WO2020185373A1

WO2020185373A1 - Coated proppants

Info

Publication number: WO2020185373A1
Application number: PCT/US2020/019428
Authority: WO
Inventors: Sachit GOYAL; Arjun RAGHURAMAN; Thomas P. WILLUMSTAD
Original assignee: Dow Global Technologies Llc
Priority date: 2019-03-11
Filing date: 2020-02-24
Publication date: 2020-09-17
Also published as: AR118189A1

Abstract

Proppant particles are made by coating the particles with a coating composition that contains a polyisocyanate, followed by applying and reacting a compound that is isocyanate-reactive and also has acrylate or methacrylate groups. The resulting proppant particles bond together when emplaced in a subterranean formation to form a proppant pack that resists flowback. Good bonding is seen even at somewhat low formation temperatures.

Description

COATED PROPPANTS

This invention relates to proppants and methods of making proppants.

Oil and natural gas are obtained by drilling into subterranean reservoirs. Often, the oil and gas products are trapped within a formation that has low porosity and low permeability and for that reason the products cannot be extracted easily. Formations such as these often are hydraulically fractured by pumping fluids into them at high pressure and velocity. Trapped oil and gas are released from the fractured formation. The fracturing forms flow channels through which those products can travel into the well bore, from which they can be extracted.

Because of high localized pressures, those fractures and fissures tend to close when the hydraulic fluid pressure is released. This shuts off the flow channels, reducing or eliminating the flow of product to the well bore. To avoid this problem, proppants often are injected into the well along with the hydraulic fracturing fluid. The proppants are solid materials that occupy space in the fractures and thus prevent them from becoming closed off when the hydraulic fluid pressure is released. The proppants are in the form of small particles. Sand is widely used because it is readily available, inexpensive and has a suitable particle size. Even though the proppant particles occupy space within the fractures, there is room between them for the oil and gas products to flow.

The flow of oil and gas can wash the proppant out of the formation and back into the well, a phenomenon known as “proppant flowback” . This is undesirable because the fractures partially or entirely close once the proppant is washed away, leading to decreased production rates and downtime The proppant needs to be separated from the product, as well. The proppants, especially silica sand, are abrasive and can damage submersible pumps and other equipment if they are washed back to the wellbore.

A common way to reduce proppant flowback is by applying a polymeric coating to the particles. At the temperature and pressure conditions in the well, the polymeric coating causes the particles to stick together and also to the underlying rock formation. This makes the particles more resistant to being washed out of the fractures without rendering the formation unduly impermeable to the flow of oil and gas out of the well. The coating also improves crush resistance in some cases. Being softer and more deformable than the underlying proppant particle, the polymeric coating can deform under load and thus absorb damaging stresses, help to distribute stresses uniformly through a proppant pack and minimize point stresses that can otherwise cause the proppant particles to fracture.

Among the polymers that have been used for this purpose are phenolic resins, various epoxy resins, and isocyanate-based polymers that have urethane, urea, carbodiimide, isocyanurate and other linkages formed in a reaction of an isocyanate group. Polymer- coated proppants of this type are described, for example, in WO 2017/003813, US Published Patent Application Nos. 2008- 0072941 and 2016-0186049 and US Patent Nos. 9,290,690, 9,725,645, 9,896,620 and 9,714,378. Deformable proppants made from certain acrylate terpolymers are described in US 7,322,411.

US 2013-0081812 describes proppant particles having a polymeric coating and a“surface wettability modifier” disposed on the polymeric coating. The surface wettability modifier imparts hydrophobic or hydrophilic properties to the proppant particles to affect surface wettability.

While good performance has been obtained in some cases, these polymer systems often perform inadequately at lower temperatures (such as 60° C or lower) due to poor bonding between the particles. This restricts their use to higher-temperature subterranean formations. There remains a need to provide proppants that bond together to form a proppant pack that is highly resistant to flowback, even when used in lower temperature formations.

This invention is a method for forming a coated proppant, comprising a) applying a coating composition to the surface of solid substrate particles, wherein the solid substrate particles are thermally stable to a temperature of at least 100°C, and wherein the coating composition comprises at least one polyisocyanate;

b) at least partially curing the coating composition to form on the surface of the solid substrate particles a polymeric coating that has free isocyanate groups and linkages selected from one or more of urethane, urea, carbodiimide or isocyanurate linkages;

c) applying at least one isocyanate-reactive (meth) acrylate- functional compound to the polymeric coating and reacting isocyanate- reactive groups of the isocyanate-reactive (meth) acrylate functional compound with free isocyanate groups of the polymeric coating to produce free (meth)acrylyl groups on the polymeric coating.

