Page 128 - Shale Shakers Drilling Fluid Systems

SHAKER APPLICATIONS 111
rigs. The cost of handling discarded material may
require drying the discard. The fine mesh screens
discharge much wetter solids than those discarded
from very coarse screens (see Chapter 13).
CASCADE SYSTEMS
The first cascade system was introduced to
the drilling industry in the mid-1970s. A scalper
shaker received fluid from the flow line and re-
moved gumbo, or large drilled solids, before the
fluid passed through the main shaker using a fine
mesh screen (at this time, 80- to 120-mesh screens
were the practical limits). The first unit combined
a single-deck, elliptical motion shaker mounted di-
rectly over a double-deck, circular motion shaker
(Figure 4-1). This combination was especially suc-
cessful offshore where space is limited.
One advantage of multiple-deck shale shakers is
their ability to reduce solids loading on the lower,
fine mesh screen deck. This increases both shaker
capacity and screen life. However, capacity may still
be exceeded under many drilling conditions. The
screen mesh and, thus, the size solids returned to
the active system, is often increased to prevent loss
of whole mud over the end of the shaker screens.
Processing drilling fluid through shale shaker
screens, centrifugal pumps, hydrocyclones, and
drill bit nozzles can cause degradation of solids
and aggravated problems associated with fine sol-
ids in the drilling fluid. To remove drilled solids as mance objectives, and overall cost.
soon as possible, additional shakers are installed
at the flow line—sometimes, as many as six to ten
parallel shakers—so that the finest mesh screen
may be used. With the finer mesh screens and
additional shakers in place, downstream equip-
ment is often erroneously eliminated. It is impor-
tant to remember that the improved shale shaker
FIGURE 4-1
still remains only one component of a complete
drilled solids removal system (see Chapter 7).
A system of cascading shale shakers is de-
signed to use one set of screens (or shakers) to
scalp large solids and gumbo from the drilling
fluid, and another set of lower screens (or shak-
ers) to receive the upper shaker underflow fluid for
removal of fine solids. This combination increases
the solids removal efficiency of high performance
shakers, especially during fast, top-hole drilling or
in gumbo-producing formations. The cascade sys-
tem is used where the solids' loading exceeds the
capacity of the fine mesh screen. The advantages
of this cascade arrangement include:
1. Higher overall solids loading on the system
2. Reduced solids loading on fine mesh screens
3. Finer screen separations
4. Longer screen life
5. Lower fluid well costs
There are three basic designs of cascade shaker
systems:
1. The separate unit concept
2. The integral unit with multiple vibratory
motions
3. The integral unit with a single vibratory
motion
The choice of which design to use depends on
many factors, including space limitations, perfor-
Separate Unit
The separate unit system mounts usable rig
shakers (elliptical or circular motion) on stands
above newly installed linear motion shakers (Fig-
ure 4-2). Fluid from the rig shakers (or scalping
shakers) is routed to the possum belly (back tank)
FIGURE 4-2