Page 43 - Air and Gas Drilling Manual

1-20 Air and Gas Drilling Manual
The heated drilling mud flowing in the annulus heats the outside of the drilling
string and this in turn heats the drilling mud flowing down the drill string. Because
of its good heat storage capabilities, the drilling mud exits the annulus with a
temperature greater than the injection temperature but less than the bottomhole
temperature. In this example, the temperature of the drilling mud exiting the
annulus is approximately 130˚F.
Figure 1-19 shows the plots of the temperature in the compressible air drilling
fluid as a function of depth. The compressed air drilling fluid is significantly less
dense than drilling mud. Thus, compressed air has poor heat storage qualities
relative to drilling mud. Also, compressed air flowing in the drilling circulation
system is flowing rapidly and therefore the flow is turbulent inside the drill string
and in the annulus. Turbulent flow is very efficient in transferring heat from the
surface of the borehole to the flowing air in the annulus and in the inside the drill
string. Assuming the compressed air entering the top of the drill string is at 60˚F
the heat rapidly transfers to heat (or cool) the air flow in the well. Under these
conditions the compressed air exiting the annulus has approximately the same
temperature as the air entering the top of the drill string.

Figure 1-19: Air drilling temperature versus depth.