Roller cutter bits use a crushing action to remove rock from the cutting face and advance the drill bit. The weight or axial force that is applied to the drill bit is transferred to the tooth or teeth on the bit. These teeth are pointed (mill tooth bit) or rounded (insert tooth bit) and the force applied is sufficient to fail the rock in shear and tension and cause particles of the rock to separate from the cutting face. The drill bits are designed to remove a layer of rock with each successive rotation of the bit. Figure 3-5a shows the tooth of a tri-cone bit being forced against the rock face. Figure 3-5b shows the rock particles created by the failure of the rock face due to the "crushing" action of the tooth. The circulation fluid entrains these rock cuttings and carries them away from the rock face.
FORCE ON BIT
FORCE ON BIT
The roller cutter element(s) of a drill bit has a series of teeth that are designed to crush rock over the entire rock face after a single rotation of the drill bit. The repeated crushing action of the teeth in conjunction with the circulation fluid allow rock particles at the rock face to be continuously removed and the drill bit advanced.
When this crushing action takes place at the bottom of a well filled with drilling mud, the hydrostatic pressure due to the fluid column compresses the rock face and places the rock face and the rock material in the immediately vicinity of the rock face in compression. This makes the crushing action less efficient and ultimately reduces the overall drilling rate of the drill bit (for a given WOB).
When this crushing action takes place at the bottom of a well filled only with compressed air, there is little hydrostatic pressure on the rock face. Further, the drilling process has removed a column of rock (above the rock face) from an semiinfinite block of pre-stressed rock. The in-situ pre-existing stresses that were in this block of rock prior to the drilling operation and the vertical cylindrical void of the new borehole create a thin tension stress field in the rock material just below the rock face (see Figure 3-6). This makes the crushing action very efficient and ultimately increases the overall drilling rate of the drill bit (for a given WOB).
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Figure 3-6: Schematic of tension rock face at bottom of borehole.
The above argument explains why the drilling rate for an air and gas drilling operation is approximately two to four times greater than that of a similar mud drilling operation (given similar geology and drilling parameters).
There are four styles of roller cutter drill bits. These are quad-cone drill bits, tri-cone drill bits, dual-cone bits, and single cone bits. Quad-cone drill bits and dual-cone drill bits are used for special mud drilling operations and have little application in air and gas drilling. Tri-cone drill bits are used extensively in air and gas drilling operations.
The most widely used roller cutter bit is the tri-cone drill bit. The tri-cone drill bit has three roller cutter cones. Each of these cones has a series of teeth that crushes rock on the rock face as they roll over the face when the drill string (and thus the drill bit) is rotated. Figure 3-7 shows a schematic of the configuration of the tri-cone bit. Figure 3-7a shows a cross-section view of a cone (for a soft rock formation). Figure 3-7b shows the three roller cones at the bottom of a borehole. This latter schematic shows the offset of the cones. The offset is the degree the cones of the bit are designed to depart from a true rolling action on the rock face. Offset indicates that two or more cones of the bits do not have their centerlines of rotation passing through the center of bit rotation. Figure 3-7b shows a bit with no cone centerline passing through the center of the bit rotation ,
Figure 3-7: Schematic of the three cones of the tri-cone bit on the bottom of the borehole, a) cross-section of cone, and b) top view of action of the three cones during rotation .
Tri-cone drill bits can be used to drill a wide variety of rock formations. Figure 1-1 in Chapter 1 shows a typical mill tooth tri-cone drill bit. These drill bits are used to drill soft to medium rock formations. The "mill tooth" term refers to the fact that the teeth on the cones are machined into the cone as an integral part of the cone. Figure 3-8 shows a typical insert tri-cone drill bit. The "insert" term for this bit refers to the fact that the teeth are tungsten carbide studs that are inserted (shrink fit) into holes drilled in the cone material. The insert tri-cone drill bits are used to drill medium to hard rock formations.
Most tri-cone drill bits are manufactured to be used with drilling mud. But most manufactures produce a few of their drill bit styles for air drilling operations. These tri-cone drill bits are designed with special internal air passages to provide the bit bearings with the appropriate cooling from the less dense compressed air or natural gas. Figure 3-9 shows a cut-a-way of a tri-cone drill bit used for air operations.
