John H. Cohen, William C. Maurer
Maurer Engineering Inc.
Houston
Paul A. Westcott
Gas Research Institute
Chicago
Optimizing the number, size, and orientation of cutters on thermally stable diamond (TSD) bits increases penetration rate and extends bit life.
The use of optimized TSD (also commonly referred to as thermally stable product or TSP) bits on high-power drilling motors can greatly reduce drilling time for harsh-environment wells, such as deep gas wells. The power delivered to the rock governs drilling rate, and at high speed the optimized TSD bits are capable of effectively delivering power to drill the rock.
In general, the larger TSD cutters (5-mm cubes) drill faster and suffer less breakage than smaller cutters (5-mm cubes). Additionally, TSD cutters set with sharp points down drill faster than face-set, or flat cutters. TSD bits are better than polycrystalline diamond compact (PDC) or roller cone bits for hard rock drilling because of their thermal stability.
This article reviews a 3-year project to develop advanced thermally stable diamond bits that can operate at a power level 5-10 times greater than that typically delivered by conventional rotary drilling. These bits are designed to operate on advanced drilling motors that drill three to six times faster than rotary drilling. These advanced bits and motors are especially designed for use in slim-hole and horizontal drilling applications.
The TSD bit design parameters which were varied during the tests were cutter size, shape, density (number of cutters), and orientation. Drilling tests in limestone,
sandstone, marble, and granite blocks showed that these optimized bits drilled many of these rocks at 5001,000 ft/hr compared to 50-100 ft/hr for conventional rotary drilling.
A Gas Research Institute project conducted by Maurer Engineering Inc. showed that approximately 70% of the time involved in a deep well is spent drilling and tripping.
For wells deeper than 15,000 ft, the time breaks down as follows: 48.45% on drilling, 21.94% on tripping, 6.48% on testing and logging, 5.22% on mud conditioning, 3.95% on trouble and problems, 3.37% on blowout preventer operations, 2.69% on waiting, and 2.48% on reaming.
A sensitivity model showed that doubling the rate of penetration significantly reduced the time to drill a well and reduced costs by 13%.
DIAMOND BITS
TSD and PDC bits are often run on high-speed (300-1,000 rpm) downhole motors because these motors deliver more power to drill the rock and increase penetration rates two to three times more than conventional rotary drilling (50-200 rpm).
PDC bits use cutters made of a thin layer of small synthetic diamonds bonded to a tungsten carbide substrate. The PDC cutters are mounted on tungsten carbide studs pressed into steel-body bits or brazed directly to matrix-body bits.
Matrix-body bits are ideal for deep drilling because the hard matrix material resists erosion even in harsh-environment deep wells. PDC bits have been used successfully to drill soft to medium-hard formations, but these bits break down because of thermal limits when used to drill hard rocks in deep wells.
PDC cutters fail at high temperatures because the cobalt binder holding the diamonds together has a higher coefficient of thermal expansion (12.0 C. x 10-6) than that of the diamonds (1.5-4.8 C. x 10-6, 3.8 C - x 10-6 for TSD). When the cutters encounter hard rocks, high frictional temperatures cause the cobalt binder to expand more than the diamonds, breaking the PDC cutter.
Thermally stable diamond cutters have the cobalt binder removed; thus, they can operate at higher temperatures. TSD cutters are made in large disks which are then cut into triangular or cubic shapes. The small shapes are cast directly into matrix bits. Bits with TSD cutters are ideally suited for drilling hard sedimentary rocks encountered in deep drilling.
TSD bits can drill in excess of 90 m (300 ft) before wearing out in granite, an igneous rock much harder than the sedimentary rocks encountered in deep oil and gas wells (Fig. 1).
ATMOSPHERIC TESTS
The nine bits shown in Fig. 2 were used in tests at atmospheric pressure. Bits 1 through 6 had TSD cutters of varying size, density, and orientation. Bit 7 had PDC cutters. Bit 8 was a roller cone bit with steel teeth, and Bit 9 was a roller cone bit with tungsten carbide teeth. The drilling tests (Fig. 3) were run on an atmospheric drilling test stand at Mauer Engineering's Drilling Research Center (DRC).
The DRC's drilling test stand uses 143/4 in. x 143/4 in. x 36 in. blocks of rock. The drill stand is controlled by a personal computer servo control system which allows preprogramming of bit weight and rotary speed. This closed-loop control system uses feedback from sensors to control the following test parameters: Drill shaft (bit) torque, bit rotary speed, drilling fluid flow rate, swivel input pressure, borehole differential pressure, displacement, and bit weight.
The following were the test conditions: 3 in. bit diameter, 7,000 lb weight on bit, 30-200 psi bit pressure, 0-800 rpm rotary speed, 60 gpm flow rate with water as the drilling fluid, and 0 psi confining pressure. The rock types were sandstone, limestone, and marble.
Laboratory tests showed that the design parameters (size, density, and orientation) have major effects on TSD bit drilling rate and life. Thus, the performance of TSD bits can be greatly enhanced by optimizing these variables.
TEST RESULTS
PDC and TSD bits drilled 9-14 times faster than roller cone bits because they can be operated at higher rotary speeds and they transmit more torque to the rock. In tests in Leuders limestone, the PDC bits drilled at 1,390 ft/hr, the TSD bits at 890 ft/hr, and the roller cone bits at 97 ft/hr. Although PDC and TSD bits drill much faster than roller cone bits, the PDC and TSD bits require more energy to remove a unit volume of rock (Fig. 4). Thus, the method of delivering greater power levels to the rock is a key to faster drilling.