The process has several important advantages. The coating process is facile and rapid. It often can be performed at moderate temperatures, which reduces energy requirements. Free-flowing or at most lightly agglomerated coated proppant particles are produced. The coated particles handle easily during packaging, transportation and use. Once emplaced within a subterranean formation, the particles pack well and bond well to each other, even at temperatures below 60°C. A proppant pack made up of the coated proppant particles bonded together in such a manner is resistant to proppant flowback.

The invention is also a coated proppant particle made using the method.

In particular embodiments, the invention is a coated proppant particle comprising a substrate particle having a polymeric coating that weighs 0.1 to 10 percent of the weight of the substrate particle, wherein the polymeric coating has linkages selected from one or more of urethane, urea, or isocyanurate linkages, and free (meth)acrylyl groups chemically bonded to the polymeric coating. The polymeric coating may also contain free isocyanate groups,

The invention is also a method of hydraulically fracturing a subterranean formation, comprising injecting a carrier fluid and coated proppant particles of the invention into the subterranean formation to cause the subterranean formation to form fractures, whereby at least a portion of the coated proppant particles are retained in the fractures.

The substrate particle can be of any material that is solid and thermally stable at a temperature of at least 100°C. Preferably, the substrate particle is heat- stable at a temperature of at least 200°C and more preferably at least 300°C. By“heat-stable”, it is meant that the substrate particle does not melt or otherwise heat-soften to form a material that flows under the conditions in the subterranean formation, does not thermally degrade and does not thermally decompose at the stated temperature. Examples of substrate particles include sand and other mineral and/or ceramic materials such as aluminum oxide, silicon dioxide, titanium dioxide, zinc oxide, zirconium dioxide, cerium dioxide, manganese dioxide, iron oxide, calcium oxide, boron nitride, silicone carbide, aluminum carbide, bauxite, aluminum oxide and glass, as well as metals such as metal shot. The substrate particles may have a particle size such that at least 90 weight-percent of the particles pass through a U.S. 15 mesh screen, which has nominal 4.0 mm openings. In some embodiments, at least 90 weight-% of the substrate particles pass through a U.S. 10 mesh screen, which has nominal 2.0 mm openings, or at least 90 weight-% pass through a 20 mesh screen, which has nominal 1.0 mm openings In some embodiments least 90 weight-% of the substrate particles preferably are retained on a U.S. 400 mesh screen, a U.S. 200 mesh screen or a U. S. mesh 140 screen, which have nominal openings of 0.037 mm, 0.074 mm and 0.105 mm, respectively. Because the coating weights are low, as described below, the coatings are thin and the coated proppants generally have similar particle sizes.

The coating composition i) contains at least one polyisocyanate and ii) is curable to produce a polymeric coating that has free isocyanate groups and linkages selected from one or more of urethane, urea, carbodiimide, or isocyanurate linkages. Urethane, urea, carbodiimide and isocyanurate groups are generated through reactions of isocyanate groups of the polyisocyanate when the coating composition cured. The polymeric coating may further contain other linkages formed in a reaction of one or more isocyanate groups.

Examples of suitable coating compositions include but are not limited to those described in WO 2017/003813, US Published Patent Application Nos. 2018- 0072941 and 2016-0186049 and US Patent Nos. 9,725,645, 9,896,620 and 9,714,378.

