WO2012104190A1 - A proppant - Google Patents

Abstract

A proppant for hydraulically fracturing a subterranean formation includes a particle and a polycarbodiimide coating disposed on the particle. The polycarbodiimide coating comprises the reaction product of an isocyanate reacted in the presence of a trialkyl phosphate. A method of forming the proppant includes the steps of providing the particle, the isocyanate, and the trialkyl phosphate. The method also includes the steps of reacting the isocyanate in the presence of the trialkyl phosphate to form the polycarbodiimide coating and coating the particle with the polycarbodiimide coating. A method of hydraulically fracturing a subterranean formation uses the proppant.

Description

A PROPPANT

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 61/438,062, filed on January 31, 2011, which is incorporated herewith by reference in its entirety.

FIELD OF THE INVENTION

[0002] The subject invention generally relates to a proppant and a method of forming the proppant. More specifically, the subject invention relates to a proppant which comprises a particle and a coating disposed on the particle, and which is used during hydraulic fracturing of a subterranean formation.

DESCRIPTION OF THE RELATED ART

[0003] Domestic energy needs in the United States currently outpace readily accessible energy resources, which has forced an increasing dependence on foreign petroleum fuels, such as oil and gas. At the same time, existing United States energy resources are significantly underutilized, in part due to inefficient oil and gas procurement methods and a deterioration in the quality of raw materials such as unrefined petroleum fuels.

[0004] Petroleum fuels are typically procured from subsurface reservoirs via a wellbore. Petroleum fuels are currently procured from low-permeability reservoirs through hydraulic fracturing of subterranean formations, such as bodies of rock having varying degrees of porosity and permeability. Hydraulic fracturing enhances production by creating fractures that emanate from the subsurface reservoir or wellbore, and provides increased flow channels for petroleum fuels. During hydraulic fracturing, specially- engineered carrier fluids are pumped at high pressure and velocity into the subsurface reservoir to cause fractures in the subterranean formations. A propping agent, i.e., a proppant, is mixed with the carrier fluids to keep the fractures open when hydraulic fracturing is complete. The proppant typically comprises a particle and a coating disposed on the particle. The proppant remains in place in the fractures once the high pressure is removed, and thereby props open the fractures to enhance petroleum fuel flow into the wellbore. Consequently, the proppant increases procurement of petroleum fuel by creating a high-permeability, supported channel through which the petroleum fuel can flow.

[0005] However, many existing proppants exhibit inadequate thermal stability for high temperature and pressure applications, e.g. wellbores and subsurface reservoirs having temperatures greater than 70°F and pressures, i.e., closure stresses, greater than 7,500 psi. As an example of a high temperature application, certain wellbores and subsurface reservoirs throughout the world have temperatures of about 375°F and 540°F. As an example of a high pressure application, certain wellbores and subsurface reservoirs throughout the world have closure stresses that exceed 12,000 or even 14,000 psi. As such, many existing proppants, which comprise coatings, such as epoxy or phenolic coatings, which melt, degrade, and/or shear off the particle in an uncontrolled manner when exposed to such high temperatures and pressures. Also, many existing proppants do not include active agents, such as microorganisms and catalysts, to improve the quality of the petroleum fuel recovered from the subsurface reservoir.

[0006] Further, many existing proppants comprise coatings having inadequate crush resistance. That is, many existing proppants comprise non-uniform coatings that include defects, such as gaps or indentations, which contribute to premature breakdown and/or failure of the coating. Since the coating typically provides a cushioning effect for the proppant and evenly distributes high pressures around the proppant, premature breakdown and/or failure of the coating undermines the crush resistance of the proppant. Crushed proppants cannot effectively prop open fractures and often contribute to impurities in unrefined petroleum fuels in the form of dust particles.

[0007] Moreover, many existing proppants also exhibit unpredictable consolidation patterns and suffer from inadequate permeability in wellbores, i.e., the extent to which the proppant allows the flow of petroleum fuels. That is, many existing proppants have a lower permeability and impede petroleum fuel flow. Further, many existing proppants consolidate into aggregated, near-solid, non-permeable proppant packs and prevent adequate flow and procurement of petroleum fuels from subsurface reservoirs.

[0008] Also, many existing proppants are not compatible with low-viscosity carrier fluids having viscosities of less than about 3,000 cps at 80 °C. Low-viscosity carrier fluids are typically pumped into wellbores at higher pressures than high-viscosity carrier fluids to ensure proper fracturing of the subterranean formation. Consequently, many existing coatings fail mechanically, i.e., shear off the particle, when exposed to high pressures or react chemically with low-viscosity carrier fluids and degrade.

[0009] Finally, many existing proppants are coated via non-economical coating processes and therefore contribute to increased production costs. That is, many existing proppants require multiple layers of coatings, which results in time-consuming and expensive coating processes. [0010] Due to the inadequacies of existing proppants, there remains an opportunity to provide an improved proppant.

SUMMARY OF THE INVENTION

[0011] A proppant for hydraulically fracturing a subterranean formation, a method of forming the proppant, and a method of hydraulically fracturing a subterranean formation are provided. The proppant includes a particle and a polycarbodiimide coating disposed on the particle. The polycarbodiimide coating comprises the reaction product of an isocyanate reacted in the presence of a trialkyl phosphate.

[0012] The method of forming the proppant includes the steps of providing the particle, the isocyanate, and the trialkyl phosphate. The method of forming the proppant also includes the steps of reacting the isocyanate in the presence of the trialkyl phosphate to form the polycarbodiimide coating and coating the particle with the polycarbodiimide coating.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention provides a proppant and a method of forming, or preparing, the proppant, a method of hydraulically fracturing a subterranean formation, and a method of filtering a fluid. The proppant comprises a particle and a coating disposed on the particle. The proppant is typically used, in conjunction with a carrier fluid, to hydraulically fracture a subterranean formation which defines a subsurface reservoir (e.g. a wellbore or reservoir itself). The proppant props open the fractures in the subterranean formation after the hydraulic fracturing. In one embodiment, the proppant may also be used to filter unrefined petroleum fuels, e.g. crude oil, in fractures to improve feedstock quality for refineries. However, it is to be appreciated that the proppant can also have applications beyond hydraulic fracturing and crude oil filtration, including, but not limited to, water filtration and artificial turf.

[0014] The proppant comprises the particle and the coating disposed on the particle. Although the particle may be of any size, the particle typically has a particle size distribution of from about 10 to about 100 mesh, more typically from about 20 to about 70 mesh, as measured in accordance with standard sizing techniques using the United States Sieve Series. That is, the particle typically has a particle size of from about 149 to about 2,000, more typically of from about 210 to about 841, μηι. Particles having such particle sizes allow less coating to be used, allow the coating to be applied to the particle at a lower viscosity, and allow the coating to be disposed on the particle with increased uniformity and completeness as compared to particles having other particle sizes.

[0015] The shape of the particle is not critical; the particle can be any shape. Typically, the particle is either round or roughly spherical. Particles having a spherical shape typically impart a smaller increase in viscosity to a hydraulic fracturing composition than particles having other shapes. The hydraulic fracturing composition is a mixture comprising the carrier fluid and the proppant.

[0016] The particle typically contains less than about 1 part by weight of moisture based on 100 parts by weight of the particle. Particles containing higher than about 1 part by weight of moisture typically interfere with sizing techniques and prevent uniform coating of the particle.

[0017] Suitable particles include any known particle for use during hydraulic fracturing, water filtration, or artificial turf preparation. Non-limiting examples of suitable particles include minerals, ceramics such as sintered ceramic particles, sands, nut shells (including crushed walnut hulls), gravels, mine tailings, coal ashes, rocks (including bauxite), smelter slag, diatomaceous earth, crushed charcoals, micas, clays (including kaolin clay particles), sawdust, wood chips, resinous particles (including phenol-formaldehyde particles), polymeric particles, and combinations thereof. It is to be appreciated that other particles not recited herein may also be suitable.

[0018] Sand is a preferred particle and when applied in this technology is commonly referred to as fracturing, or frac, sand. Examples of suitable sands include, but are not limited to, Arizona sand, Wisconsin sand, Missouri sand, Brady sand, Northern White sand, and Ottawa sand. Based on cost and availability, inorganic materials such as sand and sintered ceramic particles are typically favored for applications not requiring filtration.

