U.S. patent application number 11/042104 was filed with the patent office on 2006-07-27 for lightweight proppant and method of making same.
This patent application is currently assigned to Global Synfrac Inc.. Invention is credited to Thomas Wilhelm Urbanek.
Application Number | 20060162929 11/042104 |
Document ID | / |
Family ID | 36695500 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060162929 |
Kind Code |
A1 |
Urbanek; Thomas Wilhelm |
July 27, 2006 |
Lightweight proppant and method of making same
Abstract
A method of forming lightweight, high-strength proppants is
disclosed, comprising the steps of: homogeneously blending at least
one ceramic precursor and at least one pore former; pelletizing the
blend to form microspheres; heating the microspheres to less than
sintering temperatures, to evaporate volatile components and
pyrolyze fugitive components; further heating the microspheres to
temperatures sufficient to sinter the continuous phase of the
ceramic precursor, to form sintered particles; and then forming the
sintered particles into generally spheroid proppants. The generally
spheroid proppants, which have preferably been sintered to near
theoretical density, may then be coated. Heating of the
microspheres may comprise a series of heating stages.
Inventors: |
Urbanek; Thomas Wilhelm;
(Calgary, CA) |
Correspondence
Address: |
GOWLING LAFLEUR HENDERSON LLP
SUITE 1400, 700 2ND ST. SW
CALGARY
AB
T2P 4V5
CA
|
Assignee: |
Global Synfrac Inc.
|
Family ID: |
36695500 |
Appl. No.: |
11/042104 |
Filed: |
January 26, 2005 |
Current U.S.
Class: |
166/280.2 |
Current CPC
Class: |
C04B 38/009 20130101;
C04B 35/443 20130101; C04B 2235/77 20130101; C09K 8/80 20130101;
C04B 38/0675 20130101; C04B 35/01 20130101; C04B 38/068 20130101;
C04B 35/01 20130101; C04B 38/085 20130101; C04B 35/01 20130101;
C04B 38/02 20130101; C04B 38/009 20130101; C04B 2235/448 20130101;
C04B 38/009 20130101; C04B 2235/94 20130101; C04B 38/009 20130101;
C04B 38/009 20130101; C04B 35/01 20130101 |
Class at
Publication: |
166/280.2 |
International
Class: |
E21B 43/267 20060101
E21B043/267 |
Claims
1. A method of forming lightweight, high-strength proppants
comprising the steps of: (a) blending at least one ceramic
precursor and at least one pore former to form a blend; (b)
pelletizing the blend to form a plurality of microspheres; (c)
heating the plurality of microspheres to less than sintering
temperatures, to evaporate volatile components and pyrolyze
fugitive components; (d) heating the plurality of microspheres to
temperatures sufficient to sinter the continuous phase of the at
least one ceramic precursor, to form sintered particles; and (e)
forming the sintered particles into generally spheroid
proppants.
2. The method of claim 1 wherein the at least one ceramic precursor
is a ceramic oxide selected from the group consisting of alumina,
aluminum hydroxide, boehmite, pseudo boehmite, kaolin clay,
kaolinite, silica, clay, talc, magnesia, cordierite, and
mullite.
3. The method of claim 1 further comprising the step of selecting
the at least one pore former on the basis of particle size,
particle size distribution, morphology, specific gravity, and
reactivity at elevated temperatures.
4. The method of claim 1 wherein the at least one pore former is an
inert pore former.
5. The method of claim 1 wherein the at least one pore former is a
fugitive pore former.
6. The method of claim 1 wherein the at least one pore former is
selected from the group consisting of finely divided natural
materials, finely divided man-made materials, walnut shells,
alginates, saccharides, polymers, and carbon modifications.
7. The method of claim 1 wherein the at least one ceramic precursor
is homogeneously blended with the at least one pore former to form
a homogeneous blend.
8. The method of claim 1 wherein at least one additional component
is blended with the at least one ceramic precursor and the at least
one pore former to form the blend.
