Page 574 - Air and Gas Drilling Manual

11-12 Air and Gas Drilling Manual
small as 3 5/8 inches. The 16 inch housing outside diameter hammer can drill a
boreholes from 17 1/2 inches to 33 inches. For shallow drilling operations,
conventional air hammer bits are adequate. For deep drilling operations (usually oil
and gas recovery wells), higher quality oil field quality drill bits are required (see
Figure 3-11).
There are a variety of manufacturers of downhole air hammers. These
manufacturers use several different designs for their respective products. The air
hammer utilizes very little power in moving the piston inside the hammer housing.
For example, a typical 6 3/4 inch outside diameter air hammer with a 77 lb piston
operating at about 600 strikes per minute, will use less than 2 horsepower driving
the piston. This is a very small amount of power relative to the total needed for the
actual rotary drilling operation. Thus, it is clear that the vast majority of the power
to the drill string is provided by the rotary table. Therefore, any pressure loss (i.e.,
energy loss) due to the piston lifting effort can usually be ignored. The major
pressure loss in the flow through an air hammer is due to the flow energy losses
from the constrictions in the flow path when the air is allowed to exit the hammer
(on the down stroke of the piston). All air hammer designs have internal flow
constrictions. These flow constrictions can be used to model the flow losses
through the hammer. In most designs these constrictions can be approximately
represented by a set of internal orifice diameters in the flow passages to the drill bit.
These internal orifices are usually the ports (and associated channels) through the
piston and the orifice at the open foot valve.
Illustrative Example 11.1 Determine the injection pressure and resulting
compressor fuel consumption when an air hammer is utilized to drill the openhole
interval from 7,000 ft to 10,000 ft of the Illustrative Example 8.3 series in Chapter
8. In this illustrative example it is assumed that an 6 3/4 inch outside diameter
Halco Model D750 air hammer with a 7 7/8 inch diameter air hammer bit is used to
drill at 10,000 ft. This is a feed tube type design downhole air hammer. The
internal constrictions in this air hammer can be represented as compressed air flow
through a 1.60 inch inside diameter feed tube, then three 5/8 inch inside diameter
passages each approximately 8.0 inches in length through the hammer piston, then
through 1.4375 inch inside diameter passage approximately 11.0 inches in length
through the hammer bit, and then through three 1.125 inch inside diameter passages
each approximately 3.0 inches in length from the bottom of the main passage
through the bit to the exit orifices in the bit to the bottom of the borehole. The air
hammer piston is estimated to be operating at about 216 strokes per minute. In the
Illustrative Example 8.3 series two semi-trailer mounted Dresser Clark Model CFB-
4, four-stage, reciprocating piston compressors were selected. Each of these
compressors were driven by a Caterpillar Model D398 diesel fueled prime mover.
These two compressor units provide a total of 2,400 acfm to the borehole. The
drilling location is at 4,000 ft above sea level. The annulus solution given in
Illustrative Example 8.3b will be used as the starting point for this example.
In Illustrative Example 8.3b the bottomhole annulus pressure was determined.
This annulus bottomhole pressure was found to be
.
p a5 204 5 psia