Page 234 - Shale Shakers Drilling Fluid Systems

CHAPTE R THIRTEE N

Solids Dewatering

INTRODUCTION In many areas, the cost of drilling fluid disposal
may greatly exceed the preparation cost. These
The use of "dewatering units," as a regular part fluids may be classified as "hazardous" and, there-
of the drilling fluid treatment process on drilling fore, require disposal in hazardous disposal sites.
rigs, continues to increase. A dewatering unit, the The combined cost of transportation and disposal
final phase of a solids control program, is designed can run into the hundreds of dollars per barrel.
to discard no free liquid. A solids control program The chemicals in the fluid, such as caustic soda,
in which a dewatering unit is used is referred to oil, or salt, are usually major constituents of the
as a "closed-loop system" (CLS), "sumpless sys- drilling fluid. By removing the solids from the fluid
tem," or "chemically enhanced centrifugation" (CEC). and adjusting the chemistry, the remaining water
These units can process drilling fluid from the can be reused in the drilling fluid.
active mud system or underflow from the solids A variety of shakers, pumps, centrifuges, tanks,
removal equipment (shakers, desanders, desilters, mixers, and chemicals are used in the dewatering
and centrifuges). Chemical pH adjustments are process. The solids are removed from the waste
made to the excess mud: flocculants and coagu- fluid to minimize the volume of waste and reduce
lants are added to the input stream of the dewa- the cost of disposal. Although the final result may
tering unit. The flocculated solids are usually re- be approximately the same, the solids removal
moved by a high-speed (high-gravity) centrifuge. cost and techniques used may vary considerably.
Colloidal-rich discards typically contain approxi- The fluid from which the solids are to be re-
mately 50% by volume water but can have as little moved may be any type of drilling fluid, including
as 10% by volume solids and still be handled as a oil muds. The fluid may be excess native mud
solid. These solids can be handled with a front- from the active mud system or may contain con-
end loader and hauled on dump trucks, therefore, centrated solids from the underflow of mud clean-
disposal is typically not a problem. The recovered ers, dryer shakers, desilters, or centrifuges. The
liquid is usually treated and returned to the active mud type may be native mud, low solids non-
mud system. Treating chemicals, including acids, dispersed mud, highly treated lignosulfonate mud,
polymers, strong bases, and clarifying agents, should salt muds, or even oil muds. Generally, the more
be carefully selected and additions carefully moni- highly treated the mud system, the more compli-
tored in order to prevent personnel injury, over- cated and expensive it is to dewater and return a
treatment, and excessive treatment cost. clean, usable water.
Water-based muds vary from lightly treated "spud
mud" to highly treated, saturated salt mud. The
PROCEDURE lightly treated muds are simple to dewater and the
solids can often be disposed of without restric-
Dewatering of drilling fluids while drilling is the tions. Polyacrylamide treated muds are usually the
final step when removing solids from excess fluid. simplest to treat because they have a low pH and
All solids are removed from the drilling fluid, may contain the same type polyacrylamide that is
related sludge, and waste water. The recovered used as the flocculant. The process becomes more
water is normally reused as treatment water in the difficult and costly as the chemical content of
active mud system. This process is now relatively the mud increases. Saturated salt mud is more ex-
common in environmentally sensitive areas. Al- pensive to process because the removed solids
though the process is expensive, it can be cost ef- contain excessive amounts of salt, and the water
fective in areas where disposal costs are excessive. cannot be disposed of because of the high salt
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