An estimate of the low-gravity solids content of the mud cleaner screen discard can be made by weighing the discard. Since the solids concentration will be around 60% volume, the mud weight will be a reasonable predictor of the low-gravity solids concentration. For lowgravity solids with a specific gravity (SG) of 2.6 and a barite SG of 4.2,
the equation to determine the low-gravity solids concentration, VLG, is:
VLG = 62.5 + 2.0Vs -7.5MW
where Vs is the volume percentage of total suspended solids, and MW is
the mud weight, in ppg.
Assume that the Vs is 60% volume for a mud weight of 19.0 ppg, and the equation will calculate that the volume concentration of low-gravity solids is 40%. This means that 20% of the volume is barite. So, twice the volume of low-gravity solids is being discarded as barite. Even if the actual Vs were 57% instead of 60%, the low-gravity solids concentration would be 34% volume. In most cases the decision to continue running the mud cleaner would not be affected by this inaccuracy. Even if the barite in the discard exceeds the low-gravity concentration, the benefits of removing those larger solids will be evident. Accurate results, of course, can be obtained by retorting the solids; but this is a tedious process because the solids are difficult to handle—care must be taken to obtain a representative sample and pack it into the retort cup without leaving void spaces. A much more accurate method is to use the gravimetric procedure, in which larger quantities can be used and no volume measurements are made.
Generally the discard from a mud cleaner screen is relatively dry and contains around 60% volume of (%vol) solids. The density of this slurry can be measured with a mud balance, but the solids concentration is difficult to measure with a retort. Generally decisions about the performance of a mud cleaner or a centrifuge can be made by weighing the heavy, or underflow, discard from a centrifuge or the screen discard. Very accurate measurements are not really needed. The chart in Figure ·1 allows an estimate of the concentration of low-gravity solids.
For example, if the mud weight of a mud cleaner shaker screen discard weighs 18.0 ppg, the low-gravity solids concentration would be about 40%vol if the solids concentration were 58%vol. Barite would be 18%vol. The concentration of low-gravity solids would be about 48%vol if the discard total solids concentration is 60%vol. Barite would be only 12%vol for this condition. Note that in either case, the mud cleaner is doing a great job of removing drilled solids or low-gravity solids from the drilling fluid. An accurate measurement is not needed to make the decision to continue running the mud cleaner.