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BACKGROUND OF THE INVENTION
1. Field Of The Invention
The present invention relates to mixing of polymer gel agents and water to
form a well treatment fluid, such as a fracturing ("frac") gel or other
similar gel, and more particularly, to a method and apparatus for
continuously mixing such gels on a real time basis to achieve rapid
hydration without the necessity of an oil-based fluid or the suspension
agents normally associated therewith.
2. Description Of The Prior Art
Many treatments and procedures are carried out in industry utilizing high
viscosity fluids to accomplish a number of purposes. For example, in the
oil industry, high viscosity aqueous well treating fluids or gels are
utilized in treatments to increase the recovery of hydrocarbons from
subterranean formations, such as by creating fractures in the formation,
acidizing the formations, etc. High viscosity aqueous fluids are also
commonly utilized in well completion procedures. For example, during the
completion of a well, a high viscosity aqueous completion fluid having a
high density is introduced into the well to maintain hydrostatic pressure
on the formation which is higher than the pressure exerted by the fluids
contained in the formation, thereby preventing the formation fluids from
flowing into the well bore.
High viscosity treating fluids, such as fracturing or acidizing gels, are
normally made using dry polymer additives or agents which are mixed with
water or other aqueous fluids at the job site. Such mixing procedures have
some inherent problems, particularly on remote sites or when large volumes
are required. For example, special equipment for mixing the dry additives
with water is required, and problems such as chemical dusting, uneven
mixing, lumping of gels while mixing and extended preparation and mixing
time are involved. In addition, the mixing and physical handling of large
quantities of dry chemicals require a great deal of manpower, and when
continuous mixing is required, the accurate and efficient handling of dry
chemicals is extremely difficult.
The lumping of gels occurs because the initial contact of the polymer with
water results in a very rapid hydration of the outer layer of particles
which creates a sticky, rubbery exterior layer that prevents the interior
particles from contacting water. The net effect is formation of what are
referred to as "gel balls" or "fish eyes". These hamper efficiency by
lowering the viscosity achieved per pound of gelling agent and also by
creating insoluble particles that can restrict flow both into the well
formation and back out of it. Thus, simply mixing the untreated polymer
directly with water is not a very successful method of preparing a smooth
homogeneous gel free from lumps. A method directed to solving this problem
is to control particle size and provide surface treatment modifications to
the polymer. It is desired 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 can be achieved by coating the polymer with materials such as
borate salts, glyoxal, non-lumping HEC, sulfosuccinate, metallic soaps,
surfactants, or other materials of opposite surface charge to the polymer.
One 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 (SPS), also referred to as a liquid gel concentrate (LGC). The
liquid gel concentrate is premixed and then later added to the water. In
U.S. Pat. No. 4,336,145 to Briscoe, assigned to the assignee of the
present invention, a liquid gel concentrate is disclosed comprising water,
the polymer or polymers, and an inhibitor having the 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 of other selected
condition 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, rather than water, higher quantities
of solids can be suspended. For example, up to about five pounds per
gallon of polymer may be suspended in a diesel fuel carrier. Such a liquid
gel concentrate is disclosed in U.S. Pat. No. 4,722,646 to Harms and
Norman, assigned to the assignee of the present invention. Such
hydrocarbon-based liquid gel concentrates work well but require a
suspension agent such as an organophylic clay or certain polyacrylate
agents. The hydrocarbon-based liquid gel concentrate is later mixed with
water in a manner similar to that for aqueous-based liquid gel
concentrates to yield a viscosified fluid, but hydrocarbon-based
concentrates have the advantage of holding more polymer.
An additional problem with prior methods using liquid gel concentrates
occurs in offshore situations. The service vessels utilized to supply the
offshore locations have a limited storage capacity and must therefore
often return to port for more concentrate before they are able to do
additional jobs, even when the liquid gel concentrate is
hydrocarbon-based. Therefore, it would be desirable to be able to
continuously mix a well treatment gel during the actual treatment of the
subterranean formation from dry ingredients. For example, such an on-line
system could satisfy the fluid flow requirements for large hydraulic
fracturing jobs during the actual fracturing of the subterranean formation
by continuously mixing the fracturing gel.
