Note: Descriptions are shown in the official language in which they were submitted.
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YERSATILE PRESSURIZED CONSlSTOMETER/RHEOMETERtFLUID LO~S APPAkA~US
Background of the Invention
This invention relates to a design of an apparatus primarily inten-
ded to serve as a pressurized consistometer suitable for determining the
thickening tirne of cement slurries under simulated temperature and
pressure conditions encountered in the cementing operations involved in
oil and gas well completions. A modification is described however, which
enables the apparatus to be used for determining the rheological
properties of a wide variety of viscous fluids. A second modification is
described whereby the apparatus can also be used as an in-line visco-
meter. A third modification is also described whereby the apparatus can
be used to determine dynamic or static fluid loss values for cement
slurries or other fluids at elevated temperatures. A fourth modification
is described which permits the apparatus to be used to condition cement
slurries at high temperatures and then transfer to a standard fluid loss
apparatus.
The thickening time of the cement slurry referred to in this des-
cription is defined as 'the time required for cement slurry of the given
composition to reach a consistency of 100 Bearden Units of Consistency
(~c), determined by methods outline by API Specification 1o~.l API
Specification 102 describes a stirring apparatus used for consistency
measurements and will be discussed in 'Description of Prior Art' section.
The thickening time of cement slurries is the most critical property
invo1ved in cementing operations. Although it is desirable to check the
thickening time of slurries prepared with the actual cement and mix water
to be used on a particular well, the high cost of existing pressurized
consistometers along with their large size makes this impractical in most
cases. One of the objects ot the apparatus in this invention is to
provide small field labs with the capability of performing these
pressurized thickening time tests.
. _
1. API ~ulletin lOC, "Well Cement Nomenclature", Second Edition, April
1979.
. API Spec 1~ aterial and Testing for Well Cements", Second
Edition, June I~, 1984.
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In addition to its use for determining cement slurry thickening
times, the apparatus can also be easily modified to permit its use in
determining rheological properties of a wide variety of viscous fluids.
The determination of rheological prop?rties involves the rneasurement of
the relationship between sheariny stress and shearing rate with time.
Such a study enables dilatancy or psuedo-plasticity characteristics to be
determined as well as detecting thixotropic and rheopectic hehavior. As
in the case of the thickening time tests, these studies can be carried
out at elevated temperatures and pressures. Also, the apparatus can be
used in determining the viscosity of visco-elastic fluids since the
entire fluid sarnple is sheared uniformly and the opening around the
driveshaft is very small.
While many applications for such measurement capabilities exist in
the study of cement slurries, drilling fluids, and fracturing and
acidizing fluids in use in oil and gas well drilling, completion, and
stimulation, there are many fluids encountered in chemical processing,
mining, etc., where these same properties are of great importance and can
likewise be measured with the apparatus of this invention.
A second modification of this equipment would permit its use as a
flow-through or inline viscometer. Again, while there are many uses for
such an apparatus in oil and gas operations, it would find great utility
in other areas as well.
A third rnodification of this equipment permits its use to determine
the fluid loss of cement slurries. In the case of static fluid loss, the
cement slurry is gently agitated while being heated to temperature and
then unstirred during the fluid loss test. In the case of dynamic fluid
1OSSJ a high speed impeller assembly is used which permits cement slurry
agitation during the duration of the fluid loss test.
A fourth moditication permits the apparatus to be used to condition
a cement slurry and then transfer it under pressure to a standard high
temperature, high pressure fluid loss apparatus.
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DESCRIPTION OF DR~WINGS
. _ . .
Figure 1 depicts the general design of a
conventional cement thickening time tester or
pressurized consistometer;
Figure 2 depicts the general design of the
Lmproved cement thickening time tester or
pressurized consistometer of this invention;
Figure 3 depicts the electronic components
of the motor generator control and measuring circuit;
~igure 4 depicts a modification of the apparatus
for in-line or continuous flow ~iscometry;
Figure 5 depicts the equipment arrangement for
application of the continuous flow viscometer in the
hydraulic fracture stimulation of oil and gas wells;
Figure 6 depicts a modification of the apparatus
for measurement of static fluid loss;
Figure 7 depicts a modification of the apparatus
for measurement of dynamic fluid loss;
Figure 8 depicts an equipment arrangement for
conditioning slurries for transer to a standard fluid
loss apparatus.