[0019] A specific example of a sand that is suitable as a particle is Arizona sand, which is a natural grain that is derived from weathering and erosion of pre-existing rocks. As such, this sand is typically coarse and is roughly spherical. Another specific example of a sand that is suitable as a particle is Ottawa sand, commercially available from U.S. Silica Company of Berkeley Springs, WV. Yet another specific example of a sand that is suitable as a particle is Wisconsin sand, commercially available from Badger Mining Corporation of Berlin, WI. Particularly preferred sands are Ottawa and Wisconsin sands. Ottawa and Wisconsin sands of various sizes, such as 20/40, 30/50, 40/70, and 70/140 can be used.

[0020] Specific examples of suitable sintered ceramic particles include, but are not limited to, aluminum oxide, silica, bauxite, and combinations thereof. The sintered ceramic particle may also include clay-like binders. [0021] An active agent may also be included in the particle. In this context, suitable active agents include, but are not limited to, organic compounds, microorganisms, and catalysts. Specific examples of microorganisms include, but are not limited to, anaerobic microorganisms, aerobic microorganisms, and combinations thereof. A suitable microorganism is commercially available from LUCA Technologies of Golden, Colorado. Specific examples of suitable catalysts include fluid catalytic cracking catalysts, hydroprocessing catalysts, and combinations thereof. Fluid catalytic cracking catalysts are typically selected for applications requiring petroleum gas and/or gasoline production from crude oil. Hydroprocessing catalysts are typically selected for applications requiring gasoline and/or kerosene production from crude oil. It is also to be appreciated that other catalysts, organic or inorganic, not recited herein may also be suitable.

[0022] Such additional active agents are typically favored for applications requiring filtration. As one example, sands and sintered ceramic particles are typically useful as a particle for support and propping open fractures in the subterranean formation which defines the subsurface reservoir, and, as an active agent, microorganisms and catalysts are typically useful for removing impurities from crude oil or water. Therefore, a combination of sands/sintered ceramic particles and microorganisms/catalysts as active agents are typical for crude oil or water filtration.

[0023] Suitable particles may even be formed from resins and polymers. Specific examples of resins and polymers for the particle include, but are not limited to, polyurethanes, polycarbodiimides, polyureas, acrylics, polyvinylpyrrolidones, acrrylonitrile-butadiene styrenes, polystyrenes (including polyvinyl styrene), polyvinyl chlorides, fluoroplastics, polysulfides, nylon, polyamide imides, and combinations thereof.

[0024] As described above, the proppant also comprises the coating disposed on the particle. As used herein, the terminology "disposed on" encompasses the coating being "disposed about" the particle and also encompasses both partial and complete covering of the particle by the coating.

[0025] The coating is typically present in the proppant in an amount of from about 0.1 to about 10, more typically of from about 0.5 to about 7.5, and most typically of from about 1.0 to about 6.0, percent by weight based on 100 parts by weight of the particle. However, it should be appreciated that the coating can be present in the proppant in an amount of greater than about 10 percent by weight based on 100 parts by weight of the proppant.

[0026] The coating may be formed in-situ where the coating is disposed on the particle during formation of the coating. Said differently, the components of the coating are typically combined with the particle and the coating is disposed on the particle.

[0027] However, in one embodiment the coating is formed and some time later applied to, e.g. mixed with, the particle and exposed to temperatures exceeding about 100°C to coat the particle and form the proppant. Advantageously, this embodiment allows the coating to be formed at a location designed to handle chemicals, under the control of personnel experienced in handling chemicals. Once formed, the coating can be transported to another location, applied to the particle, and heated. Other advantages of this embodiment include quicker sand coating cycle times, less generation of volatile organic compounds during coating of the particle, and reduced use of raw materials. In addition to the advantages described above, there are numerous logistical and practical advantages associated with this embodiment. For example, if the coating is being applied to the particle, e.g. frac sand, the coating may be applied immediately following the manufacturing of the frac sand, when the frac sand is already at elevated temperature, eliminating the need to reheat the coating and the frac sand, thereby reducing the amount of energy required to form the proppant.

[0028] Multiple layers of the coating can be applied to the particle to form the proppant. As such, the proppant can comprise a particle having multiple identical or different coatings disposed on the particle.

[0029] The coating may include an additive component. Suitable additive components include, but are not limited to, surfactants, blowing agents, blocking agents, curatives, dyes, pigments, diluents, solvents, specialized functional additives such as antioxidants, ultraviolet stabilizers, biocides, adhesion promoters, antistatic agents, fire retardants, fragrances, and combinations of the group. For example, a pigment allows the coating to be visually evaluated for thickness and integrity and can provide various marketing advantages.

[0030] The adhesion promoter is also commonly referred to in the art as a coupling agent or as a binder agent. The adhesion promoter binds the coating to the particle. More specifically, the adhesion promoter typically has organofunctional silane groups to improve adhesion of the coating to the particle. Without being bound by theory, it is thought that the adhesion promoter allows for covalent bonding between the particle and the coating. [0031] In one embodiment, the adhesion promoter may be incorporated into the coating. As such, the particle is then simply exposed to the adhesion promoter when the coating is applied to the particle. In another embodiment, the surface of the particle is activated with the adhesion promoter by applying the adhesion promoter to the particle prior to coating the particle with the coating. In this embodiment, the adhesion promoter can be applied to the particle by a wide variety of application techniques including, but not limited to, spraying, dipping the particles in the coating, etc.

[0032] The adhesion promoter is useful for applications requiring excellent adhesion of the coating to the particle, for example, in applications where the proppant is subjected to shear forces in an aqueous environment. Use of the adhesion promoter provides adhesion of the coating to the particle such that the coating will remain adhered to the surface of the particle even if the proppant, including the coating, the particle, or both, fractures due to closure stress.

[0033] Examples of suitable adhesions promoters, which are silicon-containing, include, b u t a r e n o t l i m i t e d t o , g 1 y c i d o x y p r o p y 1 1 r i m e t h o x y s i 1 a n e , aminoethylaminopropyltrimethoxysilane, methacryloxypropyltrimethoxysilane, gamma- aminopropyltriethoxysilane, vinylbenzylaminoethylaminopropyltrimethoxysilane, glycidoxypropylmethy ldiethoxy s ilane, chloropropy ltrimethoxy s ilane, phenyltrimethoxysilane, vinyltriethoxysilane, tetraethoxysilane, methyldimethoxysilane, bis-triethoxysilylpropyldisulfidosilane, bis-triethoxysilylpropyltetrasulfidosilane, phenyltriethoxysilane, aminosilanes, and combinations thereof.

[0034] Specific examples of suitable adhesion promoters include, but are not limited to, SILQUEST™ A1100, SILQUEST™ Al l 10, SILQUEST™ A1120, SILQUEST™ 1130, SILQUEST™ Al l 70, SILQUEST™ A-189, and SILQUEST™ Y9669, all commercially available from Momentive Performance Materials of Albany, NY. A particularly suitable silicon-containing wett i n g a g e nt i s S I L Q UE S T™ A 1 1 00 , i . e . , g a m m a- aminopropyltriethoxysilane. The silicon-containing wetting agent may be present in the proppant in an amount of from about 0.001 to about 10, typically from about 0.01 to about 7.5, and more typically from about 0.04 to about 5, percent by weight based on 100 parts by weight of the coating.

[0035] Suitable coatings include any known coating for use during hydraulic fracturing, water filtration, or artificial turf preparation. Typically, the coating comprises a polymer which may include moieties selected from the group of isocyanate moieties, isocyanurate moieties, uretdione moieties, carbodiimide moieties, uretonimine moieties, and urethane moieties.

[0036] Typically, the coating is a polycarbodiimide coating. The polycarbodiimide coating is typically selected for applications requiring excellent coating stability and adhesion to the particle. For example, the polycarbodiimide coating is particularly applicable when the proppant is exposed to significant compression and/or shear forces, and temperatures exceeding about 500°F in the subterranean formation and/or subsurface reservoir defined by the formation. The polycarbodiimide coating is generally viscous to solid nature, and depending on molecular weight, is typically sparingly soluble or insoluble in organic solvents. Any suitable polycarbodiimide coating may be used. The polycarbodiimide coating can be formed from all known raw materials, methods, and chemical reactions. [0037] The polycarbodiimide coating is typically formed by reacting an isocyanate in the presence of a catalyst. Obviously, the polycarbodiimide coating may be the reaction product of more than two isocyanates. When one or more isocyanates are reacted to form the polycarbodiimide coating, the physical properties of the polycarbodiimide coating, such as hardness, strength, toughness, creep, and brittleness can be further optimized and balanced.

