NL1039260C2

NL1039260C2 - Method for the preparation of polymer comprising particles.

Info

Publication number: NL1039260C2
Application number: NL1039260A
Authority: NL
Inventors: Rudolf Trion; Gerard Jager
Original assignee: Resin Proppants Internat B V
Priority date: 2011-12-22
Filing date: 2011-12-22
Publication date: 2013-06-26
Also published as: WO2013095150A1; EP2794216A1

Description

Method for the preparation of polymer comprising particles

The present invention relates to a method for the preparation of particles comprising at least a polymer, wherein a melt comprising the said 5 polymer is extruded by passing the melt through at least a die into a solidifying medium having a temperature of below the melting temperature of the melt, allowing the melt to solidify in the solidifying medium into the particles. The invention further relates to particles obtainable by the said method, to a collection of such polymer comprising particles, to the use of such particles as component in 10 fracturing fluids, to a fracturing fluid comprising such particles as well as to methods for hydraulic fracturing using such particles.

Background of the Invention

Polymer comprising particles, in particular for use in fracturing of subterranean formations in oil and gas production are known. Particles such as the said 15 particles are also known as ‘proppants’. Proppants are incorporated into high-pressure fluids to help create and maintain fractures in rock, contributing to increased well production in the oil and gas field. Proppants are particulate material used in the hydraulic fracturing of subterranean formations, and they also function to keep the cracks open. In the art, sand and small ceramic beads are suspended in the fracturing 20 fluid and often used in hydraulic fracturing of oil and gas wells. Hydraulic fracturing is accomplished by pumping fluid down a well under high pressure to create fractures in the surrounding rock. The proppants flow into the fractured cracks and extend outward from the wellbore to prop the fractures open. After pumping, the proppant materials remain in the cracks of the separated rock and form an open channel to allow the 25 hydrocarbons to flow more easily to the surface. As oil and gas resources continue to deplete, there is more need for hydraulic fracturing. Fracturing may also be accomplished by the use of explosive charges and in such applications proppants may also be used.

A number of considerations are taken into account when selecting 30 proppants appropriate to the intended use such as particle size. It is often useful to have sufficient space between the proppant particles for the desired fluid to be able to easily flow between them. In addition, the size of material may also be a consideration depending on depth of field applications. For shallow depths big round particles (i.e. having a diameter of above 1.0 mm) can be favoured, while for deeper depths smaller 2 round particles (i.e. having a diameter of 0.4 - 0.8 mm or smaller) can be the material of choice.

The cracks, once formed, tend to reclose as a result of the enormous pressure within the geologic formations. Therefore, the particles should also be resistant 5 to so-called "closure stress" and should preferably retain their shape and integrity at the temperatures at which hydraulic fracturing takes place. The said temperatures are herein also referred to as ‘operational temperatures’. When particles are crushed, they can form very fine particles that decrease the permeability of oil or gas through the cracks. The hardness and resistance to deformation are therefore also essential to 10 support the burden of the rock, and proppants should have the strength to resist the stress. Regarding the above, the particles should have a compressive strength of at least 45 N/mm2 at a temperature of at least 80°C, preferably at the temperature at which hydraulic fracturing takes place, to resist the closure stress within the cracks.

At shallow depths, the minimal temperature at which hydraulic fracturing 15 takes place is often about 60°C. Temperatures of 150°C or higher are an issue at deeper depths. Materials having sufficient compressive strength at the temperature at which hydraulic fracturing takes place should be selected.

Regarding the above the size and the compressive strength at the operational temperature are important properties.

20 Among materials commonly used as proppants are sand, ceramic beads, and walnut hulls. However one of the problems using sand or ceramics is that the density of the proppant particles is high compared to the fracturing fluid. For example, while the density of a typical fracturing fluid is about 1.0 g/ml, the density of sand is about 2.65 g/ml. As a result the proppant particles settle too rapidly during the fracturing 25 process. Therefore fracturing fluids often have high viscosities in order to effectively suspend such high specific gravity proppants. A disadvantage to using high viscosity fluids is that they often do not efficiently penetrate small cracks.

