TABLE OF CONTENTS 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. 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.
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.
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 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 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 components in the drill string are designed with a...
