Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to rheometers and in
particular to an improved bob design for a high
temperature, high pressure rheometer of the Serle (rotating
bob) type.
There are many problems a~ociated with high
temperature, high pressure rheology measurements.
The first is the lengthy time required to
establish temperature uniformity within the pressure cell.
~his is due to the thermal inertia of the large, heavy bob.
Also, when measuring rheology of high viscosity fluids at
high shear rates, the di~sipation of frictional heat poses
additional temperature distribution uncertainties.
An equally serious problem to particle settling
and thermal effects is the tendency of slurries and
certain polymer solutions to develop gel structures if not
continuously sheared. While some rheological experiments
are aimed at obtaining information on the development of
these gel structures, in other cases their formation
interferes with the desired measurement of the shear
rate/æhear stress relationship o~ the fluids.
Another problem encountered with rotational
rheometry of slurry is the weight of the bob and
accompanying friction and wear on the supporting shaft.
The determination of bulk slurry rheology requires a
relatively large gap, many times the particle diameters, to
permit averaging. This in turn necessitates a large
diameter bob in order to have uniform shear stresses and
shear rates across the gap and to minimize turbulence at
high shear rates.
Attempts have been made to alleviate the above
problems by stirring ths test sample. For instance, US
Patent 4,524,611 issued June 15, 19~5 to Richon et al.
describes a rheometer wherein the test sample can be
agitated prior to a measurement of rheological property, to
maintain homogeneity. This was deemed particularly
important in the measurement of the rheological properties
of slurries, although, the same reasoning would apply to
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emulsions. From the standpoint of the present invention,
it is noteworthy that in the Richon apparatus there is no
displacement of the fluid in the measuring gap whils the
measurements are being made. This necessitates short
measurement time when rapidly settlin~ particlss are
present.
It is an object of the present invention to
improve on the ability of a rheometer to determine the
rheological properties of drilling fluids, fracturing
fluids, and cementing slurries at high temperatures and
pressures, while minimizing the aomplexity, si~e, and
associated expense of equipment re~uired to meet the
desired objectives.
In general terms, the invention provides a
rheometer bob mecha~ism for use in a stationary vessel of a
high pressure rheometer with a rotatiny bob, including
means for reactive torque measuring, the mechanism
including a rotor-stator pump means disposed internally of
the bob, wherein an internal part of the bob is a rotor
portion of said pump means, said pump means furthe~
including a stator portion adapted to be held stationary
relative to the vessel whereby, as the bob rotates,
circulation of the test sample up or down an internal
channel in the bob, and slow but positive displacement of
the test sample in a bob/vessel wall gap is effected.
The invention will now be described by way of a
preferred embodiment, with reference to the accompanying
drawings, wherein;
Figure 1 depicts, in a simplified, diagrammatic
fashion, a cross-sectional view of the general mechanical
arrangement of a known rheometer;
Figure 2 depicts a similar cross-sectional view
but showing, on an enlarged scale, the lower part of the
device of Figure 1, incorporating an exemplary embodiment
of the pre~ent invention.
The invention is described by way of describing
the modification of the rotating bob of an existing
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rheometer to accommodate a stator attached to the
stationary vessel or cell, so as to permit the construction
of a pumping mechanism within the bob. As the bob turns,
the pumping mechanism circulates the test sample up or down
the internal bore and flow channels of the rotating ~ob and
returns it via the gap between the rotating bob and the
vessel wall, i.e. through the gap at which the actual
measuring of shear rate takes place.
Two criteria must be met in the design o~ the
pump: First, the shear rate in the gap due to circulation
must be small compared to the shear rate due to the bob
rotation. Simple vector analysis indicates that a
circulation shear rate of 20% of the rotational shear rate
gives a resulting shear rate only 2% higher than the shear
rate due to rotation alone. The second criterion is that
the torque required to provide circulation must be small
compared to the torque provided by the shear of the test
sample in the bob/vessel wall gap. It is also desirable
for the shear rate in the pumping section to be similar to
the shear rats in the gap.