The above materials, while possessing the strength desired for effective use as a proppant, also deteriorate into fines under the pressure that would be 30 experienced underground. Another problem is that proppants such as walnut hulls do not possess the required resilience needed to press back against shifting subterranean pressures.

US6,059,034 describes resilient proppants by coating non-deformable material such as sand or glass beads with a deformable polymer. However such 3 proppants still have the disadvantage of high specific weights. US7,322,411 describes deformable proppants in a proppant pack to minimize fines generation in the cracks and proppant pack damage. Said proppants can be natural materials such as crushed nutshells, optionally coated with a polymer, and polymeric spheres in a partial 5 monolayer having a lower modulus than the surrounding rock.

In the art also polymeric proppants are known, e.g. from W02007/109281. Said polymeric proppants contain filler materials such as sand, glass beads or glass powder in order to reinforce the polymer and to modulate the density of the proppant material. Densities of close to that of the fracturing fluid can be achieved, therewith 10 reducing the settling rate of the proppant within the fracturing fluid, as well as the use of high viscous fracturing fluids. However the particle size is rather large. The smallest particles obtained according to W02007/109281 have a pellet weight of 0.88 g. Pellet weight is the weight of 100 particles.

Therefore, in the art there is a great need for proppant materials that have 15 a density close to that of water, having a small size and having sufficient pressure resistance at operational temperatures. Attempts have been made to downmodulate the density of sand and ceramic based proppants by coating these with a light polymeric material. Nevertheless these proppants still have a relatively high density.

The present inventors have surprisingly found that proppants having the 20 required properties can surprisingly be produced by starting from polymeric material.

Proppants of polymeric material are not very common in the art, although such materials may have the required specific weight. It has been impossible to produce particles of polymer material with the required pressure resistance and the required small particle size. At present there are no techniques available enabling the 25 preparation of small sized polymeric particles that are suitable as proppant and for that reason the skilled person is not further exploring the route of small sized polymeric proppants.

Some of the reasons therefore lie in the fact that small polymer particles are produced by underwater granulation of the polymeric melt. According to this 30 technique a polymer melt is extruded and passed through a die, which die is submersed in water. The water temperature may vary, usually between 20 and 100°C. As soon as the polymeric melt is passed through the die, the melt is cooled by the water and solidifies; the water has the function of solidifying medium. A cutting device cuts the 4 extruded material into pieces and the said pieces form particles during the solidification process.

Polymers that are suitable as proppant according to the invention usually have a high melting point Tpm (the peak melting temperature as determined 5 according to the ISO standard 11357-1:1997), e.g. significantly higher than that of water. As a result the melt will solidify instantly upon contact with the water. It is also observed that the melt tends to solidify in the outlet opening of the die, therewith clogging the die, as a result of which the extrusion apparatus becomes clogged.

10 The present invention intends to solve one or more of the above problems. To this end, the invention is characterized in that the melt is chosen such that in solidified state it is has a density of 1.0 to 2.1 g/ml and a compressive strength of at least 45 N/mm2 at 80°C, in that the die has an outlet opening with a diameter of at most 1.2 mm, and in that the temperature of the melt at the die 15 outlet opening is controlled such, that closure of the die outlet opening by solidification of the melt at the die outlet opening is reduced.

The term ‘reduced’ means that the closure of the die at the outlet opening is less than under the same process conditions without the said temperature control. Preferably, the said solidification is avoided.

20 By the above method particles comprising at least a polymer having a density of 1.0 to 2.1 g/ml, a particle size of 1.3 mm or less and a compressive strength of at least 45 N/mm2 at 80°C can be produced for the very first time. Unless indicated otherwise, the term ‘particle size’ as used herein reflects the weight based particle size, which equals the diameter of the sphere that has same 25 weight as a given particle. The weight based particle size is defined by the following formula: 2*(3*weight(particle)/4/pi/density(particle))A(1/3).

