Rock bit

The most frequently used bit types are the roller cone or rock bit (F g. 3.8) and the polycrystalline diamond cutter or PDC bit. 

If a shallow kick off in soft formation is required (e.g. to steer the borehole away underneath platforms) a technique using jet bit deflection or badgering is employed (Fig. 3.16). A rock bit is fitted with two small and one large jet. With the bit on bottom and oriented in the desired direction the string is kept stationary and mud is pumped through the nozzles. This causes asymmetric erosion of the borehole beneath the larger jet. Once sufficient hole has been jetted, the drill bit will be rotated again and the new course followed. This process will be repeated until the planned deviation is reached. 

Classification of Drilling Bits 769. Roller Rock Bit 771. Bearing Design 774. Tooth Design 776. 

The drill string is defined here as a drill pipe with tool joints and drill collars. The drill stem consists of the drill string and other components of the drilling assembly that includes the kelly, subs, stabilizers, reamers as well as shock absorbers, and junk baskets or drilling jars used in certain drilling conditions. The drill stem (1) transmits power by rotary motion from the surface to a rock bit, (2) conveys drilling fluid to the rock bit, (3) produces the weight on bit for efficient rock destruction by the bit, and (4) provides control of borehole direction. 

Roller rock bits (milled tooth bits)... 

The elements of the roller rock bit are shown in Figure 4-137 [45]. Roller rock bits have three major components the cone cutter, the bearings and the bit body. The cutting elements are circumferential rows of teeth extending from... 

Figure 4-137. Roller (rock) bit elements [45]. (Courtesy Canadian Association of Oilwell Drilling Contractors.)...

A roller rock bit is pulled out of the hole after 12 hr of rotation at the bottom. The driller estimates that the worst cone could rotate 4 hr more before being completely worn out thus total bearing life estimated is 16 hr. 

Roller rock bit nozzle sizes are given in Table 4-98. The total pressure drop across a roller rock bit, Pj (psi), is [47]... 

Figure 4-143 gives convenient graphs for nozzle selection for roller rock bits [47]. 

Diamond bits are being employed to a greater extent because of the advancements in mud motors. High rpm can destroy roller rock bit very quickly. On the other hand, diamond bits rotating at high rpm usually have longer life since there are no moving parts. 

The term fixed cutter is used as the most correct description for the broad category of nonroller cone rock bits. The cutting elements may be comprised of any suitable material. To date, several types of diamond materials are used almost exclusively for fixed cutter petroleum drilling applications. This leads to the widespread use of the term diamond bits and PDC bits in reference to fixed cutter designs. 

A tricone roller rock bit is furnished with three nozzles with the diameters of 32 od -il in. Calculate the bit pressure drop if the mud weight is 10 Ib/gal and flowrate is 300 gpm. 

The downhole turbine motors that are hydraulically operated have some fundamental limitations. One of these is high rotary speed of the motor and drill bit. The high rotary speeds limit the use of downhole turbine motors when drilling with roller rock bits. The high speed of these direct drive motors shortens the life of the roller rock bit. 

Motor speeds and, therefore, bit speeds are high, which limits the use of roller rock bits. 

For straight hole drilling the turbine motor with the highest possible torque and the lowest possible speed is of most use. Thus the turbine motor is selected such that the motor produces the maximum amount of power for the lowest possible circulation flowrate (i.e., lowest speed). The high power increases rate of penetration and the lower speed increases bit life particularly if roller rock bits are used. 

Soft, medium and hard rock formations can be drilled with a positive displacement motor using nearly any type of rock bit. The positive displacement motor is especially adaptable to drilling with roller rock bits. 

The positive displacement motor whose performance characteristics are given in Table 4-115 is a 5 6 lobe profile motor. This lobe profile design is usually used for straight hole drilling with roller rock bits, or for deviation control operations where high torque polycrystalline diamond compact bit or diamond bits are used for deviation control operations. 

Bit Nozzle Sizes. The pressure loss through the bit must be 500 psi with a circulation flowrate of 348 gal/min with 11.6-lb/gal mud weight. The pressure loss through a roller rock bit with three nozzles is (see the section titled Drilling Bits and Downholes Tools )... 

Bottom-hole assembly (BHA) Assembly composed of the rock bit, stabilizers, reamers, drill collars, subs, etc., used at the bottom of the drillstring. 

Rock bit type (tooth shape, journal angle, cone offset, fluid circulation system)... 

There are items other than tubulars that must be fished from boreholes. These are drill bit parts, pieces of downhole tools left in the borehole and hand tools accidentally dropped in the borehole. Nearly all of these items cannot be drilled by a normal rock bit. Such items are called junk. 

The rotary drill bits are generally made from quench and tempered steel alloys such as AISI 3115 to 3120, 4620, 4815 to 4820 and 8620 to 8720. The only corrosion related problem that can arise may result from a hydrogen sulfide environment. The bearing in the roller rock bits can be damaged by H S contamination of drilling fluid. However, a well conditioned drilling fluid at... 

To remove rock bit cuttings from the bottom of the hole and carry them to the surface... 

Z. Fang, S. Peterson, and R. Denton. O-ring seal with lubricant additives for rock bit bearings. Patent GB 2318139, 1998. 

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