Page 206 - Air and gas Drilling Field Guide 3rd Edition

8.2 Minimum Volumetric Flow Rate and Compressor Selection 197

Table 8-1. Open Hole Wall Absolute Surface Roughness for Rock Formation Types

Rock Formation Types Surface Roughness
Competent, low fracture 0.01 to 0.02 ft (0.003 to 0.006 m)
n Igneous (e.g., granite, basalt)
n Sedimentary (e.g., limestone, sandstone)
n Metamorphic (e.g., gneiss)

Competent, medium fracture 0.02 to 0.03 ft (0.006 to 0.009 m)
n Igneous (e.g., granite, basalt)
n Sedimentary (e.g., limestone, sandstone)
n Metamorphic (e.g., gneiss)

Poor competence, high fracture 0.02 to 0.04 ft (0.009 to 0.012 m)
n Igneous (e.g, breccia )
n Sedimentary (e.g., sandstone, shale)
n Metamorphic (e.g., schist)

roughness of the annulus can be approximated by using a surface area weight
average relationship between the open hole surface area and its roughness and
the outside surface of the drill string and its roughness. Thus, the value for e av is

2 2
p p
e r D H o þ e s D H o
h
p
4 4
e av ¼ :
p 2 p 2
D H o þ D H o
4 h 4 p
The term H 0 cancels and the aforementioned reduces to
2
e r D þ e s D 2 p
h
e av ¼ : (8-1)
2
D þ D 2 p
h
For the sandstone and limestone sequence in the section to be drilled in this well,
the open hole absolute surface roughness is assumed to be 0.01 ft (0.003 m).
Blooey Line
The calculation procedure must be initiated with the known atmospheric pres-
sure at the exit to the blooey line. For the illustrative example, it is assumed that
the air will exit the well annulus and enter the blooey line with a surface geother-

mal temperature of 44.74 F(7.08 C). It is further assumed that the temperature

of the air flow in the blooey line does not change until it exits the blooey line
(i.e., the steel blooey line will be at nearly the surface geothermal tempera-
ture at steady-state flow conditions). The standard isothermal gas pipeline flow
equation can be used to determine the major loss due to pipe wall friction [10].
This equation can be adjusted to also include the minor loss for the Tee turn at