Page 498 - Air and Gas Drilling Manual

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Surface Return Flow Line sec Chapter 10: Stable Foam drilling 10-23
The calculation procedure must be initiated with the known back pressure in the
surface return flow line. It is assumed that the foam will exit the well annulus and
enter the surface return flow line at the surface geothermal temperature of 44.74˚F (or
504.41 R). It is further assumed that the temperature of the foam in the surface
return flow line does not change as it flows through the return flow line (i.e., the
steel surface return flow line will be at nearly the surface geothermal temperature at
steady state flow conditions). Equation 6-102 can be altered to allow its application
to horizontal flow in the surface return flow line. This altered equation is
P s dP L s
dl (10-4)
P
B () 0
P g s
where
2 !
P g T r
Q g Q m
w ˙ t f sr P T g
P
B () ∀
s
P g T r 2 gD sr 2
Q g Q m 4 D sr
P T g
#
2
and P is the pressure at the entrance of the surface return flow line (lb/ft , abs),
sr
f is the Fanning friction factor for flow in the surface return flow line,
sr
D is the inside diameter of the surface return flow line (ft),
sr
L is the length of the surface return flow line (ft).
sr
In order to obtain the approximate value of the Fanning friction factor for
Equation 10-4, it is necessary to estimate the Reynolds number and, thus, the flow
conditions inside the surface return flow line. This is accomplished by determining
the Reynolds number near the exit end just upstream of the back pressure valve. It
is assumed this Reynolds number approximates the flow conditions throughout the
flow line.
The inside diameter of the surface return flow line, D , is
sr
.
d sr 5 625 inches
.
5 625
D sr
12
D sr 0 469 ft
.