Page 80 - Air and Gas Drilling Manual

3-8 Air and Gas Drilling Manual
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 semi-
infinite 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).

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.
Tri-Cone Bits
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