Page 364 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 340

[ ] Practical Wellsite Operations
3.2.1

solids removal equipment. In general, it is preferable to stop drilling,
continue to circulate at half rate over the other shaker(s), and fix the
split screen as soon as possible.
Splits can often be detected even when under a depth of mud; look
at the surface for unusual flow patterns. Sometimes it looks as if mud
is welling up from below; sometimes a pattern of mud splashing up is
in a ring around the hole. Investigate any unusual signs.
Sand trap. After the shale shakers, the mud should pass through
into the sand trap and possibly from there into a settling tank. These
tanks should not be agitated nor used as suction feed tanks for hydro-
cyclones or centrifuges. Flow from them should pass over a weir to the
next tank so that they are kept full to maximize settling.
The sand trap should have a discharge butterfly valve located at the
bottom of the tank that can be quickly opened and closed again, allow-
ing settled solids to come out while minimizing the loss of expensive
whole mud. It is detrimental to clean out the sand trap except when
necessary for changing mud systems. The solids will settle on bottom
at an angle to the discharge gate, and if these bottom solids are cleaned
out more whole mud will be lost whenever the sand trap is dumped,
until this layer has built up again.
The sand trap is an important part of the system, protecting other
equipment from shale shaker inefficiencies. If the shakers always
worked perfectly and screens never tore then the sand trap would
lose much of its purpose. However, this is unlikely to be the case for
a considerable time, therefore, the sand trap must be designed and
used properly.
Hydrocyclones: principle of operation. Once the mud passes
through the shakers and sand trap, the coarse particles should have
been removed. Finer solids will remain in the mud, which are still
capable of damaging equipment and adversely affecting mud proper-
ties: abrasive sands, fine silts, and other low gravity solids. Correctly
designed and adjusted hydrocyclone banks can remove most of these
undesirable solids at the full flow rate used while drilling.
Fluid is fed into the feed inlet at the side of the cylindrical section
on top of the cone. This flow enters the feed chamber directed along
the inside wall. The curve of the feed chamber forces the mud to swirl
around the inside. The overflow opening at the top extends down into
the feed chamber so the inside of the chamber is an annulus, not an

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