Source: http://www.google.com/patents/US20070181302?dq=6,621,746
Timestamp: 2016-06-30 10:40:17
Document Index: 522474713

Matched Legal Cases: ['art. 19', 'Application No. 60', 'Application No. 20060065398', 'Application No. 20060073980', 'Application No. 20050154221', 'Application No. 20030134928', 'Application No. 20030134927', 'Application No. 20030139489', 'Application No. 20040097655', 'art 2', 'art 5']

Patent US20070181302 - Method for the fracture stimulation of a subterranean formation having a ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsA method for fracture stimulation of a subterranean formation having a wellbore includes providing a thermoset polymer nanocomposite particle precursor composition comprising a polymer precursor mixture, dispersed within a liquid medium, containing at least one of a monomer, an oligomer or combinations...http://www.google.com/patents/US20070181302?utm_source=gb-gplus-sharePatent US20070181302 - Method for the fracture stimulation of a subterranean formation having a wellbore by using thermoset polymer nanocomposite particles as proppants, where said particles are prepared by using formulations containing reactive ingredients obtained or derived from renewable feedstocksAdvanced Patent SearchPublication numberUS20070181302 A1Publication typeApplicationApplication numberUS 11/740,589Publication dateAug 9, 2007Filing dateApr 26, 2007Priority dateDec 30, 2004Also published asCA2688665A1, EP2142759A2, EP2142759A4, WO2009005880A2, WO2009005880A3Publication number11740589, 740589, US 2007/0181302 A1, US 2007/181302 A1, US 20070181302 A1, US 20070181302A1, US 2007181302 A1, US 2007181302A1, US-A1-20070181302, US-A1-2007181302, US2007/0181302A1, US2007/181302A1, US20070181302 A1, US20070181302A1, US2007181302 A1, US2007181302A1InventorsJozef BiceranoOriginal AssigneeSun Drilling Products CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (51), Referenced by (48), Classifications (14), Legal Events (1) External Links: USPTO, USPTO Assignment, EspacenetMethod for the fracture stimulation of a subterranean formation having a wellbore by using thermoset polymer nanocomposite particles as proppants, where said particles are prepared by using formulations containing reactive ingredients obtained or derived from renewable feedstocks
US 20070181302 A1Abstract
A method for fracture stimulation of a subterranean formation having a wellbore includes providing a thermoset polymer nanocomposite particle precursor composition comprising a polymer precursor mixture, dispersed within a liquid medium, containing at least one of a monomer, an oligomer or combinations thereof having three or more reactive functionalities capable of creating crosslinks between polymer chains, wherein 1% to 100% by weight of said polymer precursor mixture is obtained or derived from a renewable feedstock; and from 0.001 to 60 volume percent of nanofiller particles possessing a length that is less than 0.5 microns in at least one principal axis direction; subjecting the nanocomposite particle precursor composition to polymerizing conditions to form the polymeric nanocomposite particle, whereby said nanofiller particles are substantially incorporated into a polymer; forming a slurry comprising a fluid and a proppant, wherein said proppant comprises the nanocomposite particles, said nanocomposite particles being formed from a rigid thermoset polymer matrix; and injecting into the wellbore said slurry at sufficiently high rates and pressures such that said formation fails and fractures to accept said slurry. Images(14) Claims(20)
1. A method for fracture stimulation of a subterranean formation having a wellbore, comprising: providing a thermoset polymer nanocomposite particle precursor composition comprising a polymer precursor mixture, dispersed within a liquid medium, containing at least one of a monomer, an oligomer or combinations thereof having three or more reactive functionalities capable of creating crosslinks between polymer chains, wherein 1% to 100% by weight of said polymer precursor mixture is obtained or derived from a renewable feedstock; and from 0.001 to 60 volume percent of nanofiller particles possessing a length that is less than 0.5 microns in at least one principal axis direction; said nanofiller particles comprising at least one of dispersed fine particulate material, fibrous material, discoidal material, or a combination of such materials, wherein said nanofiller particles are substantially dispersed within the liquid medium; subjecting the nanocomposite particle precursor composition to polymerizing conditions to form the polymeric nanocomposite particle, whereby said nanofiller particles are substantially incorporated into a polymer; forming a slurry comprising a fluid and a proppant, wherein said proppant comprises the nanocomposite particles, said nanocomposite particles being formed from a rigid thermoset polymer matrix; injecting into the wellbore said slurry at sufficiently high rates and pressures such that said formation fails and fractures to accept said slurry; and emplacing said proppant within a fracture network in said formation in a packed mass or a partial monolayer of particles, which packed mass or partial monolayer props open the fracture network; thereby allowing produced gases, fluids, or mixtures thereof, to flow towards the wellbore. 2. The method of claim 1, wherein said renewable feedstock is selected from the group consisting of soybean, sunflower, canola, castor, olive, peanut, cashew nut, pumpkin seed, rapeseed, corn, rice, sesame, cottonseed, palm, coconut, safflower, linseed, hemp, tall oil, fish oil, lard, neatsfoot oil, tallow oil, similar natural fats and oils, and mixtures thereof. 3. The method of claim 1, wherein said polymer precursor mixture comprises at least one of monomer, oligomer or combinations thereof; said at least one of monomer, oligomer or combinations thereof being used to synthesize thermoset epoxies, epoxy vinyl esters, polyesters, phenolics, melamine-based resins, polyurethanes, polyureas, polyimides, or mixtures thereof. 4. The method of claim 1, wherein said polymer precursor mixture comprises a crosslinking monomer selected from the group consisting of: Divinylbenzene, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, trimethylolpropane dimethacrylate, trimethylolpropane diacrylate, pentaerythritol tetramethacrylate, pentaerythritol trimethacrylate, pentaerythritol dimethacrylate, pentaerythritol tetraacrylate, pentaerythritol triacrylate, pentaerythritol diacrylate, bisphenol-A diglycidyl methacrylate, ethyleneglycol dimethacrylate, ethyleneglycol diacrylate, diethyleneglycol dimethacrylate, diethyleneglycol diacrylate, triethyleneglycol dimethacrylate, and triethyleneglycol diacrylate, a bis(methacrylamide) having the formula: a bis(acrylamide) having the formula: a polyolefin having the formula CH2═CH—(CH2)x—CH═CH2 (wherein x ranges from 0 to 100, inclusive), a polyethyleneglycol dimethylacrylate having the formula: a polyethyleneglycol diacrylate having the formula: a molecule or a macromolecule containing at least three isocyanate (—N═C═O) groups, a molecule or a macromolecule containing at least three alcohol (—OH) groups, a molecule or a macromolecule containing at least three reactive amine functionalities where a primary amine (—NH2) contributes two to the total number of reactive functionalities while a secondary amine (—NHR—, where R can be any aliphatic or aromatic organic fragment) contributes one to the total number of reactive functionalities; and a molecule or a macromolecule where the total number of reactive functionalities arising from any combination of isocyanate (—N═C═O), alcohol (—OH), primary amine (—NH2) and secondary amine (—NHR—, where R can be any aliphatic or aromatic organic fragment) adds up to at least three, 1,4-divinyloxybutane, divinylsulfone, diallyl phthalate, diallyl acrylamide, triallyl cyanurate, triallyl isocyanurate, triallyl trimellitate or mixtures thereof. 