Note: Descriptions are shown in the official language in which they were submitted.
CA 02494051 2005-Ol-26
LIGHTWEIGHT PROPPANT AND METHOD OF MAKING SAME
FIELD OF INVENTION
Lightweight particles, commonly referred to as proppants, are provided for use
in
oil and gas wells. The particles are useful to prop open subterranean
formation
fractures.
BACKGROUND OF THE INVENTION
Hydraulic fracturing is a process of injecting fluids into an oil or gas
bearing
formation at sufficiently high rates and pressures such that the formation
fails in
tension and fractures to accept the fluid. In order to hold the fracture open
once
the fracturing pressure is released, a propping agent (proppant) is mixed with
the
fluid and injected into the formation. Hydraulic fracturing increases the flow
of oil
or gas from a reservoir to the well bore in at least three ways: (1 ) the
overall
reservoir area connected to the well bore is increased, (2) the proppant in
the
fracture has significantly higher permeability than the formation itself, and
(3)
highly conductive (propped) channels create a large pressure gradient in the
reservoir past the tip of the fracture.
Proppants are preferably spherical particulates that resist high temperatures,
pressures, and the corrosive environment present in the formation. If
proppants
fail to withstand the closure stresses of the formation, they disintegrate,
producing fines or fragments, which reduce the permeability of the propped
fracture. Early proppants were based on silica sand, glass beads, sand, walnut
shells, or aluminum microspheres. For its sensible balance of cost and
compressive strength, silica sand (frac-sand) is still the most widely used
proppant in the fracturing business. Its use, however, is limited to depth
with
closure stresses of 41 MPa. Beyond this depth resin-coated and ceramic
proppants are used. Resin-coated and ceramic proppants are limited to closure
CA 02494051 2005-Ol-26
stresses of 55 and 83 MPa, respectively.
According to a study for the United States Department of Energy, published in
April 1982 (Cutler and Jones, 'Lightweight Proppants for Deep Gas Well
Stimulation' DOE/BC110038-22), ideal proppants for hydraulic fracturing would
have a specific gravity less than 2.0 g/cm<sup>3</sup>, be able to withstand closure
stresses of 138 MPa, be chemically inert in brine at temperatures to
200.degrees. C., have perfect sphericity, cost the same as sand on a volume
basis, and have a narrow proppant size distribution. The report concludes that
such a proppant is not likely to be forthcoming in the foreseeable future.
United States Patent No. 4,493,875 to Beck et al. discloses the manufacture of
lightweight composite particles, the core of which is a conventional proppant
particle, such as silica sand. The core has a thin coating containing hollow
glass
microspheres. Proppant particles manufactured in accordance with the invention
have apparent densities ranging from of 1.3 to 2.5 g/cm<sup>3</sup>. Proppants
manufactured according to this invention are not much stronger than the core
particle itself and are, due to the cost of the resin and hollow glass
spheres, quite
expensive to manufacture.
United States Patent No. 5,030,603 to Rumpf and Lemieux teaches the
manufacture of lightweight ceramic proppants with apparent specific gravities
ranging from 2.65 to 3.0 g/cm<sup>3</sup> from calcined Kaolin clay having particle
sizes of less than 8 micron. The clay is mixed with an organic binder, then
pelletized and sintered at 1,400.degrees. C. Disadvantages of this invention
are
that the proppants have a relative high apparent specific gravity and are
limited
to closure stresses of 55 MPa.
United States Patent No. 5,120,455 to Lunghofer discloses the manufacture of
lightweight ceramic proppants with apparent specific gravities of
approximately
2.65 g/cm<sup>3</sup> by sintering a mixture largely containing alumina and silica
at
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1,200 to 1,650.degrees. C. The proppants show significant conductivity at
closure stresses of 83 MPa. The main disadvantage of this invention is that
the
proppants still have a relative high apparent specific gravity.
United States Patent No. 6,364,018 to Brannon, Rickards, and Stephenson
discloses the manufacture of proppants with apparent specific gravities
ranging
from 1.25 to 1.35 g/cm<sup>3</sup> from resin-coated ground nut hulls. The patent
states low conductivities at closure stresses of 15 MPa. The use of the
proppants, therefore, is limited to shallow wells.
United States Patent No. 6,753,299 to Lunghofer et al. claims the use of using
quartz, shale containing quartz, bauxite, talc, and wollastonite as raw
materials.
