Note: Descriptions are shown in the official language in which they were submitted.
~ CA 02220972 1997-11-20
HOMOGENIZER/HIGH SHEAR MIXING TECHNOLOGY FOR ON-THE-FLY
HYDRATION OF FRACTURING FLUIDS AND ON-THE-FLY
MIXING OF CEMENT SLURRIES
FIELD OF THE I~VENTION
The present invention relates to the mixing of
chemical agents and base fluids to form well treatment fluids
and more particularly to a method and apparatus for
continuously mixing such fluids including, but not limited to,
fracturing and acidizing gels, polyemulsions, foams and cement
slurries, on a real time on-the-fly basis.
BACKGROUND OF THE INVENTION
High viscosity aqueous fluids, such as fracturing
gels, acidizing gels, cement slurries and high density
completion fluids, are commonly used in the oil industry for
the treatment of subterranean wells. The gels for example are
normally made using dry polymer additives or agen~ wh~ch are
mixed with water or other aqueous fluids at the job site.
Mixing procedures used in the past have resulted in a number
of problems. For example, early "batch" mixing procedures
invol~ed mixing bags of powdered polymer in tanks at the job
site. This resulted in uneven and inaccurate mixing, lumping
of the powder into insoluble balls or globules which
obstructed the flow of the gel, chemical dust hazards, and
required the transport and use of huge tanks adding greatly
t~ costs
A known method of solving the lumping, gel ball
problem is to delay hydration long enough for the individual
polymer particles to disperse and become surrounded by water
so that no dry particles are trapped inside a gelled coating
to form a gel ball This delay is achieved by coating the
polymer with material such as borate salts, glyoxal, non-
lumping HEC, sulfosuccinate, metallic soaps, surfactants, or
other materials of opposite surface charge to the polymer.
CA 02220972 1997-11-20
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Another known way to improve the efficiency of
polymer addition to water and derive the maximum yield from
the polymer is to prepare a stabilized polymer slurry, also
referred to as a liquid gel concentrate. The liquid gel
concentrate is premixed prior to transport and then later
added to the water at the well site. In Briscoe U.S. Patent
No. 4,336,145, a liquid gel concentrate ls disclosed
comprising water, the polymer or polymers, and an inhibitor
having a property of reversibly reacting with the hydratable
polymer in a manner wherein the rate of hydration of the
polymer is retarded. Upon a change in the pH condition of the
concentrate such as by dilution and/or the addition of a
buffering agent (pH changing chemical) to the concentrate,
upon increasing the temperature of the concentrate, or upon
a change to other selected conditions of the concentrate, the
inhibition reaction is reversed, and the polymer or polymers
hydrate to yield the desired viscosified fluid. This reversal
of the inhibition of the hydration of the gelling agent in the
concentrate may be carried out directly in the concentrate or
later when the concentrate is combined with additional water.
The aqueous-based liquid gel concentrate of Briscoe has worked
well at eliminating gel balls and is still in routine use in
the industry. However, aqueous concentrates can suspend only
a limited quantity of polymer due to the physical swelling and
viscosification that occurs in a water-based medium.
Typically, about 0.8 pounds of polymer can be suspended per
gallon of the concentrate.
By using a hydrocarbon carrier fluid for the slurry,
rather than water, higher quantities of solids can be
suspended. For example, up to about five pounds of polymer
can be suspended in a gallon of diesel fuel carrier. Such a
liquid gel concentrate is disclosed in Harms and Norman U.S.
Patent No. 4, 722, 646. The hydrocarbon-based liquid gel
concentrate is later mixed with water at the well site in a
manner similar to that for aqueous-based liquid gel
CA 02220972 1997-11-20
concentrates to yield a hydrated viscosified fluid, but
hydrocarbon-based concentrates have the advantage of holding
more polymer. Proppants can be added to the hydrated gel
prior to injection of the fluid down the well bore. Elevated
viscosities in the treatment fluid are required to maximize
its proppant carrying capacity and to minimize leak off into
the treated formation during the high pressure fracturing
phase of the operation.
