Page 652 - Air and Gas Drilling Manual

12-16 Air and Gas Drilling Manual
The hydraulic radius is defined for this flow path opening (area above the
horizontal dashed line) as [5]
R = flow cross - section area (12-1)
h
wetted perimeter
Note, the hydraulic radius term is not a true geometric radius.
The flow path opening cross-section area is
a fp = 11 965 in 2
.
The wetted perimeter is

s wp = 16 147 inches
.
Equation 12-1 becomes
.
R = 11 970
h
16 147
.
R h = 0 742 inches
.
The effective hydraulic diameter of the is flow path opening is [5]
d h = 4 R h

d h = 4 0 742)
(.
.
d = 2 965 inches
h
If the return flow were channeled to the annulus space above the drill string,
then the flow velocity would be higher than if the flow were channeled through an
ideal annulus space with equal openings on all sides of the drill string (assuming
constant volumetric flow rate). Thus, the carrying capacity of this higher velocity
channel above the drill string should be high. But there are protrusions of tool
joints into this “path of least resistance” which will cause cuttings to be dropped
into the low side of the borehole in spaces that do not have high velocity flows.
Therefore, saltation of cuttings into the low side of the borehole is unavoidable.
Thus, the only way to avoid unmanageable saltation accumulations in the low side
of the borehole will be to drill (directionally by sliding the drill string) for a drill
pipe joint, then pull the drill string back a stand of joints in the mast and blow the
hole clean before going back to bottom and continuing sliding and drilling. This
careful cleaning process while sliding the drill string appears to unavoidable.