The skilled person will be capable to choose the proper polymer or a combination of polymers that have the proper density and compressive strength in 30 solid state. The compressive strength can be determined using e.g. apparatus from the company Instron type 3366 and following the manufacturer’s manual. Herein the compressive strength is measured at the indicated temperature on a single particle at a compression of the particle of 20% (i.e. compressed to 80% of its original diameter).

5

In coming to the invention the inventors overcame the prejudice against the use of polymeric proppants. It has been encountered by the inventors that using the commonly known underwater granulation techniques clogging of the outlet opening of the dies could not be avoided for suitable polymers, as soon as a 5 small particle size, e.g. below 1.3 mm was envisaged. However the inventors found that solidification of the melt at the die outlet opening could be avoided by controlling the temperature of the melt at the outlet opening of the die. The term ‘controlling’ includes transferring heat to the melt at the location of the die outlet opening or reducing the loss of heat from the melt at the position of the die outlet 10 opening to the solidifying medium, therewith avoiding clogging of the die outlet opening.

There appeared to be some compensation possible by elevating the pressure or temperature of the melt. However it was found that using the conventional underwater granulation techniques at the required small diameters of 15 the die outlet opening resulted, in most cases, in clogging of the die outlet opening by solidified polymer. Additional control appeared to be necessary, which is explained further below.

In an attractive embodiment said controlling of the temperature of the melt at the die outlet opening comprises holding the temperature of the said die, at 20 least at the outlet opening thereof, at a temperature higher than 60°C below the melting temperature Tpm of the polymer, i.e. at a temperature above (Tpm - 60°C).

It was found that the die outlet openings did not anymore clog and that the obtained polymer comprising particles appeared to be very well suitable as small sized proppant particles (i.e. having a particle size of less than 1.3 mm, 25 preferably less than 1.0 mm, more preferably between 0.4 and 0.8 mm, most preferably between 0.4 and 0.6 mm). It appeared that using the conventional underwater granulation techniques, the temperature of the outlet opening was too low to avoid clogging. By holding the temperature at the above values, clogging could surprisingly be avoided, enabling, for the first time, the production of small 30 sized polymer-based proppants.

The term polymer comprising particle comprises particles of any shape, such as spherical, spheroidal, elliptical, and right cylindrical shapes. Spherical shapes are preferred as these have the best flow properties in the 6 fracturing fluid. The particle comprises at least one polymer, although polymer mixtures are also possible.

It is however advantageous to use a single polymer as the polymer with the least qualities usually determines the properties of the particle.

5 Herein, the term ‘the polymer’ also encompasses an optionally cross linked polymer, an optionally cross linked copolymer, an optionally cross linked terpolymer, an optionally cross linked block(co) polymer or any mixtures thereof unless indicated otherwise. The particle can also advantageously comprise one or more filler materials, e.g. to adjust the specific weight of the particle, to reinforce it, 10 or to reduce costs of the particle material.

Preferably the melting temperature Tpm of the polymer is at least 120°C, preferably at least 150°C, more preferably at least 180°C, most preferably at least 200°C. Polymers having a relatively low melting point usually appear to be less suitable as component for the polymer comprising particles according to the 15 invention. Further, particles of such polymer materials would easily melt at the temperatures at which fracturing takes place.

In addition to the above-mentioned compressive strength, an additional parameter for the determination of the temperature dependent integrity is the softening point, expressed as the so-called VICAT softening point, according 20 to ISO standard 306. For a polymer to be suitable as proppant material, the polymer should preferably have a softening point that is above the lowest temperature at which hydraulic fracturing takes place. If the softening point is too low, the proppant particles will be soft during the hydraulic fracturing, therewith loosing their shape and not having the pressure resistance needed to keep the 25 fractures open.

Preferably the polymer has a VICAT softening point of at least 50°C, preferably at least 70°C. A softening point in the said ranges is advantageous, as at the working temperatures during fractioning, the particles preferably remain rigid, although some deformation may take place.