The polyisocyanate preferably has an average functionality from about 1.9 to 4, and more preferably from 2.0 to 3.5. It is preferably a liquid at the application temperature. The average isocyanate equivalent weight can be from about 70 to 500, more preferably from 80 to 200 and still more preferably from 125 to 175. The polyisocyanates can be aromatic, aliphatic and/or cycloaliphatic. Exemplary polyisocyanates include, for example, m-phenylene diisocyanate, 2,4- and/or 2, 6 -toluene diisocyanate (TDI), the various isomers of diphenylmethanediisocyanate (MDI), hexamethylene- 1,6-diisocyanate, tetramethylene- 1,4-diisocyanate, cyclohexane- 1,4-diisocyanate, hexahydrotoluene diisocyanate, hydrogenated MDI (H12 MDI), naphthylene- 1,5-diisocyanate, methoxyphenyl-2, 4-diisocyanate, 4,4’-biphenylene diisocyanate, 3,3’-dimethoxy- 4, 4’ -biphenyl diisocyanate, 3,3’-dimethyldiphenylmethane-4,4’-diisocyanate, 4,4’, 4” -triphenylmethane tri-isocyanate, polymethylene polyp henylisocyanates, hydrogenated polymethylene polyp henylisocyanates, toluene-2, 4, 6-triisocyanate, and 4, 4 -dimethyl diphenylmethane-2,2’,5,5’-tetraisocyanate. Preferred polyisocyanates include MDI and derivatives of MDI such as biuret-modified “liquid” MDI products and polymeric MDI. “Polymeric MDI” is a mixture of MDI (any isomer or mixture of isomers) with one or more polymethylene polyp henylisocyanates that have three or more phenylisocyanate groups. The polymeric MDI may have, for example, an isocyanate equivalent weight of 126 to 150 and a number average isocyanate functionality of 2.05 to 3.5, especially 2.2 to 3.2 or 2.2 to 2.8.

A mixture of two or more polyisocyanates may be present in the coating composition.

Coating compositions that cure to form a polymeric coating having isocyanurate groups preferably contain at least one isocyanate trimerization catalyst. The isocyanate trimerization catalyst is a material that promotes the reaction of isocyanate groups with other isocyanate groups to form isocyanurate rings. Useful isocyanate trimerization catalysts include strong bases such as alkali metal phenolates, alkali metal alkoxides, alkali metal carboxylates, quaternary ammonium salts and the like Specific examples of such trimerization catalysts include sodium p -nonylp he no late, sodium p- octyl phenolate, sodium p-tert-butyl phenolate, sodium acetate, sodium 2- ethylhexanoate, sodium propionate, sodium butyrate, the potassium analogs of any of the foregoing, trimethyl-2-hydroxypropylammonium carboxylate salts, and the like. The isocyanate trimerization catalyst may be present in a catalytic quantity, such as from 0.05 to 10 parts by weight per 100 parts by weight of the polyisocyanate. In specific embodiments, this catalyst may be present in an amount of at least 0.1, 0.25, 0.5 or 1 part by weight per 100 parts by weight of the polyisocyanate, and may be present in an amount up to 7.5, up to 5 or up to 2.5 parts by weight per 100 parts by weight of the polyisocyanate.

In certain embodiments, the coating composition includes only a polyisocyanate and an isocyanate trimerization catalyst.

Coating compositions that cure to form a polymeric coating having carbodiimide groups preferably contain a least one carbodiimidization catalyst. Examples of carbodiimidization catalysts include, for example, phospholene compounds; phospholidine compounds such as phospholidine oxides; phosphate esters; phosphine oxides; diaza- and oxaza-phospholanes and phosphorinanes; triaryl arsines; arsine oxides; metallic derivatives of acetylacetone; metal complexes derived from a d- group transition element and a T-bonding ligand; and urea, biuret, amide, imide and/or anilide compounds. Suitable phospholene and phospholidine carbodiimidization catalysts include those described in US Patent Nos. 2,663,736, 2,663,737, 2,663,738, 2,663,739, 4,014,935, 4, 120,884, 4,260,554 and 4,743,626.

Coating compositions that cure to form a polymeric coating having urethane groups contain at least one monoalcohol (monol) or polyol, or both. The monoalcohol and/or polyol may have a hydroxyl equivalent weight of, for example, 25 to 6000 or more, although preferred monoalcohols and polyols have hydroxyl equivalent weights of 2500 or less, especially 1500 or less, 1000 or less or 500 or less. A polyol may have, for example, 2 to 8, 2 to 6 or 2 to 4 hydroxyl groups per molecule.

In some embodiments, the coating composition includes a polyol having a number average molecular weight of at least 25 g/mol, at least 50 g/mol, or at least 75 g/mol and less than 400 g/mol, especially less than 350 g/mol, less than 300 g/mol, less than 250 g/mol or less than 200 g/mol. Examples of such polyols include glycerin, ethylene glycol, propylene glycol, dipropylene glycol, diethylene glycol, sorbitol, volemitol, threitol, ribitol, mannitol, maltitol, iditol, fucitol, galactitol, arabitol, erythritol, pentaerythritol, trimethylolpropane, trimethylolethane, triethanolamine, and triisopropanolamine, as well as alkoxylates thereof having molecular weights below 400 g/mol.

