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132 Part II Gas Drilling Systems

Illustrative Example 6.1 (Continued )
The required minimum volumetric air flow rate at the collar shoulder is esti-
mated to be

2 2
Q g = π ð7:875Þ − ð4:5Þ ð21:58Þð60Þ = 295 ft /min = 8:35m /min
3
3
4 144
The required minimum volumetric air flow rate at the collar shoulder is con-
verted to standard condition as

Q go = ð520Þð85Þð144Þð295Þ = 1,431 scf/min = 40:52 scm/min
ð14:7Þð144Þð620Þ

The Minimum Kinetic Energy Criterion
The minimum kinetic energy criterion was established in the 1950s. The
mixture of gas and solid is treated as one homogeneous phase with mix-
ture density and velocity—that is, interactions between particles and fluids
are not considered. Several models have been presented, including those
by Martin (1952, 1953), Scott (1957), Angel (1957), and McCray and
Cole (1959). Although McCray and Cole’s model permits a constant-
percentage slip velocity of solid particles, it uses the same particle lift
criterion as Angel’s model.
The criterion for the minimum volume requirement is based on the
experience gained from quarry drilling with air. The minimum annular
velocity to effectively remove solid particles from the borehole is usually
assumed to be 3,000 ft/min, or 50 ft/sec (ft/s), under atmospheric condi-
tions (close to the standard condition of 14.7 psia at 60°F). This velocity
is believed to be high enough to remove dustlike particles in air drilling.
Although big cuttings that are not removed from the vicinity of the bit
by the circulating air are reground by the bit teeth, it would be unecono-
mical to lift large cuttings without first trying to control their initial size
at the bit.
The carrying power of air can be evaluated based on its kinetic energy
E o per unit volume of air:
γ
E go = 1 go v 2 (6.40)
2 g go