Page 177 - Shale Shakers Drilling Fluid Systems

160 SHALE SHAKERS AND DRILLING FLUID SYSTEMS

enhanced by coagulation and flocculation—can drilling fluid properties will be more uniform. The
provide much of the required dilution volume for centrifuge performance is enhanced with a lower
its operation. feed rate because the fluid residence time within
the machine is longer.
Unweighted drilling fluid applications. In un- When centrifuging a weighted mud, bentonite
weighted drilling fluid, a centrifuge may be used (prehydrated) and other chemicals must be added
to remove particles smaller than a desilter can back into the system. The amount of replacement
remove. The D 50 cut point is around 2 to 10 mi- materials may be calculated from mass balance
crons as opposed to 15 to 25 microns for a desilter. equations. In general, for gel/lignosulfonate drill-
The heavy slurry is discarded and the light slurry ing fluids, add one sack of (prehydrated) bentonite
is retained, identical to that of a desilter. per hour of centrifuge operation.
Centrifuging unweighted drilling fluids removes The centrifuge underflow, or heavy slurry, will
drilled solids that are too small to be removed by contain 55% to 60% solids by volume and is too
other separtion devices. A dry discharge is desired, "dry" to flow easily.
which reduces total liquid consumption and dis- Discharge of weighted drilling fluids should be
posal volumes, and is an important consideration introduced at an angle (45° or greater) into a well-
if the fluid phase contains expensive additives. agitated pit to retard settling. If discharged solids
Centrifuges can also be used to process hydro- are dry and do not move easily down the trough,
cyclone underflow. In this case, the cone apex downstream drilling fluid can be used as a wash
may be opened to produce a wetter discharge fluid. Similarly, solids discarded by two-stage cen-
yielding a lower solids concentration to be centri- trifuging, or discards from centrifuging unweighted
fuged. Frequently, additional fluid from the desilter drilling fluids, can be conveyed by wash fluid sup-
overflow is added to the underflow to assist cen- plied from the reserve pits.
trifuge processing.
Decanting centrifuges are used to process un-
weighted oil muds, discussed later in this chapter.
Other Factors of Centrifuge
Performance
Weighted drilling fluid applications. In weighted
drilling fluids, the heavy slurry from a centrifuge
contains appreciable quantities of barite. Both Pond depth is set by an adjustable weir and
streams can contain low-gravity solids and barite. controls fluid residence time. Increased residence
Usually, most of the large drilled solids and large time increases separation at the expense of flow
barite are contained in the heavy slurry, and most (and processing) capacity. For fine solids distribu-
of the colloidal barite and drilled solids are con- tion, a deep pond (at lower flow rates) will yield
tained in the light slurry. A centrifuge is effective higher separation efficiencies. For coarse solids,
in reducing the colloidal concentration in a weighted less separation efficiency is needed, and a shal-
drilling fluid. lower pond (and higher flow rates) may be desir-
able. This corresponds to shallow pond depths
In weighted mud applications, centrifuges re-
cover barite and "coarse" drilled solids from drill- generally associated with upper-hole drilling and
ing fluids, while the colloidal-size solids (barite deeper pond depths for deeper drilling, and with
and drilled solids) in the centrate, or light slurry, weighted fluids.
are discarded. This controls the high shear-rate The difference in speed between the bowl and
and low shear-rate viscosity while the recovered the conveyor controls the velocity with which
solids are returned to the active system. settled solids are conveyed through the centrifuge.
Viscosity of weighted, water-based drilling Settled solids are transported through the centri-
fluids can be controlled by feeding a portion fuge because of the small difference in rotating
of the active circulating system into a decant- speed, or rpm differential, which has little influence
on settling efficiency, provided that the differential
ing centrifuge and discarding the liquid, or colloid,
overflow. These colloidals contribute inordiante- is sufficient to keep pace with the settling rate to
ly to viscosity and gel strength problems. The avoid undue solids buildup in the beach.
heavy slurry, containing the larger particles (both
weighting agent and drilled solids) are returned to Discharge dryness. As previously discussed,
the system. larger particles have less associated surface liquid.
Centrifuge operation on weighted mud is fre- Thus, while discharge dryness may seem indica-
quently intermittent as needed to control viscos- tive of satisfactory centrifugal separations, it has
ity. If the feed rate into the centrifuge is decreased, been shown to correspond more with median par-
so that the centrifuge operates most of the time, ticle size (D 50).