An additional problem with prior methods using
liquid gel concentrates is the time required for the polymer
material in the concentrate to fully hydrate, i.e., absorb
water. Without complete or near complete hydration, fluid
viscosities will be inadequate to maximize proppant
concentrations and to minimize leak-off. As well, without
complete hydration, the addition of buffer or pH adjusters for
cro~-li~ki~ will actually prevent full hydr~tion. Without
agitation of the gel/water mixture, full hydration requires
at least 15 minutes of residence time, necessitating the use
of huge and difficult-to-transport storage vessels capable of
holding a "batch" of sufficient quantity to complete the job
at hand. Batching is expensive because of wasted time and
unused fluid resulting from treatment delays, termination of
the treatment before pumping all fluids, and fluid residues
remaining at the bottom of the storage tanks that cannot be
pumped out. The disposal of unused gelled fluid has also
become an expensive process due to stricter laws on the
disposal of chemical wastes.
More recently, it has been proposed to effect real
time or on-the-fly hydration of a gellable fluid for well
treatment operations by increasing the residence time of the
gellable flow in a flow-through operation through a series of
vertical flow tanks. The hydratable gel material is mixed
with water at the beginning of the series o~ tanks and, in
theory, the mixture passes through the tanks in a "plug flow"
which allows the gellable material sufficient time to hydrate
CA 02220972 1997-11-20
in the aqueous mixture. Such a system is described in U.S.
Patent No. 4,828,034 (Constien) for achievin~ substantially
complete hydration of the hydratable gel. Systems like that
of Constien however, as actually used in the field, still
typically require a blender tub operating volume on the order
of 200 barrels to obtain sufficient residence time for full
hydration. A 200 barrel blender tub moreover makes an
extremely large unit difficult to transport to the field.
Yet more recent approaches are described in U.S.
Patents 5,046,856 and 5,195,824. McIntire in '856 proposes
hydration of a hydratable gel by achieving near absolute
theoretical plug flow through a plurality of tanks in series
fluid communication. The plug flow is accompanied by high
shear of the hydratable gel along its flow path through the
series of tanks using a radial flow impeller positioned within
at least one of the tanks. In practice, some or even all of
the tanks are provided with mixing impellers.
Stromberg in '824 proposes to reduce the size of the
apparatus required by the McIntire process by subjecting the
hydratable/gel water mixture to "high shear rotary mixing".
High shear rotary mixing means are disposed in a blender tub
divided into first and second zones with first and second
mixers disposed in the first and second zones. The plurality
of rotary mixers provides a total circulation flow rate at
least an order of magnitude greater than the mixture flow rate
through the tub so that an average fluid particle of the
mixture passes through the mixers a total of at least 10 times
while passing through the blender tub.
A slightly modified approach is taught by Wilson in
U.S. Patent No. 5,052,486. Wilson initially applies a
relatively low level of mixing energy to the hydratable
gel/water mixture and then allows this mixture to flow through
a first compartment of a residence tank for approximately 45
seconds, after which the mixture enters via plug flow into a
second recycle compartment. The product in the recycle
CA 02220972 1998-10-14
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compartment is withdrawn in portions. The withdrawn
portion is subjected to high shear and is then returned to
the recycle compartment. This reoccurs until fully
hydrated product is introduced into an exit compartment.
Wilson claims a reduction of residence time to three
minutes or less.
It will be appreciated that all of these on-the-fly
methods still require residence times measured on the order
of minutes. Residence times of this magnitude remain
sufficiently large that storage containers or high volume
manifolds are required. Stromberg requires a blender tub,
McIntire teaches the use of a "plurality of tanks" arranged
in series for plug flow, Wilson requires a multi-
compartmented mixing chamber, and Constien requires plug
flow holding tanks. Whatever the nature of these
reservoirs, they are all prohibitively expensive, difficult
to clean, and all require specialized and expensive
mounting for transport to the field. For systems designed
to deliver 4 m3 to 10 m3 of hydrated gel per minute, a
residence volume of 12 to 30 m3 to 16 to 40 m3 is required.