9. The method of claim 8 wherein the at least one additional
component is selected from the group consisting of inorganic and
organic additives.
10. The method of claim 8 wherein the at least one additional
component is selected from the group consisting of fillers, fibres,
binders, fugitive binders, surfactants, plasticizers, and
thickeners.
11. The method of claim 1 wherein each of the plurality of
microspheres is a generally spherical body less than 5 mm in
diameter.
12. The method of claim 1 wherein the heating of the plurality of
microspheres to sinter the continuous phase of the at least one
ceramic precursor causes the at least one ceramic precursor to
reach near theoretical density.
13. The method of claim 1 wherein the blending of the at least one
ceramic precursor with the at least one pore former to form a blend
is achieved by milling the at least one ceramic precursor and the
at least one pore former in dry form.
14. The method of claim 1 wherein the blending of the at least one
ceramic precursor with the at least one pore former to form a blend
is achieved by dispersing the at least one ceramic precursor and
the at least one pore former with a liquid.
15. The method of claim 1 wherein the pelletizing of the blend to
form the plurality of microspheres is achieved by a process
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.
16. The method of claim 1 comprising the further step after step
(e) of coating the generally spheroid proppants.
17. The method of claim 16 wherein the coating of the generally
spheroid proppants employs a material selected from the group
consisting of organic coating, epoxy, furan, phenolic resins and
combinations thereof.
18. The method of claim 1 wherein the at least one pore former is
less than 5 microns in size.
19. The method of claim 18 wherein the at least one pore former is
less than 1 micron in size.
20. The method of claim 1 wherein the at least one pore former is
produced by a process selected from the group consisting of
grinding, ball milling, precipitating, dispersing, flame pyrolysis,
gas condensation, spray conversion, crystallization,
polymerization, chemical synthesis, and sol-gel techniques.
21. The method of claim 1 wherein the at least one ceramic
precursor is less than 10 microns in size.
22. The method of claim 21 wherein the at least one ceramic
precursor is less than 5 microns in size.
23. The method of claim 1 wherein the at least one ceramic
precursor is produced by a process selected from the group
consisting of grinding, ball milling, precipitating, dispersing,
flame pyrolysis, gas condensation, spray conversion,
crystallization, chemical synthesis, and sol-gel techniques.
24. The method of claim 14 wherein the liquid has a boiling point
of less than 150.degrees.C.
25. The method of claim 14 wherein the liquid is water.
26. The method of claim 14 wherein concentration of the liquid is 2
to 75 wt. percent.
27. The method of claim 10 wherein the fillers are selected from
the group consisting of fly ash, sludges, slags, volcanic
aggregates, expanded perlite, pumice, obsidian, diatomaceous earth
mica, borosilicates, clays, oxides, fluorides, sea shells, silica,
inorganic pore formers, mineral fibres, and chopped fibreglass.
28. The method of claim 27 wherein the inorganic pore formers are
selected from the group consisting of carbonates, acetates,
nitrates, silica and alumina hollow spheres.
29. The method of claim 10 wherein the binders are selected from
the group consisting of acrylic polymers, alginates, saccharides,
silicates, and monomer-catalyst combinations.
30. The method of claim 1 wherein the step of heating the plurality
of microspheres to less than sintering temperatures comprises a
series of heating stages.
31. The method of claim 1 wherein the temperatures sufficient to
sinter the continuous phase of the at least one ceramic precursor
are above 800.degrees.C.
32. The method of claim 1 wherein the heating of the plurality of
microspheres to sinter the continuous phase of the at least one
ceramic precursor causes the at least one ceramic precursor to
achieve less than theoretical density.
Description
FIELD OF INVENTION
[0001] 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
[0002] 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.
[0003] 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 stresses
of 55 and 83 MPa, respectively.
[0004] 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/BC/10038-22), ideal
proppants for hydraulic fracturing would have a specific gravity
less than 2.0 g/cm.sup.3, 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.
