Classifications

• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
  • C09K8/00—Compositions for drilling of boreholes or wells; Compositions for treating boreholes or wells, e.g. for completion or for remedial operations
  • C09K8/60—Compositions for stimulating production by acting on the underground formation
  • C09K8/80—Compositions for reinforcing fractures, e.g. compositions of proppants used to keep the fractures open
  • C09K8/805—Coated proppants
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
  • C09K2208/00—Aspects relating to compositions of drilling or well treatment fluids
  • C09K2208/08—Fiber-containing well treatment fluids

Definitions

• the invention relates to a composition and method for the production of proppants having a coating that exhibits enhanced conductivity under medium and high pressure, downhole, fractured strata conditions.
• Coated proppants are often used in hydraulic well fracturing to increase production rate of the well.
• the commercial "standard" coatings are typically a form of phenolic thermoset coating.
• precured phenolic coatings are often used due to their high load- bearing properties.
• the high crack closure stresses are usually above 6,000 psi, and often above 10,000 psi, so the proppant must resist such forces in order to keep the fracture cracks open and maintain fracture conductivity.
• each acts like a fully cured coating for purposes of handling, shipping and introduction into a fractured field yet exhibit the inherent ability to form interparticle bonds under downhole temperatures and pressures for enhanced conductivity and to minimize proppant flowback after the well is put into production.
• Commercially available proppants that use such coatings are available under the designations PEARL and GARNET from Preferred Sands, Inc. of Radnor, PA.
• temperature/high pressure strata would also exhibit some level of interparticle bond strength without the use or introduction of bond formation or polymer softening agents into the fractured strata.
• Such interparticle bonding would provide a further effect for retaining the coated proppants within the fractured strata despite the outflow of fluids and gases that can dislodge the proppant particulates and flush them from the strata.
• US Patent No. 4,493,875 relates to a composite proppant with a sand core and hollow, glass microspheres in an "adhesive" that bonds the microspheres to the core.
• a resole phenol/formaldehyde resin is used in the examples as a coating on the sand core of the proppant.
• 5,422,183 and 5,597,784 teaches a proppant having a substantially cured inner resin coating, an outer resin coating, and a reinforcing agent interspersed at the inner coating/outer coating boundary, which is used in the propping of a fracture in a subterranean formation.
• the core of the proppant is said to be glass beads; various organic materials such as walnut shells, pecan shells, and synthetic polymers; or metallic particulates such as steel or aluminum pellets.
• US Patent No. 6,406,789 describes a proppant particle made with a resin and filler material.
• the disclosed resins include epoxy, phenolic, a combination of a phenolic novolac polymer and a phenolic resole polymer; a cured combination of phenolic/furan resin or a furan resin to form a precured resin; or a curable furan/phenolic resin system curable in the presence of a strong acid to form a curable resin.
• the finely divided minerals that can be included in the resin include silica (quartz sand), alumina, mica, meta-silicate, calcium silicate, calcine, kaolin, talc, zirconia, boron and glass. Microcrystalline silica is noted as especially preferred.
• US Patent No. 6,528, 157 discloses a resin-coated proppant that contains fibers where at least a portion of the fibers protrude from the resin coating to interlock with fibers of other proppant particulates.
• US Patent No. 7,490,667 describes a proppant having a water-soluble external coating on the proppant particle substrate and a microparticulate reinforcing and spacing agent at least partially embedded in the water-soluble external coating in a manner such that the microparticulate reinforcing agent is substantially released from the proppant particle substrate when the water-soluble coating dissolves or degrades.
• US Patent No. 7,803,742 pertains to thermoset nanocomposite particulates made with carbon black, fumed silica, fumed alumina, carbon nanotubes, carbon nanofibers, cellulosic nanofibers, fly ash, polyhedral oligomeric silsesquioxanes, or mixtures thereof.
• US Patent Nos. 8,006,754 and 8,006,755 describe proppants coated by a material whose electromagnetic properties change at a detectable level under a mechanical stress such as the closure stress of a fracture.
• a preferred proppant is described as a thermoset nanocomposite particulate substrate where the matrix material comprises a terpolymer of styrene,
• ethylvinylbenzene and divinylbenzene and carbon black particulates possessing a length that is less than 0.5 microns in at least one principal axis direction incorporated as a nanofiller.
• a coating that comprises a PZT alloy manifesting a strong piezoelectric effect or Terfenol-D manifesting giant magnetostrictive behavior to provide the ability to track in a downhole environment.
• US Patent No. 8,298,667 describes the use of two ceramic layers that can contain a reinforcing agent of carbon black, fiberglass, carbon fibers, ceramic whiskers, ceramic particulates, metallic particulates, or any combination thereof.
• the present invention provides a proppant having a polymeric coating that is strengthened with reinforcing particulates that are grafted to or bonded to the polymeric proppant coating.
• these particulates are added into and become part of the coating during the coating process.
• functionalized particulates are used that become grafted into the polymer of the proppant coating through the chemical functionality imparted to the particulates.
• a coupling agent is preferably added to enhance the bond strength between the added particulates and the polymeric matrix of the proppant coating.
• the hard particulates that are integrated into the proppant coating are preferably chemically integrated and chosen to impart a greater hardness and/or deformation resistance to the coating.
• An increased hardness reduces agglomeration during storage and shipping and helps to mitigate dust.
• Reduced deformation of the proppant coating avoids pore closure due to coating deformation with the effect of maintained conductivity, even in high pressure wells.
• the added chemical bonding helps the particle to remain in the coating and avoid the formation of microcrack defect sites that could be initiation sites for cracks leading to dusting and deterioration.
• the present invention relates to a coated proppant that includes particulates that are firmly bound to or grafted to the polymeric coating. These particulates impart enhanced hardness to the proppant coating and an internal reinforcing agent linked to the polymeric matrix of the coating that resists deformation of the composite coating under medium and high pressure stress.
• the particulates added to the proppant coating in the present invention can be organic or inorganic.
• Preferred particulates for use in the present composite proppant coating are selected from among a wide variety of materials witose presence in the coating will enhance the overall strength and deformation resistance of the coated proppant.
• Reinforcing particulates can be used in any layer or layers applied to the proppant core solid.
• Organic particulates that are useful for the present invention include particulates that are relatively harder than the proppant matrix polymer and may be pre-reacted to include reactive functionalities for bonding with the polymeric matrix of the proppant coating or they may be non-reactive if a separate adhesion promoter is added to the composite to enhance bonding betw r een the polymeric matrix and the added particulates.
• Suitable organic particulates include fullerenes, activated carbon, rubber, aibber-reinforced polymers, and other organic particulates sold as "impact modifiers" for composites.
• the preferred particulates for use in the present composite coating exhibit a wet glass transition temperature (Tg) for enhanced structural reinforcement that is greater than the glass transition temperature of the cured (or as substantially fully cured as the coating becomes in use) coating resin as well as the expected operating temperature where the proppant will be used.
• Tg wet glass transition temperature
• the proppant formulator would use particulates with a Tg that is lower than that of the coating or lower than the expected operating temperature where the proppant will be used.
• the added particulate is, or can be made to be, reactive towards the chemistry of the resin coating so that the particulate remains firmly attached and/or chemically grafted into or onto the coating of the proppant.
