Page 484 - Air and Gas Drilling Manual

Chapter 10: Stable Foam drilling 10-9
w ˙
Q
g
P f g T av w ˙ s H P bh P bh P s g H Q f (10-1)
ln
T g P s
Illustrative Example 10.1 below describes the implementation of the basic
planning steps Nos. 1 through 7 in Section 10.1. The non-friction solution is
applied to a deep well example. This is only done as a demonstration. It is
instructive to compare the non-friction solution results to the full friction solution
results that will be obtained later in this chapter. The approximate values for the
volumetric flow rates of the incompressible fluid (water and surfactant) and the
compressible gas (air) can be used as initial values for the complicated trial and error
solution required for the full friction solution.
Illustrative Example 10.1 The borehole to be used in this illustrative example
is the basic example used in Chapters 8 and 9. The 7 7/8 inch diameter borehole is
to be drilled out of the bottom of API 8 5/8 inch diameter, 28.00 lb/ft nominal,
Grade H-40, casing set to 7,000 ft (see Figure 10-2 for well casing and openhole
geometric configuration). The drill bit to be used to drill the interval is a 7 7/8 inch
diameter tri-cone roller cutter insert type (nozzles 9/32 inches). The anticipated
drilling rate in a sandstone and limestone sequence (sedimentary rock) is
approximate 60 ft/hr. The openhole interval below the casing shoe is to be drilled
from 7,000 ft to 10,000 ft.
The drill string for this illustrative example is made up of 500 ft of 6 3/4 inch
by 2 13/16 inch drill collars above the drill bit and API 4 1/2 inch, 16.60 lb/ft
nominal, EU-S135, NC50 (IF) to the surface. The drilling is to be carried out at a
surface location of 4,000 ft above sea level where the actual atmospheric temperature
is 60˚F. The regional geothermal gradient is approximately 0.01˚F/ft. The borehole
is to be drilled with a stable foam drilling fluid composed of fresh water (with a
surfactant) and compressed air.
A horizontal section of API line pipe that runs 100 ft from just above the BOP
stack to the burn pit is to be used as the return flow line. The surface return flow line
is API 6 5/8 inch line pipe, 32.71 lb/ft nominal, Grade B. The inside diameter of
this line pipe is 5.625 inches. Two gate valves are installed in the return flow line
at the end of the line that is attached to the BOP stack. These valves have an inside
diameter of 5 9/16 inch. Just downstream of the two gate valves is an adjustable
ball valve that can be used to place a back pressure on the flow of stable foam from
the annulus. The back pressure is assumed to be 17.0 psig.
The stable foam is to be composed of treated fresh water (with a commercial
surfactant) and compressed air. The foam quality at the top of the annulus is to be
maintained at 0.98 and the foam quality at the bottom of the annulus is to be 0.60
or greater.
Using the non-friction method, determine the approximate volumetric flow rates
of incompressible fluid (fresh water and surfactant) and of compressible gas (air)
required to give adequate bottomhole cleaning while drilling at 10,000 ft (i.e., an
3
annulus minimum kinetic energy per unit volume of approximately 3.0 ft-lb/ft ).