Page 331 - Air and Gas Drilling Manual

8-14 Air and Gas Drilling Manual
competent low fractures to non-competent highly fractured. Table 8-1 correlates
borehole wall inside absolute surface roughness values for these rock formations
categories with the absolute surface roughness values of known engineering pressure
conduits [12].
Using Table 8-1, the borehole wall absolute surface roughness for this example
borehole is assumed to be approximately
e oh = 001 ft
.
Table 8-1: Openhole wall absolute surface roughness for a rock formation types.
Rock Formation Types Surface Roughness (ft)

Competent, low fracture
- Igneous (e.g., granite, basalt) 0.001 to 0.02
- Sedimentary (e.g., limestone, sandstone)
- Metamorphic (e.g., gneiss)

Competent, medium fracture
- Igneous (e.g., granite, basalt) 0.02 to 0.03
- Sedimentary (e.g., limestone, sandstone)
- Metamorphic (e.g., gneiss)

Poor competence, highly fracture
- Igneous (e.g, breccia ) 0.03 to 0.04
- Sedimentary (e.g., sandstone, shale)
- Metamorphic (e.g., schist)

The inner surface of this annulus section is the outer surface of the drill pipe
body which has the absolute surface roughness of commercial steel pipe, e p. The
absolute surface roughness of commercial steel pipe is
e = 0 00015 ft
.
p
The average absolute surface roughness of the annulus is approximated by using
a surface area weight average relationship between the openhole surface area and its
roughness and the outside surface of the drill string and its roughness. Thus, the
value for e avo is