Page 135 - Shale Shakers Drilling Fluid Systems

118 SHALE SHAKERS AND DRILLING FLUID SYSTEMS

sand can be used, however, sand does not have both shakers receive fluid with the same amount
similar particle sizes as shale. Additionally, the as- and type of solids. Turns in the flow line or other
pect ratio of sand is different than shale, therefore, sources of turbulence can create flow disturbances
their movement across the shaker will differ. An- (such as pipe constrictions due to barite or drilled
other concern with using laboratory testing is the cuttings settling) that alter the solids distribution
problem with reproducing the rheological proper- in a moving stream. Solids distribution not only
ties of drilling fluids. These properties can sub- controls the solids concentration but also the
stantially vary from field drilling fluids even if the rheological properties of the drilling fluid. In the
plastic viscosity and yield point are the same. Field field, accurate measurements of drilled solids ar-
drilling fluids generally have a higher percent vol- riving at the shakers indicate a significant varia-
ume of drilled solids. For this reason, field drill- tion in quantity. Changing all of the flow from one
ing fluid particle-size distributions will be skewed shaker to the other eliminates many of the prob-
toward the smaller sizes when compared to labo- lems associated with the division of solids. If this
ratory-prepared drilling fluids, which will affect method is attempted in the field, measurements
fluid movement across the shaker screen. Conse- must confirm that the flow stream and the fluid
quently, laboratory measurements of shale shaker rheological properties are relatively the same dur-
performance can be reproduced but may have no ing the testing interval. Each machine should be
significant relationship to field performance. sampled alternately three or four times to deter-
mine the effect of the uncontrolled variation in
Field testing. Field testing requires careful plan- cuttings arriving at the surface.
ning and awareness of some variations in solids
behavior. When comparing the performance of two Problem #4. It is often assumed that the fluid lost
shakers, an attempt to split the flow from the well with the cuttings is the same as the fluid in the cir-
in two equal streams for a side-by-side compari- culating system. With high-speed shakers, variable
son is difficult to obtain even under the best con- deck angles, and fine screens (flat and corrugated),
ditions. Even a slight degree of unequal flow will the fluid component discharged is generally differ-
cause uneven solids loading on the shakers. As an ent than the drilling fluid in the circulating system.
alternative, the entire circulation can be directed The fluid type, pool depth, and dry beach area on
from one shaker to the other shaker but this proce- the last screen panel are important factors that de-
dure has its share of problems. termine the fluid composition associated with the
cuttings. All discharge from the screens must be
divided into low-gravity solids, weighting material,
Example Problems water, and oil. The specific gravity of each must
be known to use the gravimetric cuttings analysis
Problem #1. Drilled cuttings are transported to in Chapter 10.
the surface by the drilling fluid, however, the rate Because of the problems associated with cut-
at which the cuttings are generated and the rate tings collection and analysis, regular sampling and
they arrive at the surface are not the same. This analysis of the shaker screen discharge can only
is caused by variations in annular flow patterns, be considered a worthwhile measurement when
mud temperature, cutting size and density, hole used to determine trends of solids removal. The
deviations, drilling methods, pipe and hole geom- equipment used for the collection of shale shaker
etry, riser boost systems, and so forth, resulting discharge ranges from the very sophisticated and
in a different rate of cuttings being presented to complex to the relatively simple and straightfor-
the machines. ward. Because of the variables discussed previ-
ously, the general consensus among participants
Problem #2. In most cases, the hole diameter in solids evaluation is that the results achieved
does not equate the bit diameter. Hole enlarge- with complex, expensive equipment yield as much
ment is dependent on many factors, ranging accuracy as those achieved with simple proce-
from fluid type and chemistry to hydraulic and dures (i.e., a split PVC pipe is as accurate as an
mechanical factors. Although the drilling rate may automatic sampler).
be relatively constant, this does not ensure that
the cuttings volume reaching the shale shakers
will be the same as the rate when the cuttings Evaluation
were generated.
Usually, tests are performed to determine the
Problem #3. If the flow is split equally between quantity of drilled solids discarded by a shale
two shale shakers, care must be taken to ensure that shaker. However, there are many important facets