CA2688665A1 - A 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 feedstocks - Google Patents

Abstract

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.

Description

A METHOD FOR THE FRACTt1RE STIMULATION OF A SUBTERRANEAN
FORMATION HAVING A WELLBORE BY liSING THERMOSET POLYMER
NANOCOiVIPOSITE PARTICLES AS PROPPANTS, Vi'HERE SAID PARTICLES
ARE PREPARED BY USING FORMULATIONS CONTAINING REACTIVE
INGREDIENTS OBTAINED OR DERIVFD FROM RENEWABLE FEEDSTOCKS
CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Patent Application No. 11i740,389, entitled "Method #'or the Fracture Stimulatioti of a Subterranean Formation Iiaving a Wellbore by Using Thermoset Polymer Nanocomposite Particles as Proppants, where Said Particles Are Prc.Ypared by Ylsing Formulations Containing Reactive Ingredients Obtained or Derived from Renewable Feedstocks", filed Aptil 26, 2007, which application is a continuation-in-part of U.S. Patent Application No.
11/323,031 entitled "Therrnoset Nanocomposite Particles, Processing For Their Production, And Their Use In Oil And Natural Gas Drilling Applications", filed December 30, 2005, which claims priority to U.S. Provisional Application No. 60/640,965 filed Deccmber 30, 2004. This application is also a continuation-in-part of U.S_ Patent Application No.
11/451,697 entitled "Thetmoset Particles With Enhanced Crosslinking, Processing For Their Production, And Their Use In Oil And Natural Gas Drilling Applications", filed June 13, 2006. This application is also a continuation-in-part of U.S. Patent Application No.

11/695,745 entitled "A Method For The Fracture Stimulation Of A Subterranean Formation Having A Wellbore By Using Impact-Modified Thermoset Polvnier Nanocomposite Particles As Proppants," filed April 3, 2007. The contents of prior application nos. 11/323,031, 11/451,697, 11/695,745, and 60/640,965 are fully incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for the fracture stimulation of a subterranean formation having a wellbore by using ultralightweight thermoset polymer _1.

SUBSTITUTE SHEET (RULE 26) nanocomposite particles as proppants, where said particles are prepared bv using fotmulations containing reactive ingrcdicnts obtaincd or dcrivcd from rencwable fccdstocks. Without reducing thc genera{ity of the invention, in its currcntly preferred cmbodiments, the thermoset polymer matrix of said particles consists of a copolymer of styrenc, ethyvinylbenzcnc, divinylbcnzene and additional monomers obtained or derived froni plant oils; carbon black is used as the nanofiller, suspension polymerization in the rapid rate polyrnerization mode is pcrfonned to prepare said particles, and post-polymerization heat treatment is performed in an tutreactive gas environment to further advance ihe 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. 1'he fracture stimulation method of the invention can be implemented by placing said particles in the fracture either as a packed mass or as a panial monolayer.
Without reducing the generality of the invention, said panicles are placed as a partial monolayer in its preferred embodiinents.

1. Introduction U.S. Patent No. 6,248,838, "Chain entanglement crosslinked proppants and related uses"; the background section of U.S. patent Application No.
111323.031 entitlcd "l'hermoset nanocomposite panicles, processing for their production, and their usc in oil and natural gas drilling applications"; the background section of U.S. Patent Application No. 11/451,697 entitled "Thermoset particles with enhanced crosslinking, processing for their production, and thcir use in oil and natural gas drilling applications";
and the background section of U. S. Patent Application No. 11 /695,745 entitled "A
method for the fracture stimulation of a subtrrranean formation having a wellbore by using impact-modified thertnoset polymer nanocomposite particles as proppants", provide background infotmation rclated to the prescnt 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.

Applicant has found no prior art in the patent litcrature, and no publications in the general scientific literature, that disclose a method for the fracture stimulation of a subterrancan formation having a wellbore by using, as proppants, ultralightweight thcmloset polymcr nanocomposite partictes where the matrix polymer phase is prepared by the reaction of'cotnponents (monomers, oligomers and/or polytncrs containing reactive functionalities) obtained or derived trom renewable feedstocks. Thc discussion below is hence intended to he 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.

Por the purposes of this disclosure, a"n:newable feedstock" is delined as a feedstock obtained from a microorganism-based, plant-based, or aniinal-based re-source that, once used, can be renewed on the time scale of a hutnan life; in other words, within no more than one century. In practice, most of the typical renewable resources (such as soybean or com plants) that can serve as a source of useful renewable feedstocks can be renewed in much shorter periods, such as yearly. By contrast, while pctrochemical (fossil fiiel) resources also have a biological ongin, they are not "renewable" in the practical sense captured by our definition since, once used, their rene.val would requtre the passage of geological time scales (thousands to millions of years).

2. Utilization of Renewable Fcedstocks as Components of Proppants a. Fundamcntal Considcrations The potcntial utilixation of rencwable feedstocks as ingredients of lightwcight and ultralightweight proppants of sufficient compressive strength to be useful for applications in fracture stimulation has been investigated for manv years.

1t is important, for the sake of clarity, to bcgin by distinguishing the general benefits that result from the ulttalightwcight characteristies (near neutral buoyancy in water) of such proppants from the benefits of using renewable f eedstocks as ingredients in their preparation.

