Page 20 - Air and gas Drilling Field Guide 3rd Edition

2.1 Rotary Drilling 11

is carried out almost exclusively in sedimentary rock. Boreholes for the recovery
of geothermal steam and water are constructed in all three rock types. The rotary
drilling method requires the use of a rock cutting or crushing drill bit. Figure 2-1
shows a tungsten carbide insert tricone roller cone bit. This type of drill bit uses
more of a crushing action to advance the bit in the rock (see Chapter 4 for more
details). These bits are used primarily to drill medium hard sedimentary rock.
To advance the drill bit in rock requires the application of an axial force on
the bit (to push the bit into the rock face), torque on the bit (to rotate the bit
against the resistance of the rock face), and circulating fluid to clear the rock cut-
tings away from the bit as the bit generates more cuttings with its advance (see
Figure 2-2). If the axial force is missing and the other two processes are operating,
then the bit will not advance. Likewise, if torque is not present and the other two
processes are operating, then again the bit will not advance. However, if circula-
tion is not present and the other two processes are present, the drill string
will likely be damaged. This short discussion emphasizes the critical nature of
the circulating system.

FIGURE 2-1. Tungsten carbide insert 7 7/8 inch (200.1 mm) tricone roller cutter bit IADC Code
627 (courtesy of Hughes Christensen Incorporated).