FIELD OF INVENTION
- BACKGROUND OF THE 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.
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 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/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.
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.
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.
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.
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.
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.
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’.
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.
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.
- SUMMARY OF THE INVENTION
The present invention seeks to address the perceived limitations in the art by providing a novel lightweight proppant and method of manufacturing the same.
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 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 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 rheology 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.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
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.
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  65, 1953 and Journal of the American Ceramic Society, 36  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 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 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.
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.
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.
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 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.
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 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’.
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.
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.
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.
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.
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.
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 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.
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 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.
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.