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 liroppants. These gencral benefits are, hence, independent of'the inbredicnts 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 fraeturing fluids of 210 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 tluids so that rnuch greater effective fracture lengths can be attained. tilickwatcr is less damaging to the reservoir permeability than the crosslinked gelled fluids requircd to carry proppants of high density.
Finally, the use of ultralight.veight proppants inakes it practical to placc 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 sumniarv, substantially smaller volumes and concentrations of proppant would be required to realize sufficient fracture width and conductivity when a partial monolayer can be emploved instead of a convcntional proppant pack. Contbined 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.

lf renewable feedstocks are used in the preparation of uliralightweight pruppants ofsufficient coinpressive strength, then they offer tx.nefits in ternis ofsustainability in addition to ot7ering 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.

b. Detailed Example of a General Approach 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 scries of U.S. patcnts (No. 6,364,018, No. 6,749,025 and 6,772,838') and U.S. patent applications (No. 20060065398 atid No.
2006007398U). 'l'his technology will be reviewed below.

The particulate material comprises a plant-based malerial selected from at least oae of ground or crushed nut (such as walnut, pecan, almond, ivory nut or brazil nut) shetts, ground or crushed secd shells of other plants (such as corn), ground or crushed fruit (such as plum. peach, cherry or apricot) pits, processed wood (for examptc, from WO 2009/005880 PCTJiJS2008/061520 oak, hickory, walnut, poplar or mahogany), or a mixture thereof. A protective and/or hardening coatirtg is also used. Additional components are also incorporated in somc embodiments, for purposes such as tailoring the densily 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.

Applications of the resulting rclatively lightweight and/or substantially neutrally buoyant particles arc claimed as proppant material in hydraulic fracturing treatments (U.S. Patent No. 6,364,018 and U.S. Patent No. 6,772,838); as enhanccrs of productivity in hydraulic fracturing of subacrranean forntations having natural fractures when used to pre-Ireat the fonnation (U.S. Patent Application No. 20060065398); as proppant material in acid fracturing treatments ((U.S. Patent Application Nlo. 20060073980); and as particulate material for sand control niethods such as gravel packing and frac packs (U.S.
Patent No. 6,749,025 and U.S. Patent No. 6,772,838).

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.
patcnts (No.

6,364,018, No. 6,749,025 and No. 6,772,838) and U.S. patettt 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 ultralight,-veight proppant used in that study "consists of a cliemically hardened %i,alnut hult core with multiple layers of epoxy resin coating as the outer shelP'.

c. Other C:xaniples -6.

WO 2009!005880 PCT/US2008l061520 In addition to the technology reviewed in the subsection above which has been fully reduced to practicc, many other patents and pateni applications also mention (albeit in a inore cursory inanner) the use of renewable ingredients in proppants.
Some of these patent documents mention the use of rencwable ingredients only in the main bndv of their text, while others also mention them in the clainis.

One typical context is in patent documents teaching coated proppant technologies. In some such technologies, the proppant particles that are being coated niay comprise renewable ingredients siniilar ro those discussed above, such as grotmd or crushed walnut shell material. ]n an alternative and less commonly proposed coated proppant approach, the coating that is placed on sand or ceramic proppant panicles may comprise renewable ingredients (such as plant oils).

The other typical context is in patent documents teaching various techniques for fracture stimulation, gravel pack completion and!or sand control; where partictes 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.
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 tltese two typical contexts include U.S.
4,585,064, U.S. 5,597,784, U.S. 7.021,379, U.S. 7,073,581, U.S. 7,128,158, U.S.

7,160.844, U.S. 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, W02005100007, W02006034298, and W02006084236.

3. Utilization of Renewable Feedstocks as Components of Reactive Mixture Used in Syrtthesis of Vlatrix Polymcrs of Thermosel Composites =7-a. Introduction A background paper on biopolymcrs, publishcd by the U. S. Congress, Officc of Technology Assessment (September 1993), suggcstcd that the usc of biologically derived polymers could emerge as an important component of a new paradigm of sustainable economic systems that rcly 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 polvmers 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.

b. Some Promising Renewable Sources of Reactive Ingredients 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 obtaincd or derived from renewable resources as coinponents of polymer composites is, therefore, anticipated to continue to increase in the future.

Among renewable feedstocks for the synthesis of polymeric products, natur,il fats and oils extracted from some cornnton types of plants [such as soybcan, sunflowcr, canola, castor, olive, peanut, cashew nut, punipkin seed, rapeseed, corn, rice, sesanie.
cottonseed, palm, coconut, safllower, linseed (also kno%vn as flaxseed), hemp, tall oil, and similar natural fats and oils; and especially soybean, sunflower, canola and Iinseed oils] appear to be very proinising as potential sources of inexpcnsivc monomers. Some animal-based natural fats and oils, such as fish oil, lard, neatsfoot oil and tallow oil, may also hold promisc as potcntiai sourccs ofincxpcnsive monomers.

c. Gcneral Classes of'lltcrmoset Cornpositcs Using [ngrcdients Obtaiited or Derived from Rencwable Feedstocks 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 inonomers obtained or derived from petrochemical fecdstocks. For exarnple, U.S. Patent No. 5,834,105 teaches siructural polyinerie composites consisting of a polymeric matrix and intact corn husks, and hence provides an example of this general type of approach.

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 andlor paniculate components that have been extracted from plants and then pressed and/or agglomerated.
For cxample, in the fabrication of panicleboard, a plant-hased cellulosic inaterial (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.

'1'he development of thermosel composites where reactive components extracted from renewablc fcedstocks are used as building blocks for the matrix polymer is a much newer area of research and development that is gaining momentuni. 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.

