Various arbitrary procedures seem to be developed by operating personnel in the field. Often a centrifuge is run for a specific number of hours per day, or a hydrocyclone bank is used for only a short period. Mud cleaners seem to also attract a variety of erroneous rules of thumb.

In heavily weighted drilling fluids, above 13–14 ppg, mud cleaners are frequently shut off because of excessive barite discards. When a mud tank system is plumbed incorrectly, specifically when mud guns transport freshly added barite fluid into the removal system, a significant amount of barite will be discarded. Barite that meets API specifications may have as much as 3% weight larger than 74 microns. If a thousand 100-lb sacks of barite are added to a drilling-fluid system during drilling of an interval (used as clean drilling fluid to dilute remaining drilled solids), 3000 lb of the 100,000 lb added will be larger than 74 microns. Now the question becomes, what damage will these large particles create? Any particles larger than 74 microns, whether they are drilled solids, barite solids, diamonds, or pieces of gold, will make a poorquality filter cake and enhance the probability of incurring all of the problems created by a large quantity of drilled solids in the drilling fluid.

In a drilling-fluid system that is arranged properly, barite will have traveled down the drill string, passed through the bit nozzles, and then traversed the borehole before reaching the mud cleaner. Any particle larger than 74 microns should be removed from the drilling-fluid system. Barite larger than 74 microns should be removed from the system. In situations in which the system is not arranged properly, the mud cleaner might be shut down for two or three circulations after a significant weight-up. This is a good test for correct mud tank arrangement. If the mud cleaner starts discarding barite immediately upon weight-up, the system has serious flaws. The objective is to remove large particles from the drilling fluid so the filter cake will be thin, slick, impermeable, and compressible. With solids larger than 74 microns in the cake, this objective will not be achieved.

One common problem with sieving hydrocyclone underflow with a mud cleaner is the tendency of the material to dewater, or deliquefy, before reaching the discard end of the screen. Clumps of material traveling down a screen with too little liquid will not separate solids properly. A small reflux of drilling fluid from the hydrocyclone overflow should be sprayed onto the screen to break these clumps into material that can be separated. A spray of water or oil (depending on the liquid phase) could be used but will generally dilute the system too much. The drilling fluid reflux enhances the screen capability to separate solids from the slurry. If the slurry remains liquid until all separation has been completed, the mud cleaner screen should not remove any more barite than would be removed on the main shaker with the same-size screen.

There are situations in which the solids disperse as they travel up the
borehole. Usually only slivers and cavings from the borehole wall are
removed by the main shaker. When this occurs, a centrifuge is needed to
remove the very small particles. A change in drilling-fluid systems might
be contemplated before drilling the next well.


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