Page 235 - Air and Gas Drilling Manual

Chapter 5: Shallow Well Drilling Applications 5-77
2

f
 R
=
b


gD −
π
a
S 
 

2
(
D )
h
p
g
2 2
  2 ( D w ˙ 2 h g 2 − D ) (5-18)
p
4
  2
 
f =   D − 1   (5-19)
D 

 2 log. 
h
p
+ 1 14
   e   
 av 
The outer wall absolute surface roughness for the limestone sequence is assumed
to be 0.005 ft (see Table 8-1) [2]. The inner surface of the borehole annulus is the
outer surface of the drill pipe. The absolute surface roughness of commercial pipe,
e p, is given as 0.00015 ft [2]. The average absolute surface roughness of the annulus
is approximated by the calculating the surface area average of these surface
roughness. Thus, the value for e av is
π
π
 
eDH + e   DH
2
2
oh   h p   p
4
4
e =
π
π
av
  DH +   2
2
  h   DH
p
4
4
The above reduces to
π
π
   
e D 2 + e D 2
oh   h p   p
4
4
e = (5-20)
π
π
  2 +   2
av
  D h   D p
4
4
Thus, e av, is
π
π
  2   2
0
(.005 )   ( .021 ) + (.0 00015 )   (.458 )
1
0
e = 4 4
π
π
av   2   2
  (. 1 021 ) +   ( . 0 458 )
4
4
.
e av = 0 0042 ft
Equation 5-19 becomes