One frequent comment while running a mud cleaner is that the mud cleaner is discarding so much barite that large quantities of it must be added to keep the mud weight constant. Actually, discarding any solids from a drilling fluid decreases density. Replacing low-gravity solids with barite will decrease the total solids concentration, make filter cakes more compressible, decrease the propensity for stuck pipe and lost circulation, and improve the possibility of faster drilling (by increasing the founder point). However, some formations that disperse significantly in a freshwater drilling fluid and very few large solids arrive at the surface. Solids discarded from the mud cleaner screen should be examined to determine whether drilled solids are being removed. If they are not, centrifuges should definitely be considered to remove low-gravity solids.

A small change in the density of the low-gravity solids and barite affects the low-gravity solids calculation. Table 12.5 indicates that 14%vol total nonsoluble solids in 11-ppg drilling fluid could have between 6.3 and 9.8%vol low-gravity solids, depending on the density of the barite and the low-gravity solids.

Table 12.5
Values for an 11-ppg Drilling Fluid with 14% Volume of Total Solids

Density of LGS Density of Barite Vol Fraction of LGS
2.4 4.2 7.1
2.9 4.2 9.8
2.4 4.0 6.3
2.9 4.0 9.1
2.6 4.2 8.0

LGS = low-gravity solids

Obviously the density of the low-gravity solids and the barite must be known for an accurate determination of the concentration of solids in the drilling fluid or the discard from any solids-control equipment. The most common error in viewing the discard from a mud cleaner screen is the conclusion that large quantities of barite are contained in the discard. This is the reason that the new API solids analysis recommends that the density of the barite and solids on the shaker screen be determined on location.

Actually, mud cleaners frequently do not discard as much barite as do the main shale shakers. Visual observations tend to give an erroneous view. The discard from a mud cleaner screen (and the underflow from a decanting centrifuge) contains around 60%vol solids, while the shale shaker discard contains around 35–40%vol solids. The more liquid (shale shaker) discard does not appear to contain as much barite. However, the shale shaker discard may concentrate barite or deplete barite from the flowline drilling fluid. The way drilling fluid discarded from a shale shaker appears does not reveal the concentration of barite.


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