5. The method of claim 1, wherein said polymer precursor mixture comprises a non-crosslinking monomer selected from the group consisting of: Styrenic monomers, styrene, methylstyrene, ethylstyrene (ethylvinylbenzene), chlorostyrene, chloromethylstyrene, styrenesulfonic acid, t-butoxystyrene, t-butylstyrene, pentylstyrene, alpha-methylstyrene, alpha-methyl-p-pentylstyrene; acrylic and methacrylic monomers, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, lauryl acrylate, lauryl methacrylate, glycidyl acrylate, glycidyl methacrylate, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, diethylene glycol acrylate, diethylene glycol methacrylate, glycerol monoacrylate, glycerol monomethacrylate, polyethylene glycol monoacrylate, polyethylene glycol monomethacrylate, butanediol monoacrylate, butanediol monomethacrylate; unsaturated carboxylic acid monomers, acrylic acid, methacrylic acid; alkyl vinyl ether monomers, methyl vinyl ether, ethyl vinyl ether, vinyl ester monomers, vinyl acetate, vinyl propionate, vinyl butyrate; N-alkyl substituted acrylamides and methacrylamides, N-methylacrylamide, N-methylmethacrylamide, N-ethyl acrylamide, N-ethyl methacrylamide; nitrile monomers, acrylonitrile, methacrylonitrile; olefinic monomers ethylene (H2C═CH2) and the alpha-olefins (H2C═CHR) where R is any saturated hydrocarbon fragment; vinylic alcohols, vinyl alcohol; vinyl halides, vinyl chloride; vinylidene halides, vinylidene chloride, or mixtures thereof. 6. The method of claim 1, wherein said thermoset polymer matrix comprises a copolymerization product of a monomer, oligomer, or mixtures thereof, obtained or derived from a renewable feedstock; with styrene, divinylbenzene, ethylvinylbenzene, or mixtures thereof. 7. The method of claim 1, wherein said nanofiller is selected from the group of nanofillers consisting of carbon black, fumed silica, fumed alumina, carbon nanotubes, carbon nanofibers, cellulosic nanofibers, natural clays, synthetic clays, fly ash, polyhedral oligomeric silsesquioxanes, metal clusters, metal alloy clusters, metal oxide clusters, or mixtures thereof. 8. The method of claim 1, wherein said nanofiller comprises carbon black, possessing a length that is less than 0.5 microns in at least one principal axis direction and an amount from 0.1% to 15% of said particle by volume. 9. The method of claim 1, wherein said polymer precursor mixture further comprises additional formulation ingredients selected from the group of ingredients consisting of: Initiators, impact modifiers, catalysts, inhibitors, dispersants, stabilizers, rheology modifiers, buffers, antioxidants, defoamers, plasticizers, pigments, flame retardants, smoke retardants, or mixtures thereof. 10. The method of claim 9, wherein said impact modifier comprises at least one of a monomer, an oligomer or a polymer having one or more reactive functionalities; obtained or derived from a petrochemical feedstock, a renewable feedstock, or a combination thereof. 11. The method of claim 10, wherein said impact modifier comprises at least one of a monomer, oligomer or polymer, having one or more reactive functionalities; selected from the group consisting of: Polybutadiene (including its solid and liquid forms, and any of its variants comprising different cis-1,4, trans-1,4, and vinyl-1,2 isomer contents), natural rubber, synthetic polyisoprene, polychloroprene, nitrile rubbers, other diene rubbers, partially or completely hydrogenated versions of any of the diene rubbers, acrylic rubbers, olefinic rubbers, epichlorohydrin rubbers, fluorocarbon rubbers, fluorosilicon rubbers, block and/or graft copolymers prepared from formulations comprising styrenic monomers and diene monomers, partially or completely hydrogenated versions of block and/or graft copolymers prepared from formulations comprising styrenic monomers and diene monomers, silicone rubbers, rubbers containing aliphatic or partially aromatic polyether chain segments, rubbers containing aliphatic or partially aromatic polyester chain segments, rubbers containing aliphatic or partially aromatic polyurethane chain segments, rubbers containing aliphatic or partially aromatic polyurea chain segments, rubbers containing aliphatic or partially aromatic polyamide chain segments, ionomer resins which may be partially or wholly be neutralized with counterions; other rubbery homopolymers, copolymers containing random, block, graft, star, or core-shell morphologies, and mixtures thereof; the monomeric or oligomeric precursors of any of the cited types of rubbery polymers; and reactive molecules obtained or derived from soybean, sunflower, canola, castor, olive, peanut, cashew nut, pumpkin seed, rapeseed, corn, rice, sesame, cottonseed, palm, coconut, safflower, linseed, hemp, tall oil, fish oil, lard, neatsfoot oil, tallow oil, and similar natural fats and oils. 12. The method of claim 1, wherein said polymerizing comprises suspension polymerizing. 13. The method of claim 12, wherein said suspension polymerizing comprises rapid rate polymerizing. 14. The method of claim 1, wherein said particle is subjected to a post-polymerizing process. 15. The method of claim 14, wherein said post-polymerizing process is heat treatment performed in a medium including a vacuum, a non-oxidizing gas, a mixture of non-oxidizing gases, a liquid, or a mixture of liquids; or in a downhole environment of a hydrocarbon reservoir. 16. The method of claim 1, wherein said particle is coated, during the polymerizing process itself, in a post-polymerizing process, or a combination thereof. 17. The method of claim 16, wherein said coating comprises an epoxy, epoxy vinyl ester, polyester, acrylic, phenolic, alkyd resin, melamine-based resin, furfuryl alcohol resin, polyacetal, polyurethane, polyurea, polyimide, polyxylylene, silicone, fluoropolymer, a copolymer thereof, or a mixture thereof. 18. The method of claim 1, wherein said particle is a bead having an average roundness of at least 0.7 and an average sphericity of at least 0.7 as measured by the use of a Krumbien/Sloss chart. 19. The method of claim 1, wherein said particle has an average diameter that ranges from 0.1 mm to 4 mm. 20. The method of claim 1, wherein said packed mass or said partial monolayer exhibits a static conductivity of at least 100 mDft after 200 hours at a temperature greater than 80� F.
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of U.S. patent application Ser. No. 11/323,031 entitled “Thermoset Nanocomposite Particles, Processing For Their Production, And Their Use In Oil And Natural Gas Drilling Applications”, filed Dec. 30, 2005, which claims priority to U.S. Provisional Application No. 60/640,965 filed Dec. 30, 2004. This application is also a continuation-in-part of U.S. patent application Ser. No. 11/451,697 entitled “Thermoset Particles With Enhanced Crosslinking, Processing For Their Production, And Their Use In Oil And Natural Gas Drilling Applications”, filed Jun. 13, 2006. This application is also a continuation-in-part of U.S. patent application Ser. No. 11/695,745 entitled “A Method For The Fracture Stimulation Of A Subterranean Formation Having A Wellbore By Using Impact-Modified Thermoset Polymer Nanocomposite Particles As Proppants,” filed Apr. 3, 2007. The contents of prior application Ser. Nos. 11/323,031, 11/451,697, 11/695,745, and 60/640,965 are fully incorporated herein by reference.