In some embodiments, the coating composition includes a monol having a number average molecular weight less than 2000 g/mol. The number average molecular weight of the monol may be at least 100 g/mol or at least 200 g/mole and less than 1750 g/mol, less than 1500 g/mol, less than 1250 g/mol, less than 1000 g/mol, less than 900 g/mol, less than 800 g/mol, less than 700 g/mol, less than 600 g/mol, less than 500 g/mol or less than 400 g/mol. The monol may be, for example, a polyether such as a polymer of an alkylene oxide such as propylene oxide, ethylene oxide, 1,2 -butylene oxide, 2,3-butylene oxide, tetrahydrofuran or a mixture of any two or more thereof. Such a polyether is conveniently prepared by polymerizing the alkylene oxide(s) in the presence of a monofunctional initiator. In some embodiments, the coating composition includes such a monol and at least one polyol, which may be or include a polyol as described in the preceding paragraph. Coating compositions containing such a mixture of monol and polyol are described, for example, in WO 2018/175515, incorporated herein by reference. The weight ratio of the monol and polyol in such embodiments may be, for example, 1:18 to 18: 1, 1:17 to 17: 1, 1: 15 to 15:1, 1: 12 to 12: 1, 1: 10 to 10: 1, 1:7 to 7: 1, 1:5 to 5: 1, 1:3 to 3: 1, or 1:2 to 2:1.

Coating compositions that cure to form a polymeric coating having urea groups contain (i) water, (ii) at least one amine compound having one or more primary and/or secondary amino groups, or both (i) and (ii). The amine compound may have an equivalent weight per primary and/or secondary amino group of, for example, about 25 to 6000 or more, although preferred amine compounds have equivalent weights of 2500 or less, especially 1500 or less, 1000 or less or 500 or less. An amine compound may have, for example, 1 to 8, 1 to 6 or 1 to 4 primary and/or secondary amino groups per molecule. Examples of amine compounds include, for example, ethylene diamine, diethylene triamine, triethylene tetraamine, piperazine, aminoethylpiperazine, phenylene diamine, diethyltoluenediamine, mono-n-propyl amine, di-n-propyl amine, di-n-butyl amine, di- sec-butyl amine, amine-terminated polyethers, and the like.

The coating composition may contain one or more aminoalcohols, which can produce both urethane and urea linkages when the coating composition is cured. Such aminoalcohols compounds have at least one primary and/or secondary amino group and at least one hydroxyl group. Useful aminoalcohol compounds include, for example monoethanolamine, diethanolamine, monoisopropanolamine, diisopropanolamine, aminoethylethanolamine, and the like.

When the coating composition contains a monoalcohol, polyol, a primary and/or second amine compound, or water, or a mixture of any two or more thereof, the isocyanate index may be at least 10, at least 25, at least 50, at least 75 or at least 90, but preferably is at least 100. The isocyanate index may be, for example, up to 1000, up to 600, up to 400, up to 250, up to 200, up to 150 or up to

125. Isocyanate index is the ratio of isocyanate groups provided by all isocyanate-containing compounds to the total number of hydroxyl, primary amino and secondary amino groups provided to the coating composition. Water is considered as having 2 hydroxyl groups for purposes of this calculation.

When the coating composition cures to form a coating that has urethane and/or urea compounds, it may further contain one or more catalysts for the reaction of an isocyanate group with water, a hydroxyl group and/or an amino group . Examples of such catalysts include, for example, tin (II) and tin (IV) catalysts, catalysts that contain other Group III to Group XV metals; tertiary amine compounds, amidines, tertiary phosphines, phospholene oxides and the like, each of which preferably are absent or if present are present only in small quantities as indicated in the previous sentence. If present at all, such catalysts are present in only very small quantities, such as no more than 0.01 part by weight per 100 parts by weight of the polyisocyanate.

The coating composition may include certain optional components. An optional component of particular interest is a finely divided particulate solid, which does not melt, degrade or decompose under the conditions of the coating step or use of the coated proppant in a subterranean formation. The finely divided particulate solid should have a particle size much smaller than that of the substrate particles. The particle size may be, for example, smaller than 100 mih, smaller than 10 mhi, smaller than 1 mhi, smaller than 500 nm or smaller than 100 nm, as measured by dynamic light scattering methods. The particle size may be at least 5 nm, at least 10 nm or at least 20 nm. Examples of such finely divided particles include fumed silica, carbon black, carbon nanotubes, various metals, various metal oxides, talc, steatite, other ceramic particles, finely divided thermoset polymers, and the like. Fumed silica is particularly preferred.