As a practical maUer, a proppant must be able to retain good performance for prolonged periods in a wide rangc ofharsh cm=ironments in order to find widespead utility. Conscquently. while therc are many potenlial applications for composites 2.5 (prepared from renewable feedstocks) where biodegradability and/or other types of envirottmental deeradability are among the key target properties, such composites are not optimal for use as proppants in implcmenting thc fracture stimulation mcthod of the invcntion, and will hcnce not be discussed further.

d. Chemical Modification for Derivation of Optimal Reactive Ingredients for Use in Polymer Synthesis It is possible to use the triglycerides obtained from plant oils directly as monomcrs in the preparation of thermoset polymers and composites. It is, however, usually preferable to modifv these triglycerides cheniically to obtain inonomers which have more attractive reactivity profiles and contribtuions to the properties of the final tbermoset system afler incorporation.

fvtany chemical modifications can be made readily to tailor the reactivity profile and the Gnal propenies. Different plant oils provide significantly different mixtures of staning 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 inteiise research.
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 developmenl.

The developinent of processes for the utilization of reactive components obtained or derived froni 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 diffcrent reactive components, and the modification of these components by breaking thetn down into snialler monomers and/or derivatizing thcm, is a crucial part of - l0-rescarch aimed towards the utilization of such reactive components as building blocks in the preparation of polymer composites.

For exarnple, U.S. Patent Application No. 20050154221 teaches intcgrated chemical processes for the indttstrial utilization ofseed oil feedstoek compositions.

Pillai (2000) discusses the wealth of high value polymers that can be produced by usino constituents cxtractcd from cashcw nut shell liquid.

Additional examples will bc provided bclow in the context of specific types of polynters and composites prepared by using reactive components obtained or derived from tiatural fats and oils extracted from plant-based sources.

c. Various Polymers and Polvmer Coniposiles Synthesized By Using Formulations Containing Reactive Ingredients Obtained or Derived from Renewable Feedstocks U.S. Patent No. 6,682,673 teaches a process for making a composite where a natural fiber is used as the reinforcing agent, and the niixture of reacrants from which the matrix potytner is synthesized via free radical copolymerization comprises a ring opening product of epoxidized fatty conipounds with olefinically unsaturated tatty acids such as acrylic acid or merhacrytic acid. The initial fatty compounds are obtained from sources such as soybean oil.

Methods arc taught for the production of radically postcured polyurethanes by reacting acrylic or methacrylic acid derivadves based on natural oils (epoxidized faity acid esters and/or epoxidized triglycerides) with aromatic and/or aliphatic isocyanates (U.S. Yatent Application No. 20030134928). In similar approaches, reactive anhydrides (U.S. I'atent Application No. 20030134927), structural coinponents such as acrolein, acrylamide, vinyl acetate and styrene (U.S. Patent Application No.
20030139489), or - ll -WO 2009/005880 PCT/i3S2008/061520 diallyl phthalates (U.S. Patent Application No. 20040097655) are included in the second stage of the preparation of the polymers.

I-lusic et al. (2005) rcported that they prcpared and compared two series of glass fiber reinforced composites, one using a polyol based on soybean oil and one using a petrochcmical polyol in the prcparation of the polyurethane matrbi. llte mechanical properties (such as tensile and Ilexural ntodulus, and tensile and flexural strength) of the two series of coniposites were comparable. It was stated that soybean oil-based composites are likely to find increasing applications beeause of tfieir superior oxidative, themial and hydrolytic stabilities.

Mosiewicki et a1. (2003) and Aranguren et al. (2005) developed composite materials forrnulated 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. Thc cured tannin matrix was found to be even niore hygroscopic than woodtlour.

Belcher et al. (2002) investigated the properties of biofiber-reinforced biobascd epoxy resins for automotive exterior applications. '1'hey considered the use of both epoxidized linseed oil and epoxidized soybean oil as nioditicrs of conventional epoxy resin compositions bascd on pctrochemical prccursors. They showed that the blending of functionalized soybean oil with petrochetnical-based resins can incrcase the toughncss of a petroleum-based thermoset resin without compromising stiffness, whilc also improving its environmctttal friendliness.

lz 1: Various Polymers and Polymer Composites Synthesized By Using Formulations Containing Petrochemical Comononiers Along With Reactive ingredicnis Obtained or Derived from Renewable Feedstocks The most extensive amount ofwrork appears to have been done on the use of monomers extractcd 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 themioset composites using such thermoset matrix polymers. This work will be summarized below.

Further details (beyond the summary that will be provided below) can be found in the following references: U.S. Patent No. 6.121,398, Warth et al. (1997), Willianis and Wool (2000), Khot et al. (2001), Can et al. (2001), Can ct al. (2002), La Scala and Wool (2002), Belcher et al. (2002) which was bric(ly discussed above, I..u et al.
(2004), O'Donnell et at. (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).
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 wcre dcrived by chemical modirication from the plant oils, rather than using the plant oils thentselves or the niononters extracted 1rorn llte plant oils directly.
In fact, research on the developinent ofclieinically modified monomers has paralleled thermoset polymcr and composite synthcsis in many rescarch groups.

Styrenc is the most comnionly ttsed petrochemical comonomer in such thermoset polymers and composites. Divinylbcnzene is also sometimcs used as a comonomer, to l3 WO 2009/005880 PCTlUS2008/061520 provide additional crosslinking sites beyond those that are present in the monomers originating from plant oils. 'fhe plant oil based monomers can readily undergo free radical copolymerization over a very broad range of amount of coinonomer with styrene andJor divinylbenzene in the presence of suitable initiators and/or catalysts.
The ntost extensively investigated composition region is front a total of 33% (a fraction of 113) to 40% (a fraction of 215) by weight of comonomers such as styrene and divinylbenzene.
't'his composition range corresponds to a common amount of such comonomers used in typical petrochemical-based resins such as epoxy vinyl esters.