FIELD OF THE INVENTION [0002] The present invention relates to a method for the fracture stimulation of a subterranean formation having a wellbore by using ultralightweight thermoset polymer nanocomposite particles as proppants, where said particles are prepared by using formulations containing reactive ingredients obtained or derived from renewable feedstocks. Without reducing the generality of the invention, in its currently preferred embodiments, the thermoset polymer matrix of said particles consists of a copolymer of styrene, ethyvinylbenzene, divinylbenzene and additional monomers obtained or derived from plant oils; carbon black is used as the nanofiller, suspension polymerization in the rapid rate polymerization mode is performed to prepare said particles and post-polymerization heat treatment is performed in an unreactive gas environment to further advance the curing of the thermoset polymer matrix. The main benefit of the use of reactive ingredients obtained or derived from renewable feedstocks is that doing so reduces the reliance on petrochemical feedstocks and hence provides advantages in terms of sustainability. The fracture stimulation method of the invention can be implemented by placing said particles in the fracture either as a packed mass or as a partial monolayer. Without reducing the generality of the invention, said particles are placed as a partial monolayer in its preferred embodiments. BACKGROUND [0000] 1. Introduction [0003] U.S. Pat. No. 6,248,838, “Chain entanglement crosslinked proppants and related uses”; the background section of U.S. patent application Ser. No. 11/323,031 entitled “Thermoset nanocomposite particles, processing for their production, and their use in oil and natural gas drilling applications”; the background section of U.S. patent application Ser. No. 11/451,697 entitled “Thermoset particles with enhanced crosslinking, processing for their production, and their use in oil and natural gas drilling applications”; and the background section of U.S. patent application Ser. No. 11/695,745 entitled “A method for the fracture stimulation of a subterranean formation having a wellbore by using impact-modified thermoset polymer nanocomposite particles as proppants”, provide background information related to the present invention and are fully incorporated herein by reference. The background discussion presented below is intended to supplement the background discussions in these four prior filings, and focuses on additional background information that is not found in these filings. [0004] Applicant has found no prior art in the patent literature, and no publications in the general scientific literature, that disclose a method for the fracture stimulation of a subterranean formation having a wellbore by using, as proppants, ultralightweight thermoset polymer nanocomposite particles where the matrix polymer phase is prepared by the reaction of components (monomers, oligomers and/or polymers containing reactive functionalities) obtained or derived from renewable feedstocks. The discussion below is hence intended to be mainly of a pedagogical nature. It provides background information that will help those in the field understand the invention by familiarizing them with key information on the use of renewable feedstocks as components of (a) proppants in the fracture stimulation of a subterranean formation, and (b) the reactive mixture (monomers, oligomers and/or polymers containing reactive functionalities) used in the synthesis of the matrix polymers of thermoset composites. Since these two types of use of renewable feedstocks do not appear to have ever been pursued simultaneously in previous work, they will be discussed below in separate subsections. [0005] For the purposes of this disclosure, a “renewable feedstock” is defined as a feedstock obtained from a microorganism-based, plant-based, or animal-based resource that, once used, can be renewed on the time scale of a human life; in other words, within no more than one century. In practice, most of the typical renewable resources (such as soybean or corn plants) that can serve as a source of useful renewable feedstocks can be renewed in much shorter periods, such as yearly. By contrast, while petrochemical (fossil fuel) resources also have a biological origin, they are not “renewable” in the practical sense captured by our definition since, once used, their renewal would require the passage of geological time scales (thousands to millions of years). [0000] 2. Utilization of Renewable Feedstocks as Components of Proppants [0006] a. Fundamental Considerations [0007] The potential utilization of renewable feedstocks as ingredients of lightweight and ultralightweight proppants of sufficient compressive strength to be useful for applications in fracture stimulation has been investigated for many years. [0008] It is important, for the sake of clarity, to begin by distinguishing the general benefits that result from the ultralightweight characteristics (near neutral buoyancy in water) of such proppants from the benefits of using renewable feedstocks as ingredients in their preparation. [0009] The general benefits of using ultralightweight proppants of sufficient compressive strength, regardless of the source of the feedstock used in their preparation, arise from their densities which are much lower than the densities of typical sand-based or ceramic-based proppants. These general benefits are, hence, independent of the ingredients used in the preparation of such ultralightweight proppants. These benefits include excellent ability to be transported (without requiring the use of very high pumping rates), without settling substantially during transport, in fracturing fluids of very low viscosity such as “slickwater”. The key benefit of efficient proppant transport is that ultralightweight proppants can be transported much further than heavy proppants into the formation by using such fluids so that much greater effective fracture lengths can be attained. Slickwater is less damaging to the reservoir permeability than the crosslinked gelled fluids required to carry proppants of high density. Finally, the use of ultralightweight proppants makes it practical to place the proppant in the fracture as a “partial monolayer”, a mode of proppant placement that was demonstrated by Darin and Huitt as far back as 1959 on theoretical grounds to be especially effective in fracture stimulation. In summary, substantially smaller volumes and concentrations of proppant would be required to realize sufficient fracture width and conductivity when a partial monolayer can be employed instead of a conventional proppant pack. Combined with a greater effective fracture length, the ability to place the proppant as a partial monolayer would result in the exposure of more of the reservoir to the conductive path and thus lead to greater hydrocarbon production over the long term. [0010] If renewable feedstocks are used in the preparation of ultralightweight proppants of sufficient compressive strength, then they offer benefits in terms of sustainability in addition to offering all of the general benefits of ultralightweight proppants. Since renewable feedstocks typically have much lower densities than materials such as sand and ceramics, it is thus natural to expect that their potential use in the preparation of ultralightweight proppants manifesting the additional advantages of sustainability has generated much interest. [0011] b. Detailed Example of a General Approach [0012] Typical of a general approach that is often used, but further along than similar technologies in its reduction to practice and hence especially useful as an example, is the technology taught in a series of U.S. patents (U.S. Pat. Nos. 6,364,018, 6,749,025 and 6,772,838) and U.S. patent applications (No. 20060065398 and No. 20060073980). This technology will be reviewed below. [0013] The particulate material comprises a plant-based material selected from at least one of ground or crushed nut (such as walnut, pecan, almond, ivory nut or brazil nut) shells, ground or crushed seed shells of other plants (such as corn), ground or crushed fruit (such as plum, peach, cherry or apricot) pits, processed wood (for example, from oak, hickory, walnut, poplar or mahogany), or a mixture thereof. A protective and/or hardening coating is also used. Additional components are also incorporated in some embodiments, for purposes such as tailoring the density and/or providing additional hardness. In a preferred embodiment, ground or crushed walnut shell material is coated with a polyurethane resin for protection and waterproofing. [0014] Applications of the resulting relatively lightweight and/or substantially neutrally buoyant particles are claimed as proppant material in hydraulic fracturing treatments (U.S. Pat. No. 6,364,018 and U.S. Pat. No. 6,772,838); as enhancers or productivity in hydraulic fracturing of subterranean formations having natural fractures when used to pre-treat the formation (U.S. Patent Application No. 20060065398); as proppant material in acid fracturing treatments (U.S. Patent Application No. 20060073980); and as particulate material for sand control methods such as gravel packing and frac packs (U.S. Pat. No. 6,749,025 and U.S. Pat. No. 6,772,838). [0015] The theoretical and practical advantages (as well as the technical challenges) of the use of ultralightweight proppants such as those taught by the cited U.S. patents (U.S. Pat. No. 6,364,018, 6,749,025 and 6,772,838) and U.S. patent applications (No. 20060065398 and No. 20060073980) are described further, and examples (including field testing results) are given of the utilization of such proppants, by Rickards et al. (2003), Wood et al. (2003), Brannon et al. (2004), Myers et al. (2004), Schein et al. (2004), Posey and Strickland (2005), Kendrick et al. (2005), and Ward et al. (2006). It is also worth noting that Kendrick et al. (2005) state that the ultralightweight proppant used in that study “consists of a chemically hardened walnut hull core with multiple layers of epoxy resin coating as the outer shell”. [0016] c. Other Examples [0017] In addition to the technology reviewed in the