Page 486 - Air and Gas Drilling Manual

t
.
t
T
at
T at at 60˚F . 459 67 Chapter 10: Stable Foam drilling 10-11
519 67˚R
at
Thus, P g and T g become
2
.
,
P P 1 826 6 lb/ft abs
g at
.
T g T at 519 67˚R
Using Equation 4-11, the specific weight of the gas entering the compressor is
1
(,827 ) ( . )
1
0
g
(53 .36 ) (519 . )
67
0 0659 lb/ft 3
.
g
Note the above specific weight value can also be obtained from Figure D-4 in
Appendix D.
The back pressure at the top of the annulus is 17 psig. The back pressure, P bp,
in absolute pressure is
p bp 17 0 12 685
.
.
p bp 29 685 psia
.
P p 144
bp bp
2
,
P 4 275 lb/ft abs
bp
Note that the back pressure can be adjusted to improve annulus bottomhole foam
quality and kinetic energy per unit volume values.
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 44.74˚F for average year round
temperature for a surface elevation location of 4,000 ft above sea level (for mid
latitudes of North America, or see Appendix D). Therefore, the absolute temperature
of the rock formations at the surface, T r, is

.
t r 44 74˚F