Page 164 - Air and Gas Drilling Manual

5-6 Air and Gas Drilling Manual
π
 
=
2
˙ w
. ) 4
D (62
 
s
s
h
( ) ( ) 
4

where K is the drilling rate (ft/hr), S    60 K 60    (5-4)
S s is the specific gravity of the solid rock cuttings.
Therefore, ˙ w is
s
π
2
.) (. ) 
˙ w =   (. 0 375 ) (62 4 2 7  30  
s     (60 ) (60 )  
4
˙ w = . 0 155 lb/sec
s
The temperature of the rock formations near the surface (geothermal surface
temperature) is estimated to be the approximate average year round temperature at
that location on the earth’s surface. Table 4-1 gives 59˚F for the sea level average
year round temperature for the mid latitudes of North America (or see Appendix D).
Therefore, the absolute temperature of the rock formations at the surface, T r, is
o
t r = 59 F
T = t + 459 67.
r
r
T = 518 67. o R
r
The bottomhole temperature is determined from the surface geothermal
temperature, the geothermal gradient, and the depth of the borehole. For this
example and the figures that follow, the geothermal gradient is approximated to be
0.01˚F/ft. Thus, T bh, is
T bh = T + 0 01 1 000. ( , )
r
o
T bh = 528 67 R
.
The average temperature of this borehole is
T + T bh
r
T =
av
2
o
.
T = 523 67 R
av
The drill pipe outside diameter, D p, is