The amount of finely divided particulate solid, when present, may be, for example, at least 1, at least 5, at least 10 or at least 25 parts by weight per 100 parts by weight of the polyisocyanate and up to 100, up to 75 or up to 50 parts by weight per 100 parts by weight of the polyisocyanate.

As discussed below, a finely divided particulate solid may be applied to the substrate particles as part of the coating composition (i.e., at the same time the polyisocyanate and other ingredients are applied, prior to curing). Alternatively, the finely divided particulate solid may be applied after the coating composition has been applied and at least partially (or entirely) cured, and before, at the same time or after the isocyanate-reactive (meth) aery late functional compound is applied to the coating.

Similarly, the coating composition may contain one or more other solvents or diluents, which may be present, for example, as a liquid phase in which the finely divided particles, the catalyst(s) (if any) or both are dispersed. Another optional ingredient is an adhesion promoter. Examples of suitable adhesion promoters include hydrolysable silane compounds such as amino silanes (for example, 3-aminopropyl triethoxysilane) and epoxy silanes.

The various ingredients of the coating composition can be combined to form a mixture that is applied to the substrate particles. Alternatively, the various ingredients can be applied sequentially to the substrate particles, or in various subcombinations. For example, it may be convenient to apply the polyisocyanate first, followed by applying the other ingredients together, singly or in some combination. In such a case, the catalyst (if any) may be applied next, followed by or accompanied by the finely divided particles (if used), which are preferably dispersed in water or other liquid phase. In other embodiments of the invention, finely divided particles may be applied after the coating composition is applied, either during the curing step or after the coating composition has cured to form the coating.

The amount of coating composition applied is, for example, sufficient to provide 0.1 to 10 parts by weight of the coating composition per 100 parts by weight of the substrate particles. A preferred amount is sufficient to provide 0.1 to 5, 0.1 to 4, or 0.1 to 3 parts by weight of the coating composition, on the same basis.

The coating composition (or any component thereof) can be applied by spraying or other suitable method. The substrate particles are preferably stirred or otherwise agitated during the coating process. They may be, for example, disposed in a fluidized bed, in a stirred container, or another device that permits the particles to be separated and individually coated. The ability to spray the coating composition onto the substrate particles is an advantage of this invention.

Curing preferably is performed at an elevated temperature, which may be, for example, up to 160°C, up to 140°C, up to 125°C or up to 100°C. The elevated temperature preferably is at least 50°C or at least 60°C and may be up to 90°C or up to 80°C. An advantage of this invention is that the coating composition in most cases cures rapidly at such moderately elevated temperatures. The curing time at such temperatures is typically no greater than 10 minutes and may be as short as one minute. A typical curing time may be 1 to 5 minutes, 2 to 5 minutes or 2 to 4 minutes. As discussed below, the subsequent step of applying and reacting the isocyanate-reactive (meth)acrylate-functional compound may be performed prior to complete curing; i.e., the isocyanate-reactive (meth)acrylate-functional compound may be applied to the coating before the curing step has completed, and the reaction thereof with isocyanate groups on the coating may proceed simultaneously with a portion of the curing reaction.

It is generally convenient to heat the substrate particles to the curing temperature before applying the coating composition. The applied coating composition in such cases may be heated to the curing temperature by transfer of heat from the substrate particles, without the need to apply further heating during the curing process. However, it is possible to apply the coating composition to unheated substrate particles and heat the substrate particles and applied coating composition together to the curing temperature.

Agitation should be provided during the curing step to avoid particle agglomeration. The coated proppant particles are generally produced in the form of free flowing particles or at most lightly agglomerated, easily broken aggregates of primary particles.

Curing produces a polymeric coating that contains one or more of urethane, urea, carbodiimide or isocyanurate linkages, or some mixture of two or more of those linkages. At the time the isocyanate-reactive (meth) acrylate - functional compound is applied, the coating has free isocyanate groups that are available for reaction with the isocyanate-reactive (meth)acrylate-functional compound. Some free isocyanate groups tend to remain even after full curing, and some free isocyanate groups may remain even after reaction with the isocyanate-reactive (meth)acrylate-functional compound. The presence of greater amounts of free isocyanate groups is favored by methods such as (1) using an excess of polyisocyanate(s) over isocyanate-reactive materials such as monoalcohols, polyols, amine compounds and water, so not all isocyanate groups are consumed in the curing step and/or (2) applying and reacting the isocyanate- reactive (meth)acrylate-functional compound before the isocyanate groups present in the coating composition have been completely consumed.

