Water and abrasive jetting, and mechanical techniques expedite hard rock drilling

Jack J. Kollé
Tempress Technologies Inc.
Kent, Wash.

Construction activities that require the placement of gas, electrical, or communication utilities in hard rock will benefit from lightweight systems capable of precisely drilling short, constant-radius arcs (Fig. 1 [66,512 bytes]).

Existing mechanical drilling systems are capable of drilling shallow directional holes, but the equipment is heavy, drilling rates are low, and costs are high.

A comparison of approaches for rapidly drilling small-diameter (25-50 mm) and near-surface holes along a short-radius (30 m) arc, in a variety of hard rock types, is described. Four approaches are considered:

1. Rotary diamond drilling with a downhole motor
2. Ultra-high pressure (UHP) water jet drilling
3. Mechanically assisted UHP water jet drilling
4. Abrasive jet drilling-abrasive water jet and abrasive slurry jet.

Data relating mechanical and hydraulic drilling parameters for each approach were compiled from literature and drilling tests for all four techniques. ¹ The drilling data are summarized in a common format to provide direct drilling efficiency comparisons for:

• Jet pressure and hydraulic power
• Thrust and torque requirements and abrasive feed.

In order to compare approaches for drilling, a specific energy parameter has been defined:

S _e = W/Q _r (1)

Where: Q _r is the volumetric rate of rock removal and W is the hydraulic or mechanical cutting power applied. This basic parameter allows a calculation of the rock-drilling rate for any approach based on the power available for drilling at the drill head.

Rotary drilling

A short-radius directional hole can be drilled in hard rock using a diamond bit mounted on a positive-displacement downhole motor (PDM). A typical small motor has a diameter of 43 mm and a length of 2.1 m (84 in.). Thus, the smallest hole diameter that can be drilled is about 50 mm (1.97 in.).

The motor can be built into a bent housing to provide directional control. At an operating pressure of 2.8 megapascal and a flow rate of 10^-3 cu m/sec (15 gpm), a 5:6 lobe PDM can deliver 80 newton-meters (60 lb force-ft) of torque at 200 rpm rotary speed with a mechanical power of 1.7 kw.

Higher rotary speeds and lower torque can be provided with a 1:2 lobe PDM. An additional 10 kw hydraulic power is required to provide turbulent chip cleaning and bit cooling. A detailed comparison of diamond drilling with surface-set and impregnated diamond bits across a wide range of high-strength rock types, including granite, quartzite, and taconite, is discussed by Clark.²

The specific energy required to drill 150-400 megapascal rock, using a surface-set diamond bit, is between 1 and 2 joules/cu mm; while a specific energy of about 10 joules/cu mm is required to drill with impregnated diamond bits.

This is consistent with the specific energy required for diamond drilling in 200 megapascal rock, reportedly in the range of 1-10 joules/cu mm.³ Based on these specific energy values, the rate of penetration for a 50-mm diameter, surface-set diamond bit with 1.7 kw of mechanical power in hard rock, would be 1.5-3 m/hr (5-10 ft/hr).

Kollé, et al., discuss two drilling tests in black granite with a compressive strength of 280 megapascal using a 38-mm (1.5 in.) diameter surface-set diamond bit.¹ At rotary speeds of 340-780 rpm and a thrust of 2,200 N (500 lb force), the penetration rate was 1.8-2.9 m/hr (5.9-9.5 ft/hr).

Penetration rates with an impregnated diamond bit should be about 0.3 m/hr (1 ft/hr). Under normal operating conditions, abrasive wear limits the bit life of diamond bits to less than 30 m in rock types such as granite that have a quartz content greater than 20%. At the low thrust and rotary speeds, a surface-set diamond bit might be capable of drilling 30 m.

Drilling with diamond bits requires a high thrust load to ensure that the cutters penetrate into the rock. Clark provides data on surface-set diamond drilling torque and thrust as a function of rock compressive strength under optimal drilling conditions.² Drilling hard rock at a bit torque of 80 newton-meters requires a thrust of 13 kilonewton (3,000 lb force).