[0005] U.S. Pat. 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. 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.
[0006] U.S. Pat. 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 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.
[0007] U.S. Pat. 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 by sintering a mixture
largely containing alumina and silica at 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.
[0008] U.S. Pat. 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 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.
[0009] U.S. Pat. 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. The invention provides some
improvements on U.S. Pat. No. 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.
[0010] U.S. patent application Ser. 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 using sol-gel processes. The
application claims the preferred use of two exothermic chemical
compositions commonly referred to as `Geopolymers` and `Phosphate
Cements`.
[0011] U.S. patent application Ser. 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 by introducing micro- and
mesopores into ceramics. The application claims the use of sol-gel
processes to form porous proppants.
[0012] At the present time, commercially available lightweight
ceramic proppants have an apparent specific gravity of around 2.7
g/cm.sup.3. The proppants are manufactured in accordance with U.S.
Pat. No. 5,120,455, cited above.
[0013] 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
[0014] According to the present invention there is provided a
composition and method useful in the manufacture of lightweight and
high-strength proppants.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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 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.
[0019] Proppants are formed by methods comprising the steps of:
[0020] (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; [0021] (b) pelletizing the homogenous blend
to form microspheres. The term `microspheres` refers to preferably
spherical bodies of less than 5 mm in diameter; [0022] (c) heating
the microspheres to less than sintering temperatures to evaporate
volatile components and to pyrolyze fugitive pore formers and other
fugitive components; and [0023] (d) heating the microspheres to
temperatures sufficient to sinter the ceramic precursors,
preferably to near theoretical density.
[0024] 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
microspheres.
[0025] 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 rheology
modifiers, such as fillers, fibres, binders, fugitive binders,
surfactants, plasticizers, and thickeners.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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 and a compressive strength of 5 to 140 MPa.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0030] 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.
[0031] 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).
[0032] 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.
[0033] Since porosity Is very sensitive to ceramic precursor
properties, sintering 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.
[0034] In U.S. Pat. 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.
[0035] In U.S. Pat. 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.
[0036] In U.S. Pat. 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 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.
[0037] 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.
[0038] 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
lower specific gravity than the continuous phase of sintered
ceramics.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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 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, slags, 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`.
[0045] Homogenously blended materials are heated in several stages
to sintering temperatures. At temperatures below 500.degree. 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.degree. C.
The remaining organic compounds are typically burned off above
temperatures of about 800.degree. 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.
[0046] The disclosed lightweight proppants may be coated with
organic coatings, such as epoxy, furan, and phenolic resins (U.S.
Pat. 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 surface
of sintered proppants. Applications may be carried out in
accordance with known methods for coating proppants or
ceramics.
[0047] 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 strong in compression,
have a low apparent specific gravity, and can be made more
economically than presently available materials.
[0048] 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 and a compressive strength of 14 to 104
MPa.
[0049] 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 pelletizing, vibration-induced dripping (U.S. Pat.
No. 5,500,162), spray nozzle formed droplets (U.S. Pat. No.
4,392,987), selective agglomeration (U.S. Pat. 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 heating techniques. Proppants manufactured according
to the present invention have an apparent specific gravity of 1.0
to 2.9 g/cm.sup.3 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.
[0050] 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.
[0051] 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 pelletizing techniques.
Staged heating of the microspheres to sintering temperatures
produces porous ceramics with repeatable accuracy. The pelletized
porous ceramics are useful as lightweight and high-strength
proppants.
EXAMPLE
[0052] The following example illustrates the use of porous ceramics
in the manufacture of lightweight proppants.
[0053] 160 litres of an aqueous solution of 8% by weight Al.sub.2
(SO.sub.4).sub.3 and 3% by weight MgSO.sub.4 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 O.sub.4 spinel, having an
apparent specific gravity of 1.8 g/cm.sup.3. and a compressive
strength of 39 MPa.
[0054] 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.
* * * * *