Page 328 - Air and Gas Drilling Manual

Chapter 8: Air, Gas, and Unstable Foam Drilling 8-11
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 reference temperatures for the mid latitudes of North America (or see
Appendix D). Therefore, the absolute reference temperature of the rock formations at
the surface, T r, is
o
t = 59 F
r
T = t + 459 67.
r
r
T = 518 67. o R
r
where t r is the reference temperature (˚F),
T r is the absolute reference temperature (˚R).
It is assumed that two thirds of the borehole depth is cased. Thus, the
temperature at a depth of 6,667 ft must be determined. This temperature is
determined from the surface geothermal temperature, the geothermal gradient, and the
depth to the bottom of the cased section of the borehole. For this example, and for
the figures in Appendix E, the geothermal gradient is approximated to be 0.01˚F/ft.
Thus, the temperature at the bottom of the cased section of the borehole, T c, is
T = T + 0 01 6 667. ( , )
r
c
T = 585 34. o R
c
where T c is the absolute temperature at the bottom of the casing (˚R).
The average temperature of this cased section of the borehole is
T + T
T avc = r c
2
o
T avc = 552 00 R
.
where T avc is the average absolute temperature over the depth to the bottom of the
cased section of the borehole (˚R).
The drill pipe outside diameter of the drill pipe in the cased section of the
borehole is
d p = 450 inches
.
d p
D p =
12