BAC~GROUND OF THE INVENTION
The operating principle of the apparatus which
today serves as the industry standard for determining
thickening time of cement slurries is essentially the
same as the instrument developed by the Pan American
Oil Company in the 1~40s. The objecti~e o~ the
pressurized consistometer or thickening time tester
is to measure the consistency of the cement slurry
which is gently agitated while being subjected to the
temperature and pressure conditions that would be
encountered in the cement operation involving an oil
or gas well. Construction details of such a thickening
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tLme tester are shown in Figure 1.
During the test~ the cement slurry being tested
is contained within cup 3 which is placed within high
pressure chamber 5 of the pressure vessel shown generally
as 1. Slurry cup 3 is rotated at a fixed 150 RPM and
agitation of the slurry is accomplished by stirring
paddle 7 within slurry cup 5. Paddle 7 is kept
~tationary by spring potentiometer mechanism 9 having
tongue spring 10. Potentiometer mechanism 9 also serves
to measure the torque being exerted on paddle 7.
Potentiometer mechanism 9 clicks into insulated electric
connectors which are drilled through the pressure vessel
wall of pressure vessel 1 and which serve to provide,
through oontact pin 33, the electrical signal for
recording the consistency of the cement slurry. Pressure
is supplied by oil from an air-driven hydraulic pump
(not shown). through oil-pressure connection 11. Oil is
separated from the cement slurry in slurry cup 3 by
flexible rubber diaphragm 13 and a Teflon O-ring assembly
(~not shown). The temperature of the cement slurry is
sensed by thermocouple 15 which enters pressure vessel
1 through top removable screw closure 17 and extends
down into the holl~w shaft of paddle 7 in slurry cup 3.
Heat is supplied by electric tubular heater coils 19
surrounding slurry cup 3 within pressure vessel 1.
Pressure-cylinder thermocouple 35, going into the oil
bath, can be used to proYide information about the rate
of heating. Again, current is supplied through electrical
connections made through contact pin 33.
Rotation of turntable 21 on which slurry cup 3
sits is provided by packed shaft 23 which extends
through removabie packing caxtridge 24 and through the
bottom of pressure vessel 1 to ~otorized gear drive
assem~ly 25 which includes thrust bearing 26, miter
gears 27 and gea.r reducer 28. Cooling jacket 29 is
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wrapped around the exterior of the pressure vessel,
and the entire assembly is then surrounded by glass wool
insulation 31 (only partially depicted). Other components
of the thickening time tester involve a complex arrange-
ment of valving to provide for oil-pressure Gontrol and
filling of the pressure Yessel (not shown in the drawing),
air supply pressure connection 34 for forcing the oil from
chamber 5 at the end of the test, and cylinder sealing O-
ring 32 for providing a pressure seal.
Over the years, se~eral improvements have been
made to this basic design in order to overcome some un-
desirable features. A magnetic drive unit is now
available which replaces packed shaft 23 and gear drive
assembly 25 for providing rotation of slurry cup 3.
This has reduced the maintenance with the packing and
eliminates the risk of fire due to leaks through the
packing. Flexible diaphragm 13 and the O-ring assembly
have now largely been replaced ~y a sLmple ~la~ high-
temperature rubber diaphragm. This minimizes ~he
problem of diaphragm 13 being compressed down against
slurry cup paddle 7 when entrained air is present inthe cement slurry. A potentiometer lock-down mechanism
has recently become available which overcomes ~he problem
of potentiometer assembly 9 sliding up off the slurry
paddle mechanical connection. Other improvements involve
the use of digital temperature displays and temperature
programmers which facilitate the control of the rate of
temperature rise during the test to simulate the
temperature rise which the cement slurry would experience
30- going down an oil or gas well.
Presently, there are several manufacturers of
thickening time testers;howe~er, they all use the same
~asic design details described above. For a short period
of time around l970, the Fann Instrument Company offered
a portable cement thickening time teste~ however it was
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not widely accepted by the industry. In that apparatus,
cement agitation was accomplished by periodically
magnetically lifting a heavy bo~ within the pressure
chamber and allo~ing it to fall through the cement
slurry. The time of rise and ~all of the bob was
electronically monitored and resulted in a rough measure
of the consistency of the slurry. It is suspected that
one of the reasons why this instrument did not gain
acceptance was the fact that the type of agitation is
different than the agitation provided by a conventional
thickening tLme tester. With some slurries the degree
of agitation has a profound effect on the thickening
time, so that results obtained with the Fann ins~rument
would not directly correlate with a conventional
thickening time tester.
