Page 40 - Air and Gas Drilling Manual

Chapter 1: Introduction 1-17
string and the surfaces of the annulus. This resistance to flow results in pressure
losses due to friction. The total losses due to friction are the sum of pipe wall,
openhole wall, and drill bit orifice resistance to flow. This mud drilling example
shows a drilling string design which has a open orifice or large diameter nozzle
openings in the drill bit. This is reflected by the approximate 700 psi loss through
the drill bit. Smaller diameter nozzles would yield higher pressure losses across the
drill bit and higher injection pressures at the surface.

Figure 1-16: Mud drilling pressures versus depth.
The air drilling calculations are carried out assuming the drilling operation is at
sea level. There are two compressors capable of 1,200 scfm each, so the total
volumetric flow rate to the drill string is 2,400 scfm. The drill bit is assumed to
have three open orifices (~0.80 inches diameter). Figure 1-17 shows the plots of the
pressures in the compressible air as a function of depth. In the figure is a plot of the