The thrust available is limited by buckling of the drill rod. Fortunately, in an inclined hole, the drill rod is stabilized by the hole.⁴ The critical buckling load for a drill rod having an OD of 38 mm, an ID of 12 mm, and laying in a 50-mm-diameter hole inclined at 45°, would be 13 kilonewton (3,000 lb force).

Drilling with a small-diameter motor is comparable to the short-radius drilling requirement since the drillstring does not rotate. The motor could then incorporate a bent sub or stabilizers that would cause it to curve upwards while the azimuth remains constant.

Over a distance of 30 m, a properly designed bent-housing assembly should provide azimuth and elevation accuracy to within 1°/30 m.^{5 6} This would allow a prediction of the exit hole location to within a few meters as long as the drill exits at an angle of 30° or more from the surface.

High-pressure, water-jet drill

Linear jet-cutting experiments in rock have been carried out by a relatively large number of researchers. Table 1 [111,069 bytes] shows the range of jet cutting and drilling specific energy for each reference as inferred from the reported operating parameters and empirical observations of rock erosion rates. Jet pressure, P_j, and unconfined compressive strength, s_c, are also listed where available. In general, high-permeability rock types such as Berea sandstone, have a low threshold pressure and specific energy.

Medium-strength, low-permeability limestones and sandstones have intermediate specific energy, whereas, high-strength, low permeability rocks such as granite, quartzite, and basalt have high threshold pressures and specific energy.

Veenhuizen and O'Hanlon developed a nonrotating standoff control collar that allows jet drilling at a constant, low-level thrust.⁷ The most successful system used a carbide collar with a pair of jets, one vertical and one angled at 20° from vertical as shown in Fig. 2. [43,692 bytes]

This system drilled uniform-gauge holes with a minimum diameter of 1 mm over the gauge. Veenhuizen and O'Hanlon found that the UHP water-jet drilling rate increases as the square root of rotary speed with the best drilling results obtained at about 1,000 rpm.

This drill was not used to drill hard rock at the time; however, the water-jet drill was used to drill black granite at a pressure of 240 megapascal in a more recent application.¹ Reported results show that granite can be drilled at pressures as low as 69 megapascal.^{8 9}

Because jet erosion requires no torque or thrust, high-pressure, water-jet drilling provides a unique capability for drilling a constant-radius directional hole without the need for steering corrections.

As shown in Fig. 2, the drill orientation could be controlled with a sleeve used to orient a nonrotating bent housing assembly. Pure water-jet drilling is less sensitive to formation changes than mechanically assisted drilling because cutting is controlled by the bit orientation.

O'Hanlon and Madonna describe the evolution of mechanically assisted drilling heads illustrated in Fig. 3 [43,301 bytes].¹⁰ The mechanical inserts ensure that the drill will not advance ahead of the full-gauge hole area. In addition, the inserts break the ridges of rock which are formed by the jets.

Torque and thrust levels are low enough to allow manual feed of the drill. Table 2 [70,447 bytes] lists drilling parameters in a variety of high-strength rock types including black granite.

Abrasive jet drilling

Abrasive jet drilling involves a supply of abrasive that is pumped with the fluid (abrasive slurry jet) or entrained at the nozzle (abrasive water jet). Entrained-abrasive water jets are widely used for manufacturing applications including production of cut stone.

This approach has been evaluated for cutting of deep slots in rock and concrete. Hashish discusses a variety of concepts for abrasive water jet rock drilling.¹¹ The basic concept is illustrated in Fig. 4 [41,858 bytes].

This approach requires a separate feed line through which abrasives are fed. High-pressure jets are discharged through carbide mixing tubes where the abrasive is entrained to form a jet. The high-pressure jet and mixing tubes rotate together.

