Page 365 - Air and Gas Drilling Manual

8-48 Air and Gas Drilling Manual
force a volumetric flow rate of 2,400 acfm through the circulation system. This
injection pressure is the pressure the compressor output must match.
Figure 8-7 shows the air flow pressures in the annulus and inside the drill string
as a function of depth for this illustrative example. The figure shows the pressure at
the bottom of the annulus is approximately 205 psia (p a5 above). If a target oil or
natural gas rock formation pore pressure at the bottom of the borehole is above this
value, the oil or natural gas will flow into the borehole as the drill bit is advanced
into the producing rock formation. This would be underbalanced drilling. If the
pore pressure is less than this value, rock cuttings from the advance of the drill bit
will be forced into the exposed pores around the bottom of the borehole resulting in
formation damage.

Figure 8-7: Pressure profile in the inside of the drill string and in the annulus for
Illustrative Example 8.3b (q g = 2,400 acfm and a drilling depth of 10,000 ft).