Plant oil based niunumers 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 ct'f'ects. The higher the level of unsaturation in the plant oil based monomer (andlor the more its functionality has been increased via chemical modification), the more its use results in an increase in the glass iransition teniperature and the less its use causes plasticization.

l'he use of styrene and/or divinylbcn=r_enc in the I'ormulation 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 ctlect.

If there is a significant reactivity difference between the monomers obtained or derived from a particular plant oil and the styrenic mononiers 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 styrcnic polymer and domains that are - l4-rich in the product of the polytneriaation of monomers obtained or derived from the plant oil.

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 niany of the matrices described above (based on thc 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 nanoGllcr. Whenever such composites can be prepared at comparabie cost so that economic factors do not discouragc their potential manufacturers and users, they can provide signiticant sustainability advantages.
SUMMARY OF T'ME INVENTION

1. Introduction The present invention relates to a method for the fracture stimulation of a subtcrranean l'ormation ltavinl; a wellbore by using ultralit;hhweight thermoset polymer nanocomposite particles as proppants, where the particles are prepared by using formulations containing reactive ingredients obtained or derived trom renewablc feed stocks.

The niain coniponents of the particles are a rigid themloset polymer matrix (Section 2) and a nanofiller which provides reinforeentent (Section 3).

Optionally, an itnpact modif.ier (Section 4) may also be present.

Additional formulation ingredient(s) may also be used during the preparation of the particles; such as, but not limited to, initiators, catalysts, inliibitots, 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.
1'he particles may be manufactured by any suitable polymerization process.
They are preferentially manufactured by suspension polymerization (Section 5).

Optionally, the particles may be postcurcd (Scetion 6) by any suitable process.
They are preferentially postcured by heat treatment after polymerization.

Optionally, the particles may be coated (Section 7) by any suitable process.
They are preferentially coated by using a fluidized bed process after polymerization.

The particles formulated and manufactured as sununarized above are used in fracture stimulation (Section 8).

2. Matrix Polymer a. General Nature of Matrix Polymer Anv rigid thennoset polymer may be used as the matrix polymer of the nanocomposite particies utili-r.,cd as proppants in implementing the fracture stimulation method of the invention, subject solely to the limitation that the formulation from witich it is synthesized comprises a renewable feedstock cotnponent.

Rigid thermoset polymers are, in general, amorphous polymers where covalent crosslinks provide a three:-dimcnsional network. However, unlike thennoset elastomers (often referred to as "rubbers") which also possess a three-dimensional network of covalent crosslinks, the rigid thennosets are. by definition, `stiff'. In other words, they have high elastic tnoduli at "room temperature" (25 C), and often up to much higher temperatures, because their cotnbinations of chain segment stiffness and crosslink density result in a high glass transition temperature.

For the purposes of this disclosure, a rigid thetmoset polynter is defined as a thermoset polymer whose glass transition temperature, as measured by difterential scanning calorimetry at a heatittg rate of 10 CJminute. equals or exceeds 45 T. Thc gradual softening of an amorphous polymcr with incrcasing tcmpcraturc accclcratcs as the tempcrature approaches the glass transition tcmperature. As discussed by Bicerano (2002), the rapid decline of the stiffness of an amorphous polymer (as quantified bv its elastic moduli) with a further increase in temperaturc normally begins at roughly 20 C

below its glass transition teinperature. Consequently, at 25 C, an amorphous polymer whose glass transition teinperarature equals or excceds 45 C will be below the temperature range at which it.s elastic ntoduli begin a rapid decline with a further increase in temperature, so that it will be rigid.

Some exaniples of rigid thermosct polymers that can be used as ntatrix materials in the nanocomposite panicies 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 gencrality of the invention, to facilitate the teaching of the invention.

Commonly ttsed: rigid thermoset polymers include, but are not limited to, crosslinked epoxies, epoxy vinyl esters, polyesters, phenolics, melamine-based resins, polyurcthancs, and polyureas. Rigid thermoset polymers that are used less oftcn because of their high cost despite thcir cxceptional performance include, but are not limited to, crosslinl:ed polyiniides. For use in proppant particles suitable for different embodiments 2U of the fracture stitnulation method of the invention, these various types of polymers can bc prepared by starting froin their monomers, from oligomers that are often referred to as "prepolvmers", or from conibinations thereot:

Many additional types of rigid ihermoset polymers can also be used. Such polymers include, but arc not limited to, various families of crosslinked copolymers ;

prepared most often by the polymcrization of vinvlic monomcrs, of vinylidene monontcrs, or ofmixtures thcrcol:

The "vinyl fragment" is commonly dcfined as the CH:=<: H- hagmcnt. So a "vinylic inonomer" is a monomer of the general structure C1=12=C1-IR where R
can be any one of a vast variety of molecular fragments or atoms (other than hydrogen).
When a vinylic nionomer CH2=CHR reacts. it is incorporated into thc polymer as the -repeat unit. Among rigid thetmosets built from vinylic monomers, the crosslinked styrcnics and crosslinked acrylics are especially familiar to workers in the field. Some other familiar types of vinylic monomers (antong others) include the olefins, vinyl alcohols, vinyl esters, and vinyl halides.

