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drill bit[′dril ‚bit]
the basic element of a drilling tool for the mechanical breakup of rock at the face of a borehole during the drilling operation. The term “bit” dates from the early period in the development of drilling technology, when the only method of sinking wells was percussion drilling, in which the drill bit resembled a carpenter’s chisel. [The Russian worddoloto means both “chisel” and “bit.”] As a rule, a drill bit is attached to the end of the drill pipe, which transmits to it the axial and rotary thrust created by the drilling rig (in the case of the percussion drilling method, the drill bit is suspended on a cable and delivers blows to the bottom of the well through the energy of free fall).
Two criteria are taken as the basis for the classification of drill bits: the purpose and the nature of the bit’s effect on rock. Drill bits are classified in three groups according to purpose or use: for continuous drilling (crushing of rock over the entire face of the borehole), for core drilling (crushing of rock in a ring around the wall of a borehole, retaining the central part of the core), and for special purposes (drilling out cement in a pipe string, reaming a borehole, and so on). Drill bits are divided into four different classes according to their effect on rock: crushing, crushing-chipping, abrasive-cutting, and cutting-chipping action. The main elements of a drill bit (Figure 1) are the body and the working (cutting) part. Three types of bits, based on the working part used, are widespread in industry: rolling cutter bits, diamond bits, and rotary-blade bits.
Rolling cutter bits are used for more than 90 percent of the drilling of wells for oil and gas; this type of bit is the most efficient for driving boreholes in geological prospecting (with a continuous face) and blastholes in hard rock. Bits with conical cutters were invented in the USA in 1909. The rolling cutter bit (or core bit) consists of one, two, three, or more conical, spherical, or cylindrical rolling cutters mounted on roller or sliding bearings, or a combination of them on the journals of the drill-bit units. The outside surface of the rolling cutter is equipped with rock-crushing elements, which may be in the form of milled teeth or press-fitted (soldered) hard-alloy bits or tooth-and-bit combinations. The milled teeth are reinforced with a hard alloy to increase wear resistance. The geometric shape and dimensions of the rock-crushing elements (the height and length of the teeth, the angle of taper and truncation of their cutting points, and the closeness of tooth spacing on the cutter crown) vary for different drilling bits and depend on the physical properties of the rock. The teeth on the cones of cutter bits are usually wedge-shaped; hard-alloy bits usually have tapered or semi-spherical working surfaces. Rolling cutter bits may be of the sectional or unit-body type, depending on the body construction. In sectional bits, the body is welded together from separate (two, three, or four) sections with freely rotating cutters mounted on their journals. In unit-body drill bits, the body is of cast construction, with welded lugs and cutters. To attach the bit to the pipe string, sectional drill bits are provided with a male taper thread (nipple), and unit-body bits have a female taper thread (coupling). The lugs and cutters of drill bits are made of chrome-nickel-molybdenum, chrome-nickel, and nickel-molybdenum steels. In the USSR rolling cutter bits are standardized according to types and diameters (46 to 490 mm); 14 types are manufactured, for drilling in soft, medium-hard, tough, hard, and very hard rocks. Tricone bits are most commonly used. Limited use is made of detachable two- and three-cone rock bits for driving boreholes by the turbine and rotary methods, which make it possible to lower a new bit and raise the worn bit within the pipe string without raising the string to the surface, and multicutter bits, for drilling holes 346-2,600 mm in diameter by the reaction-turbine method.
