Note: Descriptions are shown in the official language in which they were submitted.
WO 2014/039968 PCT/US2013/058763
PROPPANT PARTICLES FORMED FROM SLURRY
DROPLETS AND METHOD OF USE
BACKGROUND OF INVENTION
1. FIELD OF THE INVENTION
[0001] This invention relates to hydraulic fracturing of subterranean
formations in the earth.
More particularly, sintered ceramic proppant particles formed from vibration-
induced
dripping from a nozzle of a slurry of finely-divided ceramic material are
provided, along with
a method of use of the particles.
2. Description of Related Art
[0002] Hydraulic fracturing is a process of pumping liquids down a well and
into a
subterranean founation at high rate and pressure, such that a fracture is
formed in the rock
around the well. After pumping a liquid volume sufficient to widen the
fracture adequately,
solid particles, called "proppant," are added to the liquid. After pumping is
completed, the
well is opened for production of hydrocarbons. The production rate of fluid
from the well is
usually significantly increased after the fracturing treatment. Vast
improvements in the
hydraulic fracturing process have been developed since the process was
originally patented in
1949. (U.S. Pat. Nos. 2,596,843 and 2,596,844)
[0003] The material first used for proppant in hydraulic fracturing of wells
was silica sand.
As wells became deeper, sand was found to have inadequate strength. In deep
wells, stress of
the earth causes the sand to crush and become much less effective in
increasing the
production rate of a well.
[0004] Synthetic proppant materials were developed to provide higher strength
proppants.
The original synthetic sintered proppant was sintered bauxite. In later years,
a variety of
ceramic raw materials have been used to make sintered ceramic proppants,
including bauxite
containing lesser amounts of alumina and clay minerals, such as kaolin.
Generally, it has
been found that the strength of ceramic particles increases with the amount of
alumina in the
particle, all other factors remaining constant.
[0005] A general procedure for making synthetic proppant particles is to
obtain the ceramic
raw material, grind it to a fine powder, form it into pellets (called "green"
pellets), and sinter
the green pellets in a kiln. The final product is ceramic pellets in the size
range suitable for
proppants ¨ from about 70 mesh to 12 mesh (0.008 inch to 0.067 inch).
Different sizes of
pellets are used depending on well conditions.
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[0006] A variety of processes for forming the pellets of a proppant have been
proposed. In
early work, U.S. Pat. No. 4,427,068 describes a process for forming sintered
ceramic pellets
by adding dry powders of clay and alumina, bauxite, or mixtures to a high
intensity mixer
(hereinafter referred to as "dry mixing method"). Powdered fine grain ceramic
starting
ingredients (ceramic raw materials) are stirred to form a dry homogenous
mixture. Then,
sufficient water is added to cause agglomeration of the fine starting dust
particles to form
small composite spherical pellets from the powder. Continued mixing time is
allowed in
order to grow small pellets to the desired size. A broad range of sizes is
produced during the
pellet- forming stage. A preferred mixing device is obtained from Eirich
Machines, Inc., and
is known as the Eirich mixer. The resulting pellets are dried and sintered
into the final
proppant particles. Much of the ceramic proppant made in industry in past
years has been
made with this process of forming pellets.
[0007] U.S. Pat. No. 4,440,866 discloses an alternative process for producing
pellets that are
sintered to produce high strength pellets. A continuous spray/granulation of
an aqueous
binder containing aluminous ore suspension is used to form granules that are
subsequently
sintered (hereinafter referred to as "spray fluidized bed method"). All steps
of this process
may be carried out in a continuous manner. An aqueous suspension containing
the ceramic
raw material is continuously atomized and fed into a layer of already
partially dried small
starting dust particles (often called seeds) that are fluidized in a stream of
hot drying air. The
aqueous ceramic raw material suspension is continuously sprayed and dried onto
the seed
particles until the desired finished green particle diameter is achieved.
