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154 Part II Gas Drilling Systems

drill cuttings removal, this theory explains why the hole cleaning is still a
problem even though the gas flow rate is high enough to remove cuttings.
The typical values for oil‒gas interfacial tension and oil density are
3
20 dynes/cm and 45 lbm/ft , respectively. Equation (7.1) gives the
3
3
minimum kinetic energy value of 1.73 lbf-ft/ft (<3 lbf-ft/ft ). This num-
ber implies that the required minimum gas kinetic energy in oil-influx
wells is approximately half of that in water-influx wells, meaning that it is
easier to clean holes with oil influx than holes with water influx. This
explains why oil influx is not a significant problem in air/gas drilling.
The first solution to water removal is increasing the gas injection rate
to obtain the minimum kinetic energy required to lift water. The kinetic
energy per unit volume of gas can be expressed as

2
g
E k = ρ v g (7.2)
2g c
where
3
E k = kinetic energy of gas, lbf-ft/ft or N-m/m 3
3 3
ρ g = density of gas, lb/ft or kg/m
v g = gas velocity, ft/s or m/s
2 2
g c = 32.17 lbm-ft/lbf-s or 1 kg-m/N-s
To evaluate the gas kinetic energy E k in Eq. (7.2) at a given gas flow
rate and compare it with the minimum required kinetic energy E km in
Eq. (7.1), the values of gas density ρ g and gas velocity v g need to be
determined. The expressions for ρ g and v g can be obtained from the ideal
gas law:
2:7S g p
ρ = (7.3)
g
T
and

v g = 3:27 × 10 −2 TQ g (7.4)
Ap

where
S g = gas specific gravity (air = 1)
p = in situ pressure, psia or kPa
o
o
T = in situ temperature, Ror K
Q g = gas flow rate, scf/min or scm/min
2
A = cross-sectional area of annular space, in or m 2