At least one isocyanate-reactive (meth)acrylate-functional compound is applied to the partially- or fully-cured polymeric coating where it reacts with free isocyanate groups of the polymeric coating to produce (meth)acrylyl groups chemically bonded to the polymeric coating. It is preferred that at least a portion of the (meth)acrylyl groups is present at the surface of the coating

By“(meth)acrylyl groups”, it is meant an acrylyl (-C(0)-CH2=Cf½) or methacrylyl (-C-(0)CH(CH3)=CH₂) groups. The isocyanate-reactive

(meth)acrylate-functional compound is characterized in having at least one acrylyl or methacrylyl moiety and at least one isocyanate-reactive group. The isocyanate-reactive group is preferably hydroxyl, but may be a thiol, primary or secondary amino, epoxy, carboxylic acid or other group that engages in an addition reaction with an isocyanate group. It is preferred that the isocyanate- reactive (meth) acrylate- functional compound has no more than one acrylate or methacrylate group and no more than one isocyanate-reactive group, to minimize crosslinking between proppant particles and thus avoid agglomeration.

The isocyanate-reactive (meth)acrylate-functional compound in some embodiments has a molecular weight of no more than 3000, more than 2000, no more than 1500, no more than 1000, no more than 500, no more than 250 or no more than 150.

Examples of isocyanate-reactive (meth)acrylate-functional compound compounds include hydroxyethyl acrylate, hydroxyethyl methacrylate, 2- hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 2-aminoethyl acrylate, 2- aminoethyl methacrylate, 2-aminopropylmethacrylate and 2-aminopropyl acrylate, as well as alkoxylates (such as ethoxylates and/or propoxylates) of any of the foregoing.

The isocyanate group of the isocyanate-reactive (meth)acrylate-functional compound reacts with a free isocyanate groups of the polymeric coating to form a bond therebetween, thereby introducing (meth)acrylyl groups to the polymeric coating. The amount of isocyanate-reactive (meth)acrylate-functional compound that reacts is in some embodiments limited by the number of available isocyanate groups, and in that sense the reaction and consequently the amount of isocyanate-reactive (meth) aery late - functional compound consumed is self- limiting.

In some embodiments, the isocyanate-reactive (meth)acrylate-functional compound may react to form a layer at the surface of the coating. In such a case, the layer thickness may be, for example from 1 to 200 nm. In some embodiments the layer thickness is at least 2 nm or at least 5 nm and up to 150 nm or up to 100 nm. The amount of isocyanate-reactive (meth)acrylate-functional compound may be, for example, at least 0.001, at least 0.0025 or at least 0.005 part by weight per 100 parts by weight of the substrate particles, and, for example, up to 1 or up to 0.5 part by weight on the same basis.

The isocyanate-reactive (meth)acrylate-functional compound preferably is applied and reacted with the polymeric coating under non-polymerizing conditions. “Non-polymerizing conditions” are conditions under which the acrylyl or methacrylyl groups that are present do not polymerize.

Non-polymerizing conditions generally include the absence of free radicals and/or an effective amount of an acrylate or methacrylate polymerization catalyst. Non-polymerization conditions may further include a temperature below that at which thermally-induced polymerization of the acrylate and/or methacrylate groups takes place.

In general, conditions such as described before for applying and curing the coating composition are suitable. A temperature of 120°C or lower is especially preferred.

After applying and reacting the isocyanate-reactive (meth) acrylate - functional compound, the product particles may be cooled to below 40°C and ground lightly if desired to break up small agglomerates that may have formed.

The resulting coated proppant particles can be used in the same manner as conventional proppant particles. In a typical hydraulic fracturing operation, a hydraulic fracturing composition comprising a fracturing fluid, the coated proppant, and optionally various other components is prepared. The fracturing fluid can be a wide variety of fluids such as kerosene and water. Various other components that can be added to the mixture include, but are not limited to, guar and polysaccharides as well as other components as may be useful.

The fracturing fluid may contain a gelling agent to help prevent the proppant particles from settling prematurely. Such a gelling agent may be dissolved once the formation has been fractured to allow the proppant particles to deposit into the fractures.

The mixture is pumped into the subterranean formation under pressure to create or enlarge fractures in the subterranean formation. Coated proppant particles enter into the fractures and are retained there. The coated proppant holds the fractures open when the hydraulic pressure is released, thereby maintaining a flow path through the fractures to facilitate the extraction of petroleum fuels or other fluids from the formation to the wellbore.