The proppant contains as much as 65% quartz, and has yielded sufficient
strength to be used in wells to a pressure of 69 MPa. The apparent specific
gravity of the proppant is approximately 2.62 g/cm<sup>3</sup>. The invention
provides
some improvements on US patent 5,120,455, cited above, by reducing the
specific gravity of the proppants and by introducing cost savings due to an
increased use of silica.
United States Patent Application No. 10/804,868 to Urbanek, assigned to the
present applicant, teaches the manufacture of lightweight ceramic proppants
with
apparent specific gravities ranging from 1.4 to 1.9 g/cm<sup>3</sup> using sol-gel
processes. The application claims the preferred use of two exothermic chemical
compositions commonly referred to as 'Geopolymers' and 'Phosphate Cements'.
United States Patent Application No. 10/911,679 to Urbanek, assigned to the
present applicant, teaches the manufacture of lightweight ceramic proppants
with
apparent specific gravities ranging from 1.4 to 1.9 g/cm<sup>3</sup> by introducing
micro- and mesopores into ceramics. The application claims the use of sol-gel
processes to form porous proppants.
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CA 02494051 2005-Ol-26
At the present time, commercially available lightweight ceramic proppants have
an apparent specific gravity of around 2.7 g/cm<sup>3</sup>. The proppants are
manufactured in accordance with United States Patent No. 5,120,455, cited
above.
The present invention seeks to address the perceived limitations in the art by
providing a novel lightweight proppant and method of manufacturing the same.
SUMMARY OF THE INVENTION
According to the present invention there is provided a composition and method
useful in the manufacture of lightweight and high-strength proppants.
The proppants are composed of porous ceramics. Pores in proppants according
to this invention are of sufficient physical stability at high temperatures to
permit
accurate and independent control of porosity and the sintering process. Thus,
durable porous ceramics can be manufactured with repeatable accuracy, which
are useful in the manufacture of lightweight and high-strength proppants.
Porosity is achieved by homogenously blending ceramic precursors with pore
formers and sintering of the continuous phase of the ceramic precursors,
preferably to near theoretical density.
Ceramic precursors may comprise ceramic oxides, preferably selected from the
group consisting of alumina, aluminum hydroxide, boehmite, pseudo boehmite,
kaolin clay, kaolinite, silica, clay, talc, magnesia, cordierite, and mullite.
Pore formers comprise a predetermined particle size, particle size
distribution,
morphology, specific gravity, and reactivity at elevated temperatures. Pore
formers may inherently have a low thermal reactivity and are hereafter
referred to
as 'inert pore formers', or have a high thermal reactivity and are hereafter
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CA 02494051 2005-Ol-26
referred to as 'fugitive pore formers'. The term 'thermal reactivity' refers
to
chemical reactions, which may occur at elevated temperatures. Relative to
heating in air, the thermal reactivity of pore formers may be reduced by
heating
the disclosed compositions in the presence of non-oxidizing atmospheres,
hereafter referred to as 'inert atmospheres', or enhanced by heating in
oxidizing
atmospheres. If pore formers are substantially inert at elevated temperatures,
they are chosen to have a lower specific gravity than the sintered ceramic.
Pore
formers are preferably comprised of finely divided natural or man-made
materials, including walnut shells, alginates, saccharides, polymers, or
carbon
modifications, such as carbon black.
Proppants are formed by methods comprising the steps of:
(a) homogenously blending ceramic precursors and pore formers, and
other components which may improve the technical or economic
performance of proppants during the stages of manufacture, storage,
and field use;
(b) pelletizing the homogenous blend to form microspheres. The term
'microspheres' refers to preferably spherical bodies of less than 5 mm
in diameter;
(c) heating the microspheres to less than sintering temperatures to
evaporate volatile components and to pyrolyze fugitive pore formers
and other fugitive components; and
(d) heating the microspheres to temperatures sufficient to sinter the
ceramic precursors, preferably to near theoretical density.
Any process providing for homogenous mixtures may be selected to blend the
components of this invention. Thus, components may be ground or ball milled
together in dry form. Components may also be blended or dispersed with a
liquid
to improve homogeneity and the process of forming and sintering the
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microspheres.