[0038] The isocyanate may be any type of isocyanate known to those skilled in the art. The isocyanate may be a polyisocyanate having two or more functional groups, e.g. two or more NCO functional groups. Suitable isocyanates include, but are not limited to, aliphatic and aromatic isocyanates. In various embodiments, the isocyanate is selected from the group of diphenylmethane diisocyanates (MDIs), polymeric diphenylmethane diisocyanates (pMDIs), toluene diisocyanates (TDIs), hexamethylene diisocyanates (HDIs), isophorone diisocyanates (IPDIs), and combinations thereof.

[0039] The isocyanate may be an isocyanate prepolymer. The isocyanate prepolymer is typically a reaction product of an isocyanate and a polyol and/or a polyamine. The isocyanate used in the prepolymer can be any isocyanate as described above. The polyol used to form the prepolymer is typically selected from the group of ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, butane diol, glycerol, trimethylolpropane, triethanolamine, pentaerythritol, sorbitol, biopolyols, and combinations thereof. The polyamine used to form the prepolymer is typically selected from the group of ethylene diamine, toluene diamine, diaminodiphenylmethane and polymethylene polyphenylene polyamines, aminoalcohols, and combinations thereof. Examples of suitable aminoalcohols include ethanolamine, diethanolamine, triethanolamine, and combinations thereof.

[0040] Specific isocyanates that may be used to prepare the polycarbodiimide coating include, but are not limited to, toluylene diisocyanate; 4,4'-diphenylmethane diisocyanate; m-phenylene diisocyanate; 1,5 -naphthalene diisocyanate; 4-chloro-l ; 3- phenylene diisocyanate; tetramethylene diisocyanate; hexamethylene diisocyanate; 1,4- dicyclohexyl diisocyanate; 1 ,4-cyclohexyl diisocyanate, 2,4, 6-toluylene triisocyanate, 1 ,3-diisopropylphenylene-2,4-dissocyanate; 1 -methyl-3,5-diethylphenylene-2,4- diisocyanate; l ,3,5-triethylphenylene-2,4-diisocyanate; l,3,5-triisoproply-phenylene-2,4- diisocyanate; 3,3'-diethyl-bisphenyl-4,4'-diisocyanate; 3,5,3',5'-tetraethyl- diphenylmethane-4,4'-diisocyanate; 3,5,3',5'-tetraisopropyldiphenylmethane-4,4'- diisocyanate; l-ethyl-4-ethoxy-phenyl-2,5-diisocyanate; 1 ,3,5-triethyl benzene-2,4,6- triisocyanate; l -ethyl-3,5-diisopropyl benzene-2,4,6-triisocyanate and 1 ,3,5-triisopropyl benzene-2,4,6-triisocyanate. Other suitable polycarbodiimide coatings can also be prepared from aromatic diisocyanates or isocyanates having one or two aryl, alkyl, arakyl or alkoxy substituents wherein at least one of these substituents has at least two carbon atoms. Specific examples of suitable isocyanates include LUPRANATE^® L5120, LUPRANATE^® M, LUPRANATE^® ME, LUPRANATE^® MI, LUPRANATE^® M20, and LUPRANATE^® M70, all commercially available from BASF Corporation of Florham Park, NJ.

[0041] The one or more isocyanates are typically heated in the presence of the catalyst to form the polycarbodiimide coating. One or more catalysts can be used to form the polycarbodiimide coating. The catalyst may be any type of catalyst known to those skilled in the art. Generally, the catalyst is selected from the group of phosphorous compounds, tertiary amides, basic metal compounds, carboxylic acid metal salts, non- basic organo-metallic compounds, and combinations thereof.

[0042] The polycarbodiimide coating can also be formed from various catalysts disclosed in U.S. Patent No. 4,284,730 (the '730 patent), which is hereby incorporated by reference in its entirety. The '730 patent discloses formation of carbodiimide by reacting the isocyanate and/or other raw materials in the presence of the catalyst. Genuses of catalysts disclosed in the '730 patent include phosphates, phospholene oxides, diaza- and oxaza phospholenes and phosphorinanes, triaryl arsines, metallic derivatives of acetlyacetone, phosphate esters, metal complexes derived from a d-group transition element and a π bonding ligand, organotin compounds, organic and metal carbene complexes, and various titanium (IV), copper (I) and copper (II) complexes. The genuses of catalysts disclosed in the '730 patent are discussed in detail immediately below.

[0043] The polycarbodiimide coating can be formed using catalysts comprising phosphorous, such as phosphates, phospholene oxides, diaza- and oxaza phospholenes and phosphorinanes. Specific examples of such catalysts comprising phosphorous include, but are not limited to, phosphate esters and other phosphates such as trimethyl phosphate, triethyl phosphate, tributyl phosphate, tri-2-ethylhexyl phosphate, tributoxyethyl phosphate, trioleyl phosphate, triphenyl phosphate, tricresyl phosphate, trixylenyl phosphate, cresyl diphenyl phosphate, xylenyl diphenyl phosphate, 2- ethylhexyldiphenyl phosphate, and the like; acidic phosphates such as methyl acid phosphate, ethyl acid phosphate, isopropyl acid phosphate, butyl acid phosphate, 2- ethylhexyl acid phosphate, isodecyl acid phosphate, lauryl acid phosphate, isotridecyl acid phosphate, myristyl acid phosphate, isostearyl acid phosphate, oleyl acid phosphate, and the like; tertiary phosphites such as triphenyl phosphite, tri(p-cresyl) phosphite, tris(nonylphenyl) phosphite, triisooctyl phosphite, diphenyisodecyl phosphite, phenyldiisodecyl phosphite, triisodecyl phosphite, tristearyl phosphite, trioleyl phosphite, and the like; secondary phosphites such as di-2-ethylhexyl hydrogen phosphite, dilauryl hydrogen phosphite, dioleyl hydrogen phosphite, and the like; and phosphine oxides, such as triethylphosphine oxide, tributylphosphine oxide, triphenylphosphine oxide, tris(chloromethyl)phosphine oxide, tris(chloromethyl)phosphine oxide, and the like. Catalysts comprising phosphate esters and methods for their preparation are described in U.S. Pat. No. 3,056,835, which is hereby incorporated by reference in its entirety.

[0044] The polycarbodiimide coating can also be formed using catalysts comprising phospholene 1 -oxides and 1 -sulfides. Phospholene 1 -oxides and 1 -sulfides and methods for their preparation are described in U. S. Pat. Nos. : 2,663,737; 2,663,738; and 2,853,473, which are hereby incorporated by reference in their entirety. Specific examples of phospholene 1 -oxides and 1 -sulfides include, but are not limited to 1 -phenyl- 2-phospholene-l -oxide; 3 -methyl-1 -phenyl-2-phospholene-l -oxide; 1 -phenyl-2- phospholene-1 -sulfide; 1 -ethyl-2-phospholene-l -oxide; 1 -ethyl-3 -methyl -2 -phospholene- 1-oxide; l-ethyl-3-methyl-2-phospholene-l-sulfide; and the isomeric phospholanes corresponding to the above-named compounds. Also, the catalyst can comprise polymer bound phospholene oxides such as those disclosed in U.S. Patent Nos. 4, 105,643 and 4,105,642, which are hereby incorporated by reference in their entirety.

[0045] The polycarbodiimide coating can also be formed using catalysts comprising diaza and oxaza phospholenes and phosphorinanes. Diaza and oxaza phospholenes and phosphorinanes and methods for their preparation are described in U.S. Pat. No. 3,522,303, which is hereby incorporated by reference in its entirety. Specific diaza- and oxaza phospholenes and phosphorinanes include, but are not limited to, 2-ethyl-l,3- dimethyl-l,3,2-diazaphospholane-2-oxide; 2-chloromethyl-l, 3 -dimethyl-1, 3,2- diazaphospholane-2-oxide; 2-trichloromethyl-l,3-dimethyl-l,3,2-diazaphospholane-2- oxide; 2-phenyl- 1 ,3 -dimethyl- 1 ,3 ,2-diazaphospholane-2-ox i d e ; 2 -phenyl- 1 ,3 -dimethyl- 1 ,3 ,2-diaza-phosphorinane-2 -oxide; 2-benzyl- 1 ,3 -dimethyl-1 ,3 ,2-diazaphospholane-2- oxide; 2-allyl-l,3-dimethyl-l,3,2-diazaphospholane-2-oxide; 2-bromomethyl- 1,3- dimethyl- 1 ,3,2-diazaphospholane-2-oxide; 2-cyclohexyl-l ,3 -dimethyl-1 ,3,2- diazaphospholane-2-oxide; 2-cyclohexyl-l, 3-dimethyl-l,3,2-diaphospholane-2 -oxide; 2- (2-ethoxyethyll,3-dimethyl-l,3,2-diazaphospholane-2-o x i d e ; and 2-naphthyl-l,3- dimethyl-l,3,2-diazaphospholane-2-oxide, triethyl phosphate, hexamethyl phosphoramide, and the like.