• Suitable forms of particulate materials include dispersions, short fibers and powders (collectively referred to herein as "particulates") of finely divided, functionalized or non-functionalized metals, metal oxides, metalloids, and ceramics e.g., silica, silicon carbide (particles, whiskers or milled whisker forms), alumina, aluminosilicates, spent cracking catalysts, bauxite, ceramics, and the like.
• Especially preferred inorganic materials are functionalized forms of silica or dispersions or powders of silica to which an external coupling agent has been added to enhance the bond between the added silica and the surrounding polymeric matrix of the proppant coating.
• fibers may be any of various kinds of commercially available short fibers or crystalline whiskers.
• Such fibers include at least one type of milled glass fiber, milled ceramic fiber, milled carbon fiber, natural fiber, crystalline inorganic forms including forms having a ratio of length to diameter within the range of 1-100 (e.g., particles to whiskers), and synthetic fibers, e.g., crosslinked no olac fibers, having a softening point above typical starting
• the typical glasses for fibers include E-glass, S-glass, and AR-glass.
• E-glass is a commercially available grade of glass fibers typically employed in electrical uses.
• S-glass is used for its strength.
• AR-glass is used for its alkali resistance.
• the carbon fibers are of graphitized carbon.
• the ceramic fibers are typically alumina, porcelain, or other vitreous material.
• Fiber lengths range from about 6 microns to about 3200 microns (about 1/8 inch).
• Preferred fiber lengths range from about 10 microns to about 1600 microns. More preferred fiber lengths range from about 10 microns to about 800 microns. A typical fiber length range is about 0.001 to about 1/16 inch. Preferably, the fibers are shorter than the greatest length or depth of the coating on the proppant. Suitable, commercially available fibers include milled glass fiber having lengths of 0.1 to about 1/32 inch. Additional fibers include milled ceramic fibers that are typically about 6 to 250 microns long, milled carbon fibers that are within the range of 50 to 350 microns long, and KEVLAR aramid fibers of 6 to 250 microns long.
• Fiber diameter (or, for fibers of non-circular cross-section, a hypothetical dimension equal to the diameter of a hypothetical circle having an area equal to the cross-sectional area of the fiber) range from about 1 to about 20 microns.
• Length to aspect ratio (e.g., length to diameter ratio) may range from about 5 to about 250.
• the fiber may have a round, oval, square, rectangular or other appropriate cross-section.
• One source of the fibers of rectangular cross-section may be chopped sheet material. Such chopped sheet material would have a length and a rectangular cross-section.
• the rectangular cross-section has a pair of shorter sides and a pair of relatively longer sides. The ratio of lengths of the shorter side to the longer side is typically about 1 :2-10.
• the fibers may be straight, crimped, curled or combinations thereof. See McDaniels et al. US Patent No. 6,632,527 which is hereby incorporated by reference.
• Functionalized inorganic particulates that are particularly useful in the present invention are prepared by reacting the inorganic particle with one or more organic agents that bond to the surface of the underlying particle and provide one or more reactive sites over the surface of the particle that can be used to bond or enhance the bond between a polymeric phase and the functionalized particulates dispersed therein.
• Silica is one such particle that has been functionalized in a variety of ways. See US Patent Nos.
• 5,168,082 (functionalizing group attached to the silica sol is a branched or straight chain silane including at one end a hydrophilic moiety and at another end a silicon anchor group); 5,330,836 (polyfunctional silica particulates); 6,486,287 and 7,129,308 (functionalized silicon for silica surfaces); 6,809,149 (silica with 3- methacryloxypropylsilyl and/or glycidyloxypropylsilyl groups on the surface); and published US Patent Application Publication Nos.
• 2004/0138343 (colloidal silica functionalized with at least one organoalkoxysilane functionalization agent and subsequently functionalized with at least one capping agent); 2007/0238088 (functionalized silica compositions by reacting acidic silica particulates with hydrophilic organosilanes); 2008/0063868 (silica nano-sized particulates having polyethylene glycol linkages); and 2013/0005856 (amine-functionalized silica particulates coupled to at least one group chosen from primary amines, secondary amines, tertiary amines, and quaternary ammonium groups).
• particulates of silica, alumina, aluminosilicate, or ceramic particulates are preferred particulates for the composite coating.
• orthosilicates, disperse particulates of colloidal silica can be prepared.
• the surface of these particulates has been modified to stabilize them in water or organic solvents.
• Surface modified colloidal silica particulates are referred to as functionalized, as are the resulting colloidal solutions, or sols.
• the surface of a formed alumina, or aluminosilicate can also be
• a chemical moiety or chemical material such as an organic ligand, like a surfactant, and can provide surface wetting properties which can assist in grafting the added particle into the polymer of the coating or providing bonding functionalities that assist in resilient incorporation of the particle into the proppant coating.
• a chemical moiety or chemical material such as an organic ligand, like a surfactant
• particulates that have been functionalized to include isocyanate-terminated moieties are useful to add isocyanate
• the preferred functionalizing agents are those that are compatible with silica surfaces, such as the silicon compounds of US 6,486,287 and 7,129,308 that are made with a silicon compound comprising a silicon atom and a derivatizable functional group.
• the functionalized silicon compound is a functionalized silylating agent and includes an activated silicon group and a derivatizable functional group.
• the term "derivatizable functional group” refers to a functional group that is capable of reacting to permit the formation of a covalent bond between the silicon compound and another substance, such as a polymer.
• Exemplary derivatizable functional groups include hydroxyl, amino, carboxy, thiol, epoxy, amide, and isocyano, as w r ell as modified forms thereof, such as activated or protected forms. Derivatizable functional groups also include substitutable leaving groups such as halo or sulfonate.
• Another preferred embodiment uses a derivatizable group (e.g.,— Si(OMe) ;— SiMe(OMe)2 ' ,— SiMeCb; SiMe(OEt) 2 ; SiC and— Si(OEt) 3 ) that can react with hydroxyl functionalities found within the polyurethane, polyurea-type, furan, furyl alcohol and phenolic coatings on the proppant.
• a derivatizable group e.g.,— Si(OMe) ;— SiMe(OMe)2 ' ,— SiMeCb; SiMe(OEt) 2 ; SiC and— Si(OEt) 3
• an adhesion promoter is desirably used to enhance the wetting and/or surface bonding between the added particle and the polymeric coating.
• the adhesion promoter is preferably a silane or, more preferably, an organofunctionalized silane.
• Silanes are a particularly preferred type of adhesion promoter agent that improves the affinity of the coating resin for the surface of the proppant core solid and is particularly useful when sand is the proppant core.
• adhesion promoters can be used in an outer layer portion of a proppant coating to provide bonding sites for enhancing the interparticle bonding of proppants bearing a similarly functionalized external surface.
• silanes can be mixed in as adhesion promoters in the first step of the coating process, but can also be converted chemically with reactive constituents of the polyol component or of the isocyanate component.
• Functional silanes such as amino- silanes, epoxy-, aryl- or vinyl silanes are commercially available.
• the amino-silanes are preferred for silica-based core solids.
• ceramic core solids or gano functional zirconates or titanates are preferred, e.g., ethyltitanate.
• Suitable organofunctional silanes for use in the present invention as adhesion promoters include those with the structure:
• Rl , R2, R3, and R4 may the same or different and are independently selected from the group consisting of hydrogen, hydroxy, hydroxyalkyl, alkyl, haloalkyl, alkylene, alkynyl, alkoxy, alkynoxy, aryl, aryloxy, substituted aromatic, heteroaromatic, amino, aminoalkyl, arylamino, epoxide, thiol, and haloalkyl, ether, ester, urethane, amide, provided that at least one of Rl, R2, R3, and R4 comprises an organic moiety.
