Page 44 - Air and Gas Drilling Manual

Chapter 1: Introduction 1-21
Figure 1-19 shows that the temperature of the compressed air at any position in
the borehole is approximately the geothermal temperature at that depth. Thus, the
temperature of the flowing air at the bottom of the hole is the bottomhole
temperature of 160˚F. There is some local cooling of the air as it exits the open
orifices of the drill bit at the bottom of the hole. This cooling effect is more
pronounced if nozzles are used in the drill bit (when using a downhole motor ).
This cooling effect is known as the Joule-Thomson effect and can be estimated [8].
However, it is assumed that this effect is small and that the air flow returns very
quickly to the bottomhole geothermal temperature.
Figure 1-20 shows the plot of the specific weight of drilling mud for this
example calculation. The drilling mud is incompressible and, therefore, the specific
3
weight is 75 lb/ft (or 10 lb/gal) at any position in the circulation system. There is
some slight expansion of the drilling mud due to the increase in temperature as the
drilling mud flows to the bottom of the well. This effect is quite small and is
neglected in these engineering calculations.

Figure 1-20: Mud drilling specific weight versus depth.