Page 184 - Shale Shakers Drilling Fluid Systems

DILUTION 167

drilling fluid is needed to dilute the drilled solids drilling fluid is also the removal efficiency that
returned to the system, 286 bbl of drilling fluid is generates the minimum discard volume.
needed to maintain the pit levels, which will de- These three calculations are summarized in Fig-
crease the drilled solids concentration. In other ure 8-4. The minimum volume required is the low-
words, a 4% volume drilled solids level cannot be est point on the curve at the 90% removal effi-
sustained with constant pit volume and 100% re- ciency point.
moval efficiency.
At 90% removal efficiency, 90 bbl of drilled
solids and 167 bbl of drilling fluid are removed OPTIMUM REMOVAL EFFICIENCIES
from the system. In this case, the pit levels decrease
by 257 bbl (Figure 8-2). The solids returned to the The previous examples, with 35% volume sol-
system (10 bbl) must be diluted to 4% volume con- ids in the discard, illustrate that an optimum
centration by adding 240 bbl of new drilling fluid. removal efficiency of around 90% exists for the
The total addition to the pit system is 10 bbl of 4% volume target concentration of drilled solids.
drilled solids and 240 bbl of new drilling fluid. This These same calculations may be made for other
means that 7 bbl of new drilling fluid must be built discard and targeted drilled solids concentrations
to maintain the pit levels. as illustrated in Figure 8-5.
This example illustrates a system that is almost For example, if the removal efficiency is 75%,
"balanced." If the discarded volume is identical to over 6 bbls of drilling fluid is required for each
the volume required for dilution, the minimum barrel of drilled solids reporting to the surface
quantity of drilling fluid will be built. The optimum when the targeted drilled solids concentration is
removal efficiency for any targeted drilled solids level 4% by volume.
may be calculated by mathematically equating the Using the same calculation procedures for 6%
removal volume to the required dilution volume. volume targeted drilled solids, Figure 8-5 shows
In the case of 80% removal efficiency, 229 bbl that at 60% removal efficiency, over 6.2 bbls of
of drilled solids and drilling fluid will be dis- drilling fluid will be required for every barrel of
charged (Figure 8-3). Although this is only 21 bbl drilled solids reaching the surface. As the effi-
less than the 90% removal efficiency, the dilution ciency improves, the drilling fluid requirement
volumes are significantly higher. The dilution of decreases: at 70% slightly more than 4.5 bbl is
the 20 bbl of returned drilled solids to a 4% vol- required per barrel of solids; at 80% slightly more
ume level requires the addition of 480 bbl of than 3 bbl is required; and at 85% almost 2.5 bbl
new drilling fluid to the system. The reconstituted is required. For removal efficiencies greater than
500 bbl of drilling fluid will contain 20 bbl of 85%, the pit level decreases more than the addi-
drilled solids and 480 bbl of clean drilling fluid. tional new fluid required to dilute the solids to the
Since only 229 bbl of space is available, 271 bbl of 6% volume level. This additional fluid will dilute
fluid must b^ discarded to keep the pit levels con- the solids to a value lower than 6% volume.
stant. Therefore, the total discard is the 229 bbl The volume of drilling fluid required per barrel
from the solids-removal equipment plus 271 bbl of of drilled solids is similar whether the slurry is wet
drilling fluid. (25% volume solids) or relatively dry (45% volume
This situation creates a problem: if 271 bbl of solids). When the removal efficiency is below the
drilling fluid is not discarded, the drilled solids "balance point," the values are identical. As the
concentration will increase significantly above the concentration of drilled solids decreases, the bal-
targeted 4% volume concentration. When large ance point for any target concentration of drilled
reserve pits were used, drilling fluids were rela- solids increases. The volume of new drilling fluid
tively inexpensive and disposal costs were insig- required above the balance point is independent
nificant. Building excess volume was more of an of the dryness of the discarded solids.
inconvenience than a significant economic burden.
If drilling fluid disposal volumes are to be main-
tained as low as possible (i.e., no drilling fluid DETERMINING REMOVAL EFFICIENCIES
jetted from the pits), the removal efficiency must
be improved. Otherwise the 4% volume of drilled If a well is spudded with fresh water and the
solids will not be attained. pit levels remain constant during drilling, the drilled
The minimum discard volume will occur when solids returned to the system will increase the
the system is "balanced" (i.e., no excess drilling density. This analysis assumes that no whole drill-
fluid is needed to dilute the drilled solids return- ing fluid is emptied from the system or lost down-
ing to the system). The same solids removal effi- hole. As the borehole becomes deeper, the mud
ciency that provides the minimum quantity of new weight increases at different rates for different re-