PDC bits drilled faster than TSD bits; however, TSD bits are better suited for deep hard-rock drilling because the PDC cutters undergo rapid thermal degradation under such harsh conditions.
TSD cutters oriented with a sharp point downward drilled 16-25% faster than bits with cutters oriented with flat side down (Fig. 5). The sharp-set cutters had 9% breakage, and the flat-set cutters had 38% breakage. The cutters were 3-mm cubes.
The number of cutters (density) used in a TSD bit has a major effect on penetration rate and bit life. Test bits with a large number of cutters drilled 1.5-9 times faster than light-set bits drilled (Fig. 6). The lower penetration rate for the light-set bits was attributed mainly to excessive cutter breakage (75%) compared to cutter breakage on heavy-set bits (9%). These tests show that a large number of cutters is desirable on high-power TSD bits.
One of the most interesting results of the tests was the significant effect of cutter size on penetration rate and bit fife. The TSD cutters tested were either 3-mm cubes (L333) or 5-mm cubes (L555). The large L555 TSD cutters drilled 10-12% faster than the L333 cutters (Fig. 7). The most significant factor was the absence of bro-ken cutters on the bits with the large L555 TSDs; the bits with the small L333 cutters had 9% of the cutters broken.
These tests show that optimized TSD bits are designed with the following:
- A large number of cutters
- Large size cutters
- Cutters with sharp points oriented downward.
PRESSURE DRILLING TEST
In deep drilling applications, the pressure exerted by the drilling mud can have a major effect on penetration rate. Fluid pressure in the borehole is usually maintained greater than the formation fluid pressure to prevent gas or oil from flowing into the well and possibly causing a well control problem or blowout. However, a large differential pressure between the borehole and formation fluids creates cuttings removal problems and makes the formation difficult to drill.
Roller cone bits, which crush the formation, have difficulty drilling under these conditions. The fluid pressure holds the rock chips on the hole bottom, causing regrinding of cuttings and reduced drilling efficiency. This hole cleaning problem becomes worse as penetration rates increase.
Higher pressures can cause the formation to behave plastically; in other words, the rock merely deforms instead of fracturing in a brittle manner.
To study the effects of borehole pressure on drilling rate and bit design, the DRC modified its drilling test stand so borehole fluid pressures up to 3,000 psi could be applied. The drill shaft with the bit attached passes through a seal into a pressure vessel containing the rock sample. Drilling fluid pumped down the drill shaft and out the bit removes cuttings from beneath the bit. The cuttings are removed from the mud by a screen outside the drilling chamber. A choke is used to hold back pressure on the borehole fluid.
RESULTS
Three of the bits from the atmospheric tests were used in a series of pressure drilling tests: Bit 5-a dense coverage, L555, sharp-set TSD bit; Bit 7-a PDC bit; and Bit 8-a roller cone tooth bit.
The addition of borehole pressure significantly reduces penetration rate. Fig. 8 shows the reduction in penetration rate as a function of formation differential pressure (Moffit, 1991).
Tests were conducted with 2,000 psi differential fluid pressure across the rock. Table 1 shows the reduction in penetration rate in Leuders limestone caused by the differential pressure for each of the three bits tested. The PDC and TSD bits had a greater reduction in penetration rate than did the roller cone bit. Similar results were reported by E.E. Anderson and J.J. Azar. Fig. 9 shows the penetration rate for each bit in Leuders limestone and Batesville marble. These tests showed that TSD and PDC bits drilled much faster than roller bits under simulated deep hole conditions.
Bit speed also affects rate of penetration. Fig. 10 shows the effect of rotary speed on penetration rate in limestone and marble. Increasing the speed from 250 to 800 rpm increased drilling rate 40-100%.
Fig. 11 shows the drilling rate for the optimum TSD bit (L555 cutters, large number of cutters, and sharp set) in three different rocks at bit weights ranging from 3,000 to 9,000 lb. In these tests, penetration rate increased linearly with bit weight.
BIBLIOGRAPHY
Anderson, E.E., and Azar, J.J. "PDC Bit Performance Under Simulated Borehole Conditions," SPE paper 20412, presented at the SPE 65th Annual Technical Conference and Exhibition, New Orleans, Sept. 23-26, 1990.
Clark, I.E., and Shafto, G.R., "Core Drilling With SYNDAX3 PCD," Industrial Diamond Review, April 1987.
Cohen, John H., Maurer, W. C., and Westcott, P.A., "Design and Operation of a New Drilling Simulator," ASME PD Vol. 40, Drilling Technology, 1992.
Cunningham, R.A., and Eenink, J.G., "Laboratory Study of the Effect of Overburden, Formation and Mud Column Pressures on Drilling Rate of Permeable Formations," Transactions of the AIME, No. 216, 1959.
Maurer, W.C., Anderson, E., Hood, M., Cooper, G., and Cook, N., "Deep Drilling Basic Research," Report No. TR90-7 prepared by Maurer Engineering Inc. and the University of California at Berkeley, June 1990.
Moffitt, Stan, Personal communications (unpublished), Reed Tool Co. data, Houston, Sept. 5, 1991. Stewart, A., Falter, F.X., and Tomlinson, P.N., "Drilling Reef Quartzite with SYNDAX3," Industrial Diamond Review, March 1988.
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