Page 363 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 339

Drilling Fluid [ ]
3.2.1

Higher flow rates require coarser screens
Higher rate of cuttings generation (related to bit size and ROP)
requires coarser screens
Higher mud weight and viscosity both require coarser screens

It is not possible to give definite sizes to use for each hole sec-
tion due to the number of variables and the different types of shak-
ers in use; however, some general recommendations may be given as
a starting point. These assume only one bank of shakers with dou-
ble screen banks.
For a top hole of 20 in and above, high flow rates and large
amounts of cuttings require very coarse screens: 40 top and 80 bottom.
For hole sections below 20 in down to 12 in, 80 top and 100 bottom.
From 12 in down, 100 top and 120 to 150 bottom.
Shale shaker screens may blind due to solids of a size close to the
mesh size of the screen plugging up the mesh. In this case, finer screens
are called for. They may also blind due to very small cuttings coating
the wire, which effectively reduces the mesh opening. If this happens
then coarser screens should control the blinding.
Screen cloths are available with a square mesh design or a rectan-
gular design. It is generally recognized that the rectangular mesh is
superior, giving greater fluid throughput capacity while removing the
designed range of solids. “Pyramid” screens are a recent innovation
which give a greater mesh area for the same size of screen; experience
so far has been favorable.
The shale shaker screens must be held in the frame at the correct
tension as specified by the manufacturer. If the screens wave up and
down (as opposed to vibrating), then they are either insufficiently ten-
sioned or have been stretched or torn. Remedial action must be taken
quickly to avoid high levels of solids bypassing the shakers and affect-
ing the downstream equipment.
Torn screens must be recognized and replaced immediately. Where
high flow is passing over the shakers a torn screen can sometimes pass
unnoticed for some time, so the drilling supervisor and toolpusher
must both be on the ball when checking the screens. If this requires
bypassing the shaker while the repairs are made it is important that the
sand trap is not bypassed as well; this will hold the larger solids for a
short time while bypassing the shakers and therefore protect the other

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