30 Preferably in step a) the temperature of the melt is at least 180°C, preferably at least 200°C, more preferably at least 230°C, most preferably at least 260°C. The temperature of the melt also has an impact on the temperature of the die. A higher temperature of the melt assists in holding the temperature of the die at the required level. Further, the processability at higher temperatures is 7 improved, as the viscosity of the melt will be lower. A too high temperature will however result in degradation of the polymer in the melt. The temperature of the melt is therefore preferably below 400°C.

Also in step a) the temperature of the melt is at least 20°C above the 5 melting temperature Tpm of the polymer, preferably 30°C above the melting temperature Tpm of the polymer, more preferably 40°C above the melting temperature Tpm of the polymer in order to avoid clogging of the melt at the die.

In another embodiment the temperature of the solidifying medium is at 50°C or more lower than the melting point of the polymer, preferably 60°C or more 10 lower than the melting point of the polymer. The temperature of the solidifying medium is preferably 100°C or less lower, preferably 80°C or less lower than the melting point of the polymer. By choosing the temperature of the solidifying medium accordingly, the temperature difference between the melt and the solidifying medium is large enough to allow a good solidification of the melt, 15 without allowing the melt to cool off too much at the location of the die outlet opening, therewith avoiding clogging of the said outlet opening.

Preferably the temperature at the outlet opening of the die is held at a temperature higher than 40°C below the melting temperature Tpm of the polymer (i.e. Tpm - 40°C), preferably higher than 20°C below the said melting temperature 20 Tpm (i.e. TPm - 20°C), more preferably at the said melting temperature Tpm; even more preferably at least 20°C, still even more preferably at least 30°C above the melting temperature Tpm of the polymer. The higher the temperature at the outlet opening, the less chance for clogging of the die outlet opening exists. Care has to be taken that the temperature does not exceed the decomposition temperature of 25 the polymer in the melt. Preferably the temperature at the outlet opening of the die is held at a temperature of lower than 100°C above the melting temperature Tpm of the polymer, preferably lower than 100°C above the melting temperature Tpm of the polymer.

In another preferred embodiment the temperature at the outlet opening 30 of the die is held at above the melting onset temperature Tim of the polymer. The Tim of the polymer is also defined in the above ISO 11357-1 1997 standard and indicates the temperature where melting of the polymer starts. It is commonly known that polymers do not have a definite melting point but a melting range, starting at the onset temperature, here and in the ISO 11357-1 1997 standard 8 indicated by Tim, and ending at the end temperature indicated in the ISO 11357-1 1997 standard indicated by Tfm. The melting point Tpm is located between Tfm and Tpm, as indicated in the said standard.

In an attractive embodiment of the present invention the temperature 5 of the die, at least at the outlet opening thereof, is held by transferring external heat to the die, or by generating heat in the die. In underwater granulation the solidifying medium lowers the temperature of the die at the outlet opening thereof. By the term ‘controlling the temperature of the melt at the die outlet opening’ is meant that the temperature of the melt, in particular at the outlet opening of the 10 die, is controlled such that no clogging takes place. Although the temperature of the melt may be high, the temperature of the die at the outlet opening thereof is usually not sufficient to avoid clogging of the outlet opening by the melt. Therefore external heat can be transferred to the melt e.g. through the die, in particular at the outlet opening thereof or heat can be generated in the die, such that the 15 temperature is sufficient to heat the melt at the outlet opening of the die such, that clogging is avoided.

The temperature of the die, at least at the outlet opening thereof is preferably held by transferring external heat to the die by arrangement of at least a channel in the die, located in the vicinity of the die outlet opening and transporting 20 of a heat exchange medium through the said channel, by heat induction of the die at the location of the outlet opening or by the temperature of the solidifying medium or a combination thereof. By arranging one or more heat exchange channels in the die enabling heat to be exchanged from a heat-exchanging medium to the die at the outlet opening, clogging of the melt at the outlet opening 25 can be avoided. A preferred heat exchange medium comprises oil or glycol, but any other organic liquid or mixture thereof capable of transferring heat at a temperature of above 100°C, preferably above 150°C, more preferably above 180°C is suitable as long as clogging of the outlet opening is avoided. The die can also be designed such, that heat can be generated by heat induction within the die. 30 Another attractive embodiment to reduce heat loss at the die outlet opening, at least to the outlet opening thereof, is by increasing the temperature of the solidifying medium. In case water is used, the maximum temperature of the said solidifying medium is 100°C (or alternatively the system could be operated under pressure to increase the boiling temperature of water accordingly). This can 9 however be insufficient to hold the temperature of the die at the outlet opening at the required value to avoid clogging. Additional heat transfer would be needed in most cases.

