Page 73 - Air and Gas Drilling Manual

Chapter
Three

Downhole Equipment

Air and gas drilling operations require some special subsurface equipment and
drilling methods that are not normally used in rotary mud drilling operations. Deep
direct circulation operations use rotary drill strings that are similar to those used in
mud drilling. But even these drill strings are equipped with downhole tools unique
to air and gas drilling operations.
Larger diameter shallow and intermediate depth wells are usually drilled with
reverse circulation techniques. These techniques and their associated equipment are
virtually unknown to those who drill deep small diameter wells.
3.1 Rotary Drill String
There are two general types of drill strings used in air and gas drilling
operations. The standard drill string discussed below is used almost exclusively for
deep direct circulation operations. The dual wall pipe drill string is used exclusively
for intermediate and shallow depth reverse circulation operations.

3.1.1 Standard Drill String
Figure 3-1 shows a schematic of a standard rotary drill string used to drill deep
boreholes with direct circulation. Such a drill string would be used on large drilling
rigs. At the bottom of the drill string is the drill bit. The drill bit is threaded
(made-up) to a bit sub. The drill bit has a male thread or threaded pin pointing up.
The bit sub is a short thick wall pipe that has a female thread or threaded box on
both ends. Above the bit sub are the drill collars. Each of the drill collars and most

3-1
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