subsection above which has been fully reduced to practice, many other patents and patent applications also mention (albeit in a more cursory manner) the use of renewable ingredients in proppants. Some of these patent documents mention the use of renewable ingredients only in the main body of their text, while others also mention them in the claims. [0018] One typical context is in patent documents teaching coated proppant technologies. In some such technologies, the proppant particles that are being coated may comprise renewable ingredients similar to those discussed above, such as ground or crushed walnut shell material. In an alternative and less commonly proposed coated proppant approach, the coating that is placed on sand or ceramic proppant particles may comprise renewable ingredients (such as plant oils). [0019] The other typical context is in patent documents teaching various techniques for fracture stimulation, gravel pack completion and/or sand control; where particles comprising renewable ingredients are often listed among the types of proppant compositions that may be used in the implementation of the method that is being taught. [0020] Some examples of additional patent documents (beyond those that were discussed in the previous subsection) that mention the possible use of renewable ingredients in proppants in one or both of these two typical contexts include U.S. Pat. Nos. 4,585,064, 5,597,784, 7,021,379, 7,073,581, 7,128,158, 7,160,844, 7,178,596, U.S. 20050194141, U.S. 20060048943, U.S. 20060048944, U.S. 20060078682, U.S. 20060204756, U.S. 20060205605, U.S. 20060260811, U.S. 20060272816, U.S. 20070007010, U.S. 20070036977, WO2005100007, WO2006034298, and WO2006084236. [0000] 3. Utilization of Renewable Feedstocks as Components of Reactive Mixture Used in Synthesis of Matrix Polymers of Thermoset Composites [0021] a. Introduction [0022] A background paper on bipolymers, published by the U.S. Congress, Office of Technology Assessment (September 1993), suggested that the use of biologically derived polymers could emerge as an important component of a new paradigm of sustainable economic systems that rely on renewable sources of energy and materials. This concept has, indeed, gained increasing acceptance in the years that followed the publication of the background paper. The utilization of monomers obtained or derived from biological starting materials (such as amino acids, nucleotides, sugars, phenols, natural fats, oils, and fatty acids) in the chemical synthesis of polymers is an important component of this paradigm of sustainable development. This is an area of intense research and development activity because of the global drive to reduce the dependence of the world economy on petrochemical feedstocks. [0023] b. Some Promising Renewable Sources of Reactive Ingredients [0024] Suitable renewable feedstocks can be obtained or derived from a wide variety of microorganism-based, plant-based, or animal-based resources. The utilization of monomers, oligomers and polymers obtained or derived from renewable resources as components of polymer composites is, therefore, anticipated to continue to increase in the future. [0025] Among renewable feedstocks for the synthesis of polymeric products, natural fats and oils extracted from some common types of plants [such as soybean, sunflower, canola, castor, olive, peanut, cashew nut, pumpkin seed, rapeseed, corn, rice, sesame, cottonseed, palm, coconut, safflower, linseed (also known as flaxseed), hemp, tall oil, and similar natural fats and oils; and especially soybean, sunflower, canola and linseed oils] appear to be very promising as potential sources of inexpensive monomers. Some animal-based natural fats and oils, such as fish oil, lard, neatsfoot oil and tallow oil, may also hold promise as potential sources of inexpensive monomers. [0026] c. General Classes of Thermoset Composites Using Ingredients Obtained or Derived from Renewable Feedstocks [0027] Fibrous and/or particulate components extracted from plants have been used for decades as fillers in composites where the matrix polymer is prepared by using monomers obtained or derived from petrochemical feedstocks. For example, U.S. Pat. No. 5,834,105 teaches structural polymeric composites consisting of a polymeric matrix and intact corn husks, and hence provides an example of this general type of approach. [0028] Another well-established type of technology is the use of a polymeric resin based on petrochemical feedstock as a binder and/or coating for fibrous and/or particulate components that have been extracted from plants and then pressed and/or agglomerated. For example, in the fabrication of particleboard, a plant-based cellulosic material (such as wood chips, sawmill shavings, straw, or sawdust) is combined with a synthetic resin (binder) by using a process in which the interparticle bond is created by the synthetic resin under heat and pressure. [0029] The development of thermoset composites where reactive components extracted from renewable feedstocks are used as building blocks for the matrix polymer is a much newer area of research and development that is gaining momentum. This research area is of interest in the context of the present invention. It will hence be the focus of the remainder of this section. [0030] As a practical matter, a proppant must be able to retain good performance for prolonged periods in a wide range of harsh environments in order to find widespread utility. Consequently, while there are many potential applications for composites (prepared from renewable feedstocks) where biodegradability and/or other types of environmental degradability are among the key target properties, such composites are not optimal for use as proppants in implementing the fracture stimulation method of the invention, and will hence not be discussed further. [0031] d. Chemical Modification for Derivation of Optimal Reactive Ingredients for Use in Polymer Synthesis [0032] It is possible to use the triglycerides obtained from plant oils directly as monomers in the preparation of thermoset polymers and composites. It is, however, usually preferable to modify these triglycerides chemically to obtain monomers which have more attractive reactivity profiles and contributions to the properties of the final thermoset system after incorporation. [0033] Many chemical modifications can be made readily to tailor the reactivity profile and the final properties. Different plant oils provide significantly different mixtures of starting triglyceride molecular structures for use in the possible chemical modifications, thus providing a vast range of possibilities for new monomers. The development and new and improved monomers by chemical modification is an area of intense research. [0034] The use of genetic engineering to develop plants yielding oils containing monomers with especially desirable molecular structures is also an important area of research and development. [0035] The development of processes for the utilization of reactive components obtained or derived from natural fats and oils extracted from plant-based sources as building blocks for polymers and the matrix polymers of polymer composites is, therefore, an area of intense research activity. Plant-based liquids are typically mixtures of molecules with various chemical structures and various types of active sites. Consequently, the extraction of different reactive components, and the modification of these components by breaking them down into smaller monomers and/or derivatizing them, is a crucial part of research aimed towards the utilization of such reactive components as building blocks in the preparation of polymer composites. [0036] For example, U.S. Patent Application No. 20050154221 teaches integrated chemical processes for the industrial utilization of seed oil feedstock compositions. [0037] Pillai (2000) discusses the wealth of high value polymers that can be produced by using constituents extracted from cashew nut shell liquid. [0038] Additional examples will be provided below in the context of specific types of polymers and composites prepared by using reactive components obtained or derived from natural fats and oils extracted from plant-based sources. [0039] e. Various Polymers and Polymer Composites Synthesized by Using Formulations Containing Reactive Ingredients Obtained or Derived from Renewable Feedstocks [0040] U.S. Pat. No. 6,682,673 teaches a process for making a composite where a natural fiber is used as the reinforcing agent, and the mixture of reactants from which the matrix polymer is synthesized via free radical copolymerization comprises a ring opening product of epoxidized fatty compounds with olefinically unsaturated fatty acids such as acrylic acid or methacrylic acid. The initial fatty compounds are obtained from sources such as soybean oil. [0041] Methods are taught for the production of radically postcured polyurethanes by reacting acrylic or methacrylic acid derivatives based on natural oils (epoxidized fatty acid esters and/or epoxidized triglycerides) with aromatic and/or alphatic isocyanates (U.S. Patent Application No. 20030134928). In similar approaches, reactive anhydrides (U.S. Patent Application No. 20030134927), structural components such as acrolein, acrylamide, vinyl acetate and styrene (U.S. Patent Application No. 20030139489), or diallyl phthalates (U.S. Patent Application No. 20040097655) are included in the second stage of the preparation of the polymers. [0042] Husić et al. (2005) reported that they prepared and compared two series of glass fiber reinforced composites, one using a polyol based on soybean oil and one using a petrochemical polyol in the preparation of the polyurethane matrix. The mechanical properties (such as tensile and flexural modulus, and tensile and flexural strength) of the two series of composites were comparable. It was stated that soybean oil-based composites are likely to find increasing applications because of their superior oxidative, thermal and hydrolytic stabilities. [0043] Mosewicki et al. (2003) and Aranguren et al. (2005) developed composite materials formulated by using a natural polyphenolic matrix (a commercial tannin