Preferably the temperature of the solidifying medium is above 50°C, 5 preferably above 90°C, more preferably above 120°C, even more preferably above 140°C. As indicated above, the temperature of the solidifying medium can be used to hold the temperature of the die at the outlet opening at the required level to avoid clogging. In case a temperature of more than 100°C is desired, another solidifying medium than water should be contemplated, as long as the said medium 10 is inert to the melt and the solidified particles.

Preferably the temperature of the solidifying medium is below 250°C, preferably below 200°C. Higher temperatures may have a negative impact on the solidifying process of the melt, resulting in less reproducibility with regard to particle size. However, if polymeric melts having a high melting point are used, the 15 temperature of the solidifying medium can be accordingly elevated.

The atmospheric boiling point of the solidifying medium is therefore preferably above 90°C, more preferably above 100°C, even more preferably above 110°C, still more preferably above 120°C, most preferably above 150°C.

The solidifying medium is preferably chosen from the group 20 consisting of water, liquid salt, or an organic liquid or any mixture thereof.

The solidifying medium preferably comprises an organic liquid, having an atmospheric boiling point of above 90°C, more preferably above 100°C, even more preferably above 110°C, still more preferably above 120°C, most preferably above 150°C.

25 In an attractive embodiment, the solidifying medium is inert to the melt and the particles. With ‘inert’ is meant that the melt and the solidified particles do not dissolve substantially in the solidifying medium. Apolar organic liquids may therefore be less suitable, as these liquids tend to dissolve the melt.

Preferably, the solidifying medium comprises a polar organic liquid. 30 More polar organic liquids are in general more inert for many polymers as compared to less polar organic liquids. In another attractive embodiment, the solidifying medium comprises a mixture of two or more organic liquids, or a mixture of one or more organic liquids with water.

10

The polar solidifying medium preferably comprises an alcohol, ether, acid or ester. Among suitable alcohols are aliphatic alcohols like octanol with a boiling point of 195°C and decanol with boiling point 220-235°C. Also glycols can be used e.g. propylene glycol with boiling point 188°C and ethylene glycol with 5 boiling point 197°C. In general also ethers can be used, but these are less preferred because of the relatively low boiling point and tendency to react with oxygen to form peroxides and explosive mixtures. Among the large group of organic acids and esters are e.g. propionic acid with a boiling point of 141°C up to e.g. palmetic acid with boiling point 351 °C, which can also be used as attractive 10 solidifying media in the method of the invention. Preferably the polar organic liquid preferably comprises an alcohol.

In a very attractive embodiment, the solidifying medium comprises glycol, such as ethylene glycol, propylene glycol or a polyethylene glycol or mixtures thereof. Preferably, propylene glycol, ethylene glycol or mixtures thereof 15 are used; most preferably ethylene glycol is used.

Ethylene glycol has an atmospheric boiling point of 197°C, and can be used in any mixture with water. Ethylene glycol is therefore very versatile for use as solidifying medium. Polymers having a high melting point, such as PEI or PEEK can be subjected to underwater granulation resulting in small particles of high 20 sphericity and roundness using ethylene glycol, optionally in admixture with water, as solidifying medium.

