Page 68 - Air and gas Drilling Field Guide 3rd Edition

CHAPTER

4
Downhole Equipment

Air and gas drilling operations require some special subsurface equipment and dril-
ling 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. However, 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.

4.1 ROTARY DRILL STRING
Two general types of drill strings are used in air and gas drilling operations. The
standard drill string discussed here 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.

4.1.1 Standard Drill String
Figure 4-1 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 dril-
ling 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 threaded pin pointing up. The bit sub is
a short thick wall pipe that has a threaded box on both ends. Above the bit sub
are the drill collars. Each of the drill collars and most of the remainder of the com-
ponents in the drill string are designed with a threaded pin down and a threaded
box up. The bit sub is used to protect the bottom threads of the bottom drill col-
lar from the wear caused by the frequent drill bit changes typical for all deep dril-
ling operations. A drill collar is a thick wall pipe that provides the weight or
vertical axial force on the drill bit, allowing the drill bit to be advanced as it is
rotated (see Figure 2-2). Usually there are a number of drill collars in a drill string.
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