One method and apparatus for continuously mixing a fracturing gel is
disclosed in U.S. Pat. No. 4,828,034 to Constien et al., in which a
fracturing fluid slurry concentrate is mixed through a static mixer device
on a real time basis to produce a fully hydrated fracturing fluid during
the actual fracturing operation. This process utilizes a hydrophobic
solvent which is characterized by a hydrocarbon such as diesel as in the
hydrocarbon-based liquid gel concentrates described above.
Recently, however, there have been some problems with hydrocarbon-based
liquid gel concentrates because some well operators object to the presence
of these fluids, such as diesel, even though the hydrocarbon represents a
relatively small amount of the total fracturing gel once mixed with water.
Also, there are environmental problems associated with the clean-up and
disposal of well treatment gels containing hydrocarbons. These
hydrocarbon-related problems would also apply to the process of Constien
et al. Accordingly, there is a need for a process to produce a well
treatment gel in which relatively higher amounts of polymer per unit
volume can be utilized while eliminating the environmental problems and
objections related to hydrocarbon-based concentrates. There is also a need
for this process to produce the well treatment gel substantially
continuously during the well treatment operation to overcome the storage
capacity problems discussed above.
The method and apparatus of the present invention provide a solution to
these problems by providing a means for substantially continuously
producing a fracturing gel without the use of hydrocarbons or suspension
agents, while still avoiding gel balls, by feeding the polymer into an
axial flow mixer which has high mixing energy to substantially wet all of
the polymer during its initial contact with water. After initial mixing,
additional water may be added to the mixer to increase the volume of
water-polymer slurry produced thereby.
In the present invention, it is possible to use a non-coated
(non-surface-treated) gelling agent. This provides a simpler and less
expensive process, and the materials themselves are also cheaper because
raw gelling agents are less expensive than coated or treated materials.
SUMMARY OF THE INVENTION
The apparatus and method of the present invention provide for real time
mixing of well treatment fluids, such as fracturing gels, acidizing gels,
fracture-acidizing gels, gravel packing gels, weighted gels, or the like,
from powdered polymer solids in real time. This on-line system may be used
in oil field applications and eliminates conventional large volume mixing
tanks yet satisfies the fluid flow requirements for well treatment
processes such as large hydraulic fracturing jobs during the actual
fracturing of the subterranean formation. With the present invention, full
hydration of the polymer and optimum viscosity of the well treatment fluid
may be achieved in a relatively short time while avoiding the formation of
gel balls.
The preferred method of hydrating a polymer to produce a well treatment
fluid or gel comprises the steps of providing a predetermined quantity of
the hydratable polymer in a substantially particulate form to a polymer or
solids inlet of a water spraying mixer, supplying a stream of water to a
water inlet of the mixer, and mixing the polymer in water in the mixer,
thereby wetting substantially all of the solid polymer particles to form a
water-polymer mix prior to discharge from the mixer. The step of providing
a predetermined quantity of polymer preferably comprises adding bulk
polymer to a metering feeder and accurately supplying the predetermined
quantity of polymer from the feeder to the mixer. The metering feeder
preferably comprises a metering auger which rotates at a controlled speed,
thereby discharging the predetermined quantity of polymer therefrom at the
desired rate.
The polymer particles may be treated with a hydration-delaying coating, in
which case the method further comprises the step of adding a buffering
compound or other suitable agent to the stream of water for chemically
reversing the coating. Preferably, the buffering compound is added to the
stream of water prior to entry of the stream of water into the water
spraying mixer. This eliminates the previously known step of mixing the
buffering agent with a previously dispersed gelling agent. Thus, in this
embodiment, the method of hydrating a polymer of the present invention may
be said to comprise the steps of supplying a quantity of coated polymer to
a mixer, supplying a quantity of buffered water to the mixer for
substantially completely wetting the coated polymer, and discharging the
wetted water-polymer mix or slurry from the mixture substantially without
lumping. A step of supplying an additional quantity of buffered water to
the mixer after initial contact of the coated polymer with the first
mentioned quantity of buffered water may be added, thereby increasing the
volume of the mixture.
