Page 41 - Air and Gas Drilling Manual

1-18 Air and Gas Drilling Manual
pressure inside the drill string. The pressure is approximately 260 psia at injection
and 270 psia at the bottom of the inside of the drill string just above the bit orifices.
Also in the figure is a plot of the pressure in the annulus. The pressure is
approximately 260 psia at the bottom of the annulus just below the bit orifices and
14.7 psia at the end of the blooey line at the surface (top of the annulus).
As in the mud drilling example, the pressures in Figure 1-17 reflects the
hydrostatic weight of the column of compressed air and the resistance to air flow
from the inside surfaces of the drill string and the surfaces of the annulus. This
resistance to flow results in pressure losses due to friction. In this example the fluid
is compressible. Considering the flow inside the drill string, the hydrostatic weight
of the column dominates the flow (relative to friction losses) and this results in the
injection pressure at the surface being less than the pressure at the bottom of the drill
string (inside the drill string above the bit open orifices).

Figure 1-17: Air drilling pressures versus depth.