[0046] The polycarbodiimide coating can also be formed using catalysts comprising triaryl arsine. Triaryl arsines and methods for their preparation are described in U.S. Pat. No.3,406,198, which is hereby incorporated by reference in its entirety. Specific examples of triaryl arsines include, but are not limited to, triphenylarsine, tris(p- tolyl)arsine, tris(p-methoxyphenyl)arsine, tris(p-ethoxyphenyl)arsine, tris(p- chlorophenyl) arsine, tris(p-fluorophenyl)arsine, tris(2,5-xylyl)arsine, tris(p- cyanophenyl)arsine, tris(l-naphthyl)arsine, tris(p-methylmercaptophenyl)arsine, tris(p- biphenylyl)arsine, p-chlorophenyl bis(ptolyl)arsine, phenyl(p-chlorophenyl)(p- bromophenyl)arsine, and the like. [0047] In addition, catalysts comprising arsine compounds described in U.S. Patent No. 4,143,063, which is hereby incorporated by reference in its entirety, can be used to form the polycarbodiimide coating. Specific examples of such arsine compounds include, but are not limited to, triphenylarsine oxide, triethylarsine oxide, polymer bound arsine oxide, and the like.

[0048] The polycarbodiimide coating can also be formed using catalysts comprising metallic derivatives of acetlyacetone. Metallic derivatives of acetlyacetone and methods are described in U.S. Pat. No. 3,152,131, which is hereby incorporated by reference in its entirety. Specific examples of metallic derivatives of acetlyacetone include, but are not limited to, metallic derivatives of acetylacetone such as the beryllium, aluminum, zirconium, chromium, and iron derivatives.

[0049] The polycarbodiimide coating can be formed using catalysts comprising metal complexes derived from a d-group transition element and π-bonding ligand selected from the group consisting of carbon monoxide, nitric oxide, hydrocarbylisocyanides, trihydrocarbylphosphine, trihydrocarbylarsine , tr ihy dro carby l sti l b i ne , an d dihydrocarbylsulfide wherein hydrocarbyl in each instance contains from 1 to 12 carbon atoms, inclusive, provided that at least one of the π-bonding ligands in the complex is carbon monoxide or hydrocarbylisocyanide. Such metal complexes and methods for preparation are described in U.S. Pat. No. 3,406,197, which is hereby incorporated by reference in its entirety. Specific examples of metal complexes include, but are not limited to, iron pentacarbonyl, di-iron pentacarbonyl, tungsten hexacarbonyl, molybdenum hexacarbonyl, chromium hexacarbonyl, dimanganese decacarbonyl, nickel t e tr a c ar b o ny 1 , ruthenium pentacarbony 1, the complex of iron tetracarbonyl:methylisocyanide, and the like.

[0050] The polycarbodiimide coating can be formed using catalysts comprising organotin compounds. Specific examples of organotin compounds include, but are not limited to, dibutytin dilaurate, dibutyltin diacetate, dibutyltin di(2-ethylhexanoate), dioctyltin dilaurate, dibutylin maleate, di(n-octyl)tin maleate, bis(dibutylacetoxytin) oxide, bis(dibutyllauroyloxytin) oxide, dibutyltin dibutoxide, dibutyltin dimethoxide, dibutyltin disalicilate, dibutyltin bis(isooctylmaleate), dibutyltin bis(isopropylmaleate), dibutyltin oxide, tributyltin acetate, tributyltin isopropyl succinate, tributyltin linoleate, tributyltin nicotinate, dimethyltin dilaurate, dimethyltin oxide, diotyltin oxide, bis(tributyltin) oxide, diphenyltin oxide, triphenyltin acetate, tri-n-propyltin acetate, tri-n-propyltin laurate and bis(tri-n-propy ltin) oxide, dibutyltin dilauryl mercaptide, dibutyltin bis(isooctylmercaptoacetate),bis(triphenyltin)oxide, stannous oxalate, stannous oleate, stannous naphthenate, stannous acetate, stannous butyrate, stannous 2-ethylhexanoate, stannous laurate, stannous palmitate, stannous stearate, and the like. Typical organotin compounds include, but are not limited to, stannous oxalate, stannous oleate and stannous 2-ethylhexanoate, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin dilaurylmercaptide, dibutyltin bis(isooctylmercaptoacetate), dibutyltin oxide, bis(triphenyltin) oxide, and bis(tri-n-butyltin) oxide.

[0051] Various catalysts comprising organic and metal carbene complexes can also be used to form the polycarbodiimide coating. In addition, various catalysts comprising titanium(IV) complexes, copper(I) and copper(II) complexes can also be used to form the polycarbodiimide coating via living carbodiimide polymerizations. Furthermore, the polycarbodiimide coating can be formed via catalytic conversion of isocyanates to carbodiimides by cyclopentadienyl manganese tricarbonyl and cyclopentadienyl iron dicarbonyl dimer and derivatives.

[0052] Specific polycarbodiimide coatings include, but are not limited to, monomers, oligomers, and polymers of diisopropylcarbodiimide, dicyclohexylcabodiimide, methyl- tert-butylcarbodiimide, 2,6-diethylphenyl carbodiimide; di-ortho-tolyl-carbodimide; 2,2'- dimethyl diphenyl carbodiimide; 2,2'-diisopropyl-diphenyl carbodiimide; 2-dodecyl-2'-n- propyl-diphenylcarbodiimide; 2,2'-diethoxy-diphenyl dichloro-diphenylcarbodiimide; 2,2'-ditolyl-diphenyl carbodiimide; 2,2'-dibenzyl-diphenyl carbodiimide; 2,2'-dinitro- diphenyl carbodiimide; 2-ethyl-2'-isopropyl-diphenyl carbodiimide; 2,6,2',6'-tetraethyl- diphenyl carbodiimide; 2,6,2',6'-tetrasecondary-butyl-diphenyl carbodiimide; 2,6,2',6'- tetraethyl-3,3'-dichloro-diphenyl carbodiimide; 2-ethyl-cyclohexyl-2-isopropylphenyl carbodiimide; 2,4,6,2',4',6'-hexaisopropyl-di pheny l carb o di i mi de ; 2 , 2 '-diethyl- dicyclohexyl carbodiimide; 2, 6, 2', 6' -tetraisopropyl-dicyclohexyl carbodiimide; 2,6,2', 6'tetraethyl-dicyclohexy) carbodiimide and 2,2'-dichlorodicyclohexyl carbodiimide; 2,2'-dicarbethoxy diphenyl carbodiimide; 2,2'-dicyano-diphenyl carbodiimide and the like.

[0053] As disclosed in PCT Patent Application No. PCT/EP2009/064244, which is hereby incorporated by reference in its entirety, the polycarbodiimide coating can be formed by reacting the isocyanate in the presence of the catalyst. The polycarbodiimide coating can be the reaction product of one type of isocyanate. However, the polycarbodiimide coating is typically the reaction product of at least two different isocyanates. [0054] The polycarbodiimide coating can also be formed from various chemical reactions disclosed in "Chemistry and Technology of Carbodiimides", John Wiley & Sons, Ltd., Chichester, West Sussex, England (2007), which is hereby incorporated by reference in its entirety. As a specific example, the polycarbodiimide coating can be formed from:

1. Thioureas, isothioureas, and selenoureas, for example:

HgO (other catalysts can be used)

a. RNHCSNHR → RN=C=NR

-HgS, H₂0

2. Isocyanates, isothiocyanates, for example:

Catalyst

a. RN=C=E → RN=C=NR

-CE₂

Catalyst (Aza-Witting Reaction)

b. R₃P=NR + RN=C=E → RN=C=NR

-R₃P=E

Catalyst (phosphoramidates)

c. (RO)₂P(0)NHR + RN=C=E → RN=C=NR

Wherein P is N, P, As, Sb, or Bi; E is O or S; and R is alkyl or aryl halide

3. Cyanamides, for example:

R3CCI (other catalysts can be used)

a. R3CNHCN → RN=C=NR₃

4. Nitrene rearrangements, for example:

RS0₂C1 (other catalysts can be used)

a. RC(NHR)=NOH → RN=C=NR

5. Haloformamidines or carbonimidoyl dihalides, for example:

R3N (other catalysts can be used)

a. [RNHC(C1)=NHR]C1 → RN=C=NR 6. Thermolysis/exchange reactions

7. Other reactions

Fe(CO)₅

a. RN₃ or RNH₂ + RNC RN=C=NR

Phosphate Catalyst

1 -phenyl-3-methyl-2-phospholene-l -oxide

b. RNCO RN=C=NR

[0055] The proppant of the present invention comprises the particle and the polycarbodiiminde coating disposed on the particle. The polycarbodiimide coating is the reaction product of the isocyanate reacted in the presence of a trialkyl phosphate. Typically, the trialkyl phosphate is triethyl phosphate (TEP). To form the polycarbodiimide coating, the trialkyl phosphate is typically mixed with the isocyanate in an amount of from about 0.1 to about 25, more typically from about 1 to about 20, and most typically from about 3 to about 15, percent by weight based on 100 parts by weight of the isocyanate.