Particles produced in
this process have a size range that is less broad than those typically
produced by the dry
mixing method of U.S. Pat. No. 4,427,068 but are still of sufficient variation
as to require
further processing. Particles are continuously recovered from the layer and
they are separated
from oversized and undersized product fractions. Material is continuously
recycled in the
stream of drying air. This spray fluidized bed process has also been used to
produce large
amounts of ceramic proppants in industry.
[0008] The pellet-forming methods described above have intrinsic limitations.
The dry
mixing process produces an extremely wide range of green pellet sizes due to
the random
nature of the agitation of the rotor and pan. The spray fluidized bed process
produces a
somewhat tighter green pellet size distribution but still a much wider
distribution than
desired. These processes require extensive screening and recycling during the
manufacturing
process. Under the best manufacturing conditions about 30% of green particles
must be
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recycled through the pellet-forming process. Both the dry mixing and spray
fluidized bed
processes also produce a random distribution of pore sizes in pellets,
including a small
percentage of very large pores that significantly degrade pellet strength.
Strength of the
sintered pellets is a primary consideration, because if the pellets break
under high stress in a
fracture, the flow capacity of the fracture is decreased and the hydraulic
fracturing treatment
is less effective. The sphericity and surface smoothness of particles produced
by these
processes are also important, with high sphericity and a very smooth surface
traditionally
being most desirable. All of these characteristics are strongly affected by
the pellet-forming
method.
[0009] U.S. Pub. No. 2006/0016598 discloses a list of pellet-forming
techniques that may be
used for ceramic proppant formation, including agglomeration, spray
granulation, wet
granulation, extruding and pelletizing, vibration induced dripping according
to U.S. Pat. No.
5,500,162, spray nozzle-formed droplets and selective agglomeration. U.S. Pat.
No.
5,500,162 discloses producing microspheres by vibration-provoked dripping of a
chemical
solution through a nozzle plate, wherein the falling drops form an envelope
surrounded from
all sides by flowing reaction gas. The liquid chemical solution has no or low
(i.e. 20% or
less) solid particles at the time it enters the nozzle plate, exits the nozzle
plate, and passes
through the first free fall section. The reaction gas is required to cause
the precipitation
(gelling) of small solid particles (typically sub-micron) in the liquid drops
as they fall
through the second free fall zone, and thereafter fall into a reaction liquid
to further gel. The
reaction gas is necessary to cause the liquid to partially gel prior to
entering the reaction
liquid, and the droplets are decelerated into the liquid through a foam or the
reaction liquid is
directed onto the falling drops tangentially in the same direction in which
the droplets are
falling. These two features of falling through reaction gas and decelerating
the droplets into
foam are required to insure the droplets are partially gelled during a sol-gel
reaction and
therefore not deformed, for example flattened, when they strike the reaction
liquid. The
reaction gas is sucked away inside or outside the envelope. The method
according to the
invention can be used to produce, for example, aluminum oxide spheres up to
the diameter of
mm.
[0010] Vibration-induced dripping, herein called "drip casting," was
originally developed to
produce nuclear fuel pellets. Since then it has been adapted to produce a very
wide variety of
metal and ceramic microspheres, such as grinding media and catalyst supports.
Primarily, it
has been used in the food and pharmaceuticals industries. The drip casting
process is
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described on the website and in sales literature of Brace GmbH. Examples of
"microspheres"
formed by drip casting of different materials are also provided. U.S. Pat. No.
6,197,073
discloses a process for producing aluminum oxide beads from an acid aluminum
oxide sol or
acid aluminum oxide suspension by flowing the suspension through a vibrating
nozzle plate
to form droplets and pre-solidifying the droplets with gaseous ammonia and
then coagulating
the droplets in an ammonia solution. The mechanical strength of ceramic
particles formed by
sintering the drip cast particles was not a factor in any of the materials
used in these
references.
