Page 194 - Air and Gas Drilling Manual

5-36 Air and Gas Drilling Manual
The average temperature of this borehole is
T +
r
=
T
av
2 T bh
o
T av = 503 27 R
.
Using the above values, Equation 5-1 becomes
 2 (. 0 024 ) ( , 1 200 )   . 05

[
. (503 27
P bh =  ( , 1 695 ) 2 + 627 5 . ) 2 ] e 503 .27 − 627 5 . ) 2 
. (503 27
  

2
,
P bh = 4 732 lb/ft abs
P bh
p bh =
144
.
p bh = 32 9 psia
This is the pressure at the bottom of the borehole in the annulus.
With the large center opening in the drill bit, the pressure change in the air flow
from the bottom of the inside of the drill string (inside the drill bit) to the bottom of
the annulus 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 bhi = 32 9 psia
.
P bhi = p bhi 144
2
,
P bhi = 4 732 lb/ft abs
The inside of the drill string is approximated by the inside diameter of the drill
pipe. The inside diameter of the API 2 3/8 inch, 4.85 lb/ft nominal, EU, NC 26,
Grade E drill pipe is 1.995 inches (see Table B-4).

d = 1 995. inches
i
d
D = i
i
12
D = 0 166. ft
i