• the oganofunctional silane coupling agent includes an organic functionality selected from the group consisting of methyl, epoxide, epoxy/melamine, amino, mercapto, chloropropyl, methacryl, methacryloxy, vinyl, benzylamino, ureido, tetrasulfido, and C1-C4 alkoxy groups.
• the organofunctional silane is selected from the group consisting of mercaptosilanes possessing at least one hydroxyalkoxysilyl group and/or a cyclic dialkoxysilyl group, blocked mercaptosilane possessing at least one hydroxyalkoxysilyl group and/or a cyclic dialkoxysilyl group; mercaptosilanes in which the silicon atoms of the mercaptosilane units are bonded to each other through a bridging dialkoxy group, each silane unit optionally possessing at least one hydroxyalkoxysilyl group or a cyclic dialkoxysilyl group; blocked mercaptosilane dimers in which the silicon atoms of the blocked mercaptosilane units are bonded to each other through a bridging dialkoxy group, each silane unit optionally possessing at least one
• mercaptosilane unit the silicon atom of which is bonded to the silicon atom of a blocked mercaptosilane unit through a bridging dialkoxy group, each silane unit optionally possessing at least one hydroxyalkoxysilyl group or a cyclic dialkoxysilyl group; mercaptosilane oligomers in which the silicon atoms of adjacent mercaptosilane units are bonded to each other through a bridging dialkoxy group, the terminal mercaptosilane units possessing at least one
• blocked mercaptosilane oligomers in which the silicon atoms of adjacent blocked mercaptosilane units are bonded to each other through a bridging dialkoxy group, the terminal mercaptosilane units possessing at least one hydroxyalkoxysilyl group or a cyclic dialkoxysilyl group; and silane oligomers possessing at least one mercaptosilane unit and at least one blocked mercaptosilane unit, the silicon atoms of adjacent silane units being bonded to each other through a bridging dialkoxy group, the terminal silane units possessing at least one hydroxyalkoxysilyl group or a cyclic dialkoxysilyl group.
• n-octyltriethoxysilane (CAS No. 2943-75-1); bis[3-(triethoxysilyl) propyl] tetrasulfide (CAS No. 40372-72-3); vinyltriethoxysilane (CAS No. 78-08-0); 3- glycidoxypropyl-trimethoxysilane (CAS No. 2530-83-8); 3-mercaptopropyl-triethoxysilane (CAS No. 14814-09-6); 3-glycidox propyl-triethoxysilane (CAS No.
• silane-terminated polymers such as silane-terminated polyethers and polyurethanes. These polymers are formed by reaction of for instance a polyether polymer with isocyanate termination with aminosilanes or a polyether polymer with amino termination and/or hydroxyl termination with isocyanate-terminated silanes. Reactions of the reactive groups with other materials in the composition are also possible to create other cross-links.
• Silane-terminated polymers STP or silane- modified polymers (MS) can be all pre-polymers which at the chain ends - or laterally - carry silyl groups having at least one hydrolysable bond but which in the polymer framework, do not display the siloxane bond (SiR 2 0)n that is typical of silicones.
• Two preferred silane- terminated polymers are illustrated by Formulas 1 (a dimethoxy(methyl) silylmethyl carbamate- terminated polyether) and Formula 2:
• Polyether refers to a polyether chain having 1-200 carbon atoms. See also published US patent nos. 3,971,751 and 6,207,766 as well as US patent application publication number US 2007/0088137, the disclosures of which are hereby incorporated by reference.
• R is an amine group; each X in Formula 5 can each be independently selected from the group consisting of hydrogen, alkoxy, halogen, and hydroxyl; and n is an integer that is greater than zero.
• Such agents are commercially available from Wacker Chemie AG, Hanns-Seidel-Platz 4, 81737 Miinchen, Germany under the designation Geniosii ® STP-E.
• the dipodal silane- terminated polyether-based polymers of Formulas 1 and 2 are compatible or miscible with polyether polyols that can be used as the polyol component for making a polyurethane proppant coating. Such silane-terminated polyether-based polymers are easily blended with polyether polyols as a last step top-coat to provide an adhesive coating layer for coated proppants according to the invention.
• the dipodal amino silane of Formula 4 in the form of bis(trimethoxysilylpropyl)amine has been used as a coupling agent in the proppants industry for "difficult" substrates. In the present invention, this silane could provide two silane, adhesive-like, functionalities for every amine grafting moiety.
• the length of the carbon chain in the alkoxy moieties determines the rate of hydrolysis of the silane. So, the choice of the length of the alkoxy carbon chain can be used to provide control over the resulting moisture and water resistance. Increasing resistance is seen as the alkyl chain increases. Longer carbon length chains will also delay the hydrolysis and, therefore, the bonding performance of the proppant in the fracture.
• the size of the added particulates for the composite should be selected based on the coating thickness on the core solid of the proppant and can be in the form of sols, colloids, suspensions or dry powders. Preferably, the added particulates do not extend substantially above the upper surface of the coating or interfere with handling, transport and injection of the coated proppant. Suitable sizes are generally within the range from about 5 nm to about 1500 nm.
• the added particulates exhibit an average particle size within the range from about 5 nm to less than 1000 nm and more preferably within the range of about 8-20 nm.
• the average particle size of the added hard, crush-resistant, inorganic particulates may be selected from the range of about 5 nm to about 500 nm.
• the reinforcing particulates used in the present invention are added as an aqueous suspension as a separate stream or admixed with a compatible coating component. Water addition can be particularly useful for polyurethane and polyurea-based coatings. See copending US Patent Application serial no. 13/355,969 entitled "Manufacture of Polymer Coated Proppants", the disclosure of which is hereby incorporated by reference.
• the amount of added functionalized inorganic particulates can be within a substantial range, depending on the polymer and coating thickness used on the proppant. In general, useful amounts are within the range of about 2-85 vol% solids in the proppant coating based on the volume of the coating. Preferred amounts are within the range of 2-65 vol % solids and even more preferably 5-30 vol% solids in the proppant coating.
• a wide variety of polymers can be used as coating for proppants of the present invention.
• the coating can be thermoset or thermoplastic and may formed in one or more layers that are the same, different, analogues or homologues of the other and any intervening proppant coating layers.
• Suitable polymeric coatings include resins based on polyurethane, polyurea-type, phenolic, epoxy, polycarbodiimide, or polyester resins.
• a preferred, multilayer proppant uses a first coating layer made from a precured phenolic coating with a second coating layer made with a polyurethane or polyurea-based coating (for providing interparticle bond strength). The reinforcing particulates of the present invention would be on or in the second coating layer.
• Particularly preferred proppant coatings as the inner and/or outer layers are those using polyurea-based or, with the use of a polyol, polyurethane-based polymers. See copending US patent application serial number 13/355,969, entitled “Manufacture of Polymer Coated Proppants.”
• the polyurea-type coating is preferably formed on the proppant from a dynamically reacting mixture that comprises an isocyanate, water and a curing agent (preferably an aqueous solution containing a curing agent or catalyst) that have been simultaneous contacted and mixed in the presence of the proppant core.