Even though these volumes represent a substantial reduction
in holding capacity compared to "batch" systems, this
remains a substantial volume of fluid for disposal if a job
screens out. Moreover, for flow rates of 40 bbls/min.
(approximately 6 m3/minute) and water at 80~F, even with a
70 barrel hydration volume for approximately 2 minutes of
residency, 218 horsepower of agitation is required.
(Society of Petroleum Engineers Paper No. SPE 21857,
"Modeling the Effects of Time, Temperature and Shear on the
Hydration of Natural Guar Gels", Stromberg, J.J., et al.)
Lower agitation reduces horsepower requirements, but
residency times then go up and so must residence volumes.
Hutchinson in U.K. 1, 500,901 describes a method of
forming a hydrated colloidal suspension by passing the
mixture through a colloidal mixer in which the material is
subjected to shearing at a shear rate in the range of from
1,000 to 500,000 S-l. Hutchinson's method however does not
require the application of back pressure to the mixer for
CA 02220972 1998-10-14
on-the-fly hydration of liquid polymer concentrates in
accordance with the present method.
There remains therefore a need for a true on-the-fly
high shear mixing technology that achieves complete or
near-complete hydration of a hydratable gel in the
residence time available during normal fracturing
operations in a system having no residence volumes in the
form of large tanks or manifolds and which requires
significantly fewer horsepower (e.g. 40 to 100 H.P.)
SUMMARY OF THE INVENTION
The applicant has discovered that in-line high shear
rotor/stator mixers (sometimes also referred to as
homogenizers or emulsifiers) can be used to hydrate gel
slurries on the fly without the need for hydration units
having or requiring large residence volumes.
In a preferred embodiment, a base fluid, normally
water, is pumped directly into a high shear in-line mixer
with gel concentrate added to the water supply line by
means such as an injection Tee or venturi. Back pressure
is maintained in the pipeline mixer either by means for
example of an elongated hose from the mixer to the blender
where proppants are added to the hydrated gel or by means
of a restriction or gate valve downstream of the mixer or
a combination of the two.
Accordingly, it is an object of the present invention
to provide a method and apparatus for the mixing of
chemical agents with base fluids that obviate and mitigate
from the disadvantages of the prior art.
It is a further object of the present invention to
provide a method and apparatus for the complete or near-
complete on-the-fly hydration of a hydratable gel
concentrate or cement slurry without the need for residence
volumes in the form of tanks or reservoirs.
According to the present invention then, there is
provided a method of rapidly hydrating a liquid polymer
concentrate to form a fluid for treating a subterranean
formation, comprising the steps of combining an effective
amount of said concentrate with a hydrating fluid to
CA 02220972 1998-10-14
ultimately yield a treatment fluid having a viscosity
within a predetermined range; supplying said concentrate
and hydrating fluid into high shear rate mixing means for
mixing thereof at a predetermined shear rate to rapidly
hydrate said concentrate; applying a predetermined back
pressure to said concentrate and said hydrating fluid in
said mixing means; and directing the flow of hydrated fluid
away from said mixing means for eventual use treating said
formation.
According to another aspect of the present invention,
there is provided a method of rapidly hydrating a
hydratable gel to form a hydrated fluid comprising the
steps of dispersing a predetermined quantity of said
hydratable gel into a hydrating fluid to form a mixture;
supplying said mixture to high shear rate mixing means for
shear mixing of said mixture at a shear rate of at least
about 25,000 s-l to rapidly hydrate said hydratable gel; and
directing the hydrated fluid away from the mixing means at
a controlled rate to maintain a predetermined back pressure
in said mixing means.