It will be seen that in addition to use of the
apparatus of this invention as a thickening time tester,
a modi~ication will be described ~hich will permit its
use as a rotational-type rheometer. In 1963, Van Wasser3
published a review of viscosity and flow measurement
technology in which he outlines the operating principles
of all commercially available viscometers and rheometers
at that time. While many of the viscometers operate on
the rotational bob and cylin~er concept, all the
instruments rely upon either strain gauge or spring
mechanisms to monitor the torque on either the bob or
the cup. In the case of a pressurized rheometer, this
method of torque measurement complicates the design o~
the instrument.
3. J.R. ~anWaser, J.W. Lyons, K.W. Kim, R.E. Cowell,
"Viscosity and Flow Measurement, A Laboratory Hand-
book of Rheology", Monsanto Chemical Company,
Interscience Publishers, St. Louis, 1963.
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The two pressurized rheo~eters most widely used
at present are the Model 50 Rheometer manufactured by
the Fann Instrument Company and the BHC Rheometer
manufactured by OBI-Hughes Incorporated. The operating
principle and features of these instruments are described
in reference 4. It will be seen that the apparatus of
this invention is much less complex and simpler to
operate than the two described in this reference.
A modification to the invention will also be
described which will permit use of the apparatus ~or
determining fluid loss values of cement slurries.
Appendix F of Reference 4 descxibes` the test procedures
for determining fluid loss of cement slurries; however,
it will be noted that this test procedure is limited
to temperature below 194F. The problem with temperatures
greater than 194F is that the cement slurry must remain
unstirred in the ~luid loss cell while the temperature is
increased to the test temperature. It will be seen that
the apparatus of this invention oYercomes this limitation.
DISCLOSURE OF THE INVENTION
Pressurized Consistometer
The design features of the apparatus when used to
determine cement slurry thickening times or as a
pressurized consistometer are shown in Figure 2. One
essential feature of the apparatus is that slurry cup 100
which contains the cement slurry also serYes a$ the
pressure vessel. Another essential feature is that the
slurry cup/pressure vessel 100 is held stationary in
externally heated jacket 102 and mixing paddle 104 is
rotated. Studies carried out using clear acrylic plastic
slurry cups show that the degree of agitation obtained by
rotating the slurry paddle with a stationary cup is very
similar to that obtained by having a stationary paddle and
rotating the slurry cup.
4. API Bulletin 13D, 'The Theology of Oilwell Drilling
Fluids', First Edition, August 1980.
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Another essential feature of the apparatus is
DC motor generator 106, used to rotate paddle 104.
DC motor generator 106 has two sets of windings: the
generator windings are used to provide a signal for
measurement and control of rotational speed whereas
the motor windings are used to supply torque to the
unit and the motor current is used as a measurement of
consistency.
The electronic components required for the
measuring and control circuit are shown in Figure 3.
Clearly the accuracy and reliability of this apparatus
is dependent upon the quality of DC motor generator 106.
For cement slurry thickening time applications, a model
E650MG motor generator and speed control unit made by
Electrocraft Corp., Hopkins, MA, serves this purpose
very well. ~his drive system results in ~ery recisely
controlled rotational speeds which can be set to a wide
range of speeds. The current supply to the motor winding
required to maintain speed is a direct measure of the
load or torque on the motor and consequently this is
easily scaled to read directly in consistency readings.
A desirable feature of the apparatus is the use of
a magnetically coupled drive system having magnetic
coupling 108 coupled to drive shaft 109. This reduces
the background torque on paddle 104 which must be nulled
out in order to obtain consistency readings. Pressure
is imparted to the cement slurry by injection of oil
from a high pressure, air-operated pump (not shown)
through pressurizing port 110. The temperature can be
sensed either by thermocouple112 entering from the top
of the pressure vessel or by thermocouple 113 from the
temperature of heating jacket 102 surrounding slurry cup/
pressure ~essel 100. Not shown in the diagrams is the
possible use of ball bearing supports with the drive
system and a separation seal, from magnetic coupling 108
to slurry cup 100, to prevent air from getting into the
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oil-filled magnetic dri~e section.