adhesive) with pine woodflour as the reinforcing agent. These composites had attractive mechanical properties when they were dry. However, they were highly susceptible to water absorption in humid environments. Water absorption caused their mechanical properties to deteriorate significantly. The cured tannin matrix was found to be even more hygroscopic than woodflour. [0044] Belcher et al. (2002) investigated the properties of biofiber-reinforced biobased epoxy resins for automotive exterior applications. They considered the use of both epoxidized linseed oil and epoxidized soybean oil as modifiers of conventional epoxy resin compositions based on petrochemical precursors. They showed that the blending of functionalized soybean oil with petrochemical-based resins can increase the toughness of a petroleum-based thermoset resin without compromising stiffness, while also improving its environmental friendliness. [0045] f. Various Polymers and Polymer Composites Synthesized by Using Formulations Containing Petrochemical Comonomers Along with Reactive Ingredients Obtained or Derived from Renewable Feedstocks [0046] The most extensive amount of work appears to have been done on the use of monomers extracted from plant oils (and then optimized via chemical modification in most cases), as copolymerized with petrochemical comonomers, to prepare unsaturated liquid polyester resins, vinyl ester resins and epoxy resins that are capable of curing into thermoset polymers; and on the development of thermoset composites using such thermoset matrix polymers. This work will be summarized below. [0047] Further details (beyond the summary that will be provided below) can be found in the following references: U.S. Pat. No. 6,121,398, Warth et al. (1997), Williams and Wool (2000), Khot et al. (2001), Can et al. (2001) Can et al. (2002), La Scala and Wool (2002), Belcher et al. (2002) which was briefly discussed above, Lu et al. (2004), O'Donnell et al. (2004), La Scala and Wool (2005), Hong and Wool (2005), Mosiewicki et al. (two publications in 2005), Aranguren et al. (2006), and Lu and Larock (2006). [0048] Soybean oil and linseed oil have been used most often in such work. Rapeseed oil, corn oil, olive oil, cottonseed oil, safflower seed oil, sunflower oil, palm oil, canola oil and genetically engineered high oleic oil have also been used in some work. Most of the polymer and composite synthesis has been performed by using monomers which were derived by chemical modification from the plant oils, rather than using the plant oils themselves or the monomers extracted from the plant oils directly. In fact, research on the development of chemically modified monomers has paralleled thermoset polymer and composite synthesis in many research groups. [0049] Styrene is the most commonly used petrochemical comonomer in such thermoset polymers and composites. Divinylbenzene is also sometimes used as a comonomer, to provide additional crosslinking sites beyond those that are present in the monomers originating from plant oils. The plant oil based monomers can readily undergo free radical copolymerization over a very broad range of amount of comonomer with styrene and/or divinylbenzene in the presence of suitable initiators and/or catalysts. The most extensively investigated composition region is from a total of 33% (a fraction of ⅓) to 40% (a fraction of ⅖) by weight of comonomers such as styrene and divinylbenzene. This composition range corresponds to a common amount of such comonomers used in typical petrochemical-based resins such as epoxy vinyl esters. [0050] Plant oil based monomers can cause both plasticization (because of their flexibility) and an increase in the glass transition temperature (because of their ability to introduce crosslinks). The glass transition typically becomes very broad because of these two competing effects. The higher the level of unsaturation in the plant oil based monomer (and/or the more its functionality has been increased via chemical modification), the more its use results in an increase in the glass transition temperature and the less its use causes plasticization. [0051] The use of styrene and/or divinylbenzene in the formulation enhances the rigidity of the resulting thermoset polymer since these aromatic monomers introduce rigid moieties into the thermoset network. In particular, the use of the rigid crosslinker divinylbenzene increases the glass transition temperature without any competing plasticization effect. [0052] If there is a significant reactivity difference between the monomers obtained or derived from a particular plant oil and the styrenic monomers which tend to react fast, then there is a tendency towards the formation of a heterogeneous morphology. In such a morphology, one finds domains that are rich in styrenic polymer and domains that are rich in the product of the polymerization of monomers obtained or derived from the plant oil. [0053] Thermoset composites whose properties are comparable with those where the matrix polymer is obtained entirely from monomers originating from petrochemical feedstocks have been prepared with many of the matrices described above (based on the use of monomers obtained or derived from plant oils, as copolymerized with petrochemical comonomers) as reinforced by various natural or synthetic fibers or by layered silicate nanofiller. Whenever such composites can be prepared at comparable cost so that economic factors do not discourage their potential manufacturers and users, they can provide significant sustainability advantages. SUMMARY OF THE INVENTION [0000] 1. Introduction [0054] The present invention relates to a method for the fracture stimulation of a subterranean formation having a wellbore by using ultralightweight thermoset polymer nanocomposite particles as proppants, where the particles are prepared by using formulations containing reactive ingredients obtained or derived from renewable feedstocks. [0055] The main components of the particles are a rigid thermoset polymer matrix (Section 2) and a nanofiller which provides reinforcement (Section 3). [0056] Optionally, an impact modifier (Section 4) may also be present. [0057] Additional formulation ingredient(s) may also be used during the preparation of the particles; such as, but not limited to, initiators, catalysts, inhibitors, dispersants, stabilizers, rheology modifiers, buffers, antioxidants, defoamers, plasticizers, pigments, flame retardants, smoke retardants, or mixtures thereof. Some of these additional ingredient(s) may also become either partially or completely incorporated into the particles. [0058] The particles may be manufactured by any suitable polymerization process. They are preferentially manufactured by suspension polymerization (Section 5). [0059] Optionally, the particles may be postcured (Section 6) by any suitable process. They are preferentially postcured by heat treatment after polymerization. [0060] Optionally, the particles may be coated (Section 7) by any suitable process. They are preferentially coated by using a fluidized bed process after polymerization. [0061] The particles formulated and manufactured as summarized above are used in fracture stimulation (Section 8). [0000] 2. Matrix Polymer [0062] a. General Nature of Matrix Polymer [0063] Any rigid thermoset polymer may be used as the matrix polymer of the nanocomposite particles utilized as proppants in implementing the fracture stimulation method of the invention, subject solely to the limitation that the formulation from which it is synthesized comprises a renewable feedstock component. [0064] Rigid thermoset polymers are, in general, amorphous polymers where covalent crosslinks provide a three-dimensional network. However, unlike thermoset elastomers (often referred to as “rubbers”) which also possess a three-dimensional network of covalent crosslinks, the rigid thermosets are, by definition, “stiff”. In other words, they have high elastic moduli at “room temperature” (25� C.), and often up to much higher temperatures, because their combinations of chain segment stiffness and crosslink density result in a high glass transition temperature. [0065] For the purposes of this disclosure, a rigid thermoset polymer is defined as a thermoset polymer whose glass transition temperature, as measured by differential scanning calorimetry at a heating rate of 10� C./minute, equals or exceeds 45� C. The gradual softening of an amorphous polymer with increasing temperature accelerates as the temperature approaches the glass transition temperature. As discussed by Bicerano (2002), the rapid decline of the stiffness of an amorphous polymer (as quantified by its elastic moduli) with a further increase in temperature normally begins at roughly 20� C. below its glass transition temperature. Consequently, at 25� C., an amorphous polymer whose glass transition temperature equals or exceeds 45� C. will be below the temperature range at which its elastic moduli begin a rapid decline with a further increase in temperature, so that it will be rigid. [0066] Some examples of rigid thermoset polymers that can be used as matrix materials in the nanocomposite particles utilized as proppants in implementing the fracture stimulation method of the invention will be provided below. It is to be understood that these examples are provided without reducing the generality of the invention, to facilitate the teaching of the invention. [0067] Commonly used rigid thermoset polymers include, but are not limited to, crosslinked epoxies, epoxy vinyl esters, polyesters, phenolics, melamine-based resins, polyurethanes, and polyureas. Rigid thermoset polymers that are used less often because of their high cost despite their exceptional performance include, but are not limited to, crosslinked polyimides. For use in proppant particles suitable for different embodiments of the fracture stimulation method of the invention, these various types of polymers can be prepared by starting from their monomers, from oligomers that are often referred to as “prepolymers”, or from combinations thereof. [0068] Many additional types of rigid thermoset polymers can also be used. Such polymers include, but are not limited to, various