• the controlled rates of substantially simultaneous water and isocyanate are believed to allow the water to form a reactive amine species from the isocyanate, which newly- formed amine then reacts with other, unconverted isocyanate to form the desired polyurea-type coating directly on the outer surface of the proppant solid.
• the simultaneous contact among the ingredients forms a reacting mixture that polymerizes to form a thin, hard, substantially foam- free coating directly on the outer surface of the proppant core.
• the selection of different feed start times and rate for the isocyanate and water phase can be chosen to produce a gradient of polyurea-type polymers within in the coating.
• the reaction can proceed substantially to completion in less than about four minutes to form a hard, substantially fully-cured coating that does not require post-curing to form a tack- free or substantially tack-free outer surface.
• a polyurea-type coating can be formed on the proppant core by serially adding polyurea-type precursor components to the mixer.
• Such a process would likely need, however, sufficient agitation and mixing to avoid boundary layer effects from the first-added component that would cover the surface of the proppant core to a certain depth which might inhibit a complete reaction of all of the first material down to the surface of the proppant core solid.
• Sufficient agitation would be used to force the second component into the boundary layer of first component so that the first component boundary layer reacts downwardly from its outer surface towards the outer surface of the proppant core to form linkages that are tightly adhered to the proppant core surface.
• Tg glass transition temperature
• the Tg can be used as a guide to foretell whether a thermoplastic coating (such as the polyurethane and polyurea-based coating layers of the present invention) is potentially useable in the downhole conditions of a given fractured stratum. It is desirable that the Tg of the proppant coating be a temperature that is less than that prevailing downhole so that the thermoplastic coating has the ability to soften under prevailing combination of temperature and pressure.
• the Tg of the reinforcing particulates should, however, be higher than the prevailing downhole temperature so that the particulate does not soften or lessen its reinforcing effects.
• the Tg of the proppant coating is preferably greater than about 75° C but less than about 200° C and even more preferably within the range from about 100-165° C.
• the Tg of the proppant coating is desirably within the range of about 20°C to 60° C.
• the Tg values that are described can differ if one is describing a wet or dry Tg test. See US Patent Nos. 3,725,358; 5,310,825; and 2010/0222461 for testing to determine the wet Tg of a resin or material, i.e., performing the determination of Tg in a thermomechanical analyzer with water added to the sample container.
• a dry Tg could be in the range of 130-160° C, but in a wet test, it is difficult to measure a Tg that is above 1 10° C.
• the wet Tg preferably falls into the ranges described above to promote interparticle bonding without the use of an external activator.
• the isocyanate-functional component for the coatings of the present invention comprises an isocyanate-functional component with at least 2 reactive isocyanate groups.
• Other isocyanate-containing compounds may be used, if desired.
• suitable isocyanate with at least 2 isocyanate groups an aliphatic or an aromatic isocyanate with at least 2 isocyanate groups (e.g. a diisocyanate, triisocyanate or tetraisocyanate), or an oligomer or a polymer thereof can preferably be used.
• These isocyanates with at least 2 isocyanate groups can also be carbocyclic or heterocyclic and/or contain one or more heterocyclic groups.
• the isocyanate-functional component with at least 2 isocyanate groups is preferably a compound, polymer or oligomer of compounds of the formula (III) or a compound of the formula (IV):
• A is each, independently, an aryl, heteroaryl, cycloalkyl or heterocycloalkyl.
• A is each, independently, an aryl or cycloalkyl. More preferably A is each, independently, an aryl which is preferably phenyl, naphthyl or anthracenyl, and most preferably phenyl. Still more preferably A is a phenyl.
• heteroaryl is preferably a heteroaryl with 5 or 6 ring atoms, of which 1, 2 or 3 ring atoms are each, independently, an oxygen, sulfur or nitrogen atom and the other ring atoms are carbon atoms. More preferably the heteroaryl is selected among pyridinyl, thienyl, furyl, pyrrolyl, imidazolyl, pyrazolyl, pyrazinyl, pyrimidinyl, pyridazinyl, oxazolyl, isoxazolyl or furazanyl.
• cycloalkyl is preferably a C 3-1 o-cycloalkyl, more preferably a C5-7-cycloalkyl.
• heterocycloalkyl is preferably a hetero cycloalkyl with 3 to
• ring atoms (more preferably with 5 to 7 ring atoms ), of which one or more (e.g. 1 , 2 or 3 ) ring atoms are each, independently, an oxygen, sulfur or nitrogen atom and the other ring atoms are carbon atoms.
• the heterocycloalkyl is selected from among tetrahydrofuranyl, piperidinyl, piperazinyl, aziridinyl, acetidinyl, pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl, tetrahydrothienyl, octahydroquinolinyl, octahydroisoquinolinyl, oxazolidinyl or isoxazolidinyl. Still more preferably, the heterocycloalkyl is selected from among tetrahydrofuranyl, piperidinyl, piperazinyl, aziridinyl, acetidinyl, pyrrolidinyl, imidazolidinyl, morpholinyl, pyrazolidinyl, tetrahydrothienyl, octahydroquinolinyl, octahydroisoquinolinyl
• each R 1 is, independently, a covalent bond or i-4- alkylene (e.g. methylene, ethylene, propylene or butvlene).
• each R 2 is hydrogen or a covalent bond.
• each R 2 is each, independently, hydrogen, a halogen (e.g. F, CI, Br or I), a Ci-4-alkyl (e.g. methyl, ethyl, propyl or butyl) or Ci-4-alkyoxy (e.g. methoxy, ethoxy, propoxy or butoxy).
• a halogen e.g. F, CI, Br or I
• a Ci-4-alkyl e.g. methyl, ethyl, propyl or butyl
• Ci-4-alkyoxy e.g. methoxy, ethoxy, propoxy or butoxy
• each R 2 is, independently, hydrogen or a Ci-4- alkyl. More preferably each R 2 is hydrogen or methyl.
• R 3 is a covalent bond, a Ci-4-alkylene (e.g. methylene, ethylene, propylene or butvlene) or a group -(CH2)R 3 I-0-(CH2)R32-, wherein R31 and R32 are each, independently, 0, 1 , 2 or 3.
• R 3 is a -CH 2 - group or an -O- group.
• the average value of p is greater than or equal to 2, preferably greater than 2, and more preferably within the range of 2.05 and up to 3.
• each q is, independently, an integer from 0 to 4, preferably 0, 1 or 2.
• the corresponding group A has no substituent R 2 , but has hydrogen atoms instead of R 2 .
• each r and s are, independently, 0, 1, 2, 3 or 4, wherein the sum of average values of r and s is greater than 2.
• each the average of r and s are preferably greater than 2, and more preferably within the range of 2.05 and up to 3..
• Examples of the isocyanate with at least 2 isocyanate groups are: toluol-2,4- diisocyanate; toluol-2,6-diisocyanate; 1,5-naphthalindiisocyanate; cumol-2,4-diisocyanate; 4- methoxy-1 ,3-phenyldiisocyanate; 4-chloro-l,3-phenyldiisocyanate; diphenylmethane-4,4- diisocyanate; diphenylmethane-2,4-diisocyanate; diphenylmethane-2,2-diisocyanate; 4-bromo- 1,3 -phenyl diisocyanate; 4-ethoxy-l,3-phenyl-diisocyanate; 2,4' -diisocyanate diphenylether; 5,6- dimethyl-l,3-phenyl-diisocyanate; methylenediphenyl diisocyanate
• polymeric isocyanates can be used in the present invention. Suitable examples include polymers and oligomers of diphenylmethane diisocyanates (MDIs and pMDIs), toluene diisocyanates (TDIs), hexamethylene diisocyanates (HDIs), isophorone diisocyanates (IPDIs), and combinations thereof.