[00111 It is known that to produce ceramic proppant particles having maximum
strength for a
given ceramic material, the particles must contain minimum porosity, and the
pores present
must be kept as small as possible, since the strength of a given proppant
particle is limited by
its largest pore. What is needed is a method of forming green ceramic
particles that can be
fired to have reduced pore size and therefore maximum strength for use as a
proppant.
Preferably, the particles should be spherical, have a smooth surface and have
uniform size. A
method for forming the green particles without recycling of the undesired size
fraction of
green ceramic pellets is also needed.
BRIEF SUMMARY OF THE INVENTION
[0012] A method for forming proppant pellets from a slurry of ceramic raw
materials is
provided. The pellets produced have superior strength to prior proppant
pellets made from a
variety of ceramic raw materials using prior art pellet-forming methods.
Uniform sized
spherical pellets having a smooth surface may be made in commercial
quantities. The
particles are used in hydraulic fracturing treatments of wells.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0013] FIG. 1 is a sketch showing the principles of the pellet-forming
apparatus for proppant
pellets disclosed herein.
[0014] FIG. 2 is a sketch showing a single nozzle forming droplets from a
slurry stream.
[0015] FIG. 3 is a sketch showing a multi-nozzle plate forming droplets from a
slurry stream.
[0016] FIG. 4a is a showing of an alumina particle made of drip casting.
[0017] FIG. 4b is a showing of an alumina particle made by an Eirich mixer.
[0018] FIG. 4c is a showing of a bauxite particle made by drip casting.
[0019] FIG. 4d is a showing of a bauxite particle made by an Eirich mixer.
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[0020] FIG. 4e shows a kaolin particle made by drip casting.
[0021] FIG. 4f shows a kaolin particle made by a scale fluidized bed process.
[0022] FIG. 5 is a graph of long term permeability as a function of stress of
alumina pellets
formed by the pellet-forming apparatus disclosed herein and by the prior art
dry mixing
process using an Eirich mixer.
[0023] FIG. 6 is a frequency plot of pore size for particles of kaolin made by
the method
disclosed herein and by the prior art spray fluidized bed method.
[0024] FIG. 7 is a graph of long term permeability data for proppant having
different alumina
contents and made by different processes.
[0025] FIG. 8 is a graph of long-term permeability data for proppant having
different alumina
contents and made by different processes.
DETAILED DESCRIPTION
[0026] Referring to FIG. 1, pellet-forming apparatus 10 having a single
nozzle is
shown to illustrate the principles of the method disclosed herein, which is
commonly called
"drip casting." Nozzle 12 receives slurry 15from feed tank 14, which contains
the ceramic
raw materials suspended in water. Pressure applied to feed tank 14 by pressure
supply
system 16 causes slurry to flow through nozzle 12 at a selected rate ¨
preferably in laminar
flow. Below nozzle 12 is coagulation vessel 17, which receives the droplets.
Vibrator unit
18 is connected to nozzle 12 and is used to supply pressure pulses to the
nozzle or directly in
the slurry flowing to the nozzle. The resulting vibration of the slurry flow
through the nozzle
causes the stream exiting the nozzle 12 to break into droplets of unifoun
size. As droplets
fall toward coagulation vessel 17, surface tension effects tend to form the
droplets into
spheres. Spherical particles are formed without the necessity of a sol-gel
reaction, reaction
gas free fall zone, foamed layer of reaction liquid or reaction liquid
directed onto the droplets
prior to entering the reaction liquid bath.
[0027] FIG. 2 shows details of slurry 15 exiting nozzle 12 and breaking into
drops. Surface
tension of the slurry drives the drops toward minimum surface area, which is
acquired in a
spherical shape, as they fall toward coagulation vessel 17. The distance of
fall is preferably
selected to be great enough to allow the droplets to become spherical before
entering a liquid
in vessel 17.