families of crosslinked copolymers prepared most often by the polymerization of vinylic monomers, of vinylidene monomers, or of mixtures thereof. [0069] The “vinyl fragment” is commonly defined as the CH2═CH-fragment. So a “vinylic monomer” is a monomer of the general structure CH2═CHR where R can be any one of a vast variety of molecular fragments or atoms (other than hydrogen). When a vinylic monomer CH2═CHR reacts, it is incorporated into the polymer as the —CH2—CHR-repeat unit. Among rigid thermosets built from vinylic monomers, the crosslinked styrenics and crosslinked acrylics are especially familiar to workers in the field. Some other familiar types of vinylic monomers (among others) include the olefins, vinyl alcohols, vinyl esters, and vinyl halides. [0070] The “vinylidene fragment” is commonly defined as the CH2═CR″-fragment. So a “vinylidene monomer” is a monomer of the general structure CH2═CR′R″ where R′ and R″ can each be any one of a vast variety of molecular fragments or atoms (other than hydrogen). When a vinylidene monomer CH2═CR′R″ reacts, it is incorporated into a polymer as the —CH2—CR′R″-repeat unit. Among rigid thermosets built from vinylidene polymers, the crosslinked alkyl acrylics [such as crosslinked poly(methyl methacrylate)] are especially familiar to workers in the field. However, vinylidene monomers similar to each type of vinyl monomer (such as the styrenics, acrylates, olefins, vinyl alcohols, vinyl esters and vinyl halides, among others) can be prepared. One example of particular interest in the context of styrenic monomers is alpha-methyl styrene, a vinylidene-type monomer that differs from styrene (a vinyl-type monomer) by having a methyl (—CH3) group serving as the R″ fragment replacing the hydrogen atom attached to the alpha-carbon. [0071] Thermosets based on vinylic monomers, vinylidene monomers, or mixtures thereof, are typically prepared by the reaction of a mixture containing one or more non-crosslinking (difunctional) monomer(s) and one or more crosslinking (three or higher functional) monomer(s). [0072] The following are some specific but non-limiting examples of crosslinking monomers that can be used: Divinylbenzene, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, trimethylolpropane dimethacrylate, trimethylolpropane diacrylate, pentaerythritol tetramethacrylate, pentaerythritol trimethacrylate, pentaerythritol dimethacrylate, pentaerythritol tetraacrylate, pentaerythritol triacrylate, pentaerythritol diacrylate, bisphenol-A diglycidyl methacrylate, ethyleneglycol dimethacrylate, ethyleneglycol diacrylate, diethyleneglycol dimethacrylate, diethyleneglycol diacrylate, triethyleneglycol dimethacrylate, and triethyleneglycol diacrylate, a bis(methacrylamide) having the formula: a bis(acrylamide) having the formula: a polyolefin having the formula CH2═CH—(CH2)x—CH═CH2 (wherein x ranges from 0 to 100), inclusive), a polyethyleneglycol dimethylacrylate having the formula: a polyethyleneglycol diacrylate having the formula: a molecule or a macromolecule containing at least three isocyanate (—N═C═O) groups, a molecule or a macromolecule containing at least three alcohol (—OH) groups, a molecule or a macromolecule containing at least three reactive amine functionalities where a primary amine (—NH2) contributes two to the total number of reactive functionalities while a secondary amine (—NHR—, where R can be any aliphatic or aromatic organic fragment) contributes one to the total number of reactive functionalities; and a molecule or a macromolecule where the total number of reactive functionalities arising from any combination of isocyanate (—N═C═O), alcohol (—OH), primary amine (—NH2) and secondary amine (—NHR—, where R can be any aliphatic or aromatic organic fragment) adds up to at least three, 1,4-divinyloxybutane, divinylsulfone, diallyl phthalate, diallyl acrylamide, triallyl cyanurate, triallyl isocyanurate, triallyl trimellitate or mixtures thereof. [0073] The following are some specific but non-limiting examples of non-crosslinking monomers that can be used: Styrenic monomers, styrene, methylstyrene, ethylstyrene (ethylvinylbenzene), chlorostyrene, chloromethylstyrene, styrenesulfonic acid, t-butoxystyrene, t-butylstyrene, pentylstyrene, alpha-methylstyrene, alpha-methyl-p-pentylstyrene; acrylic and methacrylic monomers, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, lauryl acrylate, lauryl methacrylate, glycidyl acrylate, glycidyl methacrylate, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, diethylene glycol acrylate, diethylene glycol methacrylate, glycerol monoacrylate, glycerol monomethacrylate, polyethylene glycol monoacrylate, polyethylene glycol monomethacrylate, butanediol monoacrylate, butanediol monomethacrylate; unsaturated carboxylic acid monomers, acrylic acid, methacrylic acid; alkyl vinyl ether monomers, methyl vinyl ether, ethyl vinyl ether, vinyl ester monomers, vinyl acetate, vinyl propionate, vinyl butyrate; N-alkyl substituted acrylamides and methacrylamides, N-methylacrylamide, N-methylmethacrylamide, N-ethyl acrylamide, N-ethyl methacrylamide; nitrile monomers, acrylonitrile, methacrylonitrile; olefinic monomers, ethylene (H2C═CH2) and the alpha-olefins (H2C═CHR) where R is any saturated hydrocarbon fragment; vinylic alcohols vinyl alcohol; vinyl halides, vinyl chloride; vinylidene halides, vinylidene chloride, or mixtures thereof. [0074] b. Renewable Feedstock Component of Matrix Polymer Formulation [0075] A key aspect of the present invention is the utilization of reactive entities (monomers, oligomers and/or polymers containing reactive functionalities) obtained or derived from renewable resources as components of the formulations from which the polymeric matrix of the thermoset nanocomposite proppant particles used in implementing the fracture stimulation method of the invention is prepared. [0076] It is most desirable, from the viewpoint of sustainability, to maximize the proportion of renewable feedstock that is being used. In practice, however, this desired outcome must be balanced with the performance requirements and the economic constraints of the application. Consequently, the renewable feedstock content may be, and in most embodiments is, less than 100%. The total quantity of the component(s) obtained or derived from renewable feedstocks can range from 1% up to 100% by weight of the constituents of the formulation of the thermoset matrix polymer. If it is less than 100%, the remainder can comprise any suitable petrochemical ingredients, such as but not limited to those summarized in the preceding subsection. [0077] Any type of biological starting material (such as, but not limited to, amino acids, nucleotides, sugars, phenols, natural fats, oils, and fatty acids) can be used as the renewable resource in implementing the invention. Such renewable feedstocks can be obtained or derived from a wide variety of microorganism-based, plant-based, or animal-based resources. [0078] Without reducing the generality of the invention, among renewable feedstocks that can be used for the synthesis of the matrix polymer of the nanocomposite particles, natural fats and oils extracted from some common types of plants [such as soybean, sunflower, canola, castor, olive, peanut, cashew nut, pumpkin seed, rapeseed, corn, rice, sesame, cottonseed, palm, coconut, safflower, linseed (also known as flaxseed), hemp, tall oil, and similar natural fats and oils; and especially soybean, sunflower, canola and linseed oils] appear to be very promising as potential sources of inexpensive monomers. [0079] Again without reducing the generality of the invention, some animal-based natural fats and oils, such as fish oil, lard, neatsfoot oil and tallow oil, may also hold promise as potential sources of inexpensive monomers. [0000] 3. Nanofiller [0080] By definition, a nanofiller possesses at least one principal axis dimension whose length is less than 0.5 microns (500 nanometers). Some nanofillers possess only one principal axis dimension whose length is less than 0.5 microns. Other nanofillers possess two principal axis dimensions whose lengths are less than 0.5 microns. Yet other nanofillers possess all three principal axis dimensions whose lengths are less than 0.5 microns. Any reinforcing material possessing one nanoscale dimension, two nanoscale dimensions, or three nanoscale dimensions, can be used as the nanofiller. Any mixture of two or more different types of such reinforcing materials can also be used as the nanofiller. The nanofiller is present in an amount ranging from 0.001 to 60 percent of the total particle by volume. [0081] Without reducing the generality of the invention, to facilitate the teaching of the invention, we note that nanoscale carbon black, fumed silica, fumed alumina, carbon nanotubes, carbon nanofibers, cellulosic nanofibers, natural and synthetic nanoclays, very finely divided grades of fly ash, the polyhedral oligomeric silsequioxanes; and clusters of different types of metals, metal alloys, and metal oxides, are some examples of nanofillers that can be incorporated into the nanocomposite particles used as proppants in implementing the fracture stimulation method of the invention. Since the development of nanofillers is an area that is at the frontiers of materials research and development, the future emergence of yet additional types of nanofillers that are not currently known may also be readily anticipated. [0000] 4. Impact Modifier [0082] Optionally, the thermoset nanocomposite particles used as proppants in implementing the fracture stimulation method of the invention may contain an impact modifier. [0083] If its use is desired, an impact modifier is selected and incorporated into the particles as described in the SUMMARY OF THE INVENTION and the DESCRIPTION OF THE PREFERRED EMBODIMENTS sections of U.S. patent application Ser. No. 11/695,745 entitled “A method for the fracture stimulation of a subterranean formation having a wellbore by using impact-modified thermoset polymer nanocomposite particles as proppants”, which are fully incorporated herein by reference. [0000] 5. Suspension Polymerization [0084] Any method for the fabrication of thermoset polymer nanocomposite