In a preferred embodiment, the heat conductivity of the solidifying medium is below 0.55 W/m/°C, preferably below 0.50 W/m/°C, more preferably below 0.40 W/m/°C, most preferably below 0.30 W/m/°C. The heat conductivity of 25 ethylene glycol is 0.26 W/m/°C, whereas that of water is 0.609 W/m/°C. It may even be preferred to have an even lower heat conductivity as low as below 0.20 W/m/°C. N-butylalcohol has a very low heat conductivity of 0.167 W/m/°C, and a boiling point of 117.5°C. Heat conductivity is to be measured at atmospheric pressure and at room temperature, i.e. 20°C. The heat conductivity of many liquids 30 are known, and are published e.g. on the website www.toolbox.com. The heat conductivity can be measured according to the ASTM D2717 standard, e.g. with the LabTemp30190 equipment of PSL Systemtechnik (Clausthal-Zellerfeld, Germany) according to the instructions of the manufacturer.

11

It has been observed that the heat conductivity plays an important role in the avoidance of clogging of the die outlet openings, in particular when polymers having a high melting point of e.g. 300°C or higher are used in the underwater granulation process. A higher heat conductivity would allow to absorb a great 5 amount of heat by the solidifying medium from the melt as it passes the die outlet opening, having the risk that the melt solidifies as it is still at the die outlet opening by direct cooling in contact with the solidifying medium or indirect cooling via the die-plate surface, resulting in clogging of the die outlet opening. A lower heat conductivity of the solidifying medium ensures a slower solidification of the melt in 10 the solidifying medium, avoiding clogging at the die outlet opening. Also, when the particles are solidified slowly, there is more time to allow the particles to adapt to the thermodynamically preferred spherical state, therewith improving the roundness and sphericity of the particles.

Another embodiment of the invention comprises the use of flow-aids 15 during extrusion and/or solidification of the particles in order to facilitate the formation of small spherical and round particles. These flow-aids can be any known melt viscosity reducing agent such as wax, liquid or solid flame retardant, organic solvent, hydrocarbons in general etc. It is obvious to those skilled in the art that these flow aids may have a negative effect on the compressive strength of the 20 particles. In another preferred embodiment of the present invention these flow-aids are removed from the particles once they are solidified in whole or in part in a subsequent process step by known extraction agents such as water, organic alcohols such as glycols or any appropriate extraction medium.

In order to produce particles of the required small size, the outlet 25 opening of the die preferably has a diameter of below 1.0 mm, more preferably below 0.8 mm, even more preferably below 0.75 mm. The outlet opening of the die is preferably above 0.3 mm, preferably above 0.4 mm.

In a particular embodiment of the invention the outlet opening of the die cooperates with a cutting device, cutting the melt passing through the die. By 30 cutting the melt, portions of the melt are individually solidified. In the art methods are known for cutting the melt that exits a die. For example cutter blades circling around the die outlet opening are known for this purpose. Straight and angled cutter blades can be used and spacing between the cutter blades is critical to the process. Because of the surface tension of the melt, the melt rounds up upon 12 solidification in the solidifying medium. The melt is preferably passed through the die at a flow rate and wherein the cutting device is synchronized with the said flow rate such as to cut off equal portions of the passed melt. By synchronizing the cutting frequency with the flow rate of the melt through the die outlet opening, it is 5 possible to cut the melt at a predetermined length therewith controlling the shape of the particles to be formed upon solidification.

Preferably the portions of the melt after passing through the die and being cut by the cutting device have a length that corresponds with 0.7 to 5 times, in particular to 3 times, more in particular to 1.3 times the diameter of the outlet 10 opening of the die. More preferably, said length corresponds with the said diameter, in order to produce spherical particles.

The particles preferably have a particle size of 1.3 mm or lower, preferably of 0.9 mm or lower, more preferably 0.8 mm or lower. The particles preferably have a particle size 0.3 mm or more, preferably 0.4 mm or more. Said 15 particle sizes can be conveniently obtained by the choice of the diameter of the die outlet opening and of the length of the cut melt as indicated above.

Many polymers are known that are suitable to be used as polymer for the polymer comprising particle of the present invention. The polymer in the melt preferably comprises an optionally crosslinked polymer, an optionally crosslinked 20 copolymer, an optionally crosslinked terpolymer, an optionally crosslinked block(co) polymer or a mixture thereof.

