Page 355 - Air and Gas Drilling Manual

8-38 Air and Gas Drilling Manual
2
P
,
P
a5
p a5 = = 29 454 lb/ft abs
a5
144
.
p a5 = 204 5 psia
The above value is the bottomhole pressure in the annulus.
Drill Bit Open Orifices
The drill bit used in this example operation has three open orifices, each with an
inside diameter of 0.70 inches. Thus, the inside diameter of the open orifice, D o, is
d o = 070 inches
.
d
D = o
o
12
D = 0 0583. ft
o
The total cross-sectional area of flow through the drill bit orifices, A o, is
π
 
0 0583)
A = 3   (. 2
o
4
A = 0 00802. ft 2
o
Using Equation 4-11 the specific weight of the air flow at the bottom of the annulus,
γ a5 , is

29 454) ( .
(, 1 0)
γ =
a5
( 53 36) ( 604 41)
.
.
γ a5 = 0 913 lb/ft 3
.
Equation 6-63 is used to determine the pressure inside the drill bit just above the
open orifices. The flow through open orifices is generally subsonic. Therefore,
Equation 6-63 must be used to determine the pressure upstream of bottomhole
annulus pressure. Thus, Equation 6-63 becomes