[0028] Slurry 15 from feed tank 14 contains a finely ground (0.01 ¨ 50 microns
in size)
mineral or processed powder capable of producing a strong ceramic material
after sintering, a
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proper amount of dispersant necessary for keeping the solid particles in the
slurry well
separated, water, and a reactant that will react with a component in liquid 19
in coagulation
vessel 17 to form a semi-solid or insoluble compound. The solids content of
the slurries may
range from about 25% to about 75%. The viscosity of the slurries will normally
be from 1 to
1,000 centipoise, but may be higher. Lower viscosity of the slurry aids in
improving droplet
formation and formation of spherical particles and is an essential part of the
invention
claimed. Optimization of the dispersant type and concentration will reduce
viscosity.
Dispersants may be selected based on cost, availability and effectiveness in
reducing
viscosity of a selected slurry. Dispersants that may be used to reduce the
viscosity of slurries
include sodium silicate, ammonium polyacrylate, sodium polymethacrylate,
sodium citrate
sodium polysulfonate and hexametaphosphate.
[0029] The commonly used reactant chemical in the slurry in feed tank 14 is
sodium alginate.
This is a naturally occurring polysaccharide that is soluble in water as the
sodium salt but is
cross-linked to form a gel as the calcium salt. Alginate is typically added to
the slurry at
levels of 0.1% to 1.0% (weight percent alginate solid to total slurry).
Coagulation tank 17
normally contains a coagulation liquid 19 which gels the reactant chemical in
the slurry 15.
The commonly used coagulation liquid for sodium alginate is a calcium chloride
solution at
concentration levels of 0.5% to 10% by weight. A variety of reactants in the
slurry flowing
through nozzle 12 and in the coagulation vessel 17 may be used. This may
include other
polysaccharides and other cross-linking compounds such as polyvinyl alcohol or
borate
fluids.
[0030] The diameter of nozzle 12, the viscosity of slurry 15, the ceramic
particle content of
slurry 15, pressure to feed the slurry to the nozzle, along with the frequency
and amplitude of
vibration applied by vibrator source 17 are adjusted to produce droplets
having a desired size.
These variables are preferably set at a constant value as spheres are produced
to be formed
into a batch of pellets of propping material. Different batches may be
produced having
different size pellets. Preferably, each batch will be monosized (i.e.
contained on a single
sieve such as passing through a 20 mesh sieve but staying on a 25 mesh sieve).
The pressure
used to feed slurry to the nozzle is adjusted to create laminar flow through
the nozzle. The
feed pressure can range from 1 to 50 psi. The frequency is adjusted for each
set of slurry
conditions such that a resonance is established in the slurry stream exiting
the nozzle that
then produces spherical droplets. The frequency can range from 10 to 20,000
Hz. The
pressure and frequency are optimized iteratively to create uniform spherical
shapes. The
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amplitude is adjusted to improve the uniform shape of the spherical droplets
formed. The
flow rate of the slurry through a nozzle is a function of the nozzle diameter,
slurry feed
pressure, and the slurry properties such as viscosity and density. For
example, for kaolin and
alumina slurries through nozzles up to 500 microns in diameter the flow rate
per nozzle can
range from 0.2 to 3 kg/hr.
[0031] The distance between nozzle 12 and the top of the liquid 19 in
coagulation vessel 17
is selected to allow droplets to become spherical before reaching the top of
the liquid. The
distance can be from 1 to 20 cm, but is more typically in the range of 1 to 5
cm so as to
reduce distortion of the droplet shape upon impact with the liquid surface,
thereby
eliminating the need for a reaction gas, foam layer, or tangentially directed
reaction liquid
prior to the droplets entering the coagulation vessel 17. The reactant
chemical in the droplets
of slurry reacts with the coagulation liquid 19 in the coagulation vessel 17
and a semi-solid
surface is formed on the droplets, which helps retain the spherical shape and
prevents
agglomeration of the pellets. Preferably, the residence time of pellets in
coagulation vessel
17 is sufficient to allow pellets to become rigid enough to prevent
deformation of the
spherical shape when they are removed and dried, i.e., semi-rigid. In some
embodiments,
pellets may fall into a coagulation liquid solution flowing vertically upward
so that settling of
the particle through the liquid will be retarded to produce a longer residence
time in the
coagulation vessel.