Homogenous blends utilized in this invention have properties that allow them
to
be shaped and sintered to form proppant particles. These properties may be
controlled by varying the solids content, temperature, pH, particle size,
particle
size distribution, and particle morphology, and through the use of inorganic
and
organic additives, commonly known to be theology modifiers, such as fillers,
fibres, binders, fugitive binders, surfactants, plasticizers, and thickeners.
The method of forming the blends into 'green' proppants may be caused by
techniques selected from the group consisting of agglomeration, spray
granulation, wet granulation, spheronizing, extruding and pelletizing,
vibration-
induced dripping, spray nozzle formed droplets, and selective agglomeration.
The term 'green proppants' refers to microspheres of this invention, which
have
been shaped from the disclosed compositions but are not sintered.
Green proppants are then heated in stages to sintering temperatures. During
the
initial stages of heating evaporation and pyrolysis of pore formers and other
additives occurs. The present invention permits sintering of the continuous
phase
of ceramic precursors to less than or near theoretical density. Any economical
heating process may be selected to heat the blended materials.
The method may comprise the further step of coating the microspheres after
forming the proppants, the coating of the proppants then preferably comprising
use of a coating selected from the group consisting of organic coating, epoxy,
furan, phenolic resins and combinations thereof.
The invention provides a composition and method useful to economically
manufacture proppants with repeatable accuracy. The proppants have an
apparent specific gravity of 1.0 to 2.9 g/cm<sup>3</sup> and a compressive strength
of
5 to 140 MPa.
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CA 02494051 2005-Ol-26
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The following is a detailed description of preferred embodiments of the
present
invention wherein is described a composition and method useful in the
manufacture of particulate ceramics, commonly referred to as proppants. The
proppants comprise porous ceramics.
Porous ceramics have previously been used in many applications, such as
refractories, filters, abrasives, fuel cells, bone implants, catalyst
substrates,
catalysts, drying agents, diffusion layers, heat exchange components, thermal
insulators, sound barriers, and wicks. The utility of ceramics in these
applications
depends on material properties such as bend and compressive strength, thermal
shock resistance, thermal expansion, modulus of elasticity, fracture
toughness,
thermal conductivity, hardness, density, catalytic activity, and chemical
inertness.
Although many of these material properties are available in dense ceramics,
they
are lost once pores are introduced. It has been observed, for instance, that
compressive strength decreases exponentially with increasing pore volumes (see
Ryshekewitch and Duckworth, 'Compression Strength of Porous Sintered
Alumina and Zirconia', Journal of the American Ceramic Society, 36 [2] 65,
1953
and Journal of the American Ceramic Society, 36 [2] 68, 1953).
Pore volumes can be controlled to a certain degree through initial ceramic
particle properties and sintering profiles. Extended sintering periods and
high
temperatures, however, generally decrease the amount of pores present (see
Deng, Fukasawa, Ando, Zhang and Ohji, 'Microstructure and Mechanical
Properties of Porous Alumina Ceramics Fabricated by the Decomposition of
Aluminum Hydroxide', Journal of the American Ceramic Society, Vol. 84 (11 ),
2638, 2001 ). Sintering, therefore, must be restricted at times to achieve
certain
pore volumes while other mechanical properties are neglected.
Since porosity is very sensitive to ceramic precursor properties, sintering
7
CA 02494051 2005-Ol-26
temperatures, and hold times, it is difficult to produce consistent porous
ceramic
articles. Ideally, lightweight ceramics would be produced according to a
method
which controls pore size, pore size distribution, and total pore volume
independent of the sintering process. The method may also permit sintering of
ceramic precursors to near theoretical density and thereby improvements of
mechanical properties, including compressive strength.
In United States Patent No. 4,777,153 issued to Sonuparlak et al., entitled
'Process For The Production Of Porous Ceramics Using Fugitive Polymeric
Microspheres And The Resultant Product', colloidal suspensions of polymeric
microspheres of selected sizes and shapes are consolidated with aluminum
oxide particles to form compacts. Upon heating, the microspheres are
decomposed to leave pores. The resulting structure is then sintered to form a
porous ceramic body with a plurality of pores, substantially of the same size
and
shape. The pores are evenly distributed and noncontiguous throughout the
ceramic body. The major disadvantage of this process is that extended heating
periods are required to decompose the polymeric microspheres into stable pore
structures.