Tri-cone drill bits used for air and gas drilling are usually designed with increased gauge protection (relative to their mud drilling counterparts). This gauge protection allows air bits to drill long abrasive sections without appreciable loss of gauge. However, it should be noted that some gauge loss will always occur in hard abrasive formations. It is good practice to design the well profile (i.e., borehole diameters and associated casing diameters) in such a manner that long sections having hard abrasive formations can be drilled with either the drill bit diameter sequence of 6 inch, 6 1/8 inch , 6 1/4 inch, 6 1/2 inch, 6 3/4 inch, or the sequence of 7 1/8 inch, 7 3/8 inch, 7 5/8 inch, and 7 7/8 inch, or the sequence of 8 3/8 inch, 8 1/2 inch, 8 3/4 inch, 9 inch. Using one of these drill bit diameter sequences allows anticipation of loss of gauge. The top of a long hard abrasive section can be drilled with a 6 1/2 inch diameter drill bit and when there is a bit change, followed by a 6 1/4 inch diameter drill bit, and then near the bottom of the section followed by a 6 1/8 inch diameter drill bit for the last bit change.
Most air or natural gas drilling operations use insert tri-cone drill bits. Even though previous drilling operations with mud may have shown that a mill tooth bit had been successful in drilling a particular rock formation, the mechanics of the rock cuttings creation process at the bottom of the air borehole require that an insert bit be used in order to generate smaller rock cuttings. The smaller the rock cuttings generated by the drill bit, the more efficient the rock cutting creation and transport of cuttings particles from the bottom of the borehole.
Nearly all tri-cone drill bits are equipped to accept nozzle inserts in three open orifice flow channels in the drill bit body. Nozzles of various sizes (in 32nds of an inch) are extensively used in mud drilling operations. Standard practice for vertical direct circulation air or natural gas rotary drilling operations is to use tri-cone drill bits with open orifices (i.e., no nozzles). Thus, for well planning calculations (to be discussed in Chapters 8 to 11) it is important to ascertain from the drill bit manufacturer the open orifice minimum inside diameters for the drill bits to be used in the operation.
There are special reverse circulation tri-cone drill bits. These are fabricated using the same mill tooth and insert tooth cone designs as the direct circulation drill bits discussed above. Figure 1-9 shows the schematic of the inside flow channel of a reverse circulation tri-cone drill bit. This large orifice allows the return flow of drilling fluid and entrained rock cuttings to flow from the annulus through the large orifice in the bit body to the inside of the drill string. These reverse circulation drill bits are manufactured by geotechnical and mining equipment companies.
Single Cone Bits
There are single cone or "monocone" drill bits. Unlike tri-cone drill bits that drill by a crushing action, the single cone bits drill by a scraping action. Thus, the single cone drill bits utilize wear resistant tungsten carbide inserts in the cutting structure. These drill bits are most effective in smaller diameters (~ 2 3/4 inch to 6 1/8 inch) and, with the appropriate cutting structure, are suitable for drilling soft as well as medium and hard rock formations.
The principal advantage of the single cone drill bits is the large size of the support bearing of the cone and the tungsten carbide inserts in the small drill bit diameters. Small diameter tri-cone drill bits are very fragile and subject to pinching and bearing damage if forced into an out of gauge borehole or used to ream an out of gauge borehole. These single cone drill bits are not subject to pinching and other damage when used to ream out of gauge boreholes. It is therefore good drilling practice to use single cone drill bits to drill small diameter sections in deep wells.
Figure 3-10 shows a typical single cone drill bit. Single cone drill bits are also equipped to accept nozzle inserts in three open orifice flow channels in the drill bit body. But like the tri-cone drill bits used for air and gas drilling operations, it is standard practice to use single cone drill bits with open orifices (i.e., no nozzles).
Thus, for well planning calculations (to be discussed in Chapters 8 to 10) it is important to ascertain from the drill bit manufacturer the open orifice minimum inside diameters for the drill bits to be used in the operation.
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