Page 168 - Air and Gas Drilling Manual

5-10 Air and Gas Drilling Manual
ft −
lb
= 3 002
KE
.
bh
3
ft
The trial and error process for the above example requires an iterative selection
process to determine the value of q g that will give a kinetic energy per unit volume
3
at the bottom of the borehole in the annulus that is equal to 3.0 ft-lb/ft . In this
3
example the value of q g that will give a kinetic energy value equal to 3.0 ft-lb/ft is
320.9 scfm.
Using the calculation procedure as given above, plots can be prepared for the
above example geometry for a variety of drilling depths and drilling rates (for API
standard conditions and sedimentary rock). Figure 5-1 gives the air minimum
volumetric flow rates for a 2 3/8 inch drill pipe in a 4 1/2 inch open borehole. The
figure shows the minimum volumetric flow rate of 320.9 scfm for the drilling rate of
30 ft/hr and depth of 1,000 ft.
Figures 5-1 to 5-9 give the air minimum volumetric flow rates at API
Mechanical Equipment Standards standard atmospheric conditions and for
sedimentary rock for a variety of shallow borehole and drill string geometric
configurations. The openhole borehole sizes are 4 1/2 inch, 4 3/4 inch, 5 7/8 inch, 6
inch, 6 1/4 inch, 6 1/2 inch, 6 3/4 inch, 7 5/8 inch, and 7 7/8 inch. The drill pipe
sizes are 2 3/8 inch, 2 7/8 inch, and 3 1/2 inch. The figures are developed for
various drilling rates (e.g., 30 ft/hr, 60 ft/hr, and 90 ft/hr) and assuming an openhole
borehole wall absolute surface roughness of 0.005 ft.
Shallow depth direct circulation operations generally utilize smaller drill bit
diameters. Also, the tri-cone drill bits used in these operations have a single large
water course (air course) opening in the center of the drill bit body (as opposed to tri-
cone drill bits for oil field operations that have three small water course orifices for
jetting nozzles). Direct circulation shallow boreholes are usually drilled with small
drill pipe and small drill bits like those given in Figures 5-1 to 5-9. Larger diameter
shallow boreholes are usually drilled with reverse circulation operations. The
exception to this general rule is oil field drilling where blowout safety considerations
dictate that direct circulation be used on nearly all intervals.
Direct circulation requires lifting the rock cuttings in the annulus of the borehole
as the air flow returns to the surface (flowing vertically upward). The annulus is a
large cross-section opening relative to the cross-sectional area of the inside of the
drill string. Thus, the resistance to the upward flow in the annulus is dominated by
the weight of the rock cuttings being lifted.
Illustrative Example 5.2a Determine the direct circulation approximate
minimum volumetric flow rate of air required to drill a 4 3/4 inch openhole borehole
(4 3/4 inch drill bit diameter) with a drill string composed of 120 ft of 3 1/2 inch by
1 1/2 inch drill collars (see Table B-1) above the drill bit and API 2 3/8 inch, 4.85
lb/ft nominal, EU, NC 26, Grade E drill pipe above the drill collars to the surface
(see Table B-4). The anticipated drilling rate is 30 ft/hr and the maximum depth of
the well is 1,200 ft. The formations to be drilled are a sequence of competent
unfractured gneisses (metamorphic rock). The drilling location is at 6,000 ft above
sea level (in the mid latitudes of North America) and the day time air temperature is
approximately 70˚F. This is a typical mining or geotechnical borehole geometry.