particles known to those skilled in the art may be used to prepare the thermoset nanocomposite particles which are utilized as proppants in implementing the fracture stimulation method of the invention. [0085] Without reducing the generality of the invention, it is especially practical to use methods that can produce the particles directly in the desired (usually substantially spherical) shape during polymerization from the starting monomers. [0086] A substantially spherical particle is defined as a particle having a roundness of at least 0.7 and a sphericity of at least 0.7, as measured by the use of a Krumbien/Sloss chart using the experimental procedure recommended in International Standard ISO 13503-2, “Petroleum and natural gas industries—Completion fluids and materials—Part 2: Measurement of properties of proppants used in hydraulic fracturing and gravel-packing operations” (first edition, 2006), Section 7, for the purposes of this disclosure. [0087] Without reducing the generality of the invention, it is especially useful to produce the substantially spherical particles discussed in the paragraph above with an average diameter that ranges from 0.1 mm to 4 mm for use in fracture stimulation applications. [0088] Without reducing the generality of the invention, in a most preferred embodiment, at least 90% of the substantially spherical particles are produced with diameters ranging from 0.42 mm (40 U.S. mesh size) to 1.41 mm (14 U.S. mesh size). [0089] Without reducing the generality of the invention, suspension (droplet) polymerization, where the polymer precursor mixture is dispersed in a suitable liquid medium prior to being polymerized, is currently the most powerful manufacturing method available for producing the particles directly in a substantially spherical shape during polymerization from the starting monomers. In pursuing this approach, it is especially important for the nanofiller particles to be well-dispersed within the liquid medium so that they can become intimately incorporated into the thermoset nanocomposite particles that will be formed upon polymerization. [0000] 6. Heat Treatment [0090] Optionally, the thermoset nanocomposite particles used in implementing the fracture stimulation method of the invention may be subjected to suitable post-polymerization process steps intended mainly to advance the curing of the thermoset polymer matrix. [0091] If a suitable post-polymerization process step is applied to the thermoset polymer nanocomposite particles, in many cases the curing reaction will be driven further towards completion so that the maximum possible temperature at which the fracture stimulation method of the invention can be applied by using these particles will increase. [0092] In some instances, there may also be further benefits of a post-polymerization process step. One such possible additional benefit is an enhancement in the flow of the gases, fluids, or mixtures thereof, produced by the subterranean formation, towards the wellbore, even at temperatures that are far below the maximum possible application temperature of the fracture stimulation method. Another such possible additional benefit is an increase of such magnitude in the resistance of the particles to aggressive environments as to enhance significantly the potential range of applications of the fracture stimulation method utilizing the particles. [0093] Processes that may be used to enhance the degree of curing of a thermoset polymer include, but are not limited to, heat treatment (which may be combined with stirring, flow and/or sonication to enhance its effectiveness), electron beam irradiation, and ultraviolet irradiation [0094] Without reducing the generality of the invention, we focused mainly on the use of heat treatment as a post-polymerization process step during the manufacturing of the particles. Such heat treatment can be performed in many types of media; including a vacuum, a non-oxidizing gas, a mixture of non-oxidizing gases, a liquid, or a mixture of liquids. [0095] It is possible, in some instances, to postcure the “as polymerized” particles adequately as a result of the elevated temperature of a downhole environment of a hydrocarbon reservoir during the application of the fracture stimulation method of the invention. However, since it does not allow nearly the same level of consistency and control of particle quality, this “in situ” approach to heat treatment is generally less preferred than the application of heat treatment as a manufacturing process step before using the particles in fracture stimulation. [0000] 7. Coating [0096] Optionally, the thermoset nanocomposite particles used in implementing the fracture stimulation method of the invention may be coated; to achieve benefits such as protection from chemicals, waterproofing, hardening, and combinations thereof. [0097] It is preferable, in most cases, to use matrix polymer compositions that can withstand the downhole environment without requiring a coating on the particles. A coating may, however, sometimes be needed, to make it possible to use particles that have very attractive performance attributes, but that if left uncoated would suffer from some deficiency which can be remedied by the application of a coating. [0098] Any available method may be used to place a coating around the particles. A coating may be placed during polymerization, after polymerization, or a combination thereof. [0099] Without reducing the generality of the invention, for example, monomers and/or reactive oligomers having the tendency to undergo phase segregation from the bulk of the matrix polymer and migrate to the surfaces of the particles may be included in the polymer precursor mixture to place a coating during polymerization. With this approach, there is also a likelihood of some penetration of the coating material to the interior of the particles and/or the interpenetration of the coating phase and the matrix phase and/or the development of an “interphase” region over which the composition changes gradually from that of the matrix polymer to that of the coating. [0100] Again without reducing the generality of the invention, various types of fluidized bed processes provide familiar examples of methods for placing a coating around the particles after polymerization. [0101] It should also be obvious that the approaches summarized in the two paragraphs above can be combined so that a coating may comprise both components that have been placed during polymerization and components that have been placed after polymerization. [0102] Without reducing the generality of the invention, the use of a fluidized bed process as a post-polymerization step is a preferred method for the placement of a coating if needed, but it is most preferred to select a matrix polymer composition such that a coating will not be needed. [0103] Any suitable coating material may be used if a coating is needed. Without reducing the generality of the invention, epoxies, epoxy vinyl esters, polyesters, acrylics, phenolics, alkyd resins, melamine-based resins, furfuryl alcohol resins, polyacetals, polyurethanes, polyureas, polyimides, polyxylylenes, silicones, fluoropolymers, copolymers thereof, and mixtures thereof, are some examples of coating materials that may be used. [0000] 8. Fracture Stimulation [0104] The fracture stimulation method of the invention is implemented by using stiff, strong, tough, heat resistant, and environmentally resistant ultralightweight thermoset polymer nanocomposite particles. Such particles may be placed either as a proppant partial monolayer or as a conventional proppant pack (packed mass) in implementations of the invention. [0105] The optimum mode of particle placement is determined by the details of the specific fracture that needs to be propped. In practice, the use of ultralightweight particles as proppant particles in implementing the fracture stimulation method of the invention provides its greatest advantages in situations where a proppant partial monolayer is the optimum mode of placement. Furthermore, the development of the fracture stimulation method of the invention has resulted in partial monolayers becoming the optimum proppant placement method in many situations where the use of partial monolayers was either impossible or impractical with previous technologies. [0106] In any case, the method for fracture stimulation comprises (a) forming a slurry comprising a fluid and a proppant, (b) injecting this slurry into the wellbore at sufficiently high rates and pressures such that the formation fails and fractures to accept the slurry, and (c) thus emplacing the proppant in the formation so that it can prop open the fracture network (thereby allowing produced gases, fluids, or mixtures thereof, to flow towards the wellbore). [0107] The most commonly used measure of proppant performance is the conductivity of liquids and/or gases (depending on the type of hydrocarbon reservoir) through packings of the particles. A minimum liquid conductivity of 100 mDft is often considered as a practical threshold for considering a packing to be useful in propping a fracture that possesses a given closure stress at a given temperature. In order for a fracture stimulation method to have significant practical utility, a static conductivity of at least 100 mDft must be retained for at least 200 hours at a temperature greater than 80� F. [0108] It is a common practice in the industry to use the simulated environment of a hydrocarbon reservoir in evaluating the conductivities of packings of particles. The API RP 61 method, described by a publication of the American Petroleum Institute titled “Recommended Practices for Evaluating Short Term Proppant Pack Conductivity” (first edition, Oct. 1, 1989), is currently the commonly accepted testing standard for conductivity testing in the simulated environment of a hydrocarbon reservoir. As of the date of this filing, however, work is underway to develop alternative testing standards, such as International Standard ISO 13503-5, “Petroleum and natural gas industries—Completion fluids and materials—Part 5: Procedures for measuring the long-term conductivity of proppants” (final draft, 2006).