In United States Patent No. 5,563,212 issued to Dismukes et al., entitled
'Synthesis Of Microporous Ceramics', microporous ceramic compositions are
prepared by first forming an intimate mixture of oligomeric or polymeric
ceramic
precursors with additive particles to provide a composite intermediate,
followed
by pyrolysis of the composite intermediate under an inert atmosphere in
sequential stages. Although the addition of pore formers to produce porous
ceramics is paramount to this prior art, there is no suggestion that the
method
can be used to control pore volumes or compressive strength.
In United States Patent No. 6,156,091 issued to Casey, entitled 'Controlled
Porosity For Ceramic Contact Sheets And Setter Tiles', porosity of ceramics is
controlled by the volume percentage, particle size, and particle shape of a
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CA 02494051 2005-Ol-26
fugitive material, which is added to the original refractory material slurry.
The
method is used to fabricate setter tiles and contact sheets. The fugitive
phase is
used independently to introduce porosity or in conjunction with partial
densification. Since porosity is not solely dependent upon partial sintering,
higher
porosity levels can be achieved with less impact on subsequent mechanical
properties of the sintered refractory material. This prior art uses carbon
black as
a pore former to improve mechanical properties other than compressive strength
and to control pore volumes of ceramics containing contiguous pores after
sintering. The use of inert atmospheres to control porosity is not mentioned.
Bearing in mind the status of the prior art, it is therefore one object of the
present
invention to provide a composition and method to accurately and independently
control sintering of ceramic precursors and porosity of the sintered ceramics.
The
porous ceramics are useful in the manufacture of lightweight and high-strength
proppants. Control of porosity and sintering processes is achieved by
improving
the stability of intentionally introduced pores at high temperatures. The
invention
permits sintering of the pore-encompassing ceramic precursors to less than or
near theoretical density.
Pore formers of this invention may be fugitive or inert. Fugitive pore formers
are
substantially reactive and undergo chemical reactions at elevated
temperatures.
Such reactivity may encompass thermal and redox processes. The composition
of final reaction products therefore depends on the chemical composition of
pore
formers initially present, intermediates formed during heating, and the
reactivity
of both with optional oxidizing atmospheres at elevated temperatures. Those
skilled in the art will recognize the complexity of thermally induced
reactions
possible in presence or absence of oxidizing atmospheres and the multitude of
compounds that can occupy pores of the sintered ceramics. Thermal and
oxidative processes are jointly referred to hereafter as 'pyrolyses'. Inert
pore
formers inherently have low thermal reactivity and do not experience
substantial
pyrolyses with heating. Generally, pores are occupied by materials that have a
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CA 02494051 2005-O1-26
lower specific gravity than the continuous phase of sintered ceramics.
Relative to heating in air, pyrolyses of pore formers may be reduced by
heating in
non-oxidizing atmospheres or enhanced by heating in oxidizing atmospheres.
Inert atmospheres may be produced by replacing air with gases such as
nitrogen, argon, or ammonia. Oxidizing atmospheres may comprise oxygen by
itself or in presence of other gases, such as the composition of air.
Further to having a composition and reactivity, pore formers also have a
predetermined concentration, particle size, particle size distribution,
morphology,
including porous, foamed, or hollow particles, and specific gravity. These
parameters jointly permit accurate and independent command of pore sizes, pore
size distribution, total pore volumes, and pore connectivity from sintering.
Since
pores of this invention can be managed throughout the manufacturing cycle,
sintering of ceramic precursors can be independently controlled by choosing
methods and conditions. Thus, sintering of the continuous phase of ceramic
precursors to near theoretical density can be achieved, resulting in porous
ceramics of improved mechanical properties, such as compressive strength.
The at least one pore former may comprise finely divided natural or man-made,
organic or inorganic materials, including walnut shells, alginate,
saccharides,
polymers, or carbon modifications, such as carbon black. Although the carbon
black is the preferred pore former, other materials that have well-controlled
particle size distributions and are easily blended or dispersed, preferably as
fine
powders, may be utilized in the present invention. The particle size of pore
formers is preferably less than 5 microns, and most preferably less than 1
micron. Pore formers of appropriate particle size, particle size distribution,
and
particle morphology may be produced by any suitable and economical process,
such as grinding, ball milling, precipitating, dispersing, flame pyrolysis,
gas
condensation, spray conversion, crystallization, polymerization, chemical
synthesis, or sol-gel techniques.
