Page 324 - Air and Gas Drilling Manual

Chapter 8: Air, Gas, and Unstable Foam Drilling 8-7
assuming a minimum bottomhole kinetic energy per unit volume of no less than 3.0
3
ft-lb/ft . Also, it is assumed that the drilling is in sedimentary rock formations with
an average specific gravity of 2.7. These deep boreholes figures are developed for a
uniform borehole diameter with the top two thirds of the depth assumed to be cased
and the bottom one third assumed to be openhole. The basic equations used to
determine the minimum volumetric flow rate are derived in Chapter 6.
Illustrative Examples 8.1 and 8.2 describe the implementation of the basic
planning steps Nos. 1 through 7 given in Section 8.1.
Illustrative Example 8.1 The borehole to be used in this illustrative example
will be used as a basic example in this chapter and in Chapters 9 and 10. 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 8-4 for well
casing and openhole geometric configuration). The inside diameter of this casing is
8.017 inches. The openhole interval below the casing shoe is to be drilled from
7,000 ft to 10,000 ft. The drill string 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
at sea level (assume API Mechanical Standards standard atmospheric conditions, see
Chapter 4). The regional geothermal gradient is approximately 0.01˚F/ft. The
anticipated drilling rate of penetrations in a sandstone and limestone sequence
(sedimentary rock) is approximately 60 ft/hr. The blooey line for this drilling
operation is a single horizontal section of casing that runs 200 ft from just above the
BOP stack to the burn pit. The casing used for the blooey line is API 8 5/8 inch,
24.00 lb/ft nominal, Grade J-55, casing. The inside diameter of this casing is 8.097
inches, Two gate valves are in the blooey line at the end of the line that is attached
to the BOP stack. These valves have an inside diameter of 7 9/16 inch. This is a
typical borehole geometric configuration for an oil or natural gas recovery well. This
illustrative example will demonstrate how the approximate minimum volumetric
flow rate is determined.
The approximate minimum volumetric flow rate will be determined for the
deepest depth in the interval to be drilled. This is 10,000 ft. This will give the
largest minimum volumetric flow rate for the well. Although the details of the
borehole and drill string geometric configuration are important for determining
accurate pressures in the annulus and inside the drill string (particularly, bottomhole
and injection pressures), these details are not as important in obtaining the
approximate minimum volumetric flow rate.
For this example, this slightly larger inside diameter will be ignored relative to
the openhole 7.875 inch borehole. Thus, the entire length of the borehole will be
assumed to have a uniform inside diameter of 7.875 inch. Instead of the actual
depth to the bottom of the casing for this particular example, a depth of two thirds
of the total depth will be used for the cased section of the well (~6,667 ft). Further,
for this calculation the major and minor friction losses in the blooey line will be
ignored (ignoring the Tee at the top of the annulus and the blooey line length). The
drill pipe outside diameter changes at the tool joints will be ignored and the 500 ft
of drill collar outside diameter change will ignored. Therefore, the outside of the