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8 Part I Liquid Drilling Systems

Figure 1.8 Cutoff views of bit nozzles. (Courtesy of Schlumberger.)

The drill bit is connected to drill collars through a bit sub, which is a
short, thick wall pipe that has a threaded box on both ends. The bit sub
is used for protecting the bottom threads of the bottom drill collar from
wear due to the frequent drill bit connections. A drill collar is a thick
wall pipe that gives weight to the drill bit, allowing the drill bit teeth to
cut into the formation rock. Drill collars form a bottomhole assembly,
with stabilizers installed at different distances from the bit (Figure 1.9) to
control the well trajectory. The number of drill collars in a BHA depends
on how much weight must be applied to the bit to make it advance effi-
ciently (Bourgoyne et al., 1991). For directional and horizontal drilling,
the BHA also includes mud motors and tools for measurements, such as
measurements while drilling (MWD) and logging while drilling (LWD).
The major components of a mud motor are shown in Figure 1.10. A
pressure drop of a few hundred psi is required at the motor to generate
torque for the rotating drill bit.
The drill pipe joints are above the BHA. The threads of the drill
collar connections are usually not the same as the threads at the ends of
the drill pipe joint. A special crossover sub is used to connect the drill
pipe to the collars. The number of drill pipe joints used depends on the
depth of the borehole. Conventional drill pipe joints are not designed
to work in compression. After being pulled out of the hole for bit
changes or well logging operations, the drill pipe string is broken into
stand-by-stand sections and placed vertically between the drilling floor