• the preferred polymeric isocyanate for use in the present invention is polymers and oligomers based on diphenylmethane diisocyanates.
• Particularly preferred isocyanates with at least 2 isocyanate groups are toluol diisocyanate, methylenediphenyl diisocyanate, diphenylmethane diisocyanate, an oligomer based on toluol diisocyanate, an oligomer based on methylenediphenyl diisocyanate (poly-MDI) or an oligomer based on diphenylmethane diisocyanate and polymers thereof.
• a polyol component with polyhydroxy functionality is one of the components used in making a polyurethane coating on proppant solids in a process according to the invention, and it may be applied as the first component or the second component.
• the polyol component has two or more functional, hydroxyl moieties (such as diols, triols and higlier polyol functionality based on starter molecules like glycerine, trimethylolpropane, sorbitol, methyl glucoside and sucrose) excluding hydroxyl groups associated with carboxylic acids and may or may not have reactive amine functionality.
• Preferred polyhydroxyl polyols include polyethers (such as polyoxypropylene diols and triols), polyesters, aliphatic polyols, aromatic polyols, mixtures of aliphatic and aromatic polyols, synthetic polyols, polyhydroxyoligomers (see US 4554188 and 4465815, the disclosures of which are hereby incorporated by reference), natural oil polyols (such as cashew nut oil and castor oil) and natural oils that have been treated to introduce polyhydroxyl content in place of unsaturated bonds such as oxidized soybean oil, oxidized peanut oil, and oxidized canola oil such as polyols produced from biomass.
• polyethers such as polyoxypropylene diols and triols
• polyesters such as polyoxypropylene diols and triols
• aliphatic polyols such as polyoxypropylene diols and triols
• aromatic polyols such as mixtures of aliphatic and aromatic polyo
• a preferred polyurethane coating is made with a polyol mixture that includes 5-
• polyether 100 wt% of one or more polyether, polyester, aliphatic and/or polyhydroxyoligomers polyols and 0-95 wt% of an aromatic polyol.
• An especially preferred polyol is a polyetherpolyol containing 0-5 wt% castor oil.
• the polyol component is a phenol resin with monomer units based on cardol and/or cardanol.
• Cardol and cardanol are produced from cashew nut oil which is obtained from the seeds of the cashew nut tree.
• Cashew nut oil consists of about 90% anacardic acid and about 10% cardol.
• By heat treatment in an acid environment a mixture of cardol and cardanol is obtained by decarboxylation of the anacardic acid.
• Cardol and cardanol have the structures shown below:
• Cardol specifically refers to compound CAS-No. 57486-25-6 and cardanol specifically to compound CAS-No. 37330-39-5.
• Cardol and cardanol can each be used alone or at any particular mixing ratio in the phenol resin. Decarboxylated cashew nut oil can also be used. [0075] Cardol and/or cardanol can be condensed into the above described phenol resins, for example, into the resole- or novolak-type phenol resins. For this purpose, cardol and/or cardanol can be condensed e.g. with phenol or with one or more of the above defined compounds of the formula (I), and also with aldehydes, preferably formaldehyde.
• the amount of cardol and/or cardanol which is condensed in the phenol resin is not particularly restricted and preferably is from about 1 wt% to about 99 wt%, more preferably about 5 wt% to about 60 wt%, and still more preferably about 10 wt% to about 30 wt%, relative to 100 wt% of the amount of phenolic starting products used in the phenol resin.
• the polyol component is a phenol resin obtained by condensation of cardol and/or cardanol with aldehydes, preferably formaldehyde.
• this kind of long-chain, substituted phenol resin is comparatively hydrophobic, which results in a favorable shelf life of the coated proppants obtained by the method according to the present invention.
• a phenol resin of this kind is also advantageous because cardol and cardanol are renewable raw materials.
• the polyol component can still contain other compounds containing hydroxyl groups.
• the other compounds containing hydroxyl groups can be selected from the compounds containing hydroxyl groups that are known to be useful for making polyurethanes, e.g., hydroxy-functional polyethers, hydroxy-functional polyesters, alcohols or glycols.
• One preferred compound containing hydroxyl groups is, for instance, castor oil.
• Compounds containing hydroxyl groups such as alcohols or glycols, in particular cardol and/or cardanol, can be used as reactive thinners.
• the coatings of the invention can be cured with at least one of a variety of curing agents, including reactive, non-reactive (e.g., "catalysts") and partially reactive agents that facilitate the formation of polyurea-type linkages.
• the preferred curing agents are selected from the amine-based curing agents and are added to the reacting mixture of polyurea- type precursors at a total amount within the range from about 0.0001% to about 30 total wt%.
• the amine-based curing agents may also be used as a mixture of a fast-acting first curing agent and a second, latent curing agent if additional crosslinlang ability is desired to take advantage of downhole heat and pressure conditions. Either of these first and/or second amine-based curing agents may be reactive, nonreactive or partially reactive. If the amine curing agent is reactive, however, the amine is preferably chosen to favor the formation of polyurea by reaction with the isocyanate.
• Suitable single amine-based curing agents, catalysts or a mixture of amine-based curing agents for promoting the formation of polyurea can include, but are not limited to, 2,2'- dimorpholinodiethyl ether; bis-dimethylaminoethylether ; ethylene diamine; hexamethylene diamine; l-methyl-2,6-cyclohexyl diamine; 2,2,4- and 2,4,4-trimethyl-l,6-hexanediamine; 4,4'- bis-(sec-butylamino)-dicyclohexylmethane and derivatives thereof; 1 ,4-bis-(sec-butylamino)- cyclohexane; l,2-bis-(sec-butylamino)-cyclohexane; 4,4'-dicyclohexylmethane diamine; 1,4- cyclohexane-bis-(methylamine); l,3-cyclohexane-bis
• diaminocyclohexane isomers, and mixtures thereof; diethylene triamine; triethylene tetramine; tetraethylene pentamine; propylene diamine; 1 ,3-diaminopropane; dimethylamino propylamine; diethylamino propylamine; imido-bis-(propylamine); monoethanolamine, diethanolamine; triethanolamine; monoisopropanolamine, diisopropanolamine; isophoronediamine; 4,4'- methylenebis-(2-chloroaniline); 3,5-dimethylthio-2,4-toluenediamine; 3,5-dimethylthio-2,6- toluenediamine; 3,5-diethylthio-2,4-toluenediamine; 3,5-diethylthio-2,6-toluenediamine; 4,4'- bis-(sec-butylamino)-benzene; and derivatives thereof; l,4-bis-
• paraphenylenediamine ⁇ , ⁇ '-diisopropyl-isophoronediamine
• polyoxypropylene diamine polyoxypropylene diamine
• the amine-terminated curing agent is 4,4'-bis-(sec-butylamino)- dicyclohexylmethane.