[0032] Particles formed using the apparatus of FIG. I are washed to remove
excess
coagulation agent and conveyed to other devices where they are dried and later
sintered,
using well known processes in the industry.
[0033] FIG. 3 illustrates a multi-nozzle apparatus, which is required to apply
the process on a
commercial scale. Multiple nozzles 32 are placed in vessel 30, which operates
under a
controlled pressure to flow slurry through the nozzles. Large numbers of
nozzles are required
for commercial production of proppant particles. Vessel 30 is vibrated to
cause vibration of
nozzles, as described above. Alternatively, variable pressure may be induced
in the slurry to
cause formation of uniform sized droplets. The droplets are collected as
described before.
[0034] Pellets produced by the process described in FIGS. 1 ¨ 3 are near
uniform in size. For
example, Table 1 compares the pellet size distributions for sintered alumina
proppant
produced by the dry mixing process and by the drip casting process described
herein, without
screening of the green pellets. Without screening of the green pellets, dry
mixing produces
fired proppant with a distribution across six screens, whereas drip casting
produces fired
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proppant substantially on one screen. Therefore, in a manufacturing process
for proppant,
drip casting does not require sieving the green pellets to select the size
range desired and then
recycling the material in green pellets outside the selected size range. The
size pellets to be
sintered into proppant are selected by controlling the diameter of nozzle 12
or 32, the
viscosity of slurry 15, the ceramic particle content of slurry 15, pressure to
feed the slurry to
the nozzle, along with the frequency and amplitude of vibration applied by
vibrator source
17.
Table 1 ¨ Sieve Distribution of Sintered Pellets Formed by Dry Mixing and Drip
casting
16 20 25 30 35 40 50
Pan
Mesh Mesh Mesh Mesh Mesh Mesh Mesh
Dry
0% 17.8% 23.9% 24.3% 18.4% 10.6% 4.9% 0%
Mixing
Drip
0% 0% 0.2% 99.8% 0% 0% 0% 0%
casting
[0035] FIGS. 4(a-e) show photographs of aluminum oxide, bauxite, and kaolin
particles
produced by the apparatus of FIG 1 and by prior art methods. FIG. 4(a) shows
an alumina
particle made by drip casting, as illustrated in FIG. 1, which has high
sphericity and a very
smooth surface. FIG. 4(b) shows an alumina particle made by an Eirich mixer.
The surfaces
of the particles are rough and the shapes are generally oblate. FIG. 4(c)
shows a bauxite
particle made by drip casting and FIG. 4(d) shows a bauxite particle made by a
commercial
prior art process using an Eirich mixer (Carbo HSP, sold by Carbo Ceramics
Inc., Houston,
TX). FIG. 4(e) shows a kaolin particle made by drip casting and FIG. 4(f)
shows a kaolin
particle made by a pilot scale fluidized bed process.
[0036] The surface roughness of each whole pellet shown in FIGS. 4(a-f) was
measured. A
smooth, convex perimeter was drawn around each pellet, establishing an average
surface
level that mimicked the actual pellet surface as closely as possible while
still remaining
convex. Then the separation between the actual surface and the smooth, average
surface was
measured around the entire perimeter at intervals of 100Rm. At 100x
magnification used in
FIG. 4, the separation could be measured with a precision of about 0.5ttm. The
average of
the measurements from the entire perimeter is representative of the surface
roughness of the
pellet. Table 2 shows that pellets formed by dry mixing and spray fluidized
bed have surface
roughness from three to seven times as large as their drip cast counterparts.
