Usually, if solids are being removed with shale shakers, a mud cleaner will probably be beneficial. Solids removed by mud cleaners will cover a wide range of quantities depending on formations drilled, borehole stability, dispersion of solids as they move up the borehole, type of drill bit, type of drilling fluid, and other variables.
Drilling soft, dispersible shales with a freshwater drilling fluid usually results in drilled solids that cannot be removed with mud cleaners. In these cases, centrifuges should be planned for use in weighted drilling fluids.
Some data acquired from one well are presented in Table 1. The pressure at the entrance to the desilters was varied and the mud cleaner discard examined for drilled solids and barite. With an unweighted drilling fluid, a head of 75 feet was recommended for this brand of desilter. This head creates a balanced hydrocyclone with good separation of low-gravity solids; however, this may not necessarily be true for weighted drilling fluids.
Drilling with 97⁄8″ Bit Between 9300 Feet and 9400 Feet with an 11-ppg
Drilling Fluid and Six 4-inch Hydrocyclones Above an API 200 Screen
|Drilled Solids Removed
|First circulation: Bottoms up after TIH with new bit|
TIH=tool in hole.
In Table 1, during the first circulation after the new drill bit has reached bottom, higher quantities of drilled solids are discarded by the mud cleaner screen as the head is increased on the desilter feed. One method of analysis is to compare the concentration of barite lost with the drilled solids discarded. In Figures 1, 2, and 3, the total solids discarded and the quantity of drilled solids are shown as functions of the desilter manifold pressure. The lowest ratio of barite to drilled solids occurs when the manifold head (or pressure) is low. This might be misleading, however. Larger quantities of drilled-solids discards are much more desirable, even if some additional barite is lost. Higher manifold pressures are preferred to eliminate the largest quantity of drilled solids from the drilling fluid.
The bottoms-up sample indicates that the smallest discard rate has the
highest ratio of drilled solids to barite. However, the largest flow rate of
drilled solids, 1659 lb/hr, is still a flow rate 4.5 times as large as the barite
flow rate (Table 2).
Table 2 Discard Rates
|Drilled Solids Removed (lb/hr)||Barite Discarded (lb/hr)||Ratio of Drilled Solids to Barite|
A word of caution is appropriate here. The purpose of solid-control equipment is to remove drilled solids. Economics certainly justify sacrificing a small additional amount of barite for good removal of drilled solids. If solids concentrations are reduced by dilution only, the cost would be many times higher than sacrificing some barite to remove these drilled solids. So, evaluating performance by comparing discard ratios can be very misleading. This is discussed in more detail at the end of this section.