In order to illustrate the application oE these
design features, Figure 1 is provided to show a high
pressure rheometer suitable for utilizing the improved bob
arrangement. The rheometer contains the essential features
of Delorey in Canadian Patent 1,223,458, along with some
minor improvements.
A DC motor generator 16 provides means of applying
and monitoring the torque to a rotational bob 28. Motor
flange 8 is attached to the motor housing by screws 29, and
screwed to a motor support 9. The motor shaft is fixe~ by
means of a set screw 19 to an outer magnet assembly 5.
Non-magnetic housing equipped with oil filling port 27
provides a pressure barrier to an inner magnet shaft
assembly 4. The inner magnet assembly is supported by
bearings 17, 18 and attaches to a mechanical shaft coupling
. High pressure seals 24 and a bearing support ring 7
complete the magnet assembly.
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A low pressure sealing ring 23 retained by bushing
15 and held at its lower limit of travel by a spring 13
provides means o-f keeping the magnetic drive section filled
with hydraulic medium, such as low viscosity mineral oil.
The magnetic drive assembly attaches to a pres~ure vessel
top 10 which contains a pressuring port 25 and air venting
port 26. A retaining ring 1, a pressure vessel body 11, a
base plug 12, a shaft supporting plug 2 and high pressure
seals 21, 22 and 20 complete the high pressure vessel or
cell.
As in other high pressure rotational rheometers,
solid metal bob 28 rotates within the pressure vessel body
11 and is attached, via set screws, to the shaft 14 made
from hardened metal. A small hole 30 in the shaft permits
replacement of the metal bob by an alternate mixing paddle
for determining consistenay. The test sample surrounds the
bob within the pressuxe vessel body 11 and forms an
interface with the pressurizing oil in the vicinity of the
mechanical coupler 3 or fills the entire pressuring line to
an external sample pressuring cylinder. The pressure
vessel sits in a heating/cooling sleeve and a thermocouple
is inserted in thermocouple well 31 for temperature
monitoring and control purposes.
Referring now to Figure 2, the solid bob 32 has
been partially bored out from the top and the internal
surface so created machined with a shallow spiral groove
32a. Four large flow channels 32b to permit movement of
the fluid to the bottom of the bob are also provided. On
the hub of the bored out bob 32 sits a Teflon (TM) o-ring
37 which serves as a support for the stator portion of the
pump which consists of a Teflon ~TM) cylinder 33, the
outside surface of which also has a shallow spiral groove
with the reversed thread to the thread of the groove 32a in
internal surface of the rotary bob 32. The top surface of
the stator portion contains grooves which are used to hold
the stator in a stationary position by means of a ~ushing
assembly 34, 36, 35. The metal bushing assembly consists
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of two threaded part.s 34, 35 which, when screwed together,
expand the elastomeric o-ring 36 against the bore of the
pressure vessel top. Thus, in the embodiment just
described, the reverse spiral grooves on the stator
disposed internally of the bob and th0 internal part of the
rotary bob form a progressive cavity pump means which, when
the bob is rotated, causes up o~ down circulation of the
test sample.
The invention permits the circulation of test
fluid through the measuring gap and returning by means of
the centre of the ~ob, while a rheological property is
being measured.
An equally acceptable embodiment of this invention
would have the stator sitting on the base of the pressure
vessel and the bored out bob attached to the paadle shaft
by the hub at the top of the bob. The stator could simply
sit on pegs to hold it stationary. In both embodiments of
this invention the test fluid would be sub;ected to
shearing action similar to that in the bob wall gap. Also
these two embodiments result in a design ~hich is easily
machined and convenient to clean after use.
Similar embodiments utilizing stator blades and
rotor bladee could be envisioned and may be desirable for
fluids requiring greater agitation than that provided by
the shallow grooved stator and rotor. The cleaning and
machining difficulties, however, would be considerably
increased.
Xt can be seen from the above that many
embodiments of the present invention may exist which differ
from the described embodiment without departing
from the inventive concept. Accordingly, I wish to protect
by Letters Patent which may issue on this application all
embodiments which properly fall within the scope of my
contribution to the art.
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