Page 464 - Air and Gas Drilling Manual

9-68 Air and Gas Drilling Manual
p 144
P
in
in
2
P
,
in 34 070 lb/ft abs
Substituting the values of ˙ w , ˙ w , Q g, Q m, P g, P in, P i1, T g, T av1, D 7, f i1, and g
g
m
into the left side of Equation 6-82 gives
,
! 381 868
#
# dP
# 2 6 650
,
# 36 795 1 .
,
.
# 33 20 0 016 P 0 426
.
.
∀ 34 070 36 795 1 . 1
,
,
.
.
0 426 20 54 0 319 2
.
P 4

Substituting the values of H 1 into the right side of Equation 6-82 gives
,
6 650
,
1 dh 6 650
0
As can be seen in the above, the right and left hand sides of Equation 6-82 yield the
same answer. This shows that the lower limit pressure is correct.
The injection pressure while drilling at 10,000 ft of depth is approximately 237
psia. This is the approximate injection pressure for both the compressed air and the
drilling mud as they enter the surface flow lines that lead to the top of the drill
string. When drilling at a depth of 10,000 ft, the corresponding injection pressure
above is a air volumetric flow rate of 1,168 acfm (with a drilling mud volumetric
flow rate of 191 gal/hr). This compressed air injection pressure is the pressure the
compressor output must match.
Figure 9-5 shows the aerated drilling fluid (both air and mud) pressures in the
annulus and inside the drill string as a function of depth for this illustrative example
(while drilling at the depth of 10,000 ft). The figure shows the pressure at the
bottom of the annulus is approximately 3,512 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.
To drill this borehole with an aerated drilling fluid from 7,000 ft to 10,000 ft
and maintain a constant bottomhole pressure in the annulus of 3,500 psig will
require that volumetric flow rate of the compressed air be varied as the drilling
progresses. Therefore, while drilling at 10,000 ft of depth, the highest air
volumetric flow rate will be required. However, this will also require the lowest