• Preferred amine-based curing agents and catalysts that aid the -NCO- and water reaction to form the polyurea-type links for use with the present invention include triethylenediamine; bis(2-dimethylaminoethyl)ether; tetramethylethylenediamine;
• catalysts that promote the reaction of isocyanates with hydroxyls and amines that are known by the industry can be used in the present invention, e.g., transition metal catalysts of Groups III or IV used for polyurea-type foams.
• Particularly preferred metal catalysts include duibutyltin dilaurate that can be added to the water or polyol feeds for co- introduction during the coating process.
• catalysts that promote isocyanate trimerization over other reaction mechanisms. See, e.g., US Patent No. 5,264,572 (cesium fluoride or
• the amine-based curing agent may have a molecular weight of about 64 or greater. In one embodiment, the molecular weight of the amine-curing agent is about 2000 or less and is a primary or secondary amine. Tertiary amines will not generally be used as a reactant for forming polyurea-type coatings unless reactivity is provided by additional functionality, e.g., such as with triethanolamine.
• the saturated amine-based curing agents suitable for use to make polyurea-type coatings according to the present invention include, but are not limited to, ethylene diamine; hexamethylene diamine; l-methyl-2,6-cyclohexyl diamine; 2,2,4- and 2,4,4-trimethyl- 1 ,6-hexanediamine; 4,4'-bis-(sec-butylamino)-dicyclohexylmethane; 1 ,4-bis-(sec-butylamino)- cyclohexane; 1 ,2-bis-(sec-butylamino-cyclohexane; derivatives of 4,4'-bis-(sec-butylamino)- dicyclohexylmethane; 4,4'-dicyclohexylmethane diamine; 1 ,4-cyclohexane-bis-(methylamine); l ,3-cyclohexane-bis-(methylamine); di
• isophoronediamine ⁇ , ⁇ '-diisopropylisophorone diamine and mixtures thereof.
• the curative used with the prepolymer include 3,5- dimethylthio-2,4-toluenediamine,3,5-dimethyl-thio-2,6-toluenediamine, 4,4'-bis-(sec- butylamino)-diphenyhiiethane, ⁇ , ⁇ '-diisopropyl-isophorone diamine; polyoxypropylene diamine; propylene oxide-based triamine; 3,3'-dimethyl-4,4'-diaminocyclohexylmethane; and mixtures thereof.
• a hindered secondary diamine may be more suitable for use.
• an amine with a high level of stearic hindrance e.g., a tertiary butyl group on the nitrogen atom, has a slow r er reaction rate than an amine with no hindrance or a low level of hindrance and further adds to the hydrolytic and thermal stability of the final product.
• 4,4'-bis-(sec- butylamino)-dicyclohexylmethane (CLEARLI 1000® from Huntsman Corporation in The Woodlands, Texas) may be suitable for use in combination with an isocyanate to form the polyurea-type coating
• ⁇ , ⁇ '-diisopropyl-isophorone diamine also available from Huntsman Corporation, under the tradename JEFFLINK®, may be used as the secondary diamine curing agent.
• a trifunctional curing agent can be used to help improve cross-linking and, thus, to further improve the chemical and/or abrasion resistance of the coating.
• a diethylene triamine or triethylene tetramine are both highly reactive and are desirably added to the coating process with water.
• the curing agents of the present invention can be added to the coating
• the curing agent and the reinforcing particulates are co-applied with water at substantially the same time that isocyanate is added to form the proppant coating.
• the proppant coating compositions of the invention may also include various additives that change its appearance, properties, handling characteristics or performance as a proppant or in fracturing or breaker fluids.
• the coatings of the invention may also include pigments, tints, dyes, and fillers in an amount to provide visible coloration in the coatings.
• reaction rate enhancers or catalysts include, but are not limited to, reaction rate enhancers or catalysts, crosslinking agents, optical brighteners, propylene carbonates, coloring agents, fluorescent agents, whitening agents, UV absorbers, hindered amine light stabilizers, defoaming agents, processing aids, mica, talc, nanometer-sized fillers that add an additional function to the proppant, silane coupling agents (such as those in US Patent No.
• Adhesion promoter agents can be used to increase the bond strength between the outer surface of the proppant core solid and any applied coating.
• An adhesion promoter can also be used at the outer surface or outside of the outermost coating layer to enhance adhesion between adjacent proppants. See copending US application serial number 13/897,288 entitled “Proppant With Enhanced Interparticle Bonding", the disclosure of which is hereby incorporated by reference.
• the adhesion promoter for enhancing the bond between the proppant core solid and the applied polymeric coating may be the same or different than the adhesion promoter that might be added to help bond the added reinforcing particulates into the polymeric coating.
• the adhesion promoter for a nonfunctionalized particulate for both the core-polymer bonding as well as the polymer- reinforcing particulate bond can be added at the beginning of the coating process, throughout the coating process or towards the end of the coating process.
• An especially preferred treatment for the cured proppant is to use an anticaking to enhance the handling characteristics of the proppants.
• Suitable anticaking agents include amorphous silica (e.g., silica flour, fumed silica and silica dispersions) and silica alternatives (such as those used in sandblasting as an alternative to silica or organofunctional silane like the DYNASYLAN fluids from Evonik Degussa Corporation in Chester, PA). These materials are applied to the outer surfaces of the coated proppant solid to prevent the formation of
• An optional additional additive to the coating or in particulates blended with the proppants of the present invention is a contaminant removal component that will remove, sequester, chelate or otherwise clean at least one contaminant, especially dissolved or otherwise ionic forms of heavy metals and naturally occurring radioactive materials (NORMS), from subterranean water or hydrocarbon deposits within a fractured stratum while also propping open cracks in said fractured stratum.
• NORMS radioactive materials
• the contaminant removal component is associated with the proppant solid as a chemically distinct solid that is introduced together with the proppant solid as: (a) an insoluble solid secured to the outer or inner surface of the proppant solid with a coating formulation that binds the solids together, (b) as a solid lodged within pores of the proppant solid or (c) as a chemical compound or moiety that is mixed into or integrated with a coating or the structure of the proppant solid.
• Additional added functionality can also be in the form of fracture fluid breakers, de-emulsifiers, and bactericides.
• an auxiliary particle to the proppant may also be in the form of an ion exchange resin that is pretreated or which itself constitutes a dissolvable solid for the slow release of corrosion or scale inhibitors.
• Such slow release materials could prove beneficial and advantageous to the overall operation and maintenance of the well.
• the proppants can be virtually any small solid with an adequate crush strength and lack of chemical reactivity. Suitable examples include sand, ceramic particulates (such as aluminum oxide, silicon dioxide, titanium dioxide, zinc oxide, zirconium dioxide, cerium dioxide, manganese dioxide, iron oxide, calcium oxide, magnesium oxide, or bauxite), or also other granular materials.
• sand such as aluminum oxide, silicon dioxide, titanium dioxide, zinc oxide, zirconium dioxide, cerium dioxide, manganese dioxide, iron oxide, calcium oxide, magnesium oxide, or bauxite
• ceramic particulates such as aluminum oxide, silicon dioxide, titanium dioxide, zinc oxide, zirconium dioxide, cerium dioxide, manganese dioxide, iron oxide, calcium oxide, magnesium oxide, or bauxite
• Proppant sands are a preferred type of proppant for the present invention.
• Sand is mainly used in the hydraulic fracturing process of natural gas and oil wells to increase their productivity of valuable natural resources.
• Proppant sand is monocrystalline with a high silica content of at least 80 wt%, and more typically has a silica content of greater than about 97 wt% silica.