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Table 2 ¨ Surface Roughness of Drip cast and Conventionally-Formed Pellets
Average Surface Roughness (um)
Drip cast Alumina (FIG. 4a) 1.4
Dry Mixing-Formed Alumina (FIG. 4b) 5.8
Drip cast Bauxite (FIG. 4c) 1.6
Dry Mixing-Formed Bauxite (FIG. 4d) 4.9
Drip cast Kaolin (FIG. 4e) 0.8
Spray Fluid Bed-Formed Kaolin (FIG. 41) 5.7
[0037] FIG. 5 compares the permeability of pellets formed in the apparatus of
FIG.1
compared with pellets formed by the dry mixing process. The pellets from the
two processes
are identical in size and composition both being a high purity (99+%) alumina.
The only
variable is the pellet formation process. The permeabilities were measured in
accordance
with ISO 13503-5: "Procedures for Measuring the Long-term Conductivity of
Proppants,"
except that steel wafers were used rather than sandstone wafers. The long term
conductivity
apparatus described in ISO 13503-5 utilizes a steel conductivity cell that
contains an internal
slot of dimensions 7 inches in length by 1.5 inches in width. An open port is
placed in the
cell extending from the each end of the slot to the exterior of the cell to
allow for fluid flow
through the slot. Other ports are placed along the length of the slot also
extending to the
exterior of the cell for the measurement of the internal pressure of the slot.
Into this slot are
fitted a lower and upper piston the lengths which extend out beyond the
dimensions of the
cell such that a load may be applied directly to the pistons by a hydraulic
load frame. To load
the conductivity cell for the measurement of conductivity the lower piston is
first secured into
the cell so as not to obstruct the fluid or pressure ports. A seal ring is
installed to prevent
pressure or fluid leakage between slot and the piston wall. A slot sized metal
shim and a
sandstone wafer are then placed on the lower piston. Alternatively a steel
wafer may replace
the sandstone wafer (as was the case here). A set amount of proppant is then
placed on the
wafer. In this case equal volumes of the two proppants were loaded
representing initial pack
widths of about 0.19 inches. The proppant is leveled. Then on top of the
proppant is placed
a second steel wafer, metal shim, seal ring, and the upper piston. An initial
load is applied to
the pistons and fluid is flowed through the proppant pack while pressure is
measured. The
temperature of the fluid and cell was maintained at 250 degrees F. Measurement
of the rate
of fluid flow and pressure loss provides a measure of the proppant pack
conductivity in
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millidarcy- feet. The permeability of the proppant pack is calculated by
dividing the
conductivity by the measured width of the pack, which was about 0.16 ¨ 0.19
inch for the
data shown in FIG. 5. The flowing fluid was a silica saturated deoxygenated
aqueous
solution of 2% KC1. Conductivity was measured at stresses of 2000 psi to
20,000 psi in
increments of 2000 psi. In each case the stress was held for 50 hours before
measuring the
conductivity. Permeability of a proppant pack decreases as closure stress
increases due to
failure of the proppant grains. Stronger pellets will result in a higher
permeability. As can be
seen in FIG. 5, pellets made by dry mixing (line 2) lose 78% of their
permeability as the
closure stress increases from 2000 psi to 20,000 psi. By contrast the pellets
made from the
apparatus in FIG. 1 (line 1) lose only 31% of their permeability ¨ less than
one half of the
permeability loss of the pellets made by dry mixing. This higher permeability
of the pellets
made from the apparatus of FIG.1 is due to the improved strength of the
pellet.
