ROCK EXCAVATION TOOLS
Rock excavation tools disintegrate and remove the rock from boreholes and tunnels by four basic mechanisms: thermal spalling, fusion and vaporization, mechanical stresses, and chemical reactions, as shown in Figure 5.1. ''Novel" or "advanced" drilling tools utilize exotic systems such as lasers or electron beams to melt or vaporize rock or explosives, or electrohydraulic discharges, to impact and shatter rock.
Status of the Field
Novel Drilling Systems
Thermal spalling drills such as jet-piercing and forced-flame drills heat rocks to 370 to 540°C (700 to 1000°F) to create thermal stresses that spall the rock (Figure 5.2). These devices have limited application because most rocks will not thermally spall (Maurer, 1968, 1980). Melting and vaporization drills utilize high-temperature devices such as lasers or electron beams to melt and vaporize rock (Figure 5.3). These devices have relatively low drilling rates because of the high energy requirements to melt and vaporize rock. Chemical drills utilize highly reactive chemicals such as fluorine to drill rock (Figure 5.4). These drills have found limited application due to high costs and safety problems associated with handling large volumes of highly reactive chemicals. Mechanical stress drills disintegrate the rock by inducing mechanical stresses (Figure 5.5). References for individual novel drills shown in Figures 5.2 to 5.5 (Maurer, 1970) are given at the end of this chapter.
Because the cross-sectional area of a tunnel face is 10 to 100 times greater than a typical drillhole, it is very unlikely that thermal spalling, melting, or vaporization drills could be used as the sole rock removal process due to extremely high power requirements and very low penetration rates. Similarly, chemical drills would not be practical for tunneling because of the large volume of highly reactive chemicals required, safety problems, and problems associated with chemical treatment and disposal of contaminated spoils.
Most of the advanced thermal and high-pressure jet drills require 10 to 100 times more energy to drill rock than conventional rotary bits (Table 5.1). Low drilling rates and excessive power requirements preclude using these advanced devices as the sole rock removal mechanism except in special applications. Additional R&D, which focuses on the physics of the rock removal process, is needed to reduce the overall energy requirements of these novel drilling techniques.
Combined Novel/Mechanical Systems
Because of the high power requirements of novel drilling techniques, advances are more likely to be made on combined novel-mechanical drill bits in which the novel devices (e.g., high-pressure water jets) cut narrow slots or "kerfs" in the rock face, thereby weakening the rock and allowing
it to be removed with a conventional bit. These combined systems could yield a two- to fourfold increase in drilling rates (Figure 5.6). A similar system could be used to speed up the advance rate of tunnel boring machines (TBMs; Figure 5.7). Cutting slots in the rock with a novel device weakens the rock and allows conventional cutters to break the rock into larger fragments (Figure 5.8). Figure 5.9 shows an example in which a single slot produced a sevenfold increase in the amount of rock removed by a 75-ft-lb impact. Initial attempts at developing combined novel-mechanical cutters are probably best focused on mining or oil field bits, which are relatively small and inexpensive. Once this technology is developed, it can be scaled up to tunneling and excavation application.
Advanced Downhole Motors
Most guided drilling systems utilize high-speed downhole turbine (Figure 5.10) or positive-displacement motors (200 to 1,000 revolutions per minute [rpm]; Figure 5.11) to obtain high drilling rates. Los Alamos National Laboratory developed geothermal turbodrills that operate effectively in hot-dry-rock geothermal wells at temperatures in excess of 315°C (600°F). These turbodrills accurately guided geothermal holes at drilling rates three to ten times higher than conventional rotary bits. The technology now exists to build downhole motors that will increase drilling rates two- to fourfold by increasing power delivered to the drill bit. Figure 5.12 shows an example (Cohen and others, 1994b) in which a slim-hole motor was overpowered by increasing the fluid flow rate and the differential pressure across the motor. The overpowered motor delivered 56 horsepower (hp) compared to 23 hp for normal operation. In laboratory tests, the overpowered motor drilled marble blocks at rates up to 550 ft/h, compared to 225 ft/h with normal motor operation. Special high-power bits that utilized oversize man-made thermally stable polycrystalline diamond (TSP) cutters were used on this overpowered motor. This example shows that there is significant potential for increasing drilling rates by developing improved motors. Additional R&D is needed on improved air drilling motors and higher-power motors for hard-rock drilling.
Advanced Drill Bits
Conventional drill bits remove rock by impact or shearing processes (Figure 5.13). Impact or roller bits utilize steel or tungsten carbide cutters to impact and break the rock. New wear-resistant, diamond-coated cutters are finding increased use in hard abrasive rocks. Additional R&D is needed on these improved cutter materials and on improved high-speed bearings.
Shear-type bits utilize polycrystalline or natural diamond cutters to remove rock by shearing processes. Polycrystalline diamond cutter (PDC) bits utilize cutters consisting of a thin layer of small synthetic diamonds bonded to a tungsten carbide substrate, as shown in Figures 5.14 and 5.15. These bits have potential for very high drilling rates because they can operate at much higher rotary speeds (800 to 1,000 rpm) and power levels than roller bits (100 to 200 rpm). In laboratory tests, these bits drill 9 to 14 times faster than conventional roller bits. Further work is needed to develop wear-resistant cutter materials and multipurpose bits that will effectively drill alternating layers of soft and hard rock.
R&D is also needed on sensing tools to look ahead of the bit or a TBM so that potential problems (e.g., blowouts, lost circulation, or waterfilled fractures) can be detected early and appropriate remedial action can be taken. This information can also be used to optimize drilling variables such as the weight on the bit and the rotary speed.
