Page 222 - Air and Gas Drilling Manual

5-64 Air and Gas Drilling Manual
rates for the depths 500 ft, 1,000 ft, 1,500 ft, 2,000 ft, 2,500 ft, and 3,000 ft. In
Illustrative Example 5.4, the drill bit is assumed to have a rate of penetration (or
drilling rate) in the sedimentary rock of 78.4 ft/hr, which gives a solids weight rate
of flow in the inside of the drill pipe of 3.00 lb/sec. The example above
demonstrates how the curves in Figures 5-16, 5-17, and 5-18 were developed. The
solids weight rate of flow for the example is approximately 3.0 lb/sec. Entering
Figure 5-18 with the above example drill pipe inside diameter of 4.670 inches and
moving vertically upward in the figure to the 1,000 ft depth curve, the same
approximate minimum volumetric flow rate of 472.2 scfm can be obtained.
All of these curves are created for API standard conditions and sedimentary rock.
Figure 5-16 was created using a solids weight rate of flow of 1.0 lb/sec. Figure 5-17
was created using a solids weight rate of flow of 2.0 lb/sec. And Figure 5-18 was
created using a solids weight rate of flow of 3.0 lb/sec. It can be seen by comparing
these figures that the air minimum volumetric flow rates do not vary significantly
from figure to figure. The weight of the solids being carried to the surface do not
dominate the resistance to the air flow as in the direct circulation model. Return air
flows at high average velocity inside the drill pipe (relative to return flow in the
annulus in the direct circulation model). Therefore, reverse circulation return flow
resistance is dominated by pipe wall friction. This reverse circulation flow
characteristic allows for the simplification of minimum volumetric flow rate
engineering design plots. Thus, the plots in Figures 5-16, 5-17, and 5-18 can be
used to determine the approximate minimum volumetric flow rates for a wide variety
of well geometry configurations and drilling rates.
For minimum volumetric flow rates for solids weight rates of flow that are
greater than 3.0 lb/sec, use the calculation procedure in the above illustrative
example.

Illustrative Example 5.5a Determine the approximate reverse circulation
minimum volumetric flow rate of air required to drill a 12 1/4 inch openhole
borehole (12 1/4 inch drill bit diameter) with a drill string composed of 210 ft of 10
inch by 4 inch drill collars (see Table B-1) above the drill bit and API 5 1/2 inch,
24.70 lb/ft nominal, IEU, FH, Grade E drill pipe above the drill collars to surface
(inside diameter of drill pipe body 4.670 inches, see Table B-4). The anticipated
drilling rate is assumed to be 30 ft/hr and the maximum depth of the well is 2,400
ft. The formation to be drilled is a competent unfractured limestone sequence
(sedimentary rock). The drilling location (where the drill rig will sit on the surface)
is at 2,000 ft above sea level (in the mid latitudes of North America) and the day
time air temperature is approximately 80˚ F.
This well geometry is typical for large diameter deep water wells. Such wells
are drilled in the Great Plains of North America and particularly in the many fringe
areas around the great deserts of the world. This borehole configuration is also used
in mining and geotechnical drilling operations. Using reverse circulation with a
conventional drill string (drill collars and drill pipe) is restricted to geologic
provinces that are composed of competent rock formations and are not prone to
caving. Such drilling operations, particularly those in remote areas, are more
efficiently run with air drilling technology.