• the American Petroleum Institute specifications place the following limitations on sieve distribution for proppants suitable for use in hydraulic fracturing:
• No more than 0.1% of the material may be coarser than the next largest mesh size, e.g. for 20/40, up to 10% of the proppant may be between 16 and 20 mesh, but no more than 0.1% can exceed 16 mesh, and
• Proppants are divided into low-density, medium density, high-density when determined in bulk.
• Proppant crush strengths are divided into 52 MPa, 69 MPa, 86 MPa and 103 MPa series.
• the size specifications of proppant sand are generally 12-18 mesh, 12-20 mesh, 16- 20 mesh, 16-30 mesh, 20-40 mesh, between 30-50 mesh, 40-60 mesh, 40-70 mesh and smaller.
• the proppants to be coated preferably have an average particle size within the range from about 50 ⁇ and about 3000 ⁇ , and more preferably within the range from about 100 ⁇ to about 2000 ⁇ .
• the coating process of the present invention preferably produces a polyurethane or polyurea-type coating on the proppant core solids that is hard, durable and resists dissolution under the rigorous combination of high heat, agitation, abrasion and water found downhole in a fractured subterranean formation.
• the cured coating exhibits a sufficient resistance (as reflected by a 10 day autoclave test or 10 day conductivity test) so that the coating resists loss by dissolution in hot water (“LOI loss") of less than 25 wt%, more preferably less than 15 wt%, and even more preferably a loss of less than 5 wt%.
• LOI loss dissolution in hot water
• the substantially cured coating of the invention thus resists dissolution in the fractured stratum while also exhibiting sufficient consolidation and resistance to flow back without the use of an added bonding activator while also exhibiting sufficiently high crush strength to prop open the fractures and maintain their conductivity for extended periods.
• the temperature of the coating process is not particularly restricted outside of practical concerns for safety and component integrity.
• the preferred conditions for the coating'' curing step of the present invention are generally at conditions within the range of about 50° to about 225° C, more preferably at a temperature within the range from about 75° C to about 150° C, and most preferably at a temperature within the range from about 80° C to about 135° C. As noted above, this temperature is conveniently achieved by heating or using heated proppant solids.
• the preferred temperature range avoids a number of emissions issues, reduces the amount of energy consumed in the coating process and also reduces the cooling time for the coated proppants for further handling and packaging.
• Mixing can be carried out on a continuous or discontinuous basis in series or in several runs with a single mixer, but the specific mixer used to coat the proppants is not believed to be critical for the present invention.
• Suitable mixers include tumbling-type mixers, fluid beds, a pug mill mixer or an agitation mixer can be used.
• a drum mixer, a plate-type mixer, a tubular mixer, a trough mixer or a conical mixer can be used. The easiest way is mixing in a rotating drum.
• a worm gear can, for example, be used.
• a preferred mixer type is a tumbling-type mixer that uses a rotating drum driven by an electrical motor.
• the load on the motor can be used as a measure of the viscosity of the tumbling solids and the degree to which they are forming agglomerates or resinous deposits inside the mixer: the electrical load on the motor increases as the agglomeration and fouling increase.
• Adding water to the mixing solids or adding one or more of the polyurea precursor components in an aqueous solution, emulsion or suspension can help to reduce this load increase and retain the free-flowing nature of the mixing solids, thereby enabling even larger productivity from the mixer.
• water is preferably added to the isocyanate at a rate sufficient to form a reactive amine species which then reacts almost immediately with adjacent isocyanate to form polyurea.
• water and an isocyanate-containing component are used in an amount within the range from about 5-30% water, 95-70% ISO consistent with the demands of the catalyst to promote the hydrolysis of the ISO and temperature of the substrate during the timed additions onto the proppant substrate.
• the water and isocyanate are added at a rate sufficient to maintain a proportion of 5-30 to 95-70 so as to promote the in-situ formation of a reactive amine component from the isocyanate which then reacts with unconverted isocyanate to make the polyurea-type coating of the present invention.
• Most of the components for the coating are preferably added along with either the water or the isocyanate to facilitate proper mixing and metering of the components.
• a silane adhesion promoter is added to the heated sand or among the initial steps of the coating process.
• a colorant is added during the coating process by an injection line into the coating mixer.
• a last step includes adding a suspension of reinforcing particulates as the polymeric components are reacting and curing.
• a surfactant and/or flow aid can be added after the proppants have been coated to enhance wettability and enhanced flow properties with lower fines generation, respectively.
• the method for the production of coated proppants according to the present invention can be implemented without the use of solvents. Accordingly, the mixture obtained in step (a) in one embodiment of the method is solvent-free, or is essentially solvent-free.
• the mixture is essentially solvent-free, if it contains less than 20 wt%, preferably less than 10 wt%, more preferably less than 5 w r t%, and still more preferably less than 3 wt%, and most preferably less than 1 wt % of solvent, relative to the total mass of components of the mixture.
• the coating is preferably performed at the same time as the curing of the coating on the proppant.
• the coated proppant becomes free-flowing at a time of less than 5 minutes, preferably within the range of 1-4 minutes, more preferably within the range of 1-3 minutes, and most preferably within the range of 1-2 minutes to form a coated, substantially cured, free-flowing, coated proppant.
• This short cycle time combines with the relatively moderate coating temperatures to form a coating/curing process that provides lower energy costs, smaller equipment, reduced emissions from the process and the associated scrubbing equipment, and overall increased production for the coating facility.
• the coating material or combinations of different coating materials may be applied in more than one layer.
• the coating process may be repeated as necessary (e.g. 1-5 times, 2-4 times or 2-3 times) to obtain the desired coating thickness. Any or all of these coatings may contain the reinforcing particulates of the present invention.
• the reinforced coating of the present invention can be applied as the outermost layer over, e.g., a precured or curable phenolic coating, to take advantage of the underlying properties of the phenolic coating witile adding the bonding ability of the
• polyurethane or polyurea-type coating Such an outer coating would avoid the need for an added activator or surfactant compounds that are typically required for the phenolic coatings and thereby also avoid the potential for chemical incompatibility or interference with the formulated fracturing or breaker fluids used in hydraulic well fracturing.
• a typical size range for the final, coated proppant is desirably within the range of about 16 to about 100 mesh.
• the reinforced coating of the present invention can also be applied to a previously coated proppant or formed as an outermost "skin" layer of a substantially continuous coating.
• This skin layer of reinforced coating reduces any residual surface tackiness or unreacted moieties remaining after the coating reactions and reduces deformation of the resulting proppant coating.
• This skin is formed by waiting until less than 20%, preferably less than 10% of the time remaining in the coating and curing process remains before adding water to the process. See our copending US patent application serial number 13/355,969 filed on 23 January 2012 entitled “Manufacture of Polymer Coated Proppants", the disclosure of which is hereby incorporated by reference.
• the amount of added water, independent of any reinforcing particulates, should be small, i.e., less than 10 wt%, preferably less than 5 wt% of the total proppant mixture and just enough to maintain a free-flowing mixture without forming a slurry.
• the small amount of water is believed to encourage remaining unreacted isocyanate moieties to react and form a polyurea-type skin coating on the surface of the underlying proppant.
• a polyurea can be formed as the basecoat, followed by a reinforced topcoat of a phenolic, or epoxy, polyurethane or other coating.
• a reinforced topcoat of a phenolic, or epoxy, polyurethane or other coating.