Advanced TBM Cutters
Tunnel boring machines utilize disk or tooth roller cutters to disintegrate rock (Figure 5.16). The development of the disk cutter by Robbins in the 1980s was a major breakthrough because it allowed much higher cutter-rock contact loads and much higher TBM advance rates in hard rock. R&D on advanced TBM cutters is needed to develop improved cutter materials and longer-life bearings.
Because of its size, a TBM or mining machine excavates two to four orders of magnitude more material per foot of advance than mining or oil field bits. A TBM, therefore, is actually a system of drills, requiring a multiplicity of components. Reliability is thereby reduced because failure of any component stops the entire system.
Existing TBMs in soft rock are now capable of such high advance rates that muck removal, rather than rock cutting, is becoming the limiting process for the system. Thus, significant improvements in excavation will require parallel development of both the rock cutting and the muck removal components of the system.
High-Pressure Jet Drilling Systems
Experimental high-pressure jet drilling systems operating at 10,000 to 20,000 pounds per square inch (psi) can drill two to three times faster than conventional drills (Figures 5.17 and 5.18). A dual-wall drill-pipe system currently being field tested utilizes mud pumped at high pressure (34,000 psi) to cut slots in the rock ahead of roller bits and lower-pressure mud to remove the broken rock from the hole bottom (Figure 5.19). This system drilled two to three times faster than conventional bits in an east Texas oil field test (Figure 5.20). In another experiment, a high-pressure downhole motor operating at 10,000 psi produced drilling rates in excess of 1,000 ft/h in medium-strength rock.
Despite these high drilling rates, these systems have not been commercialized because of problems with the high-pressure equipment. These problems include erosion and leaks in the drill pipe caused by the abrasive drilling muds. A new concept being evaluated to eliminate these problems is the use of high-pressure downhole pumps powered by low-pressure pumps at the surface (Figure 5.21). Additional work on these systems is needed to develop improved pumps, drilling pipe, high-pressure motors, and jet bits.
Slim-Hole Drilling Tools
Slim-hole drilling is finding increased use in the oil and gas industry, because it lowers well costs by 40 to 70% due to reduced materials (mud, cement, and casing), smaller rigs and crews, and faster drilling rates. Improved slim-hole drilling tools are needed because space constraints limit the strength and life of these tools.
Guided Percussion Drills
Percussion or hammer drills can penetrate many hard rocks two to four times faster than rotary drills because they apply high-impact loads that shatter the rock into large fragments. Although widely used in shallow blast-hole operations, percussion drills have found limited use for deep oil, gas, and geothermal drilling because of the inability to accurately guide
these drills in hard rock. Improved percussion drills that can be guided accurately in hard rock are needed for deeper drilling applications.
Resonant Sonic Drills
Resonant sonic drills (Volk and others, 1993) use eccentric rotating weights to vibrate the drill pipe at frequencies of 80 to 150 cycles per second to fluidize soil and other unconsolidated materials, thus allowing the pipe to be advanced into the ground with minimal friction. No fluid circulation is used with these drills, so the material ahead of the bit either is pushed aside into the surrounding formation or flows into the core barrel and is retrieved back to the surface.
Sonic drills have been around since the 1950s but have never caught on owing to their relatively high cost and poor reliability (the tool is particularly susceptible to vibrational fatigue). Efforts are now under way at Hanford and Sandia Labs (Volk, 1992; Volk and others, 1993) to improve the reliability of this tool for use in environmental drilling and coring. Sonic drills are currently limited to drilling primarily unconsolidated materials to maximum depths of about 400 ft. Methods are needed to guide these drills directionally.
Priorities for R&D
As noted in Chapter 4, the fundamental rate-controlling process in drilling is the rate of rock breaking by the drilling tool. Significant improvements in rock breaking rates can be achieved through a better understanding of the physics of rock-tool interactions. Revolutionary advances in rates of rock breaking and rock removal are possible through the development of hybrid mechanical-novel drilling tools to break and remove rock. Evolutionary advancements in rock removal rates are likely through the development of stronger, wear-resistant cutters, bearings, and bits. R&D should be focused in the following areas:
Physics of rock-tool interactions: The physical and mechanical characteristics of rocks are generally well known, but much less is known about the mechanics of rock removal, that is, the interaction between the rock and the drilling tool. Research that focuses on the physics of rock
removal processes is a prerequisite for designing improved drilling tools and should have a high priority for support.
Improved cutter materials and bearings: Conventional drill bits utilize steel or carbide cutters to remove rock by shearing or impact. Wear-resistant, diamond-coated cutters are finding increased application in hard, abrasive rocks, particularly for shear-type bits. Additional R&D is needed on these wear-resistant cutter materials, and on these and other wear-resistant materials for high-speed bearings.
Novel-hybrid drilling technologies: R&D on novel drilling technologies should focus on reducing energy requirements for drilling. The development of hybrid systems, in which novel drilling tools are used to lower the strength of the rock, and conventional mechanical drilling tools are used to break and remove it, are especially promising and should be pursued. R&D on novel systems should focus initially on mining and oil field bits, which are relatively small and inexpensive. Once this technology has been developed, it should be scaled up to tunneling and excavation applications.
Improved mechanical drills: Significant improvements in rock removal rates can also be obtained through evolutionary advances in conventional mechanical drilling tools. R&D efforts should focus on the development of the following:
high-speed, high-power downhole motors;
guided percussion drills; and
slim-hole drilling tools.
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