[0038] The strength of a proppant can be indicated from the proppant crush
resistance test
described in ISO 13503-2: "Measurement of Properties of Proppants Used in
Hydraulic
Fracturing and Gravel-packing Operations." In this test a sample of proppant
is first sieved to
remove any fines (undersized pellets or fragments that may be present), then
placed in a crush
cell where a piston then used to apply a confined closure stress of some
magnitude above the
failure point of some fraction of the proppant pellets. The sample is then re-
sieved and
weight per cent of fines generated as a result of pellet failure is reported
as percent crush. A
comparison the percent crush of two equally sized samples is a method of
gauging the
relative strength. For the two samples of pellets used in the conductivity
test described above
the weight percent crush at 15,000 psi of the pellets produced by dry mixing
was 2.7% as
compared to 0.8% for the drip cast pellets. This again indicates that drip
casting produces a
stronger pellet.
[0039] Relative pellet strength can also be determined from single pellet
strength
measurements. Strength distributions of forty pellets from each of the two
samples of pellets
used in the conductivity test described above were measured, tabulated, and
analyzed using
Weibull statistics for the determination of a characteristic strength. The
characteristic strength of
the drip cast pellets so determined was 184 MPa as compared to 151 MPa for the
pellets made by
dry mixing.
[0040] Fracture mechanics teaches that pellets fail under stress from the
largest flaw in the
pellet. In proppant pellets, the largest flaw is believed to be the largest
pore. Therefore, the
stress at failure is inversely proportional to the square root of the size of
the largest flaw. So,
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the ratio (R) of the stress at failure of a drip cast proppant (DC) formed by
the apparatus
disclosed herein to a conventionally (CONY) made proppant (dry mixing or spray
fluid bed
processes) would be:
R = (Max pore sizenmp/Max pore sizecoN0112
[0041] Pellets made by the drip casting process and prior art processes were
examined by a
scanning electron microscope (SEM) at a magnification of 500x. To measure pore
size
distribution in particles, cross-sections of sintered alumina, bauxite and
kaolin pellets made
by each process were examined in the SEM. For each sample, a random area of
approximately 252 ,tin x 171 gm from each of ten different pellets was
photographed. The
ten largest pores in each area were measured and the equation above was used
to calculate the
theoretical ratio of stress at failure of drip cast pellets versus
conventionally made pellets.
The results are presented in Table 3. For example, the average maximum pore
size in the
drip cast alumina pellets was 16.3 m and for the dry mixing process alumina
pellets average
maximum pore size was 40.8 um. Using the equation above, the ratio of the
stress to failure
of the drip cast pellets to the dry mixing process pellets is 1.6. Thus
fracture mechanics
predicts that drip cast high alumina pellets should withstand approximately
1.6 times more
stress without fracturing than dry mixing process made pellets.
Table 3 ¨ Pore Sizes of Pellets Formed by Drip casting, Dry Mixing, and Spray
Fluid Bed
Alumina Bauxite Kaolin
Spray
Dry Dry
Drip cast Drip cast Drip cast Fluid
Mixed Mixed
Bed
Average Largest Pore
16.3 40.8 14.3 37.5 11.1 56.0
(ilm)
Average of 10 Largest
10.4 19.1 9.1 20.5 6.0 18.4
Pores (um)
Theoretical Ratio of
Drip cast Strength to 1.6x 1.6x 2.2
Conventional Strength
[0042] Additional measurements were carried out on the kaolin samples. In
these, every
visible pore was measured and the composite data from all ten areas was used
to calculate
average pore size, standard deviation in pore size, and number of pores per
square millimeter,
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as well as the largest pore data, which are presented in Table 3. A summary of
the data is
presented in Table 4, and FIG. 6 shows plots of the pore size distributions
for drip cast kaolin
(Curve 1) and spray fluid bed kaolin (Curve 2). The small percentage of very
large pores
generated by the spray fluid bed process shown in FIG. 6 (Curve 2) are readily
visible in the
microstructures in FIG. 4f. The lack of large pores in the drip cast material
provides the
strength advantage discussed above.