The polymer is preferably thermoplastic in order to minimize abrasion in the fractures. The importance of the thermoplastic nature of proppants is e.g. explained in US7,322,411, herein incorporated by reference.

25 The polymer is preferably chosen from the group consisting of polyolefines, polystyrenes, acrylonitrile-butadiene-styrene polymers, aromatic or aliphatic partially crystallized polyesters, polycarbonates, polyamides, polymethylmethacrylate, polyacetals, polyphenyleneoxides, polyphenylesulfides, polyetheretherketons, polyetherketonketons, polysulphones, polyarylates or a 30 mixture thereof.

Below examples of suitable polymers are given. It should be noted however, that the said exemplified polymers must have a softening point, melt temperature and compressive strength as described above in order to be usable as a proppant.

13

As indicated above, the polymer preferably comprises a thermoplastic polymer. Examples of suitable thermoplastic polymers include, but are not limited to, polyamides, polyacetals, polyesters (including aromatic polyesters and aliphatic polyesters), liquid crystalline polyesters, polyolefins, polycarbonates, acrylonitrile-5 butadiene-styrene polymers (ABS), poly(phenylene-oxide)s, polyphenylene-sulfide^, polymethylmethacrylate, polysulphones, polyarylates, polyetherether-ketones (PEEK), polyetherketoneketones (PEKK), polystyrenes, and syndiotactic polystyrenes. Preferred thermoplastic polymers include polyamides and polyesters.

The density of unfilled polyamide 6,6 is e.g. about 1.1 g/ml. As 10 densities of a typical fracturing fluid are often about 0.8 to 1 g/ml, the polymer may include reinforcing materials and still be a suitable as proppant.

Polyamides may be homopolymers, copolymers, terpolymers, or higher order polymers. Blends of two or more polyamides may be used. Suitable polyamides can be condensation products of dicarboxylic acids or their derivatives 15 and diamines and/or aminocarboxylic acids, and/or ring-opening polymerization products of lactams. Suitable dicarboxylic acids include, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, isophthalic acid and terephthalic acid. Suitable diamines include tetramethylenediamine, hexamethylenediamine, octamethylene-diamine, nona-methylenediamine, dodecamethylenediamine, 2-methyl penta-20 methylenediamine, 2-methyl-octamethylene-diamine, trimethylhexa-methylene- diamine, bis(p-amino-cyclohexyl)methane, m-xylylenediamine and p-xylylene-diamine. A suitable aminocarboxylic acid is 11-aminododecanoic acid. Suitable lactams include caprolactam and laurolactam.

Preferred aliphatic polyamides include polyamide 6, polyamide 6,6, 25 polyamide 4,6, polyamide 6,9, polyamide 6,10, polyamide 6,12, polyamide 10,10, polyamide 11 and polyamide 12. Preferred semi-aromatic polyamides include poly(m-xylylene-adipamide) (polyamide MXD.6), poly(dodecamethylene-terephthalamide) (polyamide 12,T), poly(decamethyleneterephthalamide) (polyamide 10,T), poly-(nonamethylene-terephthalamide) (polyamide 9,T), the 30 polyamide of hexamethyleneterephthalamide and hexamethylene-adipamide (polyamide 6,T/6,6); the polyamide of hexamethyleneterephthalamide and 2-methylpentarnethylene-terephthalarnide (polyamide 6,T/D,T); the polyamide of hexamethylene-isophthalamide and hexamethylene-adipamide (polyamide 6,1/6,6); the polyamide of hexa-methyleneterephthalamide, hexamethylene- 14 isophthalamide, and hexamethylene-adipamide (polyamide 6.T/6, 1/6,6) and copolymers and mixtures of these polymers.