The "vinylidene fragment" is commonly deGned as the. CH2=CR"- fragment. So a"vinylidene monomer" is a niononier of the general strticture CH2=CR'R" where R' and R" can each be any one of a vast varicty of molecular fragments or atoms (other than hydrogen). When a vinylidene monomer C1-IZ=CR'R" reacts, it is incorporated into a polymer as the -CH2-CR'R"- repeat unit. Among rigid thermosets built [Tom vinylidene polvmers, the crosslinked alkyl acrylics [such as crosstink.ed poly(methyl methacryiate)]
are especially familiar to workers in the field. However, vinylidene monomers similar to cach type of vinyl monomer (such as the styrenics, acrylates, olelins, vinyl alcohols, vinyl esters and vinyl halides, among others) can be prepared, One example of particular interest in the contcxt of styrenic monomcrs is alpha-methyl styrene, a vinylidene-type monomer that difTers froni styrene (a vinyl-type monomer) by having a inethyl (-CH3) group serving as the R" fragment replacing the hydrogen atom attached to the alpha-carbon.

rhermasets based on vinylic monomers, vinylidene monomers, or ntixtures thereof, are typically prepared by the reaction of a mixturc containing one or more non-crosslinking (difunctional) monomer(s) and one or more crosslinking (three or higher functional) monomer(s).

'fhe follovving are some specific bul non-limitink examples of crosslinking monomers that can be used: Divinylbenzene, trimethylolpropane t.tintethacrytote, triniethylolpropane triacrylate, trimethylolpropane dimethacrylate, trimethyloipropane diacrylatc. pcntacrythritol tetramethacrylate, pentaerylhritol trimethacrylate, pentaerythritoi diFnethacrylate, pentaerythritol tetraacrylate, pentaerythritol triacq=late, pcntacrythritol diacrylate, bisphenol-A diglycidyl methacrylate, ethylcneglycol dimethacrylate, ethyleFteglycol diacrylato, diethyleneglycol dimethacrylate, diethvlcneglycol diacrylate, triethyleneglycol dimethacrylate, and triethyleneglycol diacrylate, a bis(Fnethacrylaniide) having the formula:

a bis(acrylamide) having the formula:

11, R=
CU =Clt-~--t-(G1{~,-A'-~`.^-C1PC1l.
~O pU

a polyolern having the f=ormula CI-1Z=CH-(Cl-h),;-CI-1=C1-l. (wherein x ranges tiom 0 to 100, inclusive), a polyethyleneglycol dimethylacrylate having the formula:

Q
IF,~c;---r-ci-,~II:-CIF_C1-f~FF:-ial;~ Ci-CI!=-c:'I!;-~CY--<1---(:--C'Ii=
U iII;

a polyethyleneelycol diacrylatc having the formula:

CF
u <'11 =('Fi-I' t~-c II;-l FI.-:o-<N:-Ctl: t---cfF=rl17 ci a molecule or a macromolecule containing at least thrcc isocyanatc (-N=C=O) groups, a molecule or a macrotnolecule containing at least thrce alcohol (-OH) groups, a moiccule or a macromolecule containing at least three reactive amine functionalities where a primary amine (-NH2) contributes nvo to the total number of reactire functionalities while a secondary aniine (-NI-IR-, wherc R can be any aliphatic or aromatic organic fragment) contributes one to the total number of reactivc functionalities: and a molecule or a macromolecule wherc the total number of reactive functionalities arising from any combination of isocyanate (-N=C=0), alcohol (-01-1), primary amine (-NH2) and secondary amine (-N1-YR-, 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.

The following are some specific but non-limiting examples of non-crosslinking monomers that can be used: Slyrenic monomers, styrene, methylstyrcnc, ethylstyrcne (ethylvinylbenzenc), chlorostyrcnc, chloromcthvlsryrene, styrenesulfonic acid, t-butoxystyrene, t-butylstyrenc, pentylstyrenc, alpha-methylstyrene, alpha-methyl-p-pentylstvrene; acrylie and methacrylic monomers, methyl acrylate, methyl methacrylate, ethyl acrylatc, ethyl mcthacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrvlate, lauryl acrylate, lauryl methacrylate, glycidyl acrylate, glycidyl methacrylate, dimethylarninoethyl acrylatc, dimethvlaminocthyl methaerylate, hydroxyethyl acrylate, hydroxycthyl niethacrylate, diethylene glycol acrylate, diethylene glycol mcthacrylate, glycerol monoacrvlate, glycerol monomethacrylate, polyethylene glycol monoacrylate, polyethylene blycol monomethacrylate, butanediol monoacrylate, butanediol monomethacrylate; unsaturated carboxylic acid monomers, acrylic acid, methacrvlic acid; alkyl vinyl ether ntonomers, methvl vinvl ether, ethyl vinyl ether;
vinyl ester monomcrs, vinyl acctate, vinyl propionatc, vinyl butyrate; N-alkyl substituted acrylamidcs and mcthactylamides, N-methylacrylamide, N-ntethylmcthacrylamide, i\-ethyl acrylamide. N-ethyl methacrylamide; nitrile monomers, acrylonitrile.
methacr-ylonitrile: olefinic monomers, ethylene (H2C=CI-I2) and the alpha-olefins ~ (I-hC=CI=IR) wltere R is any saturated hydrocarbon fragment; vinylic alcohols, vinyl alcohol; vinyl halides, vinyl chloride; vinylidene halides, vinylidene chloride, or mixtures thereof.

b. Rcnewable Feedstack Component of Matrix Polynier Fomtulatian A key aspect of the present invention is the utilization of reactive entities (monomers, oligomers and/or polymers wntaining reactive functionalities) obtained or derived from rencwable resources as components of the fonnulations from which the polymeric matrix of the thermoset nanocomposite proppant particles used in implementing the fracture stimulation method of the invention is prepared.