Supplying the polymer preferably comprises the steps of feeding bulk
polymer to the metering feeder, and discharging an accurately controlled
predetermined quantity of polymer from the feeder to the mixer. The
polymer may be supplied without a suspension agent.
The method of the present invention further comprises flowing the slurry or
mix through a high shear device after it is discharged from the mixer for
increasing the rate of viscosification of the mix.
The method may also comprise the step of providing an air inlet opening for
preventing formation of a vacuum in the feeder.
The method may further comprise discharging the water-polymer mix from the
mixer into a tank and agitating the mix in the tank.
The apparatus of the present invention in a preferred embodiment comprises
the metering feeder, the discharge of which is connected to the polymer
inlet of the mixer. This connection may be made by a tee wherein one of
the tee connections is left open so that air can enter the system. A water
supply is connected by a water line to the water inlet of the mixer. The
buffer may be injected into this water line. The mixer is preferably
mounted adjacent to the upper portion of a mixing or primary tank, and an
agitator may be provided in the mixing tank to further agitate and stir
the slurry. The slurry may be transferred from the mixing tank to a
holding or secondary tank after which it is discharged to the fracturing
process. The high shear device may be disposed in the holding tank. A pump
may be used for transferring the slurry from the mixing tank to the
holding tank.
One embodiment of the water spraying mixer is an axial flow mixer
substantially identical to that disclosed in prior U.S. patent application
Ser. No. 07/412,255, assigned to the assignee of the present invention and
incorporated herein by reference. This prior art mixer has been used for
mixing cement, and in this embodiment, two additional ports in the mixer
are used for recirculating the slurry. In the present invention, these
ports are used as additional inlets branched from the main water line,
thereby providing a means for directing additional water to the mixer
after the polymer is first contracted by water in the mixer. This
increases the mixing energy within the mixer and provides an increased
volume of water-polymer mix.
The mixer comprises a valve means for controlling the amount of water
entering the mixer through the main water inlet and further comprises a
means for directing the water in a substantially spiralling flow which
wets the polymer as it falls through the mixer.
It is an important object of the present invention to provide a method of
rapid hydration of polymer when the polymer is added to water to produce a
viscous well treatment fluid, such as a fracturing gel, gravel packing
fluid, viscous acidizing gel, or similar fluid.
It is another object of the invention to provide a method of rapid
hydration of polymer in producing a viscous fluid in an on-line real time
basis by continuously producing the fluid during a well treatment process.
It is an additional object of the invention to provide a method and
apparatus of producing a viscous fluid such as fracturing gel while
eliminating the need to batch-mix the polymer in large volume tanks,
although the method can be used to prepare batches of gel to be held in
storage tanks.
It is a further object of the invention to provide a method and apparatus
for producing a fracturing gel and eliminate the formation of gel balls
without requiring the production of an aqueous-based or hydrocarbon-based
liquid gel concentrate.
Still another object of the invention is to provide a method and apparatus
for mixing a polymer with water utilizing a water spraying mixer.
Another object of the invention is to provide a method and apparatus for
rapidly hydrating a non-coated or non-surface treated gelling agent
without necessarily adding a buffering agent.
Additional objects and advantages of the invention will become apparent as
the following detailed description of the preferred embodiment is read in
conjunction with the drawings which illustrate such preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. I presents a schematic of the apparatus of the present invention for
continuously mixing polymers with water.
FIG. 2 is a partially cross-sectional and partially elevational view of the
water spraying mixer used in the present invention.
FIG. 3 is a plan view of an orifice plate of a valve of the mixer shown in
FIG. 2.
FIG. 4 is a cross-sectional view taken along lines 4--4 in FIG. 3.
FIG. 5 is a plan view of a valve plate of the valve of the mixer.
FIG. 6 is a cross-sectional view taken along lines 6--6 in FIG. 5.
