Page 330 - Air and Gas Drilling Manual

2
2

2 019)
(.
0 017
.
 53 36.
b

ac
[
]
2
π
 
.
.
(
− 0 375.
 10.
)
2 32 2) (
2
2
2
− 0 375)
(.
(.
0 656)
 
4
b ac = = 209 1 0 656 Chapter 8: Air, Gas, and Unstable Foam Drilling 8-13
.
Equation 6-57 is used to obtain the pressure at the bottom of sequential sections
of a borehole that has a different geometric configuration and/or friction loss
characteristics. Thus, for this illustrative example, Equation 6-57 can be used to
find the pressure in the annulus at the bottom of the cased section of this borehole.
This pressure is
 2 (. 0 028 ) ( ,000 )  . 05
10
[


. (552
. (552
P =  ( , 2 116 ) 2 + 209 1 .00 ) 2 ] e 552 .00 − 209 1 .00 ) 2 
ac
 


2
,
P ac = 8 309 lb/ft abs
P ac
p =
ac
144
.
p ac = 57 7 psia
The bottom one-third of the borehole depth is an openhole. Thus, the
temperature at a depth of 10,000 ft must be determined. This temperature is
T bh = T + 0 01 10 000. ( , )
r
o
.
T bh = 618 67 R
The average temperature of this openhole section of the borehole is
T + T
T avo = c bh
2
o
T avo = 602 01 R
.
The bottom one third of this example borehole is an openhole section. The
openhole annulus wall absolute surface roughness for the sandstone and limestone
sequence, e oh, can be assumed to be similar to the inside surface of a commercial
concrete pipe. Similar correlations can be made for openhole borehole walls in a
variety of rock types and rock physical characteristics. Rock types can be igneous,
sedimentary, and metamorphic. The rock physical characteristics vary between