Another desirable feature of this apparatus is
the lack of a separation diaphragm to prevent oil
contamination. The height of the slurry sample in
6 slurry cup 100 is adjusted on filling so that the oil/
cement interface ~ill ~e within the unmixed, restricted
area at the top of slurry cup 100. Also, because of
the low volume of space remaining above the cement
slurry, the use of high pressure ni~rogen as a
pressurizing media is quite feasible. This feature is
quite desirable when it is suspected that the oil would
have an undesirable effect on the cement slurry additives.
By eliminating the diaphragm, an important source of
frictional resistance which leads to errors in the
measured consistency is eliminated. Another desirable
feature of the apparatus is the fact that the slurry
cup dimensions and the mixing paddle configuration is
identical to that of the conventional thickening time
testers and since the speed can also be adjusted to 150
RPM. The degree of agitation of the two instruments and
the measured consistency should be in good agreement.
Weight reduction in the apparatus shown is
maximized by restricting the pressure rating of the
apparatus to appxoximately 15,000 PSI. While there will
be a small percentage of wells being drilled which
require a higher pressure for true simulation, it is felt
that correlation factors can be determined using
conventional thickening time testers. Weight reduction
is also accomplished by the use of high strength metals
such as titanium or high-strength iron-nickel alloys metals
as the pressure barrier in the magnetic coupling suction.
Also, the weight of slurry cup 100 is reduced by using metals
such as titanium or Series 400 stainless steel as the
material of construction. The vertical spacing between
3S slurry cup lOQ and magnetic coupling 108 is preferably
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an easily machined stainless steel which reduces
~abrication costs. The material for heating jacket 102
is preferably solid brass or aluminum surrounded by a
choice of heating elements. Because of the small volume
of oil required to pressurize slurry cup 100 and the
drive assembly, no valving is required for araining and
transferring of oil, thereby simplifying piping and
valving arrangements, this provides a further reduction
in cost.
Pressurized Rheometer
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When it is desirable to use the apparatus as a
pressurized rheometer, illustrated qenerally as 107 in the
embodiment depicted in Figure 4, mixing paddle 104 is
replaced by hollow oil-filled corrosion resistant metal
bob 105. The shape of bob lQ5 is designed so that at
any rotational speed, the shear rate on the sample in the
gap between rotating bob 105 and the inside ~alls of
slurry cup lOO.,s essentially constant. Because of the
length:diameter ratios, most of the measured torque will
result from the shearing of the fluid at the vertical
bob walls, so that end effects which are normally
difficult to correct for in rotational viscometers, will
be minimized. Also, when used as a rheometer, the
electronic controller for motor generator 106 is provide~
with a speed xeadout and a torque readout scaled to give shear
rate and shear stress in convenient units. The electronic
controller meter can be modified to facilitate the
determination of shear stress/shear rate relationships
using a multi-position rotary switch designed to provide
a range of fixed shear rates, each position having its
own no-load zero adjusting potentiometers. Alternatively,
the reference voltage used in speed control can be re-
placed by a ramping voltage which will continuously cycle
the shear rate upwards and downwards to investigate
thixotropic or rheopectic phenomenon.
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When a filling port is desirable, bottom high
pressure plug 111 in the thickening time arrangement
can be replaced by a valved fill-port. Since slurry
cup 100 is not rotating, excess fluids can be vented
through oil pressuring port 110. In some instances
where it is desirable to measure the rheology of low
viscosity fluids, a wide assor~ment of bob and cup
designs can be used. In these cases, the cups could
be inserted into slurry cup lOQ and the particular bob
substituted for the cylindrical bob.
In the case where the apparatus is not to be used
~or cement slurry thickening times but rather, strictly
as a pressurized rheometer, the dimensions of slurry
cup or pressure vessel 100 can be greatly modified to
achieve more suitable ranges of torque for measurement.
Figure 4 shows one 2mbodiment where the pressure vessel
100 is equipped with inlet ~01 and outlet 103 and
rotational bob 105 is elongated to permit continuous
flow or on-line measurements.
Figure 5 shows the details of the possible use
of a pressurized rheometer on the high pressure lines
of an oil or gas well hydraulic fracturing stimulation
treatment. A sample line is taken from the high pressure
lines which extend to the pumpers. Low pressure lines
extend from the ~lender which is in fluid connection
with the fracturing fluid tanks and a proppant tank.
