Page 249 - Air and Gas Drilling Manual

gallons. It is standard practice to assume a 20 percent additional volume of fuel for
blowing the hole between connections and other operations on the drill rig.
Therefore, the approximate total diesel fuel needed for this compressor at the drilling
location is 432 gallons. Chapter 5: Shallow Well Drilling Applications 5-91
This example shows the advantages of the combination of reverse circulation
drilling operations and the reciprocating piston compressor. The low injection
pressure is the result of low velocity of the air as it moves down the large annulus
space between the inside of the openhole borehole and the outside of the drill string
(drill pipe and drill collars). When the air velocity is low, pipe friction losses are
low. Therefore, the weight of the air column in the annulus dominates the pressure
change from the top of the annulus to the bottom of the annulus. This is illustrated
by the fact that the injection pressure at the top of the annulus is slightly less than
the pressure at the bottom of the annulus.
On the other hand, the resistance to the return air flow which flows up the inside
of the drill string (with the entrained rock cuttings) is dominated by pipe friction
loss. The weight of the air (with the cuttings) column contributes to pressure
changes in the inside of the drill string, but pipe friction loss is the main component
of losses.
This illustrative example demonstrates the calculation procedure used to plan a
typical “deep” water well drilling operation. This particular drilling operation uses
standard API drill string components (drill collars and drill pipe) and a tri-cone drill
bit that can be used for reverse circulation operations. This is an openhole reverse
circulation operation. The injected air (at low volumetric flow rate) moves very
slowly down the openhole annulus and has very little cleaning power to counter
caving problems. Therefore, openhole reverse circulation drilling operations should
be restricted to locations that have highly competent rock formations.

Illustrative Example 5.6c Determine the total diesel fuel needed by the
portable single Tamrock Driltech Model D25K drilling rig with the on-board, Sullair
Model 840 two-stage rotary screw primary compressor unit (see Figure 4-19) used in
Illustrative Example 5.6b. This compressor system has a volumetric flow rate of
840 acfm. The prime mover for this compressor is a Caterpillar Model 3406, diesel
fueled, turbocharged motor. This prime mover is also used to operate the hydraulic
pump that in turn operates the rotary top drive system. To estimate total diesel fuel
needed at the drilling location to drill the 1,200 ft deep, 5 7/8 inch borehole, it will
be necessary to also estimate the power requirements for the operation of the
hydraulic rotary top drive system. The anticipated drilling rate of penetration is
estimated to be 40 ft/hr. Since the vertical depth to be drilled is 1,200 ft, then the
estimated actual drilling time to reach this depth is approximately 30 hours.
In Illustrative Example 5.6b the derated fixed pressure output from the fixed
ratio Sullair Model 840 rotary screw compressor with a volumetric flow rate of 840
acfm was found to be 284.1 psia. This will be the pressure output of this rotary
compressor regardless of the drilling depth (and, therefore, regardless of the drilling
time). Also in Illustrative Example 5.6b for the depth of 1,200 ft the injection
pressure (into the top of the dual wall pipe annulus) was found to be 94.1 psia.