It is most desirable, from the viewpoint of sustainability, to maximize the proportion of renewable fcedstock that is being used. In practice, however, this desired outcome inust be balanced with the performancc requirements and the economic constraints of the application. Consequently, the renewable feedstock content may be, and in most embodinients is, less than 100%. The total quantity of the componcnt(s) obtained or derived trom renewable feedstocks can range from 1% up to 100% bv weight of the constituents of the formulation of the thermoset matrix polymcr. If it is less than 100%, the remainder can coinprisc any suitable petrochemical ingredients, such as but not limited to those sunuttarized in the preceding subsection.

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 implenienting the invention. Such renewable feedstocks can be E

obtained or derived from a widc varicty of microorganism-bascd, plant-bascd, or animal-bascd resourccs.

Without reducing the gcnerality of the invention, among renewable tcedstocks that can be used for the synthesis of the matrix polymer of the nanocomposite particles, natural fats and oils cxtracted from some comnion types of plants [such as soybean, sunflower, canola, castor, olive, peanut, cashew nut, pumpkin seed, rapcserd, corn, rice, sesame, cottonseed, palm, coconut, safflower, linseed (also known as flaxseed), hemp, tall oil, and similar natural fats and oils; and especially soybcan, sunflower, canola and linseed oils] appear to be- very promising as potential suurces of inexpensive mononiers.

Again without reducing ttte 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.

3. Nanofiller By definition, a nanofiller possesses at least one principal axis diniension 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 nanotillers possess all three principal axis dimensions whose lengths arc less than 0.5 microns. Any reinforcing material possessing one nanoscale dimension, two nanoscale dimcnsions, or thmc nanoscale dimensions. can bc used as the nanofiller. Anv mixture of hvo or more diff'erent types of such reinfore.ittg materials can also be used as the nanofiller. The nanoliller is present in an amotint ranging from 0.001 to 60 percent of the total particle by volume.

Without redticing the generality of the invention, to facilitate the teaching of the invention, we note that nanoscale carbon black, fumed silica, fumed alumina, carbon nanotubcs, carbon nanofibcrs, ccllulosic nanofibcrs, natural and synthetic nanoclays, vcry finely dividcd gradcs of 11y ash, the polyhcdral oligomeric silscsquioxancs; and clttstcrs of different types of mctals, metal alloys, and metal oxid'es, are some examples of nanofillers that can be incorporated into the nanocomposite partictes used as proppants in implementing the fracture stimulation metltod 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 nanotillers that are not currently known inay also be readily anticipated.

4. Impact Modifier Optionally, the thermoset nanocomposite particles used as proppants in implementing the fracture stimulation method of the invention may contain an impact modificr.

If its t~5e is desired, an impact modifier is sclccted and incorporated into the particles as described in the SUMMARY OF THE iNVr~ITiON and the DESCRIPTIO-NI OF "I'HE PREFERRED EMBODIMENTS sections of U.S. Patent Application 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.
5. Suspension Polymerization Any method lor lhe fabrication of thermoset polymer nanocomposite particles known to those skilled in the art may be used to preparc the themtoset nanocompositc particlcs which are utilized as proppants in implementing the fracture stimulation method of the invention.

Without reducing the generality of the invention, it is especially practical to use methods that can prodttce ilte panicles directly in the desired (usually substantially spherical) shape during polymerization from the starting monomers.

A substantially spherical particle is defined as a pariicle 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 fnternational Standard ISO
13503-2, "Petroleum and natural gas industries - Completion fluids and materials - Part 2: Mcasurcmcnt of properties of proppants used in hydraulic fracturing and gravel-packing operations" (first cdition, 2006), Section 7, for the purposes of this disclosure.

Without reducing the generality of the invention, it is cspecially useful to produce the substantially spherical particles discussed in the paragraph above with an average diamcter that ranges 1rom 0.1 mm to 4 nim for use in fracture stimulation applications.
Without reducing the generality of the invetttion, in a tnost preferred embodiment, at least 90% of the substantially spherical particles are produced with diameters tanging from 0.42 mm (40 U.S. mesh size) to 1.41 mm (14 U.S. mesh size).
Ntfithout reducing the generality of the invention, suspension (droplet) polymerization, where the polymer precursor mixture is dispersed in a suitable liquid tnediutn prior to being polymerized, is currently the most powerful manufacturing method available for producing the particles directly in a substantially spherical shape during polvnieriz.ation front the starting monomers. In pursuing this approach, it is especially inlportant for the nanofiller particles to be well-dispersed within the liquid medium so that they can becotne intimately incorporated into the thermoset rianocomposite particles that will be forttted upon polymerization.

6. !-leat Treatment Optionallv, the thetYrtoset nanocomposite particles used in implementing the ti-acture stimulation mcthud of the invention may be subjected to suituble post-polymerization process steps intended mainly to advartce the curing of the thermoset polymer matrix.

If a suitablc post-polymcri=ration process stcp is applicd to the thermoset polymer nanocomposite particles, in many cases the curing reaction will be driven funher to%vards completion so that the maximum possible temperature at which the fracture stimulation method of tfie invention can be applied by using these particles will increase.

In soine instances, there may also bc further benefits of a post-polymeri2ation process step. One such possible additional henefit is an enhancement in the flow of the gascs, fluids, or mixtures thereof, produced by the subterranean formation, towards the wellborc, even at temperatures that are far below the niaximuin possible application temperature ot'thc fracture stimulation method. Another such possible additional bcneCit is an increase of such magnitude in the resistance of the particles to aggressivc environments as to enhance significantly the potential range of applications of the fracture stimulation method utilizing the particles.

