Page 172 - Shale Shakers Drilling Fluid Systems

SOLIDS CONTROL EQUIPMENT 155

The underflow of the desilters does not contain As with desanders and desilters, mud cleaner
a significant quantity of viscosifiers. The mate- hydrocyclones typically receive drilling fluid through
rial that passes through the screen cannot trans- a centrifugal pump. Although there have been re-
port weighting material. Care must be exercised to ports of solids degradation due to the centrifugal
return the material to a well-agitated location in pump, particle size measurements at a variety of
the pit. rig sites fail to confirm this. Solids will eventually
With the advent of 200-mesh screens on linear degrade, however, this occurs slowly and in a
motion shale shakers, mud cleaner usage signifi- variety of other places as well. These include the
cantly decreased. Slowly, they have returned to the liquid pool of linear motion shale shakers (particu-
removal system because trial usage indicates that larily deep pools), the turbulent zone of decant-
they still remove a large quantity of solids. Plug- ing centrifuges, mud guns, crushing by the drill-
ging of the lower apex of mud cleaner hydrocyclones pipe, and the high-shear through drill bit nozzles.
indicates that very large solids frequently bypass
the shale shaker screens. Solids large enough to
plug a 4-inch cone indicate the presence of a wide CENTRIFUGES
range of other solids. Even when linear motion
shale shakers are operating properly, mud clean- A centrifuge works on the principle of acceler-
ers remove a significant quantity of drilled solids. ated settling. By imparting additional "G"-forces to
In unweighted drilling fluids, the mud cleaner the content of a centrifuge, solids (in a solids-
can be used to salvage an expensive liquid phase, laden fluid) will settle much quicker.
such as oil or a high concentration of KCl. Usu- To understand how a decanting centrifuge works,
ally, better results are obtained if the hydrocyclone first look at a simple sedimentation vessel (Figure
underflow is fed to a decanting centrifuge instead 7-4). Solids in this fluid will settle to the bottom
of being screened. A centrifuge processing the of the vessel over a period of time. One way to
underflow from hydrocyclones is a better choice speed the process of settling is to reduce the height
to return the liquid phase. A mud cleaner can be of the vessel so the solids do not have as far to
used but not as effectively. drop. If a specific volume is required, the vessels
Currently, the hydrocyclones and screens are dimensions can be lengthened or widened (Fig-
used to diminish the liquid discharge from a sys- ure 7-5). Depending on the Theological properties
tem. This combination is called a "dryer" and is of the fluid and the size and density of the solids,
discussed in Chapter 4. settling time can still prove quite slow.
Most cut point curves of 4-inch desilters show
that only about 5% of the underflow from the
cones is less than 5 microns. Viscosifiers are in Stokes' Law
this size range. The underflow that passes through
the mud cleaner screen contains such a small Particles will settle in a given fluid according to
amount of viscosifier and collodial material that it Stokes' Law, which is expressed as follows:
will not support the other solids. For this reason,
the mud cleaner should be viewed as another
method of removing drilled solids larger than the
fine mesh size on the shaker screen. If this mate-
rial remains in the drilling fluid, it will eventually
become undesirable ultra-fine solids. Colloidal
material will increase at an even faster rate be-
cause of the deterioration of larger solids.
Mud cleaners do not compete with centrifuges.
Centrifuges remove colloidal-size particles, usually
smaller than the weighing agent, from the drilling
fluid. Mud cleaners remove drilled solids larger
than the weighing agent. If improperly used, mud
cleaners will also discard large quantities of bar-
ite. The slurry on the screen must not "dewater"
too soon or the barite will "clump" together and
be discharged. Frequently, a small stream of drill-
ing fluid from the desilter overflow line can pre-
vent the loss of too much barite. FIGURE 7-4. Simple sedimentation vessel.