• any or all of these can include functionalized reinforcing particulates or, with an adhesion promoter, non-functionalized particulates.
• the amount of coating resin is preferably between about 0.5 and about 10 wt%, more preferably between about 1% and about 5 w r t%, resin relative to the mass of the proppant as 100 wt%.
• proppants can be coated at temperatures between about 50°C and about 225°C, preferably within the range of about 75°- 125° C and preferably in a solvent-free manner.
• the coating process requires a comparatively little equipment and if necessary can also be carried out near the sand or ceramic substrate source, near the geographically location of the producing field or at/near the well itself.
• the coated proppants can additionally be treated with surface -active agents, anticaking agents, or auxiliaries, such as talcum powder or stearate or other processing aids such as fine amorphous silica to improve pourability, wettability (even to the extent that a water wetting surfactant can be eliminated), dispersability, reduced static charge, dusting tendencies and storage properties of the coated product.
• auxiliaries such as talcum powder or stearate or other processing aids such as fine amorphous silica to improve pourability, wettability (even to the extent that a water wetting surfactant can be eliminated), dispersability, reduced static charge, dusting tendencies and storage properties of the coated product.
• the coated proppants can be baked or heated for a period of time sufficient to further enhance the ultimate performance of the coated particulates and further react the available isocyanate, hydroxyl and reactive amine groups that might remain in the coated proppant.
• Such a post-coating cure may occur even if additional contact time with a catalyst is used after a first coating layer or between layers.
• the post-coating cure step is performed like a baking step at a temperature within the range from about 100° - 200° C for a time of about 1 minute to 4 hours, preferably the temperature is about 125° - 200° C for about 1-30 minutes.
• the coated proppant is cured for a time and under conditions sufficient to produce a coated proppant that exhibits a loss of coating of less than 25 wt%, preferably less than 15 wt%, and even more preferably less than 5 wt% when tested according to simulated downhole conditions under ISO 13503-5 :2006(E).
• the coated proppant of the present invention exhibits the low dust and handling characteristics of a conventional pre-cured proppant (see API RP 60) but also exhibits a crush test result at 10,000 psi of less than 10%, more preferably less than 5%, and especially less than 2%.
• coated proppants of the invention preferably also have an unconfmed compressive strength of greater than 20 psi and more preferably more than 500 psi with a fracture conductivity at a given closure stress that is substantially equal to, or greater than, the conductivity of a phenolic coating used in the same product application range.
• the invention also includes the use of the coated proppants in conjunction with a fracturing liquid to increase the production of petroleum or natural gas.
• Techniques for fracturing an unconsolidated formation that include injection of consolidating fluids are also well known in the art. See U.S. Patent No. 6,732,800 the disclosure of which is herein incorporated by reference.
• a fluid is injected through the wellbore into the formation at a pressure less than the fracturing pressure of the formation.
• the volume of consolidating fluid to be injected into the formation is a function of the formation pore volume to be treated and the ability of the consolidating fluid to penetrate the formation and can be readily determined by one of ordinary skill in the art.
• the formation volume to be treated relates to the height of the desired treated zone and the desired depth of penetration, and the depth of penetration is preferably at least about 30 cm radially into the formation.
• the consolidation fluid is injected through the perforations, the treated zone actually stems from the aligned perforations.
• the fracturing liquid is not particularly restricted and can be selected from among the fracturing liquids known in the specific field. Suitable fracturing liquids are described, for example, in WC Lyons, GJ Plisga, "Standard Handbook Of Petroleum And Natural Gas
• the fracturing liquid can be, for example, liquefied petroleum gas (LPG), water gelled with polymers, an oil-in-water emulsion gelled with polymers, or a water- in-oil emulsion gelled with polymers.
• LPG liquefied petroleum gas
• the fracturing liquid comprises the following constituents in the indicated proportions: 1000 1 water, 20 kg potassium chloride, 0.120 kg sodium acetate, 3.6 kg guar gum (water-soluble polymer), sodium hydroxide (as needed) to adjust a pH-value from 9 to 1 1, 0.120 kg sodium thiosulfate, 0.180 kg ammonium persulfate and optionally a crosslinker such as sodium borate or a combination of sodium borate and boric acid to enhance viscosity.
• a crosslinker such as sodium borate or a combination of sodium borate and boric acid to enhance viscosity.
• the invention relates to a method for the production of petroleum or natural gas which comprises the injection of the coated proppant into the fractured stratum with the fracturing liquid, i.e., the injection of a fracturing liquid which contains the coated proppant, into a petroleum- or natural gas-bearing rock layer, and/or its introduction into a fracture in the rock layer bearing petroleum or natural gas.
• the method is not particularly restricted and can be implemented in the manner known in the specific field.
• the concentration of proppant in the fracturing fluid can be any concentration known in the art, and will typically be in the range of about 0.5 to about 20 pounds of proppant added per gallon of clean fluid.
• the fracturing fluid can contain an added proppant-retention agent, e.g. a fibrous material, a curable resin coated on the proppant, platelets, deformable particulates, or a sticky proppant coating to trap proppant particulates in the fracture and prevent their production through the wellbore.
• Fibers in concentration that preferably ranges from about 0.1 % to about 5.0% by weight of proppant, for example selected from natural organic fibers, synthetic organic fibers, glass fibers, carbon fibers, ceramic fibers, inorganic fibers, metal fibers and mixtures thereof, in combination with curable resin-coated proppants are particularly preferred.
• the proppant- retention agent is intended to keep proppant solids in the fracture, and the proppant and proppant-retention agent keep formation particulates from being produced back out from the well in a process known as "flowback.”
• Table 1 shows a sequence of actions, times of addition and ingredients for making a reinforced, urea-type proppant coating that takes advantage of water used for urea formation to incorporate a dispersion of functionalized silica into the coating in a substantially even distribution throughout the polymeric coating.
• Table 2 shows a sequence of actions, times of addition and ingredients for making a reinforced urea-type proppant coating that takes advantage of water used for urea formation to incorporate a dispersion of non-functionalized silica into the coating.
• the use of the present invention enables chemical integration of reinforcing particulates into the proppant coating to form a reinforced, hybrid coating.
• This hybrid should be harder and outperform what might be expected by adding only a silica "filler" into the proppant coating that is merely dispersed in the coating but is not otherwise grafted into the polymer or chemically bound to it. This suggests that ability to adjust and control the amount of proppant deformation and conductivity characteristics that are exhibited by the proppant to more closely tailor the proppant to the demands of the fractured field with only minor adjustments to the coating process and formulation.
• the advantages of using a waterborne dispersion of reinforcing particulates include: (a) the tolerance of urea-type and polyurethane polymers for the water used in the dispersion, and (b) the benefit of dealing with discrete very small, nanometer-size particulates to provide high surface area for chemical interactions with the developed and reacting polymers of the coating. [0124] Some possible reasons for how and/or why the invention w r orks as well as it does might include:
• the amount of water used is determined by considering the efficiency of the water/catalyst mix while also avoiding excessive cooling by water loss;
• the urea reaction accommodates the presence of water, it is equally tolerant of a water dispersion containing other additives, such as functionalized reinforcing particulates, especially silica, while the blowing catalyst promotes the reaction of -NCO groups with hydroxyl functionalities from the surface of hydrated or adhesively promoted silica or chemically modified silica.

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