Another advantage of the invention is that the coated proppant particles bond to each other under conditions of elevated temperature and pressure. This property permits the coated proppants to form agglomerated masses within the subterranean fracture. The agglomerated masses are more resistant to proppant flowback than are the individual proppant particles.

In particular, the coated proppant particles have been found to bond well to each other even at only mildly elevated temperatures, such as from 40 to 80° C, and especially 40 to 65°C or 40 to 60°C. Thus, in some embodiments, a fracturing fluid containing the coated proppant is injected into a subterranean formation that is at such a temperature to cause the subterranean formation to form fractures. The coated proppant is then retained in fractures having such temperatures.

The ability of the coated proppant to bond to itself can be measured in accordance with the unconfined compressive strength (UCS) test described in the following examples. When bonded together under conditions of 1000 psi (6.89 MPa) and 50°C for 24 hours, the unconfined compressive strength of the resulting bonded mass, as measured by the USC test, is in preferred embodiments at least 40 kPa. The unconfined compressive strength on this test may be at least 70 kPa or at least 100 kPa, at least 200 kPa or at least 300 kPa and may be up to 2000 kPa, up to 1500 kPa or up to 1000 kPa.

The following examples are provided to illustrate the invention but are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated.

The sand used in the following experiments is a 40/70 mesh sand product.

Examples 1-6 and Comparative Sample A

Comp. Sample A: 750 grams of the sand are preheated to 105°C and loaded into a laboratory mixer equipped with a heating jacket. 0.6 mL of gamma-aminopropyltriethyoxysilane (Momentive Silquest™ A- 1100) coupling agent is add to the stirring sand. Starting 15 seconds after the coupling agent is added, a mixture of 2.8 grams glycerin, 2.8 grams of a 350 molecular weight polypropylene glycol) monomethyl ether (MPEG) and 0.3 g of a commercially available organotin catalyst (Dabco™ T12 from Evonik) is added over 75 seconds simultaneously with 13.2 grams of a polymeric MDI (PAPI™ 27 from the Dow Chemical Company, 2.7 average isocyanate functionality, isocyanate content of

30.4-32.0%). After 150 additional seconds, the mixing is discontinued and the coated material is emptied to a tray to cool.

The product is sand particles coated with a polyurethane coating.

Example 1: Comp. Sample A is repeated, except this time, starting 75 seconds after the glycerin/MPEG/catalyst and polymeric MDI feeds are completed, 0.6 gram of hydroxyethyl acrylate (HEA) is added over 5 seconds and mixing is continued thereafter for another 75 seconds, at which time the coated material is emptied to a tray to cool. The HEA reacts with the polyurethane coating produced in the glycerin/MPEG/polymeric MDI reaction to introduce acrylyl groups to the coating.

Example 2: Example 1 is repeated, increasing the weight of HEA to 1.2 grams. The HEA reacts with the polyurethane coating produced in the glycerin/MPEG/polymeric MDI reaction to introduce acrylyl groups to the coating.

Examples 3 and 4: Examples 1 and 2 each are repeated, in each case substituting an equal weight of hydroxyethylmethacrylate (HEMA) for the HEA used to prepare Examples 1 and 2. The product in each case is sand particles coated with a polyurethane coating and having methacrylyl groups produced by the reaction of the HEMA with the underlying polyurethane coating.

Example 5: Example 3 is repeated, increasing the weight of HEMA to 3 grams. The HEMA reacts with the polyurethane coating produced in the glycerin/MPEG/polymeric MDI reaction to introduce methacrylyl groups to the coating.

Example 6: Example 3 is repeated, except the amounts of glycerin and MPEG each is reduced to 1.1 grams and the amount of polymeric MDI is reduced to 5.3 grams. Starting 75 seconds after the glycerin/MPEG/catalyst and polymeric MDI feeds are completed, 0.6 gram of hydroxyethyl methacrylate (HEMA) is added over 5 seconds. Mixing is continued thereafter for another 75 seconds, at which time the coated material is emptied to a tray to cool. The HEMA reacts with the polyurethane coating produced in the glycerin/MPEG/polymeric MDI reaction to introduce methacrylyl groups to the coating. The product of each of Examples 1-6 is a free flowing powder that may form light agglomerates upon standing. The agglomerates are easily broken in each case in which they form.

