Page 160 - Shale Shakers Drilling Fluid Systems
P. 160
SOLIDS CONTROL EQUIPMENT 143
GUMBO REMOVAL were dumped once or twice per hour. Today, with
the use of fine-mesh screens and expensive waste
Gumbo is formed in the annulus from the ad- disposal, such dumping is cost prohibitive.
herence of sticky particles to each other. It is usu-
ally a wet, sticky mass of clay, but finely ground
limestone can also act as gumbo. Enough gumbo DEGASSERS
can arrive at the surface to lift a rotary bushing
from the rotary table. This sticky mass is diffi- Drilling fluids usually encounter hydrocarbon
cult to screen and, in areas where gumbo is preva- gasses. At the bottom of a borehole, these gasses
lent, it is sometimes removed before it reaches may partially dissolve in the drilling fluid or com-
shale shakers. press to occupy very small volumes. As the fluid
Most gumbo removal devices are fabricated at rises to the surface, the hydrostatic pressure is
the rig site, have many different shapes, and are reduced and these gasses expand and evolve from
usually slides. Because gumbo does not stick to the drilling fluid. They must be removed from the
stainless steel, one effective device is a series of surface mud system or pump operations will be-
•f- to 1-inch diameter stainless steel rods arranged come erratic.
to slope downward from the end of the flow line. Shale shakers are not effective mechanisms to
The rods are separated by one to two inches and separate gas bubbles from a viscous drilling fluid.
are about four to six feet long. Gumbo leaving Degassers should be installed immediately down-
the flow line slides down the rods and is sent stream of the shale shakers, and gas should be re-
to disposal waste. Drilling fluid easily passes moved before drilling fluid enters centrifugal pumps.
through the bars and is sent to the scalping or the Hydrocyclone performance requires a continuous
main shakers. fluid volume and head generated by the centrifu-
Devices and machines designed specifically to gal pumps. A gaseous drilling fluid reduces cen-
remove gumbo are available from several manu- trifugal pump performance and may even "vapor-
facturers. One of these machines uses a series of lock" the pump so that it prevents the movement
steel bars formed into an endless belt. The bars of fluid. Even positive displacement rig pumps are
are separated by a space of 1-2 inches and are affected by gaseous drilling fluid.
disposed perpendicular to the fluid flow. The unit Two types of degassers are available: atmo-
moves gumbo to the discharge end of the ma- spheric and vacuum-type. Atmospheric degassers
chine. Another machine uses a 5 or 10 mesh syn- have a submerged centrifugal pump integral with
thetic belt run at an uphill pitch to convey gumbo the unit. It is placed into a spray chamber through
from a pool of drilling fluid. A counter rotating a disc valve where it strikes the inside wall of the
brush is used to clean gumbo from the underside chamber. The thin spray, combined with the im-
of the belt. pact of the fluid on the wall of the chamber, sepa-
Some shale shakers are also used as gumbo rates the gas from the fluid.
removal devices. Shakers are now available that Vacuum-type degassers separate gas from drill-
combine gumbo removal (for example, a scalping ing fluids by spreading the gas-cut fluid into thin
shaker) and a main shaker all on one skid. layers in a reduced atmosphere. The fluid usually
flows over a series of baffles, or plates. Degassed
drilling fluid is pumped through an eductor to re-
SAND TRAPS move drilling fluid from the vacuum chamber.
Equalization between degasser suction and dis-
After the drilling fluid passes through the main charge compartments is through a high weir at the
shaker, it enters the mud pit system. When top of the tanks. Degasser suctions should be lo-
80-mesh screens and coarser were routinely used, cated at the bottom of the compartments.
the sand trap performed a very useful function.
Large, sand-size particles would settle and could
be dumped overboard. HYDROCYCLONES
The bottom of a sand trap should be sloped at
about 45° to facilitate quick dumping. The trap Hydrocyclone is a general term used to de-
should not be agitated and should overflow into scribe a device where liquid swirls inside of a
the next compartment. Linear and balanced ellip- cone. The centrifugal force of the swirling liquid
tical motion shale shakers have all but eliminated moves the solids to the outside wall. In drilling
this technique. Small drilled solids generally do operations, hydrocyclones use these centrifugal
not have sufficient residence time to settle. When forces to separate solids in the 15- to 80-micron
inexpensive drilling fluid was used, sand traps range from the drilling fluid. This solids-laden fluid
