Page 234 - Air and Gas Drilling Manual

5-76 Air and Gas Drilling Manual
.
This is the pressure at the bottom of the inside of the drill string just above the drill
bit. p ai = 37 7 psia
With the large center opening in the drill bit, the pressure change in the air flow
from the bottom of the annulus to the bottom of the inside of the drill string (inside
the drill bit) is assumed to be negligible. The inside of the bottom of the drill
string at the drill bit is assumed to have the same pressure as that at the bottom of
the annulus. Thus,
.
p bh = 37 7 psia
P bh = p bh 144
2
P bh = 5 421 lb/ft abs
,
The annulus of this borehole is approximated by the diameter of the borehole
(drill bit diameter) and the outside diameter of the drill pipe. The 210 ft of drill
collars is ignored. The drill pipe is API 5 1/2 inch, 24.70 lb/ft nominal, IEU, FH,
Grade E drill pipe (see Table B-4). The outside diameter of the drill pipe, D p, is

d p = 550 inches
.
d p
D =
p
12
.
D p = 0 458 ft
The injection pressure, P in, into the top of the annulus space is determined from

  2 aH   05 .
a
 2 + 2 e  T av 
 P bh b T av  −  1  
a
   
P in =  2 aH  (5-16)
 a 
 e T av 
 
 
where
S g
a a = (5-17)
R