[0056] The polycarbodiimide coating can also be the reaction product of the isocyanate reacted in the presence of the trialkyl phosphate and a phospholene oxide. Suitable non limiting examples of the phospholene oxide include, but are not limited to, 3 -methyl- 1- phenyl-2-phospholene oxide (MPPO), l -phenyl-2-phospholen-l-oxide, 3-methyl-l -2- phospholen-l-oxi d e , 1 -ethy 1 -2-phospholen-l-ox i d e , 3 -methyl-l-pheny 1 -2-phospholen-l- oxide, 3-phospholene isomers thereof, and 3-methyl-l-ethyl-2-phospholene oxide (MEPO). In one embodiment, the polycarbodiimide coating is the reaction product of the isocyanate reacted in the presence of TEP and MPPO. In another embodiment, the polycarbodiimide coating is the reaction product of the isocyanate reacted in the presence of TEP and MEPO.

[0057] If the polycarbodiimide coating is the reaction product of the isocyanate reacted in the presence of the trialkyl phosphate and the phospholene oxide, the trialkyl phosphate and the phospholene oxide are typically collectively mixed with the isocyanate in an amount of less than about 25, more typically in an amount of from about 0.1 to about 25, more typically from about 0.2 to about 20, and most typically from about 0.3 to about 10, percent by weight based on 100 parts by weight of the isocyanate. Of course, the percent by weight of the collective amount of the trialkyl phosphate and the phospholene oxide can include various ratios of the trialkyl phosphate and the phospholene oxide.

[0058] The polycarbodiimide coating is typically formed by heating the isocyanate in the presence of a trialkyl phosphate. The polycarbodiimide coating can be formed at any temperature. Typically, formation of the polycarbodiimide coating occurs at a temperature greater than about 25, more typically at a temperature of from about 50 to about 250, and most typically from about 60 to about 230, °C.

[0059] The polycarbodiimide coating can also be formed from a modified isocyanate component. The modified isocyanate component can be formed by heating the isocyanate in the presence of a trialkyl phosphate. The modified isocyanate component can be formed and the polycarbodiimide coating can formed immediately therefrom or the modified component can be formed and the polycarbodiimide coating can be formed some time later therefrom. The modified isocyanate component can be used in all applicable reactions and references cited in this disclosure to form the polycarbodiimide coating. U.S. Patent No. 3,384,653 (the '653 patent), which is hereby incorporated by reference in its entirety, discloses formation of the modified isocyanate component by heating the isocyanate in the presence of the trialkyl phosphate.

[0060] The modified isocyanate component is formed by heating the isocyanate, such as methylenebis (phenyl isocyanate), in the presence of a trialkyl phosphate such as, triethyl phosphate (TEP). The trialkyl phosphate catalyzes the chemical reaction of the isocyanate and subsequent formation of carbodiimide moieties and polycarbodiimide. Upon heating, the modified isocyanate component comprises various isocyanate and carbodiimide moieties. Upon cooling, the isocyanate moieties react with the carbodiimide moieties to form uretonimine moieties. As such, the modified isocyanate component comprises the isocyanate moieties, the carbodiimide moieties, and the uretonimine moieties. The modified isocyanate component is chemically stable and exhibits excellent shelf-life. Once the modified isocyanate component is formed, the polycarbodiimide coating can be formed therefrom and disposed about the particle to form the proppant.

[0061] Notably, the modified isocyanate component is merely a reaction intermediate. The polycarbodiimide coating can be formed simply by heating the isocyanate in the presence of the TEP as necessary to react the isocyanate and form the polycarbodiimide coating. In such a case, the modified isocyanate component is not present as a discrete and independent component.

[0062] The modified isocyanate component is formed by mixing and heating the isocyanate, TEP, and optionally a catalyst and/or other additives. One or more isocyanates can be mixed with TEP to form the modified isocyanate component. [0063] In a particularly preferred embodiment, the first isocyanate is further defined as a polymeric isocyanate, and the second isocyanate is further defined as a monomeric isocyanate. As such, a mixture of LUPRANATE^® M20 and LUPRANATE^® M may be reacted to form the polycarbodiimide coating. LUPRANATE^® M20 comprises polymeric isocyanates, such as polymeric diphenyl methane diisocyanate, and also comprises monomeric isocyanates. LUPRANATE^® M comprises only monomeric isocyanates. As is known in the art, a monomeric isocyanate includes, but is not limited to, 2,4'-diphenylmethane diisocyanate (2,4'-MD I ) a n d 4 , 4 '-diphenylmethane diisocyanate (4,4'-MDI). As is also well know in the art, polymeric isocyanate includes isocyanates with two or more aromatic rings. LUPRANATE^® M20 has an NCO content of about 31.5 weight percent and LUPRANATE^® M has an NCO content of about 33.5 weight percent.

[0064] Increasing an amount of LUPRANATE^® M20 in the mixture increases the amount of polymeric MDI in the mixture, and increasing the amount of polymeric MDI in the mixture affects the physical properties of the polycarbodiimide coating. For example, in one embodiment, a mixture of LUPRANATE^® M20 and LUPRANATE® M is reacted to form the polycarbodiimide coating. Generally, increasing an amount of LUPRANATE^® M20 and decreasing an amount of LUPRANATE^® M in the mixture forms a polycarbodiimide coating which is harder, stronger, and does not creep significantly; however, the polycarbodiimide coating may also be brittle. Likewise, decreasing the amount of LUPRANATE^® M20 and increasing the amount of LUPRANATE^® M in the mixture generally decreases the brittleness but increases the creep of the polycarbodiimide coating. [0065] As one example, LUPRANATE^® M20 can be mixed with TEP and the resulting mixture can be heated to form the modified isocyanate component. As another example, a mixture of LUPRANATE^® M20 and LUPRANATE^® M can mixed with TEP and the resulting mixture can be heated to form the modified isocyanate component. The TEP is typically mixed with the isocyanate in an amount of from about 0.1 to about 25, more typically from about 1 to about 20, and most typically from about 3 to about 15, percent by weight based on 100 parts by weight of the isocyanate to form the polycarbodiimide coating. However, it should be appreciated that TEP can be mixed with the isocyanate in an amount of greater than about 25 percent by weight based on 100 parts by weight of the isocyanate.

[0066] As set forth above, the present invention also provides the method of forming, or preparing, the proppant. For this method, the particle, the isocyanate, and the trialkyl phosphate are provided, the isocyanate is reacted in the presence of the trialkyl phosphate to form the polycarbodiimide coating, and the particle is coated with the polycarbodiimide coating. The step of coating the particle with the polycarbodiimide coating is described additionally below.

[0067] The isocyanate is reacted in the presence of the trialkyl phosphate, such as TEP, to form the polycarbodiimide coating. As with all other components which may be used in the method (e.g. the particle), the isocyanate and the trialkyl phosphate are just as described above with respect to the polycarbodiimide coating. Reacting the isocyanate forms the polycarbodiimide coating. The isocyanate may be reacted to form the polycarbodiimide coating simultaneous with the actual coating of the particle; alternatively, the isocyanate may be reacted to form the polycarbodiimide coating prior to the actual coating of the particle. The isocyanate can be partially reacted to from the modified isocyanate component, whi ch can b e further heated to form the polycarbodiimide coating; alternatively, the isocyanate can be completely reacted to form the polycarbodiimide coating in a single step.

[0068] The method optionally includes the step of heating the particle to a temperature greater than about 150°C prior to or simultaneous with the step of coating the particle with the polycarbodiimide coating. If heated, the particle is typically heated to a temperature of from about 150 to about 250, more typically from about 180 to about 220, and most typically from about 190 to about 210, °C.

[0069] The method also optionally includes the step of heating the isocyanate in the presence of the trialkyl phosphate to a temperature of greater than about 150°C to from the polycarbodiimide coating. The amount of time and temperature required to form the polycarbodiimide coating varies. The isocyanate component is typically heated in the presence of the trialkyl phosphate to a temperature of greater than about 25, more typically greater than about 150, and most typically greater than about 180, °C to form the polycarbodiimide coating. Typically, the isocyanate component is heated to a temperature of from about 190 to about 230, °C.