Abrasive waterjet drilling has been used for deep kerfing of concrete and rock but has never been demonstrated for hole drilling in rock because of feed control limitations. Table 3 [63,031 bytes] indicates the jet-pressure abrasive usage and specific energy for slot cutting and surface erosion tests in sandstone, granite, and concrete.

The abrasive water jet drilling test described by Kollé is consistent with previous results.¹ Abrasive usage is given as the mass of abrasive used, ma, divided by the mass of rock removed, mr. As indicated, abrasive jet cutting requires an order of magnitude more abrasive than material removed.

For example, a 30-m deep, 25-mm diameter hole in granite would require about 1 ton of abrasive. Direct injection abrasive jet (abrasive slurry jet) cutting has been applied as an alternative to entrained abrasive jets.¹²

In this process, abrasives are suspended in a polymer solution in a pressurized tank. The abrasive slurry tank is pressurized using a conventional high-pressure pump, and the slurry is fed through an erosion-resistant nozzle as shown in Fig. 5. [39,291 bytes]

This process requires periodic mixing and filling of a large pressurized tank with abrasive and polymeric additives used to suspend the abrasive. A 100-gal tank, weighing several hundred kilograms, would only last for a few minutes of drilling. The available data on abrasive-slurry drilling is summarized in Table 4 [45,121 bytes]. Abrasive-slurry jet systems have a specific energy that is comparable to that of high-pressure abrasive water jet systems, but requires much higher water flow rates and extremely high-abrasive flow rates as indicated by the ratio of abrasive to rock mass removed.

Drilling technique comparison

A comparison of various aspects of high-pressure jet drilling techniques is provided in Table 5 [132,754 bytes]. The first three columns compare jet-drilling systems operating with a hydraulic power of 38 kw (50 hydraulic hp) at the surface.

The estimated drilling rates were obtained using the specific energy data along with the bit hydraulic or mechanical power. The UHP water-jet systems have little loss of pressure or power through a 30-m drill rod, and the bit power is essentially equal to the power available at the surface.

An abrasive-slurry jet system would lose about half its power to pressure losses in the drill rod. Abrasive water-jet drilling is not listed since this approach has never been demonstrated. The characteristics of a small-diameter, rotary, diamond drilling system, using a downhole motor, are also provided.

In this case, the power available at the bit is limited by the mechanical power capacity of a PDM. The development status of each approach is indicated by meters of rock drilled, along with thrust and torque requirements and compatibility with a passive directional control system such as a bent sub assembly.

UHP jet drilling flow rates are relatively small, so hole cleaning becomes a concern. Okranji and Azar show that cleaning of horizontal holes requires turbulent flow.¹³ The critical flow rate for a 25-mm diameter hole with a 19-mm drillstring is 9x10-5 cu m/sec (1.4 gpm).

A 38 kw, 240 megapascal water-jet drilling system would have a flow rate of 1.6 x 10-6 cu m/sec (2.5 gpm), which is sufficient to ensure the turbulent transport of cuttings out of a hole inclined at less than 45° from horizontal.

Rotary mechanical drilling, using a downhole, positive-displacement motor would be capable of drilling a short-radius, small-diameter hole. Even though diamond drilling is more efficient than jet drilling, the mechanical power available at the bit is limited, and a larger hole must be drilled to accommodate the motor, so the drilling rate is slower than any of the jet-drilling approaches.

The lowest rate given is for impregnated diamond bits in abrasive 400-megapascal rock while the highest rate is for a surface-set diamond bit in 200-megapascal rock. The penetration rate is limited by the torque and rotary speed capacity of a small downhole motor.

Abrasive suspension jet drilling is not considered a viable approach for this application because of the difficulty in handling and delivering massive amounts of abrasive and because of uncertainties during hole steering.

UHP jet drilling offers high penetration rates because the power available at the bit is extremely high. Mechanically assisted jet drilling provides slightly higher drilling rates, but this approach generates torque loads that could cause the hole trajectory to spiral.