According to yet another aspect of the present
invention, there is also provided an apparatus for
hydrating a hydratable fluid to form a hydrated fluid
comprising high shear rate mixing means for shear mixing
said hydratable fluid and a hydrating fluid therein to
rapidly hydrate said hydratable fluid, said mixing means
having an inlet and an outlet; supply means for introducing
a mixture of said hydratable fluid and said hydrating fluid
to said inlet; means for regulating the flow of said
mixture through said mixing means to create a predetermined
back pressure in said mixing means; and conduit means from
said outlet for directing said hydrated fluid away from
said mixing means.
According to yet another aspect of the present
invention, there is also provided apparatus for rapidly
hydrating a polymer gel slurry to form a hydrated and
viscosified fracturing fluid for the treatment of a
subterranean formation, comprising high shear rate mixing
CA 02220972 1998-12-03
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means for the mixing therein of said gel slurry and a fluid
for hydrating said gel slurry to form said hydrated fluid,
said mixing means having an inlet and an outlet; pump means
for supplying a mixture of said gel slurry and said
S hydrating fluid to said inlet; and means in fluid
communication with said outlet for directing said hydrated
fluid away from said mixing means at a controlled rate so
that the fluid in said mixing means is subjected to a
predetermined back pressure during the mixing thereof.
CA 02220972 1997-11-20
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments o~ the present invention will
now be described in greater detail and will be better
understood when read in conjunction with the appended drawing
which is a schematical block diagram illustrating the present
method and apparatus.
DETAILED DESCRIPTION
With reference to Figure 1, there is shown
schematically the present system for the mixing o~ a
stabilized polymer slurry or concentrate with a base fluid
which normally will be water. Although the ~ollowing
description is limited by way o~ example to the ~ormation o~
fracturing fluids ~or the treatment of underground
stratigraphic formations ~rom the mixture of a polymer
concentrate and a base fluid, it will be understood that this
same system can be used to mix together other fluids,
including fluids having particulate suspensions therein.
Examples include the mixing o~ polyemulsions, ~oams, cement
slurries, drilling ~luid and so ~orth. The use o~ the pre'sent
system ~or the hydration of gelled ~racturing ~luids is
particularly signi~icant however in view of its ability to
completely or nearly completely hydrate the ~luids in a
su~iciently short period o~ time to obviate the need ~or
prior art residence volumes.
As can be seen ~rom Figure 1, a base ~luid, normally
water, is pumped ~rom a reservoir 20 by means of a centrifugal
pump 30 through a supply line 25 into a high shear
rotor/stator type pipeline mixer/homogenizer 50 at a pressure
in the range of, but not restricted to, 250 to 300 psi. A
polymer slurry is injected at a metered predetermined rate
into supply line 25 via an injection Tee 40 ~or dispersal of
the concentrate in the water stream. A venturi can be used
for dispersal o~ the slurry into the water stream if
preferred. Tee 40 can be located as shown in the drawing
between pump 30 and mixer 50 but may also be located
CA 02220972 1997-11-20
.~
advantageously between reservoir 20 and pump 30 for better
premixing of the water/slurry mixture. An exhaust line 60
from the mixer directs the viscosified hydrated fracturing
fluid usually to a blender 100 for the introduction of
proppants into the fluid prior to injection down the wellbore
125 for treatment of the subterranean formations using
conventional high pressure frac pumps 110. Blenders and frac
pumps are well known in the art and are not therefore
described in detail herein.
The residence time of the hydrated gel/water mixture
in pipeline mixer 50 can range from almost instantaneous flow
through (i.e. near 0) to even a minute or more but will
normally be in the range of 1 to 10 seconds and as a practical
matter, the residence time is likely to be in the range of 1
to 2 seconds depending upon fluid throughput. In this
context, mixer residence time refers to the amount of time a
particular particle or sample of the gel/water mixture
requires to travel from the mixer inlet, through the
rotor/stator to the mixer outlet Residence times can-be
controlled by regulating the mixer's outflow. In one
prototypical system tested by the applicant, this is
accomplished by subjecting the mixer's shear cavity to a back
pressure in the range of 40 to 200 psi but preferably in the
range of approximately 120 to 150 psi. Optimum back
pressures, which could be inside or outside this range, may
have to be determined on a job-by-job basis depending upon
equipment used, ambient temperatures, fluid characteristics,
desired throughputs, and possibly other factors as well.
