Page 179 - Shale Shakers Drilling Fluid Systems
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162 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
liquid from fine particles. Chemical enhancement, as close to the upstream compartment wall as
used to increase effective particle size (see Chap- possible. Sand traps are very effective when wa-
ter 13), reduces water and waste volumes. When ter or seawater are used as drilling fluid. When
used for processing hydrocyclone discards or other drilling fluids are treated to provide a large value
"waste," centrifuges are operated continuously and of the low-shear-rate viscosity, solids will not
the lighter "free liquid" is retained for dilution. settle efficiently. This is most noticeable when the
shale shaker possum belly is emptied into the sand
In closed-loop systems, larger high-capacity (75- trap during a trip. Shortly after the startup of the
250 gpm) decanting centrifuges are used, some- next drill bit, the unsettled solids reach the apex
times with "standard" centrifuges, to maximize of the desilters and plug them. At this time, all of
fine solids separations. the drilling fluid processed by the plugged cones
retain their drilled solids. This makes it difficult to
maintain a low level of drilled solids in the drill-
Operating Reminders ing fluid.
The degasser suction compartment tank should
1. Before startup, rotate bowl or cylinder by equalize overflow from the next compartment down-
hand to make sure it rotates freely. stream. The degasser should process more drill-
2. Start centrifuge first, before starting drilling ing fluid than is entering its suction compartment
fluid feed pump or dilution water. from the sand trap. This will cause some drilling
3. Set drilling fluid mud and dilution rates ac- fluid to flow from the downstream compartment
cording to manufacturer's recommendations, into the degasser suction compartment.
which usually vary according to mud weight. The equalizing line from the removal section to
4. Turn drilling fluid feed off before turning di- the additions tank should be through an "L"-shaped
lution water off and prior to turning the ma- pipe that allows the discharge end to be raised
chine off. or lowered. Normally, the discharge end will be
raised so that the removal section maintains a
constant level of fluid.
Solids removal equipment is arranged so that
DEWATERING UNIT
the larger solids are removed before the smaller
solids. Each piece of equipment should discharge
More and more rigs are incorporating dewater-
into the next compartment downstream from the
ing units in their drilling fluid treatment process. suction compartment. Each compartment in the re-
Dewatering units are designed to inhibit free liq- moval section should backflow from the downstream
uid discard. The units can process drilling fluid compartment into the upstream compartment, ex-
from the active system or discard material from cept for the sand trap. A flow analysis should show
the solids removal equipment (shakers, desanders, that all fluid entering the suction compartment of the
desilters, and centrifuges). Chemical pH adjust- degasser, desilters, or desanders, from whatever
ments, flocculants, and coagulants are added to source, is processed through the equipment.
the input stream of the dewatering unit. The sol-
ids are usually removed with a centrifuge. Colloi-
dal-rich discards can have as little as 10% volume Compartment Agitation
solids and still be treated as a solid. The liquid
recovered from the unit is usually returned to the Each compartment, except the sand trap, should
active system. Chemical overtreatment creates be well stirred to provide uniform solids-loading
problems in the active system, therefore, the liquid for the removal equipment. Failure to agitate these
recovery stream must be carefully monitored. For compartments can result in short-term overload-
more information on Dewatering, see Chapter 13.
ing of the hydrocyclones. This, in turn, results in
bottom orifice plugging that prevents solids re-
moval. Removal efficiency less than 50% is com-
REMOVAL SECTION ARRANGEMENT mon in unstirred compartments.
All of the compartments, except three, use bot-
tom equalizers. The sand trap should overflow into Single-Purpose Centrifugal Pumps
the next compartment downstream, which pro-
vides an opportunity for the solids to settle. Ob- Solids removal equipment should be installed so
viously, to increase residence time to allow set- that operation is straightforward and consistent.
tling, the drilling fluid should enter the sand trap There should be a single starter (for an electric
