Page 362 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 338

[ ] Practical Wellsite Operations
3.2.1

Effective solids control requires a chain of processes that remove
progressively finer solids. Failure of any particular piece of equip-
ment will adversely affect the equipment downstream by overloading
it and therefore reducing its efficiency. If the shale shaker screens
become torn and go undetected, the desanders, desilters, mud clean-
ers, and centrifuge can become overloaded and can discharge heavy
amounts of mud—or plug up completely and pass solids back into
the active system.
Ineffective solids control adversely affects mud rheology, plugs
lines, damages pumps, and fills pits. Downhole the problems become
large filter cake buildups (increasing the risk of stuck pipe), large surge
and swab pressures leading to wellbore instability, and higher annular
circulating losses (increasing the risk of lost circulation). Mud treat-
ment becomes more difficult and expensive. Clearly, the solids control
equipment is a key component of an efficient drilling operation and it
must be kept operating at top efficiency. Each part of the system is dis-
cussed in detail.
Shale shakers. The first and most important item of solids control
equipment is the shale shakers. Correctly set up, they remove the bulk
of the solids, from the coarsest cuttings and cavings down to fine par-
ticles. If they are working correctly, they reduce the load on the hydro-
cyclones and centrifuges, allowing them to work more efficiently and
with less wear.
The ideal shale shaker setup is a “cascade” system whereby the
flowline routes to a header tank through a set of coarse screen shakers,
then onto another header tank and the main bank of fine screen shak-
ers. This allows finer final screens to be used without blinding off,
which may occur if only one set of shakers were used, therefore opti-
mizing the performance of the rest of the equipment.
Each set of screens should be as fine as possible without blinding
off, avoiding the loss of large quantities of mud over the end. The top
set of screens should be coarser than the bottom set (if the shaker has
double screen banks), so that the work of removing the solids is shared
between the screens, leading to greater efficiency.
Choice of screens will of course be limited to what is available on
the rig, which in turn may be determined by the contract with the rig
contractor. The following factors affect screen sizes:

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