Pinching the flow of fluid from the mixer to create back
pressure prevents or at least inhibits cavitation and
maintains the system, at least upstream of the pinch point,
full of fluid. It's felt this helps ensure that the water and
gel are mixed immediately and that each particle of fluid is
exposed to the high shear rates developed by the mixer Back
pressure can be developed by inserting a choke or gate valve
~ ~ CA 02220972 1997-11-20
--10--
61 and back pressure meter 62 into exhaust line 60. In
addition or in the alternative, the diameter and/or length of
the exhaust line can be varied. For example, in tests
conducted by the applicant, good results were obtained using
a 4 inch diameter line 60 feet in length measured from the
mixer to blender 100 Better results however seem to be
available from longer lines on the order of 100 to 125 feet
or at least a combination of line length and diameter that
provides on the order of 20 seconds or more of residence time
in the line for fluids sheared at the rate of 56,000 s-l in
mixer 50. In thls context, line residence time refers to the
amount of time a particle or sample of the gel/water mixture
requires to travel from the mixer outlet to blender 100. This
amount of residency appears to be appropriate not so much for
hydration of the gel/water mixture, but rather to allow the
mixture time to recover from the high rates of shear in mixer
50. It appears that if the sheared fluid is not allowed
sufficient recovery time, the polymers will not fully cross-
link when cross-linking agents are added usually at the
proppant blending stage. A FannTM 35 Sample Port 68 and an in-
line viscometer 69 can be installed in line 60 for sampling
and testing the fluid in line 60. The residency required for
shear recovery can be obtained using a relatively small
holding tank, but this would at least partially defeat one of
the advantages of the present system which otherwise requires
no such residence volumes of this sort.
It is not yet fully known whether back pressure
directly affects gel viscosity and the speed of hydration
(i.e. with back pressure, does the gel hydrate more quickly
than without back pressure) or whether its effect is indirect
resulting from the control or regulation of mixer residence
times and avoidance of cavitation and fluid by-pass of the
mixer's rotor.
Mixer 50 is a commercially available product. In
tests conducted by the applicant, good results have been
CA 02220972 1997-11-20
obtained using either a Greerco Corp. 4" Tandem-Shear Pipeline
Mixer or a Silverson Machines, Inc. 3" 600 LSH High Shear
Mixer. Larger units are available from both companies. Two
or more such mixers can be used connected together for example
in series although to date this has not been found to result
in significantly improved hydration times or viscosities.
In tests conducted by the applicant, a batch of
polymer slurry was prepared using 590 liters of diesel fuel
mixed with 5 kilograms of SA-lX mixed together and sheared for
10 to 15 minutes. One liter of methanol was then added and
mixed in for 10 to 15 minutes. This was followed by the
addition of 10 liters of S-11 mixed in for an additional 10
to 15 minutes. Guar gum (WG-15) was added in the amount o~
550 kilograms mixed in for 25 minutes. The direction of
rotation of the mixing auger was reversed for each bag of the
powdered polymer. The resulting gel slurry achieves a
viscosity of 14 to 16 cP at a shear rate of 511 s-l when
loaded to water in the ratio of 6 litres of slurry (equivalent
to 3 kg of WG-15) per cubic meter of water. Based therefore
on these tests, 100% hydration was considered to have been
achieved at a viscosity of 14 cP @ 511 s-1.