Diamond drill bits and points consist of a hard-alloy working part (matrix), on which the diamonds are mounted, and a steel body with a female taper lock joining thread. Diamond bits are differentiated according to the shape of the working part, the diamond quality, and the flushing system. The matrices of these drill bits are made from various metal powders by powder metallurgy. The powders provide good retention of the diamonds and make possible the production of matrices with various hardness and wear-resistant qualities. Matrices made from tungsten, tungsten carbide, and copper provide sufficient strength, wear resistance, and heat conductivity of the matrix material. Drill bits and points are manufactured with 0.05-0.34-carat industrial diamonds (400-650 carats of diamond, or 2,000-2,500 grains, are used for drill bits 188 mm in diameter). In the USSR diamond drill bits and points are manufactured with diameters of 140, 159, 188, 212, 241, and 267 mm, in two versions—the single-layer DR, DT, DK, KT, and KR types, with the diamonds set in the surface layer of the matrix cutting edges according to definite patterns, and the impregnated DI type, with approximately uniform distribution of fine diamond grains in the matrix material. The use of diamond drill bits is expedient at great depths (more than 3,000 m). As a rule, the durability of diamond drill bits is 20-30 times greater than the durability of rolling cutter bits. The ISM bits, whose cutting surfaces are equipped with inserts made of
extrahard Slavutich material (the upper part of the inserts, or 30 percent of the height, is made of Slavutich material, and the lower part is made of hard-alloy powders sintered by powder metallurgy), have been widely used in the USSR since 1967. The inserts are soldered flush with the ribs of the body. The ISM bits (Figure 2) are manufactured in diameters of 138, 142, 158, 188, 212, 241, 267,293, 317, 344, and 392 mm and operate efficiently during drilling in rock of soft and medium hardness and low and medium abrasiveness.
Rotary-blade drill bits (Figure 3) consist of a forged body onto which are welded three or more blades. In a two-blade bit, the body and blades are stamped as a single unit. The front blade faces are reinforced with rectangular hard-alloy plates and a granular relit alloy (a tungsten carbide composition), and the lateral faces are reinforced with small, cylindrical hard-alloy teeth. Rotary-blade bits are used for drilling in soft and medium-hard rock. The USSR maintains standard specifications for the manufacture of 3L three-blade bits 118-445 mm in diameter, 2L two-blade bits 76-161 mm in diameter, IR multiblade bits 76-269 mm in diameter, and PTs and PR spear-pointed bits 97-445 mm in diameter. The RKh (rybii khvost, “fishtail”) drill bit, a variation of the two-blade bit, is also manufactured in a number of cases. The bits used for churn drilling are also of the rotary-blade type. Milled FR cutter bits, in the form of flat-bottomed milling cutters whose lower cutting surface is equipped with hard-alloy teeth or plates that protrude from the body of the cutter bit, are manufactured in the USSR for auxiliary drilling operations.
Well-drilling efficiency is increased by the periodic removal of rock debris from the face of the hole and by transporting them to the surface, as well as by cooling the working elements of the drill bit. This is usually done by blowing air into them (Figure 4,a) during the drilling of shallow holes (to 20 m) and by flushing (washing) in the drilling of deep and superdeep holes. Two flushing methods are used in rolling cutter bits: central channel flushing (Figure 4,b) and hydraulic-monitor flushing between the rolling cutters (Figure 4,c); a combination of methods may be used in large
body-type bits. The shape, size, arrangement, and number of flushing channels depends on the purpose, type, and size of the drill bit. Interchangeable mineral or metal-ceramic nozzles of various diameters, which convert the pressure differential on the drill bit to a velocity head that increases rock-crushing efficiency, are used in hydraulic-monitor rolling cutter and rotary-blade bits to increase the fluid power.
Developments in drill-bit technology are directed toward the improvement of designs (the hydraulic-monitor system of flushing the faces, development of new sliding bearings, and so on), the use of new materials (high-strength hard alloys, electroslag and vacuum-arc smelted steels), and the equipping of cutting elements with diamonds, high-strength hard alloys, and superhard materials.
REFERENCESMal’kov, I. A. Teoriia i praktika primeneniia gidromonitornykh dolot v SShA. Moscow, 1958.
Volkov, S. A., and A. S. Volkov. Spravochnik po razvedochnomu bureniiu. Moscow, 1963.
Korneev, K. E., and P. A. Palii. Burovye dolota, 3rd ed. Moscow, 1971.
Brandy, J. E. Rotary Drilling Handbook, 6th ed. Los Angeles, 1961.
G. I. MATVEEV