STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Air and Gas Drilling 841
Air and Gas
Air and natural gas are often used as a drilling fluid with no additives placed
in the injected stream of compressed fluid. This type of drilling is also often
referred to as “dusting” because great dust clouds are created around the drill
rig when no formation water was present. However, modern air and gas drilling
operations utilize a spray at the end of the blooey line to control the dust ejected
from the well. Figure 4-185 shows a typical site plan for air drilling operations.
In air drilling operation, large compressors and usually a booster compressor
are used to compress atmospheric air and supply the required volumetric
flowrate to the standpipe in much the same way that mud pumps supply mud
for drilling. The volumetric flowrate of compressed air needed (which is usually
stated in SCFM of air) depends upon the drilling rate, the geometry of the
borehole to be drilled and the geometry of the drill string to be used to drill
the hole [64,65].
Natural gas drilling is carried out in regions where there is significant natural
gas production, and it is extremely useful in drilling into potential fire or
explosive zones such as coal seams, or oil and gas production formations. Instead
of utilizing atmospheric air and compressing the air for supply to the standpipe,
natural gas is taken from the nearby gas collection pipeline and supplied to
the standpipe, often at pipeline pressure. If high standpipe pressures are needed,
a booster is used to raise the pressure. The pressure needed at the standpipe,
and thus the need for a booster, depends upon the volumetric flow rate
required, the geometry of the borehole to be drilled, and the geometry of the
drill string to be used to drill the hole [65,66].
The volumetric flowrates required for air and natural gas drilling are basically
determined by the penetration rate and the geometry of the borehole and drill
string. There must be sufficient compressed air (or gas) circulating through the
drill bit to carry the rock cuttings from the bottom of the borehole.
The actual engineering calculations to determine the required volumetric
f lowrate and various pressure calculations will be discussed later in this section.
Figure 4-1 86. Air drilling surface equipment site plan.