For that reason the resulting solids removal equipment efficiency required is called the optimum solids-removal efficiency. It is independent of the volume of drilled solids reaching the surface, or the volume of the drilling-fluid system. Equating the volume of clean drilling fluid needed to the volume of discard results in the minimum volume of clean drilling fluid needed and, as a consequence, the minimum volume of drilling fluid disposal.

If the solids removal equipment efficiency is less than the value required to achieve the minimum volume, the amount of fluid increases rapidly as the removal efficiency decreases. Figures 6 and 7 present the results of calculations similar to the ones for the targeted 4%vol drilled solids concentration previously described. As the targeted drilled-solids concentration is increased, the volume requirements decrease for any particular solidsremoval efficiency. For example, at 70% solids removal equipment efficiency, maintaining 4%vol drilled solids requires adding 7 bbl of clean drilling fluid for every bbl of drilled solids, whereas maintaining 10%vol drilled solids requires adding 2.7 bbl of clean drilling fluid for every bbl of drilled solids.

The consequences of permitting 10%vol drilled solids, however, will usually completely eliminate any economic advantage of the lower volume requirement. The most effective, economical procedure is to improve solids-removal processing so that the efficiency approaches the minimum value for any targeted drilled-solids concentration. The targeted solids concentrations must be carefully selected. Artificially large values result in trouble costs and rig downtime; artificially low values will significantly increase drilling fluid and disposal costs (Figure 8).

Frequently, some rules of thumb are discussed in the literature about how much drilling fluid is required per barrel of drilled solids. In Figure 9 a common value of 3 is shown. This value depends on the targeted drilled-solids concentration and the solids removal equipment efficiency.

Some evaluation techniques involve not only accounting for the new solids drilled but also the resident drilled solids in the system before and after drilling. In the case of the 80% removal efficiency described, the discarded slurry contained 80 bbl of new drilled solids with 149 bbl of the drilling fluid (a total of 229 bbl). The 149 bbl of drilling fluid contained 4%vol drilled solids, or 6 bbl. Since this 6 bbl did not reduce the drilledsolids concentration in the system (it did reduce the total volume, but not the concentration), it was not part of the removal efficiency of the equipment.

To account for the 4%vol resident drilled solids, the volume of the system must be known. In this case, assume that a volume of 1000 bbl was available in the system before the drilling started. The drilling fluid had a 4%vol drilled-solids concentration, or 40 bbl of drilled solids. If new rock of 100 bbl is drilled, the system would now have a volume of 1100 bbl, with 100 bbl of new drilled solids and 40 bbl of the original drilled solids. The drilled-solids concentration in the system would now be 140 bbl / 1100 bbl, or 12.7%vol. Since the new rock occupies the same volume before and after drilling, the pit levels remain constant (except for the volume of drill pipe that enters the borehole). The total volume of drilled solids in the system is now 140 bbl.

These new drilled solids arrive at the surface and 80 bbl of them are discarded in a 35%vol slurry, or a total of 229 bbl of material leave the system. The pit levels drop by 229 bbl.

How many drilled solids are left in the system now? The discard was 80 bbl of new drilled solids and 6 bbl of the original drilled solids, or 86 bbl. This leaves 140 bbl – 86 bbl, or 54 bbl of drilled solids in the system. The system volume is now 1100 bbl – 229 bbl, or 871 bbl. The system contains 20 bbl of the new drilled solids and 40 bb – l6 bbl, or 34 bbl, of the original drilled solids, for a total of 54 bbl of drilled solids. The drilled-solids concentration at this time is 54 bbl / 871 bbl, or 6.2%vol.

These 54 bbl of drilled solids must be diluted to the requisite 4%vol drilled solids concentration. Mathematically, this could be stated: 54 bbl=0.04 (total new drilling fluid volume). So, the new drilling fluid volume must be 1350 bbl. Currently the system volume is 871 bbl, so 479 bbl of clean drilling fluid must be added to dilute the remaining 54 bbl of drilled solids. This is almost the same (within roundoff errors) as the 480 bbl originally calculated in the preceding section.

Another way to evaluate this would be to observe that the original 4%vol drilled solids that was in the drilling fluid originally did not have to be diluted. Only the new 20 bbl of drilled solids that remained in the system had to be diluted to 4%vol.

In the field, the solids removal equipment efficiency is not known. The discard volumes and solids concentrations in the system can be measured. From these numbers, the solids removal equipment efficiency can be calculated.

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