CA 02494051 2005-O1-26
Pore formers are homogenously blended with at least one ceramic precursor,
which may comprise a finely divided ceramic oxide, preferably selected from
the
group consisting of alumina, aluminum hydroxide, boehmite, pseudo boehmite,
kaolin clay, kaolinite, silica, clay, talc, magnesia, cordierite, and mullite.
Those
skilled in the art will recognize the extensive list of ceramic oxides used in
the
manufacture of ceramics. It is apparent that ceramic oxides of lower specific
gravity require lower concentrations of pores than those of higher specific
gravity
in order to produce porous ceramics of equal specific gravity. Because of the
logarithmic relationship between compressive strength and pore concentration,
the use of ceramic oxides of lower specific gravity in the manufacture of
porous
ceramics of high compressive strength, therefore, may be preferred. The
particle
size of ceramic precursors is preferably less than 10 microns, and most
preferably less than 5 microns. Ceramic precursors of appropriate particle
size,
particle size distribution, and particle morphology may be produced by any
suitable and economical process, such as grinding, ball milling,
precipitating,
dispersing, flame pyrolysis, gas condensation, spray conversion,
crystallization,
chemical synthesis, or sol-gel techniques.
Any process providing for homogenous mixtures may be selected to blend the
components of this invention. Thus, components may be blended by grinding,
ball milling, or pulverizing together in dry form. Components may also be
blended
or dispersed with at least one liquid to improve homogeneity and the process
of
forming and sintering the microspheres. For the purpose of this invention, the
liquid preferably has a boiling point less than 150.degrees. C. More
preferably,
the liquid is water. Concentrations of liquid may range from 2 to 75 wt.
percent.
Homogenous blends utilized in this invention have properties that allow them
to
be shaped and sintered to form proppant particles. These properties may be
controlled by varying the solid content, temperature, pH, particle size,
particle
size distribution, and particle morphology, and through the use of inorganic
and
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CA 02494051 2005-O1-26
organic additives, commonly known to be rheology modifiers, such as fillers,
fibres, binders, fugitive binders, surfactants, plasticizers, and thickeners.
Fillers
may be added to achieve desired economic targets, specific mechanical and
chemical properties during mixing of the chemical components, forming and
sintering of green proppants, and the field performance of the final product.
Compatible fillers include waste materials, such as fly ash, sludges, stags,
volcanic aggregates, expanded perlite, pumice, obsidian, diatomaceous earth
mica, borosilicates, clays, oxides, fluorides, sea shells, silica, inorganic
pore
formers, mineral fibres, or chopped fibreglass. Inorganic pore formers may be
added to increase porosity and are preferably selected from the group
consisting
of carbonates, acetates, nitrates, silica and alumina hollow spheres. The
addition
of binders may improve the process of dispersing, shaping, or sintering of the
composition. Binders may include natural or man-made materials such as acrylic
polymers, alginates, saccharides, silicates, and monomer-catalyst combinations
used in processes commonly known as 'reactive bonding'.
Homogenously blended materials are heated in several stages to sintering
temperatures. At temperatures below 500° C., liquids are volatilized.
At
higher temperatures, pyrolysis of polymers occurs and low-molecular-weight
organics are volatilized. Pyrolysis is also performed at temperatures below
500° C. The remaining organic compounds are typically burned off above
temperatures of about 800° C. Sintering and densification may also
occur
above these temperatures. Any economical heating process may be selected to
heat the blended materials. While partial densification produces even higher
levels of porosity, full densification of ceramic precursors is preferred. The
resulting porous ceramics are lightweight, have high compressive strength, and
can be produced with repeatable accuracy.
The disclosed lightweight proppants may be coated with organic coatings, such
as epoxy, furan, and phenolic resins (United States Patent No. 5,639,806), and
combinations of these coatings to improve their performance characteristics
and
utility. Specifically, coatings may be used to seal open pores connecting to
the
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CA 02494051 2005-O1-26
surface of sintered proppants. Applications may be carried out in accordance
with known methods for coating proppants or ceramics.
Thus, through careful selection of raw materials and manufacturing conditions,
essential properties of porous ceramics, such as compressive strength and
specific gravity can be accurately and independently controlled. The selection
of
raw materials and manufacturing conditions would be clearly evident to those
skilled in the art. It is therefore another object of this invention to
provide durable
porous ceramics, which can be manufactured with repeatable accuracy, and are
useful in the manufacture of proppants for oil and gas wells. The proppants
are
l0 strong in compression, have a low apparent specific gravity, and can be
made
more economically than presently available materials.