FIG. 7 is a plan view of a water jet member of the valve of the water
spraying mixer.
FIG. 8 is a cross section taken along lines 8--8 in FIG. 7.
FIG. 9 is a cross-sectional view of a corner of the water jet member taken
along lines 9--9 in FIG. 7.
FIG. 10 presents a cross section of a part of the water jet member taken
along lines 10--10 in FIG. 7.
FIG. 11 is a plan view of a diffuser of the mixer shown in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, and more particularly to FIG. 1, the
apparatus for continuously mixing well treatment gels or similar fluids of
the present invention is shown and generally designated by the numeral 10.
The polymer is introduced into the system by pouring it in bulk form into a
hopper portion 14 of a feeder 16. Feeder 16 is preferably of a type which
discharges an accurately metered quantity of polymer over time. The feeder
illustrated is a metering feeder, such as an Acrison feeder. It should be
understood, however, that the invention is not intended to be limited to
this particular Acrison feeder. The important feature is that a device be
used which provides an accurately metered quantity of polymer discharged
therefrom.
The Acrison feeder has a large conditioning auger or agitator 18 adjacent
to the bottom of hopper 14. Conditioning auger 18 of this prior art feeder
"conditions" or stirs the polymer and breaks up any clumps of polymer that
might be stuck together. After being stirred by conditioning auger 18, the
polymer falls through an opening 20 into a feed chamber 22. A smaller
metering auger 23 rotates within chamber 22, and the polymer is discharged
from feeder 16 through an outlet 24. In the Acrison feeder, conditioning
auger 18 and metering auger 23 rotate at dissimilar speeds. A control box
26 drives conditioning auger 18 and metering auger 23. A speed transducer
28 may be engaged with control box 26.
Outlet 24 of feeder 16 is connected to branch 30 of tee 32. In a preferred
embodiment, one end 34 of the run of tee 32 is connected to polymer inlet
36 of a high shear flow mixer 38, the details of which will be further
discussed herein. Mixer 38 is preferably a water spraying device. In
operation, mixer 38 can draw a vacuum in feeder 16 if not vented, so the
opposite end 40 of the run of tee 32 is open to the atmosphere to allow
the entry of air as necessary.
A water line 42 is connected to a water inlet 114 of mixer 38. Water line
42 may include a flow meter 44, such as a Halliburton turbine flow meter.
Water line 42 is also connected by branches 46 and 48 to additional or
auxiliary water inlets 206 and 208, respectively. Water may be supplied to
water line 42 from a water tank or reservoir 50, or the water supply may
be connected directly to the water line. A pump 51 may be used to pump
from reservoir 50 as necessary.
A buffering compound or any other desired additive may also be introduced
to water line 42 through a metering means 52. A pump 53 may be used as
necessary to pump the buffering compound or other additive. When a buffer
is required, the compound preferably is thus introduced or injected
directly into the system with the water.
A controller 55 may be connected to speed transducer 28, flow meter 44, and
pumps 51 and 53, thus providing a feedback means for controlling the flow
rates of the polymer, water and any buffering compound or other additives.
In this way, the polymer/water concentration and throughput are
controlled.
Mixer 38 is mounted to the upper portion of a mixing tank or tub 54. Mixing
tank 54 may also be referred to as primary tank 54. As will be further
discussed herein, the wetted polymer will be discharged from mixer 38 as a
water-polymer mix or slurry into mixing tank 54. The slurry in mixing tank
54 may be further stirred by an agitating means 56 of a kind generally
known in the art, although this may not be necessary. The agitating means
may be characterized as any known type of fluid shear device.
The slurry is discharged from mixing tank 54 through an outlet 58 and flows
through a slurry line 60 to inlet 62 of a holding tank 64. Holding tank 64
may also be referred to as secondary tank 64. The slurry may flow by
gravity, but generally, a pumping means, such as centrifugal pump 66 will
be installed in slurry line 60 to move the slurry. Pump 66 may also be
described as a shear device 66 which applies shear to the fluid.