[0070] Various techniques can be used to coat the particle with the polycarbodiimide coating. These techniques include, but are not limited to, mixing, pan coating, fluidized- bed coating, co-extrusion, spraying, in-situ formation of the polycarbodiimide coating, and spinning disk encapsulation. The technique for applying the coating to the particle is selected according to cost, production efficiencies, and batch size. [0071] The amount of time and temperature required to form the polycarbodiimide coating, i.e., the amount of time and temperature required to convert the isocyanate component into the polycarbodiimide coating, varies. Typically, the higher the temperature at which the proppant is heated, the less time required to form the polycarbodiimide coating. In this method, the step of reacting the isocyanate in the presence of the trialkyl phosphate to form the polycarbodiimide coating and the step of coating the particle with the polycarbodiimide coating are collectively conducted in about 60 minutes or less, typically in about 30 minutes or less, more typically in about 15 minutes or less, and even more typically in about 3 to about 5 minutes.

[0072] In this method, the proppant can be further heated, i.e., post cured, to further crosslink the polycarbodiimide coating. Generally, further heating of the proppant will improve the performance of the proppant. Where the proppant is heated to further crosslink the polycarbodiimide coating, the step of reacting the isocyanate to form the polycarbodiimide coating, the step of coating the particle with the polycarbodiimide coating, and of the step of heating the proppant to further crosslink the polycarbodiimide coating are typically collectively conducted in about 60 minutes or less, more typically in about 30 minutes or less, and most typically in about 5 minutes or less.

[0073] In one embodiment, the isocyanate is mixed with TEP and heated to a temperature of greater than about 200°C for up to about 60 minutes to form the modified isocyanate component with carbodiimide and/or uretonimine moieties. The modified isocyanate component is mixed with the catalyst, such as 3 -methyl- 1 -phenyl-2-phospholene oxide, to form a reaction mixture. The reaction mixture and the particle, comprising Ottawa sand, are added to a reaction vessel and agitated at a temperature of about 170°C for about 1 to about 5 minutes. During agitation at these conditions, the modified isocyanate component polymerizes to form the polycarbodiimide coating on the particle, i.e., the proppant. In this example, the polycarbodiimide coating is formed in-situ, i.e., the polycarbodiimide coating is disposed on the particle simultaneous to the formation of the polycarbodiimide coating. After formation of the proppant, the proppant, i.e., the particle having the polycarbodiimide coating formed thereon, can be heated (post-cured) at various temperatures and for various times to further cure the polycarbodiimide coating.

[0074] In another embodiment, the particle comprising Ottawa sand is activated with the adhesion promoter, gamma-aminopropyltriethoxysilane. The particle, now activated with gamma-aminopropyltriethoxysilane, is pre-heated to a temperature of greater than about 200°C and added to a reaction vessel. The isocyanate is mixed with TEP to form the modified isocyanate component. The modified isocyanate component is added to the reaction vessel, which contains the particle, i.e., Ottawa sand activated with the gamma- aminopropyltriethoxysilane and pre-heated, to form a reaction mixture. The reaction mixture is heated to a temperature of from about 215 to about 230, °C and agitated for about 2 minutes. During agitation at these conditions, the modified isocyanate component polymerizes to form the polycarbodiimide coating on the particle and thus form the proppant. During agitation, a silicone lubricant is sprayed on the proppant to further ensure that the proppant does not agglomerate. The proppant, i.e., the particle having the polycarbodiimide coating formed thereon, can be heated (post-cured) at a temperature of about 125°C for about 10 minutes to further cure the polycarbodiimide coating. Alternatively, the proppant, i.e., the particle having the polycarbodiimide coating formed thereon, can be used as is (without the post-cure) and the polycarbodiimide coating will further cure if it is exposed to elevated temperatures in the subterranean formation.

[0075] In yet another embodiment, the isocyanate, TEP, and optionally a catalyst, such as 3 -methyl- 1 -phenyl-2-phospholene oxide, are added to a reaction vessel, mixed, and heated to a temperature of about 190°C for about 60 minutes to form the modified isocyanate component. The modified isocyanate component, in a molten state, is cooled to a solidified, thermoplastic-like, crystalline state and is powderized. In this example, the modified isocyanate component later applied to, e.g. mixed with, the particle and exposed to temperatures exceeding about 100°C to form the polycarbodiimide coating on the particle, i.e., to form the proppant.

[0076] In one embodiment, the polycarbodiimide coating is disposed on the particle via mixing in a vessel, e.g. a reactor. In particular, the individual components of the polycarbodiimide coating, e.g. the isocyanate, the trialkyl phosphate, the particle, and optionally the catalyst, are added to the vessel to form a reaction mixture. The components may be added in equal or unequal weight ratios. The reaction mixture is typically agitated at an agitator speed commensurate with the viscosities of the components. Further, the reaction mixture is typically heated at a temperature commensurate with the polycarbodiimide coating technology and batch size. For example, the components of the polycarbodiimide coating are typically heated from a temperature of about 190°C to a temperature of about 230°C in about 10 minutes or less, depending on batch size. It is to be appreciated that the technique of mixing may include adding components to the vessel sequentially or concurrently. Also, the components may be added to the vessel at various time intervals and/or temperatures. [0077] In another embodiment, the polycarbodiimide coating is disposed on the particle via spraying. In particular, individual components of the polycarbodiimide coating are contacted in a spray device to form a coating mixture. The coating mixture is then sprayed onto the particle to form the proppant. Spraying the coating mixture onto the particle can result in a uniform, complete, and defect-free polycarbodiimide coating disposed on the particle. Spraying the coating mixture can also result in a thinner and more consistent polycarbodiimide coating disposed on the particle as compared to other techniques, and thus the proppant is coated economically. Spraying the particle even permits a continuous manufacturing process. Spray temperature is typically selected by one known in the art according to coating technology and ambient humidity conditions.

[0078] In another embodiment, the polycarbodiimide coating is disposed on the particle in-situ, i.e., in a reaction mixture comprising the components of the polycarbodiimide coating and the particle. In this embodiment, the polycarbodiimide coating is formed or partially formed as the polycarbodiimide coating is disposed on the particle. In-situ polycarbodiimide coating formation steps typically include providing each component of the polycarbodiimide coating, providing the particle, combining the components of the polycarbodiimide coating and the particle, and disposing the polycarbodiimide coating on the particle. In-situ formation of the polycarbodiimide coating typically allows for reduced production costs by way of fewer processing steps as compared to existing methods for forming a proppant.

[0079] The formed proppant is typically prepared according to the method as set forth above and stored in an offsite location before being pumped into the subterranean formation and the subsurface reservoir. As such, spraying typically occurs offsite from the subterranean formation and subsurface reservoir. However, it is to be appreciated that the proppant may also be prepared just prior to being pumped into the subterranean formation and the subsurface reservoir. In this scenario, the proppant may be prepared with a portable coating apparatus at an onsite location of the subterranean formation and subsurface reservoir.

[0080] The proppant is useful for hydraulic fracturing of the subterranean formation to enhance recovery of petroleum and the like. In a typical hydraulic fracturing operation, a hydraulic fracturing composition, i.e., a mixture, comprising the carrier fluid, the proppant, and optionally various other components, is prepared. The carrier fluid is selected according to wellbore conditions and is mixed with the proppant to form the mixture which is the hydraulic fracturing composition. The carrier fluid can be a wide variety of fluids including, but not limited to, kerosene and water. Typically, the carrier fluid is water. Various other components which can be added to the mixture include, but are not limited to, guar, polysaccharides, and other components know to those skilled in the art.

[0081] The mixture is pumped into the subsurface reservoir, which may be the wellbore, to cause the subterranean formation to fracture. More specifically, hydraulic pressure is applied to introduce the hydraulic fracturing composition under pressure into the subsurface reservoir to create or enlarge fractures in the subterranean formation. When the hydraulic pressure is released, the proppant holds the fractures open, thereby enhancing the ability of the fractures to extract petroleum fuels or other subsurface fluids from the subsurface reservoir to the wellbore. [0082] As such, a method of hydraulically fracturing a subterranean formation which defines a subsurface reservoir with a mixture comprising a carrier fluid and the proppant, the proppant comprising the particle and the polycarbodiimide coating disposed on the particle is also disclosed. In a typical embodiment, this method comprises the step of pumping the mixture into the subsurface reservoir to cause the subterranean formation to fracture, wherein the polycarbodiimide coating comprises the reaction product of the isocyanate reacted in the presence of the trialkyl phosphate.

