Since that is unlikely in the near future, the settling tank is extremely important. If the shaker screen were always adequate, never developed a tear that passed oversize solids through, never had to be bypassed during drilling, etc., the major justification for a settling tank (or “shale trap,” or “sand trap”) would disappear. Some information has been published on the subject, but the following points are cardinal:
- The settling tank usually receives the liquid slurry passing through the shale shaker, and should receive all liquid slurry bypassing the shale shaker and going to the active mud tanks.
- Being a gravity settling compartment, it is not to be stirred and it must not be used as a suction compartment for any removal process.
- A settling tank must have a discharge control easily and quickly opened and closed, so the settled solids can be dumped with minimum whole mud losses.
- The settling tank should only be dumped, not “washed out.” If the bottom
is not sloped to the solids pile angle, the settled solids should be left to form their own sloped sides. “Cleaning the bottom,” other than possibly at moving time, serves no purpose but increases loss of mud and mud cost.
- Since Stokes Law applies in a settling tank, large quantities of barites (as well as API sand) may be settled from weighted drilling fluids. Provision for bypassing the undersize screen discharge slurry from the carrying pan direct to the next processing compartment is also advisable. As all compartments except the sand trap are stirred in a well-designed active system, this will prevent settling out of barite. The settling tank must not be bypassed if there is any problem with any other solids removal apparatus.
- The mud exit from the srttling tank should be over a retaining weir to a stirred compartment.
Rig operators who have engineered a proper sand trap into their removal systems have thereby increased the utility of the other removal equipment and many times have been able to continue drilling progress under unexpectedly severe solids conditions.