In preferred embodiments of the present invention, a composition and method to
accurately and independently control sintering of ceramics precursors and
porosity of the sintered ceramic is disclosed. The resulting porous ceramics
are
lightweight and high in compressive strength. The ceramics are suitable for
the
manufacture of proppants and have an apparent specific gravity of 1.0 to 2.9
g/cm<sup>3</sup> and a compressive strength of 14 to 104 MPa.
The method of the present invention may comprise the step of homogenously
blending or dispersing the at least one finely divided ceramic precursor and
pore
former, and other additives using conventional blending or dispersing
techniques.
The properties of the disclosed blends permit use of sphere-forming techniques
such as agglomeration, spray granulation, wet granulation, spheronizing,
extruding and palletizing, vibration-induced dripping (United States Patent
No.
5,500,162), spray nozzle formed droplets (United States Patent No. 4,392,987),
selective agglomeration (United States Patent No. 4,902,666), the use of which
is
incorporated herein by reference. The techniques allow manufacture of green
proppants from the disclosed compositions. Green proppants are heated in
stages to sintering temperatures. The continuous phase of ceramic precursors
may be sintered to less than or near theoretical density using conventional
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CA 02494051 2005-O1-26
heating techniques. Prvppants manufactured according to the present invention
have an apparent specific gravity of 1.0 to 2.9 g/cm<sup>3</sup> and a compressive
strength of 5 to 140 MPa. The disclosed lightweight proppants may be coated
with organic coatings, such as epoxy, furan, and phenolic resins, and
combinations of these coatings to improve their performance characteristics
and
utility. The coating may be carried out in accordance with known methods of
coating proppants or ceramics.
When compared on volume bases to presently manufactured lightweight
proppants, high pore volumes and lower heat capacities of the porous ceramics
both permit reduction in manufacturing costs. The properties of the disclosed
blends permit production of highly spherical and near monodisperse particles.
Proppants manufactured according to the present invention can meet a wide
range of economic and mechanical requirements. As porosity of the ceramics is
increased, proppants show less compressive strength, but also material and
energy costs to manufacture the same volume of proppants are significantly
reduced. Highly porous proppants, therefore, can be manufactured according to
this invention to compete with frac-sand, and denser proppants can be tailored
to
be competitive with current ceramic proppants. This range is not readily
adapted
by other techniques.
A lightweight, high-strength proppant is disclosed, comprising the formation
of
porous ceramics by sintering ceramic precursors in the presence of pore
formers.
A method of manufacturing such a proppant is also disclosed, comprising the
steps of preferably blending ceramic precursors, pore formers, and additives
homogenously. These blends have properties, which permit the shaping of
spheres using conventional palletizing techniques. Staged heating of the
microspheres to sintering temperatures produces porous ceramics with
repeatable accuracy. The palletized porous ceramics are useful as lightweight
and high-strength proppants.
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EXAMPLE
The following example illustrates the use of porous ceramics in the
manufacture
of lightweight proppants.
160 litres of an aqueous solution of 8% by weight ALsub.2 (SO<sub>4</sub>)<sub>3</sub> and
3% by weight MgSO<sub>4</sub> are intensively blended with 0.06% carbon and 120
litres of 8% NaOH. The precipitate is filtered under vacuum and carefully
washed with water. The cake is partially dried. Conventional sphere forming
and
sintering at 1,400.degrees. C. in an atmosphere of Argon results in
lightweight
proppants made of MgAl<sub>2</sub> O<sub>4</sub> spinet, having an apparent specific
gravity of 1.8 g/cm<sup>3</sup>. and a compressive strength of 39 MPa.
While particular embodiments of the present invention have been described in
the foregoing, it is to be understood that other embodiments are possible
within
the scope of the invention and are intended to be included herein. It will be
clear
to any person skilled in the art that modifications of and adjustments to this
invention, not shown, are possible without departing from the spirit of the
invention as demonstrated through the exemplary embodiments. For
example, while porous ceramics may solely be used to manufacture proppants,
the use of fillers may improve the economical and physical properties of the
proppants, so the embodiments described above are therefore meant to be
merely illustrative. The invention is therefore to be considered limited
solely by
the scope of the appended claims.
15