DESCRIPTION OF THE PREFERRED EMBODIMENTS [0109] Details will now be provided on the currently preferred embodiments of the invention. These details will be provided without reducing the generality of the invention. Persons skilled in the art can readily imagine many additional embodiments that fall within the full scope of the invention as taught in the SUMMARY OF THE INVENTION section. [0110] The fracture stimulation method of the invention is preferably implemented by placing the ultralightweight thermoset polymer nanocomposite particles in the fracture as a partial monolayer. We have found, under standard laboratory test conditions, that the use of particles of narrow size distribution such as 14/16 U.S. mesh size (diameters in the range of 1.19 to 1.41 millimeters) is more effective than the use of broad particle size distributions. We have also found, under standard laboratory test conditions, that 0.02 lb/ft2 is an especially preferred level of coverage of the fracture area with a partial monolayer of thermoset nanocomposite particles of sufficient stiffness and strength that possess an absolute density of 1.054. However, real-life downhole conditions in an oilfield may differ significantly from those used under laboratory test conditions. Consequently, in the practical application of the fracture stimulation method of the invention, the use of other particle size distributions, other coverage levels, or combinations thereof, may be more appropriate, depending on the conditions prevailing in the specific downhole environment where the fracture stimulation method of the invention will be applied. [0111] The thermoset polymer matrix comprises a copolymerization product of monomers derived from soybean oil (a renewable resource), with three vinylic petrochemical monomers [styrene (S), divinylbenzene (DVB) and ethylvinylbenzene (EVB)]. The current preference for the use of soybean oil as a renewable resource is a result of its widespread availability and low cost, along with the fact that the derivation of useful monomers from soybean oil is at a more advanced stage than the derivation of monomers from other suitable renewable feedstocks. The current preference for the use of all three of S, DVB and EVB, instead of just using S and DVB, is a result of economic considerations related to monomer costs. The performance attributes of the particles can be tailored over broad ranges by modifying (a) the proportion of the matrix polymer originating from monomers derived from soybean oil over the range of 1% to 100% by weight, (b) the mixture of monomers derived from soybean oil, and (c) the relative amounts of the three vinylic monomers (S, DVB and EVB). [0112] Carbon black, possessing a length that is less than 0.5 microns in at least one principal axis direction, is used as the nanofiller at an amount ranging from 0.1% to 15% of the total particle by volume. [0113] Suspension polymerization, preferably in its “rapid rate polymerization” mode, is performed to synthesize the particles. The most important additional formulation ingredient (besides the reactive monomers) that is used during polymerization is the initiator. The initiator may consist of one type molecule or a mixture of two or more types of molecules that each have the ability to function as initiators. We have found, with experience, that the “dual initiator” approach, involving the use of two initiators which begin to manifest significant activity at different temperatures, often provides the best results. [0114] Additional formulation ingredients, such as impact modifiers, catalysts, inhibitors, dispersants, stabilizers, rheology modifiers, buffers, antioxidants, defoamers, plasticizers, pigments, flame retardants, smoke retardants, or mixtures thereof, may also be used when needed. Some of the additional formulation ingredient(s) may become either partially or completely incorporated into the particles in some embodiments of the invention. An example of an additional formulation ingredient which becomes incorporated in the particles is the optional impact modifier, when it is used in a particular embodiment. [0115] The suspension polymerization conditions are selected such that the particles to be used in the fracture stimulation method of the invention are selectively manufactured to have the vast majority of them fall within the 14/40 U.S. mesh size range (diameters in the range of 0.42 to 1.41 millimeters). The particles are sometimes separated into fractions having narrower diameter ranges for use in an optimal manner in proppant partial monolayers. [0116] Post-polymerization heat treatment in an unreactive gas environment is performed as a manufacturing process step to further advance the curing of the thermoset polymer matrix. This approach works especially well (without adverse effects such as degradation that could occur if an oxidative gaseous environment such as air were used and/or selling that could occur if a liquid environment were used) in enhancing the curing of the particles. The particles undergo a total exposure to temperatures in the range of 130� C. to 210� C. for a duration of 5 minutes to 90 minutes, inclusive, in an unreactive gas environment. The specific selection of an optimum temperature (or optimum temperature range) and optimum duration of heat treatment within these ranges depends on the formulation from which the particles were prepared. Nitrogen is used as the unreactive gas environment. [0117] Finally, it will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but is intended to cover modifications within the spirit and scope of the present invention as defined in the appended claims. EXAMPLES [0118] Some theoretical examples of preferred embodiments of the fracture stimulation method of the invention will now be given, without reducing the generality of the invention, to provide a better understanding of some of the ways in which the invention may be practiced. Workers skilled in the art can readily imagine many other embodiments of the invention with the benefit of this disclosure. Example 1 [0119] The fracture stimulation method of the invention is applied in a situation where it will provide the maximum possible benefit as compared with prior fracture stimulation methods. The downhole environment is one where the use of a proppant partial monolayer would be very effective in the extraction of hydrocarbons from a reservoir but has not been practical previously because of the unavailability of proppant particles of near neutral buoyancy in water along with sufficient stiffness, strength and environmental resistance. The ultralightweight thermoset polymer nanocomposite particles used in implementing the fracture stimulation method of the invention overcome this difficulty. Detailed consideration of the downhole environment results in the determination that 14/16 U.S. mesh size particles would be optimal. Particles in this size range are placed into the fracture as a partial monolayer by using slickwater as the carrier fluid. [0120] The thermoset polymer matrix of the nanocomposite particles used in this example consists of a copolymer of styrene (S), ethylvinylbenzene (EVB), divinylbenzene (DVB), and acrylated epoxidized soybean oil (AESO). The quantities of these ingredients in the reactive mixture are 51.55% S, 8.45% EVB, 15% DVB and 25% AESO by weight. In addition, the particles contain 0.5% by volume of carbon black as a nanofiller. [0121] The particles are prepared in the 14/40 U.S. mesh size range by rapid suspension polymerization. They are then postcured in a nitrogen environment for 20 minutes at a temperature of 185� C. Particles falling within the 14/16 U.S. mesh size range are separated from the broader distribution of 14/40 U.S. mesh size range by standard sieving techniques. Example 2 [0122] As in Example 1, but the quantities of the ingredients in the reactive mixture are 61.86% S, 10.14% EVB, 18% DVB and 10% AESO by weight. Example 3 [0123] As in Example 1, but the quantities of the ingredients in the reactive mixture are 41.24% S, 6.76% EVB, 12% DVB and 40% AESO by weight. Example 4 [0124] As in Example 1, but maleinized acrylated epoxidized soybean oil (MAESO) is used instead of AESO as the formulation ingredient originating from a renewable resource. Example 5 [0125] The same types of particles are used as in Example 1. However, detailed consideration of the downhole environment shows that the use of the full available 14/40 U.S. mesh size range of the particles will be optimal. Particles in this size range are placed into the fracture by using slickwater as the carrier fluid. 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