The weighted mud flows to the inlet head section of the desander and/or desilter entering the hydrocyclones for separation of particles. Mud leaving the underflow is further screened with fine mesh to separate larger particles allowing only barite size particles to pass through the screen returning and recovering then the clean mud.
LOCATION OF MUD CLEANERS IN A DRILLING-FLUID SYSTEM
Mud cleaners are normally positioned in the same location as desilters in a drilling-fluid system. Frequently, the desilters, or hydrocyclones, are used in the unweighted portion of a borehole by diverting the underflow away from the mud tanks. When a weighting agent, barite or hematite, is added to the system, screens are placed on the mud cleaner shakers. Solids discarded from the hydrocyclones are sieved to discard solids mostly larger than barite and return solids smaller than the screen size with most of the liquid phase. Practical tip: Barite goes in, screens go on.
Another method has been used to create a mud cleaner using the main shale shaker, mostly on offshore rigs. When several linear, or balancedelliptical, shale shakers are needed to handle flow in the upper part of a well bore, fewer shakers can handle the flow after the hole size decreases and the mud weight is increased. Rigs have been modified so that as many as twenty 4-inch hydrocyclones have been mounted above one of the main shakers. The feed and overflow (light slurry) from the hydrocyclones are plumbed as usual (see Chapter 5 on Tank Arrangements). All of the desilter underflow is discarded in the unweighted part of the hole as usual, while the shale shaker is screening drilling fluid from the flow line. This is normally the largest flow rate expected while drilling the well. As the well gets deeper, a weighted drilling fluid is required, and usually the flow rate is lower. When barite goes in, a valve prevents flowline drilling fluid from going to one of the main shale shakers. The desilter underflow is diverted onto the shale shaker screen so that the shale shaker becomes a mud cleaner.
OPERATING MUD CLEANERS
When the first mud cleaners were introduced into the field, they had to be shut off during weight-up. A significant amount of barite was discarded during the first circulation. Actually, this revealed that the mud tanks were plumbed improperly. Drilling fluid was frequently pumped through mud guns from the additions or suction section back upstream to the removal tank. Barite can meet American Petroleum Institute (API) specifications and still have a large amount that will be removed with an API 200 (74-micron) screen. If the barite has passed through a drill-bit nozzle, the particles are split so that it will not be removed with such a screen.
One comment frequently heard when a weighted drilling fluid is initially passed through a mud cleaner is: ‘‘It’s throwing away all my barite!’’ What creates such a comment? The mud weight is decreasing, and more barite than normal is required to maintain mud weight. When solids are removed from a drilling fluid (barite or drilled solids), mud weight will decrease. Actually, removal of solids larger than 74 microns is beneficial to drilling a trouble-free hole, whether those solids are drilled solids, barite, gold, silver, or diamonds. Those solids make a poor, incompressible filter cake and lead to stuck drill strings. The appearance of the screen discard from a mud cleaner is similar to the underflow, or heavy slurry, from a centrifuge. Although visually it appears to have mostly barite, tests will reveal that this is not true.
API specifications for barite state that 3% by weight may be larger than 74 microns. If 100,000 lb of barite is added to a drilling fluid during a weight-up, 3000 lb of barite could be removed by an API 200 screen. This is one reason that fluid from the additions compartment should not be circulated upstream. The main shale shaker will also discard most of this size of barite from an API 200 screen. The barite is not as noticeable because of the quantity of drilling fluid normally clinging to the shaker discard.
The screen discard from a mud cleaner looks like the underflow from a decanter centrifuge. The solids concentration is usually around 60% volume, and that of liquid about 40%. This initially appears to be an irrationally large quantity of liquid. Researchers frequently pack columns with loose sand to examine various oil recovery procedures. Dry sand is poured into a cylinder while vibrating the pack. If the porosity of the sand pack is 33–35% volume, the packing is about as tight as can be achieved. Loose sand on the beach, immediately after a wave has washed back out to sea, has about 40% volume of water in it. It can be scooped up without water draining from the sand pile. Mud cleaner screen discards and the heavy, or underflow, slurry from a decanter centrifuge have about the same volume percentage of liquid.
The mud cleaner is designed to continuously process drilling fluid just like the main shale shakers. The mud cleaner screen keeps larger particles from entering the system. Operating the equipment for only part of the time allows solids to remain in the drilling fluid system. These solids grind into smaller particles that become more difficult to remove. Centrifuges will be able to remove these solids from a weighted drilling fluid, but they generally do not process all of the rig flow. The mud cleaner can remove these solids before they grind into smaller particles if the mud cleaner is used continuously.
Again, note that the mud cleaner and the centrifuge are complementary to each other—not competitive with each other. The mud cleaner removes solids larger than barite; centrifuges remove solids smaller than most barite.