The literature review and testing discussed here show that a UHP drill is capable of rapidly drilling small-diameter holes in a wide range of erosion-resistant rock types. The system can be steered by providing a nonrotating bent housing that directs the drill.

Finally, a UHP jet drilling system could be made very lightweight because thrust and torque requirements are nominal.

Acknowledgments

The author would like to acknowledge support for this work by Hugh Thomsom of the Naval Facilities Engineering Service Center and Rick Stagi and Ken Theimer of Waterjet Technology Inc.

References

1. Kollé, J.J., Theimer, K., and Stagi, R., "Shore bypass drilling system," TR-002, Tempress Technologies Inc., Kent, Wash., 1997.
2. Clark, G.B., Principles of Rock Fragmentation, John Wiley & Sons, New York, 1987.
3. Jaeger, J.C., and Cook, N.G., Fundamentals of Rock Mechanics, Chapman and Hall, London, 1976.
4. Dawson, R., and Paslay, P.R., "Drillpipe buckling in inclined holes," JPT, October 1984, pp. 1734-1738.
5. Dech, J.A., Hearn, D.A., Schuh, F.J., and Lenhart, B., "New tools allow medium-radius horizontal drilling," OGJ, July 14, 1986.
6. Yost, A., Overbey, W.K., and Carden, R.S., "Drilling a 2,000-ft horizontal well in the Devonian Shale," SPE paper 16681, 62nd Annual Technical Conference and Exhibition, Dallas, Richardson, Sept. 27-30, 1987.
7. Veenhuizen, S.D., and O'Hanlon, T.A., "Development of a system for high-speed drilling of small-diameter roofbolt holes," Unpublished technical report, TR117, Water Jet Technology Inc., Kent, Wash., 1978.
8. Vijay, M.M., Grattan-Bellow, P.E., and Brierley, W.H., "An experimental investigation of drilling and deep slotting of hard rocks with rotating high pressure water jets," Seventh International Symposium on Jet Cutting Technology, Ottowa, pp. 419-438, BHRA Cranfield, Bedford, England, June 26-28, 1984.
9. Cable, B., "Horizontal drilling system (HDS) field test report-FY91," TR-2002-OCN, Naval Facilities Engineering Service Center, Port Hueneme, Calif., 1993.
10. O'Hanlon, T.A., and Madonna, P.L., "Development of a system for high-speed drilling of small diameter roofbolt holes," Unpublished technical report, TR233, WaterJet Technology Inc., Kent, Wash., 1982.
11. Hashish, M., "The potential of an ultrahigh-pressure abrasive-waterjet rock drill," 5th American Water Jet Conference," Toronto, Aug. 29-31, 1989.
12. Summers, D.A., "Waterjetting Technology," E & FN Spon, London, 1995.
13. Okranji, S.S. and Azar, J.J., "The effects of mud rheology on annular hole cleaning in directional wells," SPE Drilling Engineering, pp. 297-308, August 1986.

Bibliography

Anonymous, "Development of a Waterjet Drill/Kerf Jumbo Phase I," Unpublished technical report, TR215, Water Jet Technology Inc., Kent, Wash., 1981.
Hashish, M., Echert, D., and Marvin, M., (1987) "Development of abrasive-waterjet concrete deep kerf tool for nuclear facility decommissioning," International Water Jet Symposium, Beijing, pp. 11-33, Sept. 9-11, 1981.
Maurer, W.C., "Advanced Drilling Techniques," The Petroleum Publishing Co., Tulsa, 1980.
Momber, A.W., and Kovacevic, R., "Test parameter analysis in abrasive water jet cutting of rocklike materials," International Journal of Rock Mech. and Mining Science, Vol. 34, No. 1, pp. 17-25, 1997.
Pols, A.C., "High-Pressure jet-drilling experiments in some hard rocks," Journal Pressure Vessel Technology, pp. 353-361, May 1977.

The Author