In one embodiment, the fluid passes through another shear device 68. It is
well known that applying shear to the fluid will increase hydration and
reduce the time necessary for the fluid to reach its maximum viscosity.
Therefore, when time is a critical factor, shear device 66 and/or 68 may
be necessary. The slurry will eventually reach its maximum viscosity after
a certain period of time anyway, and if time is not critical, such as when
the fluid is held for a lengthy period in holding tank 64, then shear
devices 66 and/or 68 may be eliminated. Shear device 68 may be any device
which provides a high shear to the fluid. Examples of such high shear
devices include, but are not limited to, centrifugal pumps, rotating
turbine paddles, static flow mixers or the like. These devices may be used
singly, in series, and/or in combination.
The fluid is discharged from holding tank 64 through an outlet 70, and the
fluid then flows to other devices known in the art and then to the well.
For example, fluid flowing from outlet 70 of holding tank 64 may enter a
fracturing blender which mixes sand with the slurry. Such downstream
devices are known in the art and are therefore not illustrated in FIG. 1.
Referring now to FIG. 2, the details of water spraying mixer 38 will be
discussed. This description of mixer 38 is substantially the same as that
presented in prior U.S. patent application Ser. No. 07/412,255 which has
already been incorporated herein by reference. Mixer 38 is illustrated as
an axial flow device which conveys the polymer axially from the inlet to
the outlet thereof. That is, there are no elbows or horizontal conduits
through which the polymer must be conveyed during its mixing with water
prior to being discharged into mixing tank 54.
Water inlet 114 of mixer 38 is characterized as a water inlet member 114 or
water inlet manifold 114. Water inlet manifold 114 includes an annular top
plate 116, an annular bottom plate 118 having a central opening with a
larger diameter than the central opening of the plate 116, and a
cylindrical side wall 120 connected, such as by welding, to and between
top plate 116 and bottom plate 118. These components are disposed relative
to each other as shown in FIG. 2 so that an axial opening 122 is defined.
The bottom of axial opening 122 provides an exit port 124 through which
the water received by water inlet manifold 114 flows in a downward path
prior to mixing with the polymer. This water is received through an entry
port or inlet 126 defined by a horizontal sleeve 128 connected to side
wall 120 in communication with an opening 130 defined therein. Exit port
124 communicates with entry port 126 through an annular interior region
132 defined by the connection of water inlet member 114 with polymer inlet
134, which is received in axial opening 122. Polymer inlet 134 is
characterized as a polymer inlet member 134 which is connected to water
inlet manifold 114 by any means known in the art such as by welding.
Polymer inlet member 134 may also be referred to as sleeve 134 which has a
cylindrical wall 136 defining an axial passageway 138 between top and
bottom ends 140 and 142 of the sleeve. Top end 140 is connectable to tee
32 as previously described so that sleeve 134 receives polymer through top
end 140 and directs it in a downward flow through bottom end 142. In
particular, sleeve 134 provides a straight flow path for the polymer
between tee 32 and bottom end 142 of sleeve 134 where the polymer enters a
valve 144 of mixer 38.
Valve 144 meters the water to be mixed with dry polymer coming from sleeve
134. Valve 144 includes an orifice plate 146, a valve plate 148 and means
150 for jetting water into admixture with the polymer. The illustrated
design of orifice plate 146 contains eighteen orifices or holes, and valve
plate 148 is designed so that it opens six of the eighteen orifices first
and then an additional six holes as valve plate 148 is further rotated and
ultimately the final six holes are opened upon further rotation, although
the number and sizes of holes may vary. This design allows a maximum hole
dimension or passage diameter for a given flow rate as compared to a
system which may have the entire passageway opening simultaneously. This
controlled opening is important for contaminate passage which could block
metering orifices. In some applications, adjustable water flow may not be
required. In such cases, valve plate 148 may be eliminated.