UCS is measured for each of Comparative Sample A and Examples 1-6 by first sieving the coated sand through a 1 mm metal screen. The sieved sand is mixed with a solution of 2% potassium chloride in water at a volume ratio of 4 parts sand to 3 parts solution. 1 drop of dish soap is added to eliminate air entrainment. The resulting slurry is allowed to stand for 5 minutes and then loaded into a 1.125 in (28.6 mm) interior diameter steel cylindrical cell with removable top and bottom assemblies. Excess water is drained from the cell. A piston is placed at the top of the sample chamber and hammered into the cell. A top assembly equipped with a pressure relief valve and a nitrogen inlet is attached to the cell. The cell is pressurized to 1000 psi (6.89 MPa) with nitrogen, then kept 24 hours in a 50°C oven. The cell is then cooled to room temperature. The sand plug is removed from the cell and dried under ambient conditions for a day to remove absorbed water. The plug is then broken into 2 -inch (5.08 cm) pieces that are filed at the edges to smooth them. Plugs are tested for compressive strength using an MTS Insight electromechanical testing system with a 2000 kilonewton load cell and a compression rate of 0.01 in/minute (0.254 mm/minute). The peak stress value is reported in the following table as the USC.

This data demonstrates the large positive effect of introducing acrylyl and methacrylyl groups to sand particles already coated with a polyurethane coating. Even at low applied weights, USC increases by 40% or more and at the higher weights USC more than doubles. Comparative Samples B and C

Comp. Sample B: 750 grams of the sand are preheated to 105°C, and loaded into a laboratory mixer equipped with a heating jacket. 3.75 mL of 3- trimethyoxysilyhpropyl acrylate) is added to the stirring sand. After 120 seconds, the mixing is discontinued and the coated material is emptied to a tray to cool. Comp. Sample C is made in the same way, except mixing is discontinued 150 seconds after adding the trimethyoxysilyl(propyl acrylate)

Trimethoxysilyhpropyl acrylate) is a compound having free acrylate groups and a reactive silyl group through which the compound can couple to the sand particles. Thus, the treated sand particles are expected to have free acrylate groups on their surfaces. Nonetheless, in each case, the treated particles fail to bond together on the UCS test. This data demonstrates that merely adding acrylate functionality directly to the proppant particles does not produce the intended benefit, without an underlying coating of the isocyanate-based polymer.

Claims

WHAT IS CLAIMED IS:

1. A method for forming a coated proppant, comprising

a) applying a coating composition to the surface of solid substrate particles, wherein the solid substrate particles are thermally stable to a temperature of at least 100°C, and wherein the coating composition comprises at least one polyisocyanate;

c) applying at least one isocyanate-reactive (meth) acrylate- functional compound to the polymeric coating and reacting isocyanate-reactive groups of the isocyanate-reactive (meth)acrylate or poly(meth)acrylate compound with free isocyanate groups of the polymeric coating to produce free (meth)acrylyl groups on the polymeric coating.

2. The method of claim 1 wherein the coating composition is sprayed onto the substrate particles.

3. The method of claim 1 or 2 wherein the amount of the coating composition applied to the surface of the substrate particles is sufficient to provide 0.1 to 10 parts by weight of polyisocyanate per 100 parts by weight of substrate particles.

4. The method of any preceding claim wherein the isocyanate-reactive (meth)acrylate-functional compound has a molecular weight of no more than 250.

5. The method of any preceding claim wherein the isocyanate-reactive (meth)acrylate-functional compound is selected from one or more of hydroxyethyl acrylate, hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 2-aminoethyl acrylate, 2-aminoethyl methacrylate, 2- aminopropylmethacrylate and 2-aminopropyl acrylate, or an alkoxylate of any of the foregoing.

6. The method of any preceding claim wherein the substrate particles are sand.

7. A coated proppant particle made in the method of any preceding claim.

8. A coated proppant particle comprising a substrate particle having a polymeric coating that weighs 0.1 to 10 percent of the weight of the substrate particle, wherein the polymeric coating has linkages selected from one or more of urethane, urea, or isocyanurate linkages, and free (meth) acrylate groups on and chemically bonded to the polymeric coating.

9. The coated proppant particle of claim 8 which further contains free isocyanate groups.

10. A method of hydraulically fracturing a subterranean formation, comprising injecting a carrier fluid and coated proppant particles of any of claims 7-9 into the subterranean formation to cause the subterranean formation to form fractures, whereby at least a portion of the coated proppant particles are retained in the fractures.