Drill bit is a drilling tool main element for rock bottomhole disintegration on the unit in the process of hole making. Drill bit term remains from drilling technological expansion morning when the only method of hole making was cable drilling and drill bit resembled carpenter’s tool with the same name. Generally drill bit is fixed at the drill string end, drill string imparts twisting and axial force created by drilling rig to the drill bit (in case of cable drilling drill bit hangs on the cable and impacts bottom owing to free-fall energy).
Drill bit classification is based on two features: function and nature of effect upon rock. There are 3 grades of drill bits distinguished by purpose: for full hole drilling (rock breaking complete bottom wide), for core drilling (annular rock breaking over hole walls with bleeding core left in the center) and for special purpose (cement drilling over in the casing, hole reaming etc.). There are 4 grades of drill bits by nature of their effect upon rock: crushing, chipping-crushing, scraping-cutting and cutting-shearing. Drill bit main elements (fig. 1) — bit body and operating (breaking down) part; the latter determines 3 drill bit types widely used in the industry: rock, diamond and drag bits.
Above 90% of oil and gas wells volume is made with rock bits; these drill bits are the most productive for test (full hole drilling) and shot holes in hard rocks. First drill bit with conical cutters was invented in the USA (1909). Rock drill bit (or core-cutting head) consists of one, two, three and more conical, spheric or cylindrical cutters, mounted on ball or friction bearings or their combination on drill bit elements journals. Cutter outer surface has rock breaking elements – milled teeth or press-fitted (sealed) hard alloy tips or combination of teeth and tips. To increase abrasion resistance milled teeth are hard-faced. Rock breaking elements geometry and parameters (teeth height and length, angle of tapering and flat, tooth pattern on each cutter tip) are various for different types of drill bits and depend on physical properties of rocks. Generally cutting tip teeth are tapered taper; hard-alloy tips have tapered taper or hemispheric working surface. Depending on the body design rock bits are divided into bodyless and box-type rolling-cutter drilling bits. Bodyless rolling-cutter drilling bit body is welded of separate (two, three and four) elements, on element journals floating cutters are mounted. Box-type rolling-cutter drilling bit body is cast, arm with cutters is welded to the body. Bodyless rolling-cutter drilling bit has an external taper thread (pin) for drill bit connection to the drill string, box-type rolling-cutter drilling bit has internal taper thread (collar). Elements (arms) and cutters are made of chrome-nickel-molybdenum, chrome-nickel and nickel-molybdenum steels. In the Russia rock bits are standardized by types and diameters (from 46 to 490 mm) and are made of 14 types for soft, medium and hard formations, hard and very hard rocks drilling over. Tricone drilling bits are prevailing. At limited range two-cone and tricone drilling bits are used for rotary and turbine hole drilling that makes it possible to move a new drill bit down and used one up inside the drill string without the drill string lifted to the surface, as well as multi-cone drilling bits are used for reactive turbodrilling of 346—2600 mm diameter holes.
Diamond drill bits (and drill head) consist of hard-alloy diamond-bearing business end(matrix) and steel body with internal connecting conical tool-joint thread. Drill bits differ from each other by business end shape, diamonds quality and flushing system. These drill bit matrixes are manufactured of different metal powders by powder metallurgy. Metal powders provide good retention of diamonds and enable different hardness and resistance to abrasion matrixes creation. Tungsten, tungsten carbide and copper base matrixes provide adequate strength, resistance to abrasion and high heat transfer of matrix material. For drill bits (drill heads) manufacture industrial diamonds of 0,05 – 0,34 carat weight are used (for drill bit of 188 mm diameter it is consumed 400—650 carat or 2000—2500 diamond grains). In the Russia diamond drill bits and drill heads are produced with 140, 159, 188, 212, 241 and 267 mm diameter in two modifications: single layer DR, DT, DK, КТ and КR types when diamonds are placed in matrix working edge surface layer according to special layout and impregnated DI type with approximate uniform diamond small grains distribution in matrix material. Diamond drill bits are reasonable to be used for deep (over 3000 m) holes. Generally diamond drill bit hardness 20-30 times exceeds rock bit hardness. Since 1967 in the USSR drill bit ISM type ISM is widely used, the drill bit business ends are reinforced with Slavutich superhard material insertion pieces (insertion top part or 30% of height is made of Slavutich, bottom part is made of hard-alloy powder, powder metallurgy by powder metallurgy). Insertion pieces are fixed by soldering flush in the body fins. ISM drill bits (fig. 2) are manufactured of 138, 142, 158, 188, 212, 241, 267, 293, 317, 344, 392 mm diameter and perform well for soft and medium slightly abrasive and medium abrasive formations drilling.
Drag bit (fig. 3) consists of forged body with three or more
blades welded to the body. Fish tail bit body and blades are forged as a unit.
Blade faces of drilling bit are reinforced with hard alloy square plate and
grain alloy relit, side fаces are reinforced with hard alloy cylindrical tips.
Drag bits are used for soft and medium formations drilling. In the Russia
according to technical standards drag bits are produced of three-bladed 3L type
with diameter from 118 up to 445 mm, two-bladed 2L type with diameter from 76 up
to 161 mm and multi-bladed IR type with diameter from 76 to 269 mm, Pts and PR
types spear-shaped with diameter from 97 to 445 mm. In some cases drill bits RKh
type (fish tail) are manufactured, this drill bit type is a modification of
two-blade bits. Drill bits for cable drilling are also referred as drag bits.
For non-productive operations in the Russia milling bits FR type in the
form of flat-bottomed mills are produced, mill bottom working surface is
furnished with hard alloy tips or plates overhanging drill bit body. Efficient
hole drilling is provided by timely bottomhole cleaning and borings lifting up
as well as drill bit business end cooling. Usually bottom-hole scavenging
(fig. 4, а) is practiced for shallow (up to 20 m) holes
drilling, for deep and ultradeep holes - flushing. There are two types of
flushing applied for rock bits: a) center channeled (fig. 4, b)
and water jet — between cutters (fig. 4, c), for big box-type
drilling bits a combination of flushings is used. Shape, cross section and
number of flushing channels depends on drill bit purpose, type and size. In
water jet cone and drag bits different diameter detachable mineral ceramic and
metal ceramic jet nozzles are used for expansion of transferred by them
hydraulic horsepower, they convert drill bits pressure differential to gravity
head increasing rock breaking efficiency. Drill bit development is realized by
bit design improvement (water jet bottomhole cleaning system, new sleeve
bearings creation and others), new materials application (high tension hard
alloys, ESR and vacuum arc remelted steels), drill bit business ends fit with
diamonds, high tension hard alloys and ultrahard materials.