Page 178 - Shale Shakers Drilling Fluid Systems

SOLIDS CONTROL EQUIPMENT 161

Other Applications 2. Operating in the manner previsouly described,
using two centrifuges
Two-stage centrifuge operations. Two-stage 3. Cleaning or separating cuttings from a wash
centrifuge operations are becoming more common. fluid
These units can be operated in series or parallel
depending on whether liquid reclaimation is re-
quired. In parallel operations, two centrifuges in- Centrifuging to clean and recondition an oil-
crease the quantity of fluid that can be processed. based drilling fluid is often performed at a central
In series operations, the overflow (or light slurry) "mud plant." Eventually, 100% of the drilling fluid
from the first machine is fed to the second ma- is processed. When the ambient temperature is
chine. The underflow (or heavy slurry) from the low, the feed fluid will have a high viscosity. This
second machine is discarded and the overflow is increases the processing time required to remove
returned to the drilling fluid. This discards the solids from the drilling fluid. The problem of in-
majority of the drilled solids larger than the cut creased slurry viscosity related to cold temperature
point of the second machine and smaller than the is also encountered when drilling wells in ultra-
first machine. The objective of this process is to save depth waters, where many wells are drilled with
the expensive liquid phase at the cost of returning synthetic-oil drilling fluids. For example, seawater
the small colloidal particles to the drilling fluid. temperature below 2,000 feet asymptotically ap-
The most frequent application of two-stage cen- proaches 36°F (2°C). Drilling fluid circulating through
trifuging is on oil-based muds when cost and/or large diameters risers in deep water undergoes sig-
environmental factors prohibit discard of the liq- nificant cooling and consequent gellation. The cost
uid phase. The first centrifuge recovers the weight- of these drilling fluids can be on the order of sev-
ing material and the centrate is directed to the sec- eral hundred dollars, so economics dictate that as
ond centrifuge, which opeates at a higher "G"-force. much liquid as possible be recovered.
The efficiency of this operation depends on the Decanting centrifuges are used to process un-
efficiency of each stage. The first centrifuge must weighted oil-based drilling fluids when:
adequately separate the solids because the dis-
cards from the second centrifuge depend on the 1. Drilling fluid is brought from another loca-
solids received. The second centrifuge should op- tion and may contain a significant amount of
erate at the highest possible "G"-force with the drilled soilids
deepest pond depths. 2. Slow, hard drilling yielding a buildup of
ultra-fine solids is anticipated
• Centrifuge stage one—The first stage employs a 3. The liquid drilling fluid phase is valuable
variable speed (1800 to 2800 rpm) conical unit.
Weighted oil-based mud applications. In weighted
• Centrifuge stage two—Second-stage units should oil-based mud applications, decanting centrifuges
be a high rpm (2800 or more), contour de- are operated in series. The first unit—a "standard"
sign. This processes the overflow (normally machine—returns the dense, coarse solids to the
the discard) of the first-stage separator. This system and routes the light, fine solids discharge
overflow stream contains mostly solids smaller to a holding tank. A second unit—usually a high-
than 3 to 5 microns and the liquid fraction capacity machine—removes and discards the sol-
containing drilling fluid treatment chemicals. ids with the effluent available for return to the
The high-speed unit separates this overflow active system.
into "heavy" and "light" streams. The heavy This process is not as effective as a single unit
stream is discarded and the light stream can for viscosity control. A significant portion of the
be used as dilution fluid—thereby reducing colloidal-size solids are returned to the active sys-
the volumes discarded during drilling opera- tem in the overflow of the second unit. The over-
tions. This removes solids between 1 and 10 flow of the first unit is too valuable to discard,
microns and recovers most of the liquid.
especially with synthetic, oil-based drilling fluids.
Disposal costs can be significant for many of these
Centrifuges in oil drilling fluids. Centrifuges drilling fluids. Two decanting centrifuges can de-
have extensive application in oil-based drilling crease the total waste volume from the well.
fluids, including:
Closed-loop systems. As part of closed-loop
1. Allowing the fluid phase of hydrocyclone dis- systems, decanting centrifuges process underflow
charge to be returned to the active system from hydrocyclones to separate the discarded free