The on-line viscometer 107 depicted in Figure 4 can be
calibrated and placed near the wellhead, which is a
hazardous area, and the electrical shear stress or
viscosity signal run by cable to a recorder in an
instrumentation van. This arrangement would avoid
time delays in the monitoring of the viscosity during
the fracturing treatment. In the use of the apparatus
as an on-line viscometer during fracturing treatments,
the gap between the rotor and the pressure vessel wall
can be selected so that the influence of proppant
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particles will not result i~ erroneous readings.
In some rheological studies on thi~otropic or
rheopectic fluids, it is desira~le to follo~ the shear
rate at a constant shear stress with tLme. As indicated
by the dash-line in Figure 3, the electronic control
measuring circuit can be readily modified to accomplish
this end. In the case where rh~ological studies are not
re~uired to ~e conducted under pressurized conditions,
magnetic coupling 108 is not required,so that motor
generator 106 and the electronic control circuit can be
used with a ~ide variety of slurry cups and bobs or with
bar or impeller-type stirrers. The diameter and length
geometry of these devices can be modified to cover an
extremely wide range of shear stress and shear rate
conditions.
In addition to the use of apparatus as a
consistometer or rheometer, since slurry cup/pressure
vessel 100 is not being rotated and has connection port
111 at the bottom, it is apparent that simple side-sealing
~ilter screen assembly 115 can be lowered into pressure
vessel 1-00 so that Yessel 100 can he converted into a fluid
loss cell. It is also apparent that, if retaining
Teflon washer 117 is incorporated at the top of the
slurry cup, the pivot point from the botto~ of
paddle 104 can be eliminated so that a variety of paddle
designs can be accommodated. Alternatively, paddle 104
can be threaded to drive shaft 109 and thereby could be
suspended above filter screen 115. Figures 6 and 7 show
two possible mixing paddle arrangements.
In Figure 6, standard slurry paddle 104 is used
except that the bottom cross member and central shaft
are eliminated. By so doing, the build-up of filtercake
during the fluid loss test will not be interfered with
by the presence of the paddle. This particular design
is well suited to the performance of static fluid loss
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tests above 194F, as described in Reference 2. In this
case, the 1urry can be agitatecl a~ 15a RPM as the
temperature is raised to temperatures in excess of 194F.
The slurry is maintained under pressure by nitrogen
applied through oil-filling port 110. Once the final
desired temperature is obtained, the stirring action
can be discontinued and a static fluid loss test
performed. Since the main application for this apparatus
involves temperatures above 194F, the use of back-
pressure receiver 117 pressured to 500 PSI with nitrogen
gas through valve 118 is used to prevent the water
filtrate from boiling. In order to compensate for the
5Q0 PSI back pressure, slurry cup/pressure vessel 100
is pressurized using 1500 PSI nitrogen. This gives the
same lQ00 PSI pressure differential across 325-mesh screen
115 which is used in the fluid loss test below 194F. If
fluid loss values are anticipated to be high, the tests
can be carried out for a short period of time taPProximately
10 minutes) and after this period, isolation valve 119 can
~e closed and following a sufficient cool-down period, the
water present in the back pressure receiver can be drained
through valve 120 and measured. The filtrate volume
measured must be adjusted to correlate to the area
described in the high pressure, high temperature cell
in Reference 2.
When it is desired to conduct dynamic fluid loss
tests, an impeller design such as shown in Figure 7 is
more desirable. In this case, impeller 121 is rotated
at high speed (approximately 1000 to 2000 RPM) during
the heat-up stage of the fluid and during the fluid loss
test. With such an apparatus, the effect of slurry flow on
on fluid loss should ~e evident.
In the case where dynamic or static fluid loss
tests are to be performed on drilling fluid, it is
desirable to use hardened filter paper in place of 325-
mesh screen 115. In this case, an O-ring seal on the
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filter paper is required which necessitates the
inclusion of a second threaded ring-type seal in
the screen assembly.
Since most labs involved in oil and gas well
ce~enting already have a fluid loss apparatus for
static fluid loss testing, the arrangement shown in
Figure 8 is presented. In this case, the consistometer
is used to condition the slurry to the desired temperature
and after a specified time, the slurry is transferred through
flexible hose ~nd valve system 1~2 to the preheated
cell of standard high-temperature high pressure fluid
loss apparatus 123. The fluid loss is then measured
immediately.
