Page 201 - Air and Gas Drilling Manual

T =
r
o
T = 504 37.
R
r r t + 459 67. Chapter 5: Shallow Well Drilling Applications 5-43
T bh = T + 0 01 2 400. ( , )
r
o
T = 528 37 R
.
bh
The average temperature of this borehole is
T + T
T av = r bh
2
o
.
T av = 516 37 R
Using the above values, Equation 5-1 becomes
 2 (. 0 029 ) ( , 2 400 )  . 05


[
. (516 37
. (516 37
P bh =  ( , 1 827 ) 2 + 617 6 . ) 2 ] e 516 .37 − 617 6 . ) 2 
 


2
P bh = 7 474 lb/ft abs
,
P bh
p =
bh
144
.
p bh = 51 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 = 51 9 psia
.
P = p 144
bhi bhi
2
P bhi = 7 474 lb/ft abs
,