Page 32 - 4925

role it plays in stabilizing the hole is vital.

Circulating Equipment

Mud circulates through many pieces of equipment, all of which play an
important role. Circulating equipment includes the mud pump, the discharge line, the
standpipe, the rotary hose, the swivel (or top drive), and the kelly (on rigs with a
rotary- table system), the drill pipe, the drill collars, the bit, the annulus, the return
line, the shale shaker, the desilter, the desander, the mud tanks, and the suction line.
The mud pump takes mud from the mud tanks and sends it out a discharge line to a
standpipe. The standpipe is a steel pipe mounted vertically on one leg of the mast or
derrick. Mud flows out of the standpipe and into the rotary hose, which is connected
to the swivel on rotary-table system rigs or to the top drive. Mud goes down the kelly
on rigs with a rotary table; on rigs with a top drive, mud goes through passageways
inside it. Once it leaves the kelly or the top drive, mud flows down the drill stem, out
the bit. It does a sharp U- turn and heads back up the hole in the annulus. The annulus
is the space between the outside of the drill string and sides of the hole. As it flows up
the annulus, the mud carries the cuttings made by the bit.
Finally, the mud leaves the hole through a steel pipe called the “mud return
line” and falls over a vibrating, screenlike device called the “shale shaker”. The shale
shaker is appropriately named, for it rapidly vibrates or shakes as the mud returning
from the hole falls over it. The shale shaker acts like a sifter and screens out the
cuttings. Except in environmentally sensitive areas on land, the cuttings fall into the
reserve pit, the earthen pit excavated when the site was being prepared. In areas
where the contractor cannot use a reserve pit because of environmental reasons, the
shaker dumps the cuttings into a special receptacle.
Later, the cuttings are properly disposed of. Offshore, the cuttings are usually
dumped into a barge to be transported to a land site for proper disposal. In either case,
the mud drains back into the mud tanks where the mud pump recycles it downhole.
The circulating system is essentially a closed system. The system circulates the mud
over and over throughout the drilling of the well. From time to time, however, crew
members may add water, clay, or other chemicals to make up for losses or to adjust
the mud’s properties as the hole drills into new and different formations.

Auxiliary Equipment

Several pieces of auxiliary equipment keep the mud in good shape. The shale
shaker sifts out the normal-sized cuttings. Sometimes, though, the bit creates
particles so small that they fall through the shaker with the mud. So, after the mud
passes through the shale shaker, the system sends the mud through desanders,
desilters, mud cleaners, and mud centrifuges. These pieces of equipment remove fine
particles, or small solids, to keep them from contaminating the drilling mud.

A degasser removes small amounts of gas that enter the drilling mud as it
circulates past a formation that contains gas. A degasser is used when the amount of
gas is not enough to make the well a producer; instead, it is just enough to contami-

31