Page 84 - Air and Gas Drilling Manual

3-12 Air and Gas Drilling Manual
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.

Figure 3-10: Single cone roller cone drill bit (courtesy Rock Bit International
Incorporated).
3.2.3 Air Hammer Bits
Percussion air hammers have been used for decades in shallow air drilling
operations. These shallow operations have been directed at the drilling of water
wells, monitoring wells, geotechnical boreholes, and mining boreholes. In the past
decade, however, the percussion air hammers have seen increasing use in drilling
deep oil and natural gas wells. Percussion air hammers have a distinct advantage
over roller cutter bits in drilling abrasive, hard rock formations.
The use of percussion air hammers (or down-the-hole air hammers) is an
acceptable option to using rotating tri-cone or single cone drill bits for air and gas
drilling operations. The air hammer utilizes an internal piston (or hammer) that is
actuated by the compressed air (or other gas) flow inside the drill string. The
internal piston moves up and down in a chamber under the action of air pressure
applied either below or above the piston through ports in the inside of the air
hammer. In the downward stroke, the hammer strikes the bottom of the upper end
of the drill bit shaft (via a coupling shaft) and imparts an impact load to the drill bit.
The drill bit in turn transfers this impact load to the rock face of the bit. This
impact load creates a crushing action on the rock face very similar to that discussed
above at the beginning of Section 3.2.2. But in this situation, the crushing action is