Page 550 - Air and Gas Drilling Manual

Chapter 10: Stable Foam drilling 10-75
Figure 10-7 shows the back pressure at the top of the annulus as a function of
drilling time for the interval from 7,000 ft to 10,000 ft. The back pressure at the top
of the annulus can be held constant at 17 psig while drilling the interval.

Figure 10-4: Drill string injection pressure as a function of drilling time (drilling depth)
for Illustrative Example 10.2.

The data in Figures 10-4, 10-5, and 10-6 allow for the selection of the primary
compressor unit to be used in the drilling operation. The primary compressor unit
for a stable foam drilling operation is used in a different manner than its traditional
use in an air drilling operation. As can be seen in Figures 10-5 and 10-6, the stable
foam drilling operation requires varying both the water and air volumetric flow rates
as the drilling progresses. These rates increase as the drilling depth is increased.
Since primary compressors output a constant volumetric flow rate, it is necessary to
measure (with a flow meter) the volumetric flow rate of air flowing to the top of the
drill string and vent the remainder of the compressor output to the atmosphere
through a vent line (assuming the required volumetric flow rate is less than the rated
compressor output).