Page 187 - Shale Shakers Drilling Fluid Systems

CHAPTE R NIN E

Cut Points

Cut points are used to indicate the performance The screen throughput sample and the discard
of solids control equipment. Cut point curves are sample are sieved through a stack of standard
determined by examining the mass of a variety of ASTM screens. The solids retained on each size
size ranges that are discarded relative to the same screen are dried and weighed. The mass percent
mass and size range presented to the removal of solids in the throughput sample in each size
equipment. Curves can be calculated by determin- range larger than 400 mesh is then determined.
ing the ratio of the mass flow rate of solids within The flow rate through the shaker screen is ap-
each size range in the discard stream, divided by proximately 4800 Ib/min (500 gpm x 9.6 Ib/gal).
the mass flow rate of solids of the same size range The 5-gallon bucket used to sample the drilling
in the input stream. fluid passing through the screen, samples only
Cut points for shale shakers may be measured about 0.01 minute of flow. The weight of the drill-
by first determining the particle size distribu- ing fluid in the bucket is measured and if the fluid
tion of the input stream and the overflow stream. weighs 48.0 pounds, the volume in the bucket is
Generally, the distributions may be determined, 5.0 gallons. This fluid is then sieved through a
with a stack of sieves. The flow rate of each stream stack of standard ASTM screens and the dry weight
is then determined and the mass flow rate for each of the solids on each standard screen is deter-
selected size range in both streams is calculated. mined. The mass flow rate of the dry solids for
The mass flow rate of the discard stream for each each sieve can be determined by dividing the weight
size range is divided by the mass flow rate of the of the solids by the time interval of the collection
same size range presented to the shale shaker. (or 0.01 min.)
Usually, the sample from the input stream repre- All of the shale shaker discard is collected for
sents a small fraction of the total flow. This creates a specific period of time and weighed. This discard
a problem with material balances. A better method is sieved through the same set of standard ASTM
is to sample the discard and the screen through- screens. The dry weight of solids on each screen
put streams. Combining these two solids distri- is determined and divided by the time interval
butions will yield a more accurate cut point curve. required to capture these solids. For example, if
This technique is also used on hydrocyclones where the collection period is two minutes, each dry
the input stream flow rate is much greater than weight would be divided by two minutes, which
the discard stream. yields the weight per minute c f J oL.ard.
As an example, assume all of the discard from The mass flow rate of the discard and through-
a shale shaker is collected during a one-minute put streams are combined for each size range.
period while drilling with a 9.6 ppg drilling fluid This creates the mass distribution of the input
circulated at 500 gpm. At the same time, the flow stream. All solids calculations are gravimetric since
stream through the shale shaker (also referred to volumetric measurements are inaccurate.
as the "screen unders") is sampled. The weight of The mass flow rate onto and off the screen is
the drilling fluid and the solids content is deter- known, therefore a "cut point" curve may now be
mined gravimetrically. Since this drilling fluid con- established. The ratio of mass flow onto the screen
tains no barite, the fraction of solids in the fluid divided by the mass flow off the screen can be
is specified by its density and the specific gravity calculated for the solids remaining on each sieve.
of the low-gravity solids. In this case, the solids A typical set of data is shown in Table 9-1.
content is 9.5% volume, indicating a low-gravity These data, presented in a graph form, create the
solids specific gravity of 2.6. cut point curve illustrated in Figure 9-1.

170