Page 81 - Air and Gas Drilling Manual

Chapter 3: Downhole Equipment 3-9
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 [4].

a
b
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 [4].

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