[0083] The proppant typically exhibits excellent thermal stability for high temperature and pressure applications, e.g. temperatures greater than about 100°C, typically greater than about 200°C, more typically greater than about 300°C, and even more typically greater than about 400°C, and/or pressures (independent of the temperatures described above) greater than about 7,500 psi, typically greater than about 10,000 psi, more typically greater than about 12,500 psi, and even more typically greater than about 15,000 psi . The proppant does not typically suffer from complete failure of the polycarbodiimide coating due to shear or degradation when exposed to such temperatures and pressures.

[0084] For the method of filtering a fluid, the proppant is provided according to the method of forming the proppant as set forth above. In one embodiment, the subsurface fluid can be an unrefined petroleum or the like. However, it is to be appreciated that the method may include the filtering of other subsurface fluids not specifically recited herein, for example, air, water, or natural gas.

[0085] To filter the subsurface fluid, the fracture in the subsurface reservoir that contains the unrefined petroleum, e.g. unfiltered crude oil, is identified by methods known in the art of oil extraction. Unrefined petroleum is typically procured via a subsurface reservoir, such as a wellbore, and provided as feedstock to refineries for production of refined products such as petroleum gas, naphtha, gasoline, kerosene, gas oil, lubricating oil, heavy gas, and coke. However, crude oil that resides in subsurface reservoirs includes impurities such as sulfur, undesirable metal ions, tar, and high molecular weight hydrocarbons. Such impurities foul refinery equipment and lengthen refinery production cycles, and it is desirable to minimize such impurities to prevent breakdown of refinery equipment, minimize downtime of refinery equipment for maintenance and cleaning, and maximize efficiency of refinery processes. Therefore, filtering is desirable.

[0086] For the method of filtering, the hydraulic fracturing composition is pumped into the subsurface reservoir so that the hydraulic fracturing composition contacts the unfiltered crude oil. The hydraulic fracturing composition is typically pumped into the subsurface reservoir at a rate and pressure such that one or more fractures are formed in the subterranean formation. The pressure inside the fracture in the subterranean formation may be greater than about 5,000, greater than about 7,000, or even greater than about 10,000 psi, and the temperature inside the fracture is typically greater than about 70°F and can be as high as about 375°F depending on the particular subterranean formation and/or subsurface reservoir.

[0087] Although not required for filtering, it is particularly desirable that the proppant be a controlled-release proppant. With a controlled-release proppant, while the hydraulic fracturing composition is inside the fracture, the polycarbodiimide coating of the proppant typically dissolves in a controlled manner due to pressure, temperature, pH change, and/or dissolution in the carrier fluid in a controlled manner, i.e., a controlled- release. Complete dissolution of the polycarbodiimide coating depends on the thickness of the polycarbodiimide coating and the temperature and pressure inside the fracture, but typically occurs within about 1 to about 4 hours. It is to be understood that the terminology "complete dissolution" generally means that less than about 1 % of the polycarbodiimide coating remains disposed on or about the particle. The controlled- release allows a delayed exposure of the particle to crude oil in the fracture. In the embodiment where the particle includes the active agent, such as the microorganism or catalyst, the particle typically has reactive sites that must contact the fluid, e.g. the crude oil, in a controlled manner to filter or otherwise clean the fluid. If implemented, the controlled-release provides a gradual exposure of the reactive sites to the crude oil to protect the active sites from saturation. Similarly, the active agent is typically sensitive to immediate contact with free oxygen. The controlled-release provides the gradual exposure of the active agent to the crude oil to protect the active agent from saturation by free oxygen, especially when the active agent is a microorganism or catalyst.

[0088] To filter the fluid, the particle, which is substantially free of the polycarbodiimide coating after the controlled-release, contacts the subsurface fluid, e.g. the crude oil. It is to be understood that the terminology "substantially free" means that complete dissolution of the polymeric coating has occurred and, as defined above, less than about 1% of the polymeric coating remains disposed on or about the particle. This terminology is commonly used interchangeably with the terminology "complete dissolution" as described above. In an embodiment where an active agent is utilized, upon contact with the fluid, the particle typically filters impurities such as sulfur, unwanted metal ions, tar, and high molecular weight hydrocarbons from the crude oil through biological digestion. As noted above, a combination of sands/sintered ceramic particles and microorganisms/catalysts are particularly useful for filtering crude oil to provide adequate support/propping and also to filter, i.e., to remove impurities. The proppant therefore typically filters crude oil by allowing the delayed exposure of the particle to the crude oil in the fracture.

[0089] The filtered crude oil is typically extracted from the subsurface reservoir via the fracture, or fractures, in the subterranean formation through methods known in the art of oil extraction. The filtered crude oil is typically provided to oil refineries as feedstock, and the particle typically remains in the fracture.

[0090] Alternatively, in a fracture that is nearing its end-of-life, e.g. a fracture that contains crude oil that cannot be economically extracted by current oil extraction methods, the particle may also be used to extract natural gas as the fluid from the fracture. The particle, particularly where an active agent is utilized, digests hydrocarbons by contacting the reactive sites of the particle and/or of the active agent with the fluid to convert the hydrocarbons in the fluid into propane or methane. The propane or methane is then typically harvested from the fracture in the subsurface reservoir through methods known in the art of natural gas extraction.

[0091] The following example illustrates the nature of the invention and is not to be construed as limiting of the invention.

EXAMPLE

[0092] Example 1 is a proppant comprising a particle having a polycarbodiimide coating disposed thereon. The composition of Example 1 is disclosed below in Table 1. To form Example 1, a Particle A is first activated with the Adhesion Promoter, i.e., Particle A is pre-coated with the Adhesion Promoter at a concentration of 400 ppm by weight Particle A. More specifically, the Adhesion Promoter is dissolved in deionized water to form a solution comprising 0.5% weight percent Adhesion Promoter. The solution is mixed with the Particle A such that the Particle A is thoroughly wet-out by the solution. Once mixed, the particle having the solution thereon is heated, i.e., dried, in an oven set at 105°C. During drying, the solution evaporates leaving the Particle A having the Adhesion Promoter coated thereon. That is, Particle A is activated with the Adhesion Promoter. After drying, the Particle A, now activated with the Adhesion Promoter, is pre-heated to a temperature of 215°C and added to a reaction vessel (a 1 -pint aluminum can.)

[0093] Isocyanate A, Isocyanate B, and TEP are mixed in a beaker to form a reaction mixture. The reaction mixture is added to the reaction vessel, which contains the Particle A. The reaction mixture and the Particle A are mixed with a 3" Jiffy mixer at 480 rpm on a drill press for 2 minutes. During agitation at these conditions, the reaction mixture forms a polycarbodiimide coating on the Particle A.

[0094] While still in the reaction vessel, the Particle A having the polycarbodiimide coating formed thereon is heated, i.e., post-cured for a total of 10 minutes in an oven at 105°C. More specifically, after 4 minutes in the oven, the reaction vessel is extracted from the oven and mixed again with the 3" Jiffy mixer at 480 rpm on a drill press and placed back in the oven for 6 more minutes. As such, Example 1 is a proppant that comprises the Particle A having a polycarbodiimide coating disposed thereon.

[0095] Example 1 is described below in Table 1. The amounts in Table 1 are in grams. Table 1

[0096] Particle A is Ottawa sand, commercially available from U.S. Silica Company of

Berkeley Springs, WV, under the trade name OTTAWA WHITE .

[0097] Adhesion Promoter is a silane commercially available from Momentive

Performance Materials of Albany, NY, sold under the tradename SILQUEST™ Al 100.

[0098] Isocyanate A is polymeric isocyanate, commercially available from BASF Corp. of Corporation of Florham Park, NJ, under the tradename LUPRANATE^® M20.

[0099] Isocyanate B is 4,4'-diphenylmethane diisocyanate, commercially available from

BASF Corp. of Corporation of Florham Park, NJ, under the tradename LUPRANATE^®

M.

[00100] TEP is tnethyl phosphate.