The mixing water, as it exits orifice plate 146, flows in an axial
direction and is subsequently turned and directed toward the polymer flow
path coming from sleeve 134. This turning of the water flow direction is
produced by the jet means 150 which in the preferred embodiment has
grooves coinciding with the orifice plate 146 orifices. Thus, jet means
150 changes the direction of the mixing water from axially downward to
slightly tangential and downward. This produces a downwardly spiraling
column of fluid circulating about an open center or iris. In a preferred
embodiment, the depths of the grooves of jet means 150 are staggered so
that with high flow rates, backflow up passage 138 is prevented.
Referring now also to FIGS. 3 and 4, orifice plate 146 includes an annular
member 152 having a central opening 153 defined by an inner periphery 154
about which the plurality of orifices 156 is defined. The orifices of the
preferred embodiment include three sets of differently sized orifices
156a, 156b, 156c. Each set includes six orifices of the same size. In the
illustrated embodiment, the orifices 156a have the smallest diameter,
orifices 156b have a larger diameter, and the orifices 156c have the
largest diameter of the three sets. These are spaced sequentially and
equiangularly around the inner periphery 154 as best seen in FIG. 3. The
orifices can be the same size or of different sizes and different
arrangements.
Also defined about inner periphery 154 is a notch or shoulder defined by an
annular surface 158 and an adjoining, perpendicularly extending
cylindrical surface 160.
Annular member 152 also has an outer periphery through which holes 164 are
defined. Holes 164 receive retaining bolts 166, two of which are shown in
FIG. 2, extending through spacers 186.
When orifice plate 146 is connected to water inlet manifold 114 by the
retaining bolts 166, orifices 156 are disposed below exit port 124 of
water inlet manifold 114. Orifice plate 146 is also concentrically
disposed about inlet sleeve 134. A seal ring 168 seals orifice plate 146
and inlet sleeve 134. Thus, orifice plate 146 is disposed below and
adjacent to valve plate 148.
The disposition of valve plate 148 concentrically about inlet sleeve 134
adjacent to exit port 124 of water inlet manifold 114 is shown in FIG. 2.
As disposed, valve plate 148 is pivotably connected to orifice plate 146
so that the position to which valve plate 148 is pivoted determines which
of orifices 156 are open to pass liquid. The overall construction of valve
plate 148 is more clearly shown in FIGS. 5 and 6. The preferred embodiment
of valve plate 148 includes a ring 170 from which an actuating arm 172
extends radially outwardly. Arm 172 can be engaged by a suitable actuating
device (not shown).
Ring 170 has an outer periphery from which arm 172 extends. Ring 170 also
includes a central opening 173 defined by an inner periphery which has a
notched or toothed configuration as most clearly seen in FIG. 5. This
configuration includes a set of teeth 174a, a set of teeth 174b and a set
of teeth 174c. Each of the teeth within a respective set has the same
width, and the width of each of teeth 174c is larger than the width of
each of teeth 174b. Each of teeth 174b has a width larger than the width
of each of teeth 174a. This sizing corresponds to the different size
orifices 156a, 156b, 156c of orifice plate 146 and the desired sequencing
for opening orifices 156a, 156b, 156c. Thus when water metering valve 144
is fully closed, each of teeth 174a overlies a respective orifice 156a,
each of teeth 174b overlies a respective orifice 156b, and each of teeth
174c overlies a respective orifice 156c. This position is obtained by
pivoting valve plate 148 counterclockwise as shown in FIG. 5 or outwardly
from the page as shown in FIG. 2. The next respective bolt 166 limits
rotation of valve plate 148 in this direction.
The sets of orifices 156a, 156b, 156c are progressively opened as actuating
arm 172 of valve plate 148 is moved clockwise for the orientation shown in
FIG. 5 or into the page for the orientation shown in FIG. 2. This
direction of rotation is limited when actuating arm 172 abuts the
corresponding bolt 166. Opening of an orifice 156a, 156b, 156c occurs when
a corresponding aperture or space 176a, 176b, 176c defined between teeth
174a, 174b, 174c overlies or registers with the respective orifice of
inner periphery 154 of orifice plate 146. Thus these elements of valve
plate 148 define means for simultaneously opening orifices 156a, 156b,
156c of a respective set in response to pivotation of valve plate 148. In
the preferred embo | | |