Processes that may be used to enhance the degree of curing of a thermoset polymer include, but are not limited to, heat treatnient (which may be combined with stirring, flow and/or sonication to enhance its effectiveness), electron beam irradiation, and ultraviolet irradiation.

Without reducing the generality of the invention, we focused mainly on the use of heal treatment as a post-polymerization process step during the manufacturing, of the particles. Such heai treatment can be performed in manv types of rnedia;
including a vacuum, a non-oxidizinS gas, a mixturc ot'non-oxidizino gases, a liquid, or a niixture of liquids.

It is possible, in some instances, to postcure the "as polymerized" particles adequately as a resutt of the etevat.ed temperature of a downhole environment of a hydrocarbon rescrvoir 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 trcatmcnt is gencrally less prcferred than the application of heat treatment as a manufacturing process stcp bcfore using the particles in fracture stiniulation.

7. Coating Optionally, the thermoset nanocomposite particles used in iinplenienting the fracture stimulation method of the invention may be coated; to achieve benefits such as proteetion &om chemicals, waterproofing, hardening, and combinations thereof.

It is preferable, in most cases, to use matrix polymer cotnpositions that can withstand the downholc environment without requiring a coating on the particles. A
coating may, however, sonietimes 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.

Any available method may be used to place a coating around the particles. A
coating may be placed during polymerization, after polvmerization, or a conibination thereof 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 niay be included in the polymer prectirsor rnixture to place a coating during polymerization. With this approach, there is also a likelihood of some pcnctration of the coating ntaterial to the interior of the particles and/or the interpenttration of the coating phase and the matrix phase and/or the - 2(i -WO 2009/005880 PCTlUS20081061520 development of an "interphase" region ovcr which thc composition changes gradually from that of the matrix polymer to that of the coating.

Again uithout rcducing the gcncrality of the invcntion, various types of fluidized bed processes provide familiar examples of methods for placing a coating around the particles after polyntcrization.

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 afier polymerization.

Witliout reducing the generality of the invention, the use of a fluidized bed process as a post-polyme=rization step is a preferred method for the placement of a coating if needed, but it is most preferred to select a matrix polymer cotnposition such that a coating will not lx needed.

Any suitable coating niaterial may be used if a coating is needed. Without reducing the generality of the invention, epoxies, epoxy vinyl esters, polyesters, acrylics, phenolics, alkyd resins, rnelamine-based resins, furfuryl alcohol resins, polvacetals, polyurethancs, polyureas, polyimides, polyxylylencs, silicones, fluoropolyniers, copolymers therc:ol; and mixtures thereof; arc some examples of coating niaterials that tnay be used.

8. Fracture Stimulation Ute fracture stimulation method of the invention is implemented by using stitt;
strong, tough, heal resistanl, and anvironment:ally resistant ultrtrlightweight thermoset polymer nanocompositc particles. Such particles may be placed cither as a proppant partial inonolayer or as a conventionat proppant pack (packed mass) in implementations of the invention.

The optimum mode of panicle placement is determined by thc details of the specific fracture that needs to bc propped. In practice, the use of ultralightwcight particles as proppant particles in implementing the fracture stimulation method of the invention provides its greatcst advantages in situations where a proppant partial monolayer is the optimum mode of placcment. Curthermore, the development of the fracture stimulation method of the invention has resulted in partial monolayers becoming the optimum proppant placentcnt method in ntany situations where the use of panial monolavers was either impossible or impractical with previous technologies.

In any case, the inethod 1'or fracture stiniulation 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 sluny, and (c) thus etnplacing the proppant in the fomiation so that it can prop open the fracturc network (thereby allowing pmduced gases, fluids, or mixtures thereof, to tlow towards the wellbore).

The most comnionly used measure of proppant performance is the conductivity of liquids andior gases (depending on the type of hydrocarbon reservoir) through packings of the particles. A minimum liquid conductivity of 100 mllft is often considered as a practical threshold lor considering a packing to be useful in propping a fracture that possesses a given closure stress at a given tempcrature. In order for a fracture stimulation niethod to have significant practical utility, a static conductivity of at least 100 mDll ntust be retained for at least 200 hours at a temperature greater than 80 oP

It is a common practice in the industry to use the simulated environment ot' a hydrocarbon reservoir in evaluating the conductivities of packings of partic{es. The API
RP 61 niethod, described by a publication of the American Petroleum Institute titled "Recommended Practices for Evaluating Short Term Proppant Pack Conductivity"
(first edition, Octobcr 1. 1989), is currcntly the commonly acccpted testing standard for conductivity tcsting in the simulated cnvironment of a hydrocarbon reservoir.
As of the date of this filing, hoa-ever, work is underuay to develop alternative testing standards, 3 such as International Standard [SO 13503-5, "Petrolcum and natural gas industries -Completion fluids and materials - Part 5: Procedures for measuring the long-term conductivity of proppants" (final draft, 2006).

DE'SCRIP'I'ION OF THE PRFFER.RED 6M.BODIMENTS

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 an can readily imabine niany additional embodimcnts that fall within the full scope of the invention as taught in Ihe SUMMARY OF
THE
II~TVENTION section.

The fracture stimulation method of the invention is preferably implemented by placing the uitralibhteveight thermoset polymer nanocomposite particles in the fracture as a panial monolayer. We have found, under standard laboratory test conditions, that the use of particles of narrow size distribution such as 14/16 U.S. mesb 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/ItZ is an cspccially preferred lcvcl ot'covcrage of the liacturc arca with a partial monotayer of therinoset nanocomposite particles of sutficient siiffiness and strength that possess an absolute density of 1.054. I-Iowever, real-life dommhole conditions in an oilfield may differ significantly fronl those used under laboratory test conditions.
Cottst:quently, in the practical application of the 1racture stimulation method of the invention, the use of other panicle size distributions, other coverage levels, or -29.

combinations thereof, mav be more appropriate, depending on the conditions prevailing in the specific downhole environment wbere the fracture stimulation methnd of the invention will be applied.