Table 4 ¨ Additional Pore Size Measurements for Drip cast and Spray Fluid Bed
Kaolin
Drip cast Kaolin Spray
Fluid Bed Kaolin
Average Pore Size ( m) 2.0 2.8
Standard Deviation in Pore Size (tm) 1.8 6.4
Average Number of Pores Per Square
2121 5133
Millimeter
[0043] Proppant made from kaolin has a cost advantage over proppants
containing higher
alumina contents, which are made from higher-cost ores containing higher
percentages of
alumina. Four proppant products having three ranges of alumina content are
sold by Carbo
Ceramics, for example (data from www.carboceramics.com , searched 12/19/2011).
Higher
alumina content proppants generally sell for higher prices and cost more to
manufacture. The
lowest alumina contents are in the products ECONOPROP and CARBOLITE, in which
the
alumina content is about 48 and 51 per cent, respectively. A higher alumina
content is in
CARBOPROP, in which the alumina content is about 72 per cent. The CARBOPROP is
a
more expensive product to make primarily because of higher raw material costs.
[0044] The property of a proppant that is most directly related to its
performance in hydraulic
fractures is permeability under stress. Long-term permeability data for pure
alumina
proppant made by a prior art method and by the drip-casting process disclosed
herein are
shown in FIG. 5. FIG. 7 shows long-term permeability data, measured using the
same
procedures as used to obtain the data in FIG. 5, for proppant having different
alumina
contents and made by different processes. Curve 1 represents published
permeability of
20/40 mesh ECONOPROP proppant (made from kaolin, having an alumina content of
about
48 per cent) made by the Eirich-mixer process described above. Curve 2
represents
permeability of 20/40 mesh CARBOPROP proppant (made from a mixture of ores
having an
alumina content of about 72 per cent). Curve 3 represents the average
permeability vs stress
of 15 samples of proppant (made from kaolin, having an alumina content of
about 48 per
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Date Recue/Date Received 2021-05-05
WO 2014/039968 PCT/US2013/058763
cent) made by the drip cast method disclosed herein. The drip cast process
produces a
proppant made from kaolin that has about the same permeability under stress as
the higher-
cost product containing 72 per cent alumina. The average long-term
permeability measured
at 10,000 psi stress of 15 samples was 173 darcies. This is far above the
published long-term
permeability at 10,000 psi stress (85 darcies) of the commercial proppant
(ECONOPROP)
having about the same alumina content, as can be seen by comparing Curve 3 and
Curve 1.
[0045] FIG. 8 shows long-term permeability data, measured by the same
procedures as used
to obtain the data in FIGs, 5 and 7, for proppant having different alumina
contents and made
by different processes. Curve 1 represents published permeability data for
20/40 mesh
CARBOPROP proppant formed by the Eirich mixer process described above (made
from a
mixture of ores having an alumina content of about 72 percent). Curve 2
represents
permeability data for proppant (primarily sieved on a 25-mesh screen) made by
the drip cast
method disclosed herein using bauxite with an alumina content of 70 percent.
Curve 3
represents permeability data for 20/40 mesh proppant made by the Eirich mixer
process and
having an alumina content of about 83 percent alumina. The permeability of the
proppant
made by the drip cast method and having an alumina content of only 70 per cent
exhibits
practically the same permeability behavior as the prior art proppant made with
an Eirich
mixer and having about 83 per cent alumina. Since alumina is a more expensive
component
of proppants, there is considerable saving by using lower cost raw materials
and the drip cast
process disclosed herein. Comparison of Curves 1 and 2 shows the benefits of
the drip cast
process with about the same alumina content in the proppant.
[0046] It is understood that modifications to the invention may be made as
might occur to
one skilled in the field of the invention within the scope of the appended
claims. All
embodiments contemplated hereunder which achieve the objects of the invention
have not
been shown in complete detail. Other embodiments may be developed without
departing
from the spirit of the invention or from the scope of the appended claims.
Although the
present invention has been described with respect to specific details, it is
not intended that
such details should be regarded as limitations on the scope of the invention,
except to the
extent that they are included in the accompanying claims.
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Date Recue/Date Received 2021-05-05
