The term solids removal equipment efficiency is frequently used to describe solids-control equipment performance. This term may be somewhat confusing. American Petroleum Institute (API) Recommended Practice (RC) 13C, ‘‘Solids Control,’’ refers to the solids-removal process in terms of system performance.
Solids-control equipment is designed to remove drilled solids, which are not dry when they are removed from the system. For example, the underflow discharge from a properly operating 4-inch desilter or hydrocyclone contains around 35%vol solids and 65%vol drilling fluid. The liquid concentration of the discard from a shale shaker depends on the shaker screens. An API 200 screen discards a much wetter discard stream than an API 20 screen. The liquid drilling fluid that accompanies these separated solids comes from the drilling-fluid system. After screening, however, the solids in the discarded drilling fluid cannot be expected to have the same solids distribution as the drilling fluid in the tanks. Quantities of barite in the liquid phase of the discard from a fine screen may not be in the same concentration as barite in the pits. The discard obviously contains more drilled solids than the drilling fluid in the pits. The barite, or weighting agent, will probably also have a different concentration in the liquid phase of the discard than in the drilling fluid in the pit. Measuring the quantity of barite in the discard will not reveal the amount of drilling fluid discarded. The drilling fluid accompanying the drilledsolids discard will, however, contain drilled solids that have remained in the system after the fluid was originally processed by the solids-control equipment.
Efficiency is defined by the ratio of output to input. For example, if a 100-hp motor drives a rotary table and produces 85 hp to rotate a drill string, the efficiency of the system is 0.85, or 85%. Drilled solids removal efficiency would imply a ratio of output (or discard) to input (circulating volume).
Consider the situation in which 6 of the 9 bbl of rock described in the previous example reach the surface and are removed by the solidscontrol equipment. Not all of these 6 bbl of solids will be removed from the well bore in the same interval of time that they were drilled. Drilling at 20 ft / hr, solids from the 100 feet of hole would enter the system during a 5-hour period. Assume that these solids arrive at the surface during a 10-hour period (or longer). Circulating at 10 bbl / min, 6000 bbl of drilling fluid would reach the surface and be processed through the solids-control equipment. If this drilling fluid contained 4%vol drilled solids in addition to the 9 bbl of rock, a total of 249 bbl (240 bblþ9 bbl) of solids would pass through the equipment.
What would be the removal efficiency of all of the drilled solids removed? Assume that the 6 bbl of drilled rock were discarded in a slurry containing 35%vol solids. This means that the total discard volume would be 17.1 bbl (6 bbl of drilled rock and 11.1 bbl of drilling fluid). The 11.1 bbl of drilling fluid with 4%vol drilled solids would have 0.44 bbl of drilled solids. So, the input would be 249 bbl of solids, and the output would be 0.44 bbl. Clearly, the resulting 0.2% removal efficiency reveals nothing.
If only the removal of the drilled rock entering the system is considered in the process calculation, no credit would be taken for the resident drilled solids present in the drilling fluid. The input volume of drilled solids would be the 9 bbl of drilled rock, and the output volume would be the 6 bbl. The system performance would be at 67% efficiency. This is the solids removal equipment efficiency of the system, also referred to as solids removal equipment performance or drilled-solids removal system performance.
Only solids removed that decrease the solids concentration in the drilling fluid are considered in calculating solids removal equipment efficiency (SREE). If a valve is opened and 200 bbl of drilling fluid are removed from a system, some drilled solids are obviously removed with the liquid. However, the concentration of drilled solids in the remaining drilling-fluid system does not change. Removal of the 200 bbl of drilling fluid provides space for clean drilling fluid to be added. The addition of the clean drilling fluid will decrease the concentration of drilled solids in the system, but this is a very expensive method—that is, dilution—of maintaining a low concentration of drilled solids. The calculation of SREE considers only the drilled-solids removal that decreases the drilledsolids concentration in the system compared with the new drilled solids introduced during that interval.