T'hc thermoset polynier matrix comprises a cnpolymeriiation product of mnnnmers derived froni 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 uscful monomers from soybean oil is at a morc advanced stage than the derivation of nionomers from other suitable renewable feedstocks. The current preference for the use of all three of S, DVB and CVI3, 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 niodifying (a) the proportion of the matrix poly-ner originating from tnonomers derived from soybean oil over the range of t% to 100 !o by weight, (b) the mixture of nionomers derived froin sovbean oil, and (c) the relative amounts of the three vinylic monomers (S, DVB and EVB).

Carbon black, possessing a length that is less than 0.5 niicrons in at least one prittcipal axis direction, is used as the nanofillcr at an amount ranging from 0.1% to 15%
of the total particle by voluntc.

Suspension polymerization, preferably in its "rapid rate polymcrization" modc, is pcrformcd to synthesize the particics. The most important additional formulation ingredient (besides the reactive monomers) that is used during polymerization is the initiator. 'rhe initiator may consist of one type molecule or a niixture of two or more t,vpes of niolecules that each have the ability to function as initiators. We have found, with experiem;e, that ttte "dual initiator" approach, involving the use of thvo initiators which begin to manitcst significant activity at different temperatures, often providcs the best results.

Additional formulation ingrcdicnts, such as impact modifiers, catalysts, inhibitors, dispersants, stabilizers, rheology modifiers, buffers, antioxidants, defoamers, plasticizers, pigments, 8anie retardants, smoke retardants, or mixtures thercof, may also bc 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 fonnu{ation ingredient which becomes incorporated in the particles is the optional impact mudilier, when it is used in a partictilar embodimerit, 'fhe 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. niesh size range (diameters in the ratige of 0.42 to 1.41 millimeters). The panicles are sometimes separated into Iractions having narrower diameter ranges for use in an optimal matuter in proppant partial monolavers.

Post-polymerization heat treatment in an unreactive gas environment is performed as a manufacturing process step to further advance the curing of the thermosct polymer nlatrix. "fhis approach works especially well (vrithout adverse effects such as dcbradation that could occur if an oxidative gaseous environment such as air were used and/or sweiling that could occur if a liquid environmcnt wcre used) in enhancing the curing of the particles. The particles undergo a total exposure to teniperatures in the range of 130 C to 210 C for a duration of 5 minutes to 90 minutes, inclusive, in an unreactivc gas envirortment. The specific selection of an optimum temperaturc (or ?5 optimuni temperaturc range) and optimuni duration of heat treatment within these ranges depends on the formulation fmm which the particles were prepared. Nitrogen is used as the unreactive gas environment.

Finally, it .vill be appreciated by those skilled in the art that changes could be niade to the embodimcnts described above without departing from the broad inventive 3 concept thcrcof It is understood, lhcrefore, that this invention is not limited to thc particular cmboditnents disclosed, but is iatended to cover modifications within thc spirit and scope of the present invention as defined in the appended claims.

EXAMPLES
Sorne eheoretical examples of preferred embodiments of the fracture stimulation niethod 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.

F:xample t The fracture stiniulation method ol'the invention is applied in a situation where it will provide the maximum possible benefit as compared with prior i'racture stimulation methods. The downhole environment is one where the use of a proppant partial monolayer would be very effeclive 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 environntental resistance. The ultralightweight thermoset polymer nanocotnposite particles used in iinplcmenting lhe fracture stintulation rnethod of the invention overconle this difficulty. Uetailed consideration of the downhole environment results in the dctcrmination that 14!16 U.S. ntcsh sizE: particles would be optintal.
Panicies in this size range are placed into the fracture as a partial monolayer bv using slickwater as the carrier fluid.

The thermoset polymer inatrix of the nanocomposite particles used in this examp{c consists of a copolymer of styrene (S), ethyvinylbenzene (EVB), divinylbenzcnc (DVB), and acrylated epoxidized soybean oil (AESO). The quantities oi' these ingredients in the reactive mixture are 51.55% S. 8.45% EVB. 15% DVB and 25%
AESO by wcibht. In addition, the particles contain 0.5% by volume of carbon black as a nanofiller.

The particles are prepared in the 14140 U.S. mesh size range by rapid rate suspension polymeriiation. 'fhey are then postcured in a nitrogen environment for 20 minutes at a temperature of 185 T. Particles faliing ti=ithin the 14/16 U.S.
mesh size range arc separated from the broader distribution of 14/40 U.S. mesh size range by standard sieving techniques.

Example 2 As in Example 1, but the quantities of the ingredients in the reactive tnixture are 61.86% S, 10.14% EVB, 18% DV13 and 10% AESO by weight.

Example 3 As in Example 1, but the quantities of the ingredients in the reactive mixture are 41.24% S. 6.76% EVB, 12% llVB and 40% AESO by weight.

Example 4 As in Example 1, bui maleinized acrvlated epoxidized soybean oil (M.AESO) is used instead of AESO as the formulation ingredient originating from a renewable n;sotirce.

Example 5 WO 2009/005880 PCTlUS2008/061520 The sanie types of paniclcs arc used as in Example I. 1-lowever, detailcd considcration of the downhole cnvironment shows that the use of the full available 14,140 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.

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 line paniculate 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, pentacrythritol 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 (-0H) 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.