50 msec/ft, compared to the standard cutoff of 75 msec/ft for flat PDC cutters.1

Recent field data from slim hole wells drilled in the Permian basin have shown successful applications of the 3/8-in. Dome cutter in the Grayburg dolomite with its sonic travel times as low as 50-55 msec/ft and compressive strengths significantly greater than the standard operating range for PDC bit applications.

These field data indicate that the Dome cutters can successfully drill hard rock.

The convex cutter shape has good impact resistance, cuttings removal, heat dissipation, and wear resistance. 2-4 Fig. 1 is a schematic of Dome cutters with slightly convex faces.

A thorough analysis of available offset well data helped in the selection of PDC bit profile, cutter size, cutter density, and hydraulic design to improve the number of successful bit applications.

Rock mechanics and log and core data from off set wells were correlated with data from six wells deepened with the 3/8-in. miniDome PDC bit design. Additionally, four wells in New Mexico and two in West Texas were underreamed with 6-in., three-arm underreamers built with 3/8-in. Dome cutters.4

The geological correlation showed that the PDC bits and underreamers effectively drilled rock with compressive strengths ranging from 16 to 82% greater than those for typical PDC bit applications in the area.

WELL CORRELATION

Bit records, gamma ray log data, sonic log data, density and neutron porosity log data, and geological formations from numerous wells in Lea County, New Mexico, were compiled in a data base program for geological mapping and well log correlations.

The depth-based data were manually digitized from paper logs into well files for each offset well at the approximate geographic locations.

Profiles of the Queen and Grayburg formations were based on the digitized well log data and the top-of-formation data from Petroleum Information Corp. This information was correlated to the No. 242 well. Tables 1 and 2 summarize the average log curve response for these formations.

The core samples and log data from the offset wells were then correlated to the intervals drilled with PDC bits to validate the lab tests and the offset sonic log data.

The lithology data were used to make a geologic cross-section hung from the top of the Queen formation, which trends roughly north to south in Lea County, The percentage lithology components were calculated from the available log curve coefficients at each foot of measured depth (Fig. 2).

Fig. 3 shows the sonic log data for the three offset wells. No sonic data were available for the PDC drilled intervals. In Fig. 3, all travel time values less than 75 msec/ft are shaded.

Because 75 msec/ft is the currently accepted cutoff for economical use of 1/2-in. PDC cutters, the sonic data do not indicate that bits with 1/2-in. PDC cutters would be suitable.1 However, Domeshaped PDC cutters were used successfully in these zones.

Based on these geological correlations, the core samples and offset sonic log data are representative of the PDC drilled intervals.

ROCK MECHANICS

Core samples from three wells were analyzed for porosity, permeability, water and oil saturations, grain density, and lithology (Table 3). The cores were analyzed for rock mechanics: compressive and shear strengths, Poisson's ratio, and angle of internal friction.

Triaxial compressive tests were run on all three samples at a range of confining pressures to determine the failure envelope for the dolomite and sandstone samples. In situ confining pressure, or the horizontal component of the matrix stress was calculated with the following equation:

[SEE FORMULA]

In this equation, sH is the confining stress, v is Poisson's ratio, sa is the overburden stress in psi/ft, and Pp is normal pore pressure (0.464 psi/ft).

This equation yielded the following in situ confining stresses: 670 psi for well No. 242, 710 psi for well No. 855, and 760 psi for well No. 887.

Additional triaxial tests were run at 50% and 200% of each value to determine the rock mechanical properties (Table 4). Mohr-Coulomb failure criteria were determined for the three samples (Table 5). (Note: Two possible failure criteria for the No. 855 sample are likely because the core had existing fractures.) From this, the Grayburg formation can be characterized according to the following:

Compressive stress at in situ pressure, 19,230-40,070 psi
Poisson's ratio, 0. 13-0.21
Angle of internal friction, 36-53
Porosity, 3.8-12.9%
Density, 2.72-2.87 g/cc.

Table 6 lists the currently accepted operating ranges for 1/2-in. flat and Dome PDC cutters compared with the Grayburg test results.' The Grayburg dolomites and sands exceed the standard application range for 1/2-in. PDC cutters for compressive and estimated shear strengths by approximately 82% and 16%, respectively.

BIT DESIGN

The Grayburg formation was drilled with 3 1/4-4 3/4 in. M19 PDC bits (IADC M649 class). The bits had tungsten carbide matrix bodies, 3/8-in. Dome cutters, and three fixed total flow area fluid ports.

The bits were run at about 100 rpm with penetration rates of 20-60 ft/hr. For these bit runs, the wear was limited to minor chipping of the PDC diamond layer with no recognizable wear flats developed. Although most of the cutters on the face of the bits showed some minor chipping, the bits remained in excellent condition.4

Because no appreciable wear flats developed on the cutters, the effective backrake, or inclination from vertical, at the edge of the Dome cutters remained constant throughout the run.

A conventional flat PDC cutter has a backrake established by the cutter mounting in the body. In contrast, the Dome PDC cutter has a variable backrake equal to the fixed angle of the cutter mount at its center, increasing towards the cutter edge.

Generally, increased cutter backrake provides greater durability in hard rock and in interbedded formations with hard streaks.2-3

In theory, the variable backrake of the Dome cutter allows the bit to adjust its depth of cut to the formation hardness. For the bit types used in the Grayburg, each individual face cutter engaged the formation with an effective backrake angle varying from 30.8 (edge of new cutter) to 26.0 (effective angle with a depth of cut of 0.0834 in., corresponding to a penetration rate of 60 ft/hr).

The Dome cutters dissipate heat, resist impact loads, and reduce torque and vibrations better than conventional 1/2-in. flat PDC cutters. These attributes were demonstrated in the underreamer runs in the Grayburg formation. The Dome cutters had no problems, whereas conventional 1/2-in. flat PDC cutters failed in an underreaming operation in the Grayburg dolomite.

The success of the 3/8-in. Dome PDC cutter in the Grayburg included improvements in penetration rates of 100-114% more than rock bit runs in adjacent wells. Table 7 compares the performance of various types of bits and underreamers used in the Grayburg formation.

ACKNOWLEDGMENT

The author thanks Joe Carter for the data, Chevron U.S.A. Inc. for permission to use the data, Smith International Inc. for permission to publish the article, Terra-Sciences Inc. for the database program, and Terra Tek Inc. for the rock mechanics testing.

REFERENCES

Bond, D.F., "The Optimization of PDC Bit Selection Using Sonic Velocity Profiles Present In The Timor Sea," paper OSEA 90077, presented at the Eighth Offshore South East Asia Conference, Singapore, Dec. 4-7, 1990.
Knowlton, R.H., and Kesler, R., "Curved Cutters Extend Range of Formations Drilled With PDC Bits," paper SPE/IADC 18634, presented at the 1989 SPE/IADC Drilling Conference, New Orleans, Feb. 28-Mar. 3, 1989.
Knowlton, R.H., and Russell, M., "Diamond Shape Technology Improves Cutting Structures," paper MR91-156, presented at Superabrasives '91 Conference, Chicago, June 11-13, 1991.
Carter, J.A., and Akins, M.E., "Dome PDC Technology Enhances Slim-Hole Drilling and Underreaming Opportunities in the Permian Basin," paper SPE 24606, presented at the SPE Annual Technical Conference and Exhibition, Washington, D.C., Oct. 5-7, 1992.
Mason, K.L., "Three-Cone Bit Selection With Sonic Logs," SPE Drilling Engineering, June 1987.
Knowlton, R.H., personal communication.