The influx of equipment and ideas concerning liquid-solids separation in the last decade requires some classifications of process for any ordered discussion. Primary separation, as used here, refers to the use of liquidsolids separators to remove either a liquid-undersize solids fraction or an oversize solids fraction from the active mud system. Any separator taking its feed directly from an active system and returning either the liquid (less
solids than the feed stream) or solids (more solids than the feed stream) back to the active system has in effect removed a selective fraction from the system, and is performing primary separation. Figure 1 is a schematic diagram of typical primary separation equipment related to a system.
From prior to 1952 to the present, improvement in primary removal equipment has had considerable impact on general mud weighting practices, as shown in Figure 2. The lowest drilling mud weights economically possible, the cost of maintaining a given weight mud (assuming the same type chemical system), the lowest weight at which barites are used, and the highest weight at which barites are not used, have all moved to the left and downward on this diagram. At the same time, the high limit of drilling mud weight that can be maintained, as a matter of practicality, has moved to the right . It is not possible to insert actual cost figures because of innumerable variables, however for a specific area one could be built easily from actual records.
This reduction in cost at any given weight is because of the greater efficiency of mechanical removal devices over dilution , or “watering-back,” which was once the mainstay of solids control after mud had passed the conventional shale shaker. The technique of dilution is to increase mud pit level with liquid addition, discard a fraction of whole mud , add liquid, discard mud, etc . etc. Obviously this procedure effects no change in particle size distribution-the good goes with the bad.
Mechanical devices selectively remove solids, permitting us to alter the particle size distribution . They permit the removal of more of the unwanted solids with less loss of beneficial (for the purpose at a specific time ) solids than is possible by dilution.