In testing, concentrate was introduced into supply
line 25 at the rate of 1~ 1/min. to 100 l/min. to be mixed
with water introduced at the rate of 1~ to 10 m3/min. for a
system throughput o~ % m3/min. to ~11 m3/min. At each of the
lower and higher throughputs, ~ull hydration to achieve
initial viscosities at or about 14 cp at 511 s-l were obtained
in mixer residence times of near instantaneous to a few
seconds.
In most of the tests performed by the applicant,
100% hydration (i.e. viscosities ~~ 14 cP) and fluid shear
recovery were achieved with system residence times from the
inlet of mixer 50 to blender 100 of approximately 20 seconds
and thereabouts depending upon total throughput. This result
CA 02220972 l997-ll-20
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was unexpected particularly having regard to Stromberg in '824
who indicates at column 5, line 5 that:
We have discovered, as further explained
. below, that for a specific energy input
into a gelled fracturing fluid, the
energy is much more efficiently used to
increase hydration o~ the fluid if the
energy is input at lower levels over a
longer period o~ time rather than an
intense burst over a very short period of
time. Thus, large agitation tanks have
been determined to be much more energy
e~ficient viscosity producers than are
small volume devices such as centri~ugal
pumps, static mixers and the like which
are inefficient viscosity producers.
In contrast, the applicant has found the opposite to be true.
More specl~ically, the applicant has found that the use of a
low volume pipeline (inline) high shear mixer that applies an
intense burst of shear over a short period of time measured
in seconds provides the needed on-the-fly volumes of
completely or near-completely hydrated gel for commercial
fracturing operations in such a short period of time that
residence tanks, blending tubs and the like are no longer
required. No complete explanation o~ this phenomenon is as
yet available but it'~ possible that the high shear pipeline
mixers as used by the applicant which expose the particles of
polymer in the concentrate gel to a specific shear- rate o~
25,000 s-1 and preferably 50,000 to 150,000 s-1 and up to
1,000,000 s-1, may actually be fragmenting the particles into
even smaller pieces, exposing more surface area for faster and
more complete hydration to provide higher yields and
viscosities. Specific shear rates in excess of 400,000 s-1
may necessitate the use o~ longer exhaust lines 60 to allow
a greater tlme ~or shear recovery. For example, fluids
subjected to a speci~ic shear rate o~ 400,000 s-1 appear to
need as much as 40 to 50 seconds to recover for cross-linking
purposes.
" CA 02220972 1997-11-20
Shear rates can be varied in 3 ways or a combination
thereof: change the rotor diameter; change the rotor speed;
and adjust the gap between the rotor and stator. During
operations, the shear rate will normally be adjusted by
changing the rotor speed. For smaller (3"/4") mixer sizes,
speeds of 3600 to 4800 rpm are typical. If using a 6"
Greerco, for example, speeds of around 2500 rpm seem adequate.
In tests performed by the applicant, a gap between the rotor
and stator of 0.001" has produced good results.
In typical fracture operations, it is common to use
a base or polymer gel concentration of 3 to 7 kg of polymer
per cubic metre of water. At final system throughputs of 2
m3 or less, the final concentration and rate requlred at the
blender can be achieved by adding the slurry concentrate to
the water and treating this mixture through high shear mixer
50. If higher throughputs are required, there are several
options. One is to use a larger mixer capable of treating the
mixture at the desired shear and flowthrough rates. A second
option is to add a higher concentratlon of slurry (polymer)
than the finally desired concentration through the mixer and
adding makeup water through a supply line 59 that taps into
line 60 downstream of mixer 50 and prior to the sample port.
This makeup water can be metered to dilute the polymer
concentrations back to the desired final level. This second
option may be the more practical as it does not require that
a second higher volume mixer be on site.
The above-described embodiments of the present
invention are meant to be illustrative of preferred
embodiments of th=e present invention and are not intended to
limit the scope of the present invention. Various
modifications, which would be readily apparent to one skilled
in the art, are intended to be within the scope of the present
invention. The only limitations to the scope of the present
invention are set out in the following appended claims.