[00101] A Control Sample of Example 1 is stored at ambient conditions. Three samples of Example 1 are formed. A sample, Sample 1 of Example 1 is aged in an oven at 100°C for 19 hours while being exposed to air. A sample, Sample 2 of Example 1 is aged in an oven at 100°C for 33 days while being exposed to air. A sample, Sample 3 of Example 1 is submerged in deionized water and aged in an oven at 100°C for 33 days. The Control Sample, Sample 1, Sample 2, and Sample 3 are evaluated by crush testing and thermal gravimetric analysis. [00102] The crush testing results are set forth in Table 2 below. The appropriate formula for determining percent fines is set forth in API RP60. The Samples are sieved for ten minutes prior to testing crush strength to ensure that each Sample comprises individual proppant particles which are greater than sieve size 35. The crush strength is tested by compressing a test weight of each Sample (sieved to > sieve size 35) in a test cylinder (having a diameter of 1.5 inches as specified in API RP60) at a loading density of 4 lb/ft . E a ch Sample is compressed for 1 hour at 10,000 psi and 121°C (approximately 250°F). After compression, agglomeration is determined, and after each Sample is removed from the test cylinder, the Sample is sieved for 10 minutes and the percent fines is calculated.

[00103] Agglomeration is an objective observation of a Sample after crush strength testing as described above. The Sample is assigned a numerical ranking between 1 and 10. If the Sample agglomerates completely, it is ranked 10. If the Sample does not agglomerate, i.e., it falls out of the cylinder after crush test, it is rated 1.

[00104] The crush testing results, including percent fines and agglomeration, are set forth in Table 2 below.

Table 2

Note: Sample 3 was tested twice, represented by Repetitions 1 and 2.

[00105] Samples 2 and 3 were also analyzed via thermal gravimetric analysis

(TGA) over a temperature range of 5 to 750, °C in air at a heating rate of 10°C/min, using a TA Instruments Q5000 TGA. The TGA results are set forth in Table 3 below.

Table 3

Note: Samples 3 and 4 were tested twice, represented by Repetitions 1 and 2.

[00106] As described above, the crush strength, agglomeration, and thermal stability of Example 1 is excellent. Further, as is also described above, the performance properties of Example 1 , such as crush strength, agglomeration, and thermal stability, are consistent even after exposure to various conditions. [00107] It is to be understood that the appended claims are not limited to express and particular compounds, compositions, or methods described in the detailed description, which may vary between particular embodiments which fall within the scope of the appended claims. With respect to any Markush groups relied upon herein for describing particular features or aspects of various embodiments, it is to be appreciated that different, special, and/or unexpected results may be obtained from each member of the respective Markush group independent from all other Markush members. Each member of a Markush group may be relied upon individually and or in combination and provides adequate support for specific embodiments within the scope of the appended claims.

[00108] It is also to be understood that any ranges and subranges relied upon in describing various embodiments of the present invention independently and collectively fall within the scope of the appended claims, and are understood to describe and contemplate all ranges including whole and/or fractional values therein, even if such values are not expressly written herein. One of skill in the art readily recognizes that the enumerated ranges and subranges sufficiently describe and enable various embodiments of the present invention, and such ranges and subranges may be further delineated into relevant halves, thirds, quarters, fifths, and so on. As just one example, a range "of from 0.1 to 0.9" may be further delineated into a lower third, i.e., from 0.1 to 0.3, a middle third, i.e., from 0.4 to 0.6, and an upper third, i.e., from 0.7 to 0.9, which individually and collectively are within the scope of the appended claims, and may be relied upon individually and/or collectively and provide adequate support for specific embodiments within the scope of the appended claims. In addition, with respect to the language which defines or modifies a range, such as "at least," "greater than," "less than," "no more than," and the like, it is to be understood that such language includes subranges and/or an upper or lower limit. As another example, a range of "at least 10" inherently includes a subrange of from at least 10 to 35, a subrange of from at least 10 to 25, a subrange of from 25 to 35, and so on, and each subrange may be relied upon individually and/or collectively and provides adequate support for specific embodiments within the scope of the appended claims. Finally, an individual number within a disclosed range may be relied upon and provides adequate support for specific embodiments within the scope of the appended claims. For example, a range "of from 1 to 9" includes various individual integers, such as 3, as well as individual numbers including a decimal point (or fraction), such as 4.1 , which may be relied upon and provide adequate support for specific embodiments within the scope of the appended claims.

[00109] The present invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that the present invention may be practiced otherwise than as specifically described.

Claims

CLAIMS What is claimed is:

1. A proppant for hydraulically fracturing a subterranean formation, said proppant comprising:

A. a particle; and

B. a polycarbodiimide coating disposed on said particle and comprising the reaction product of an isocyanate reacted in the presence of a trialkyl phosphate.

2. A proppant as set forth in claim 1 wherein said trialkyl phosphate comprises triethyl phosphate (TEP).

3. A proppant as set forth in claim 2 wherein said TEP is mixed with said isocyanate in an amount of from about 0.1 to about 25 parts by weight based on 100 parts by weight isocyanate to form said polycarbodiimide coating.

4. A proppant as set forth in claim 3 wherein said isocyanate is heated in the presence of said TEP to a temperature of greater than about 150°C to form said polycarbodiimide coating.

5. A proppant as set forth in any of claims 1-4 wherein said isocyanate is also reacted in the presence of a phospholene oxide.

6. A proppant as set forth in any of claims 1-5 wherein said isocyanate is further defined as a first isocyanate comprising a polymeric isocyanate and a second isocyanate different from said first isocyanate and comprising a monomeric isocyanate such that said polycarbodiimide coating comprises the reaction product of said polymeric and monomeric isocyanates.

7. A proppant as set forth in claim 6 wherein said polymeric isocyanate comprises polymeric diphenylmethane diisocyanate and has an NCO content of about 31.5 weight percent and wherein said monomeric isocyanate comprises 4,4'- diphenylmethane diisocyanate and has an NCO content of about 33.5 weight percent.

8. A proppant as set forth in any of claims 1-7 wherein said particle is selected from the group of minerals, ceramics, sands, nut shells, gravels, mine tailings, coal ashes, rocks, smelter slag, diatomaceous earth, crushed charcoals, micas, sawdust, wood chips, resinous particles, polymeric particles, and combinations thereof.

9. A proppant as set forth in any of claims 1-8 wherein said polycarbodiimide coating is present in said proppant in an amount of from about 0.1 to about 10 percent by weight based on 100 parts by weight of said particle.

10. A proppant as set forth in any of claims 1-9 that is thermally stable at temperatures greater than about 400°C.

11. A method of forming a proppant for hydraulically fracturing a subterranean formation, said method comprising the steps of:

A. providing a particle;

B. providing an isocyanate;

C. providing a trialkyl phosphate;

D. reacting the isocyanate in the presence of the trialkyl phosphate to form a polycarbodiimide coating; and

E. coating the particle with the polycarbodiimide coating.

12. A method as set forth in claim 11 wherein the trialkyl phosphate comprises triethyl phosphate (TEP).

13. A method as set forth in claim 1 further comprising the step of heating the TEP and the isocyanate to a temperature of greater than about 150°C to from the polycarbodiimide coating.

14. A method as set forth in any of claims 11-13 wherein the step of reacting the isocyanate in the presence of the trialkyl phosphate to form the polycarbodiimide coating is further defined as reacting the isocyanate in the presence of the trialkyl phosphate and a phospholene oxide to form the polycarbodiimide coating.

15. A method as set forth in any of claims 11-14 wherein the isocyanate is further defined as a first isocyanate comprising a polymeric isocyanate and a second isocyanate different from the first isocyanate and comprising a monomeric isocyanate such that the polycarbodiimide coating comprises the reaction product of the polymeric and monomeric isocyanates.

16. A method as set forth in any of claims 11-15 further comprising the step of heating the particle to a temperature greater than about 150°C prior to or simultaneous with the step of coating the particle with the polycarbodiimide coating.

17. A method as set forth in any of claims 11 -16 wherein the steps of reacting the isocyanate in the presence of the trialkyl phosphate to form the polycarbodiimide coating and of coating the particle with the polycarbodiimide coating are collectively conducted in about 60 minutes or less.

18. A method as set forth in any of claims 11-17 further comprising the step of heating the proppant to further crosslink the polycarbodiimide coating.

19. A method as set forth in any of claims 11-18 wherein the particle is selected from the group of minerals, ceramics, sands, nut shells, gravels, mine tailings, coal ashes, rocks, smelter slag, diatomaceous earth, crushed charcoals, micas, sawdust, wood chips, resinous particles, polymeric particles, and combinations thereof.

20. A method of hydraulically fracturing a subterranean formation which defines a subsurface reservoir with a mixture comprising a carrier fluid and a proppant, the proppant comprising a particle and a polycarbodiimide coating disposed on the particle, said method comprising the step of pumping the mixture into the subsurface reservoir to cause the subterranean formation to fracture, wherein the polycarbodiimide coating comprises the reaction product of an isocyanate reacted in the presence of a trialkyl phosphate.