Examples of suitable aliphatic polyamides include polyamide 6/6 copolymer, polyamide 6,6/6,8 copolymer, polyamide 6,6/6,10 copolymer, 5 polyamide 6,6/6,12 copolymer, polyamide 6,6/10 copolymer, polyamide 6,6/12 copolymer, polyamide 6/6,8 copolymer, polyamide 6/6,10 copolymer, polyamide 6/6,12 copolymer, polyamide 6/10 copolymer, polyamide 6/12 copolymer, polyamide 6/6,6/6,10 terpolymer, polyamide 6/6,6/6,9 terpolymer, polyamide 6/6,6/11 terpolymer, polyamide 6/6,6/12 terpolymer, polyamide 6/6,10/11 10 terpolymer, polyamide 6/6,10/12 terpolymer and polyamide 6/6,6/PACM (bis-p-(aminocyclohexyl)methane)terpolymer.

The polymer preferably comprises polyethylene terephtalate, preferably crosslinked, the polyethylene terephtalate preferably being recycled polyethylene terephtalate, most preferably crosslinked recycled polyethylene terephtalate. 15 Polyetylene terephtalate, in particular crosslinked polyethylene terephtalate has appeared to be a very suitable polymer to be used as proppant, i.e. having the desired properties as discussed above. In a very attractive embodiment the polyethylene terephtalate is recycled polyethylene terephtalate, preferably crosslinked.

In another attractive embodiment, the polymer comprises polycarbonate.

20 Preferably the polymer melt comprises a filler. The filler can be used to reinforce the particles, and also to modulate the specific weight of the particles and reduce cost. The filler should be capable of reinforcing the polymer, while also reducing the potential for crush as exemplified below.

The filler is preferably chosen from the group of sand, carbon black, 25 graphite, mica, silica, silicon carbide, alumina, quartz, nanotubes, coconut, walnut, natural fibre, glass fibre, glass beads, hollow glass spheres, glass powder, glass fibers, ceramics, grits, clays (e.g., kaolin), staurolite (including staurolite sand) and wollastonite or a combination thereof. The fillers may be in a variety of forms such as ground particles, flakes, needle-like particles and the like. The size and form of the particles 30 should be selected such that they may easily be incorporated into the polymeric carrier and allow for the formation of proppants having the desired size.

The fillers preferably have a Mohs hardness of at least about 3, or more preferably of at least about 5, or yet more preferably of at least about 6, or still more preferably of at least about 7.

15

The fillers may optionally be pre-treated with one or more compatibilising and/or coupling agents that facilitate adhesion to or other compatibility with the polymer. Compatibilising and/or coupling agents may also be added to the filler and polymer mixture prior to or during melt blending to form the proppants. The compatibilising 5 and/or coupling agents may be used in about 0.01 to about 1 weight percent when they are added prior to or during melt blending. Examples of coupling agents suitable for use with sand or glass are silane coupling agents such as gamma-aminopropyltriethoxysilane (silane A-1100).

Preferably the particles according to the invention comprise about 20 to 10 about 100 weight percent of polymer and about 80 to about 0 weight percent filler, wherein the weight percentages are based on the total weight of the particles. In a particular embodiment of the invention the polymer comprises 100% polymer. In another embodiment the particles according to the invention comprise about 20 to about 80 weight percent of polymer and about 80 to about 20 weight percent filler.

15 The melts for the particles according to the invention that comprise one or more fillers can be formed by melt blending the fillers with the polymers. Any melt blending technique known in the art may be used. For example the component materials may be mixed using a melt-mixer such as a single- or twin-screw extruder, blender, kneader, roller, Banbury mixer, etc.

20 In a very attractive embodiment the melt is passed through a die plate having a plurality of dies. In this way, many particles can be produced in parallel simultaneously. A suitable die plate is e.g. described in US7,776,244. Therein the die plate is used for the preparation of expandable polystyrene. However proppants according to the present invention are preferably not expandable. Preferably the outlet 25 openings of the dies of the plate are equal, enabling the preparation of particles of equal size. In case a plurality of dies are used, the term ‘reduced’ with respect of the closure of the die outlet opening by solidification of the melt at the die outlet opening may also relate to the number of die outlet openings that are closed or partially closed as compared to the number of dies (partially) closed under the same process conditions 30 without the above described temperature control.

The die preferably has an inlet having an inlet opening having a diameter being larger than that of the outlet opening. The outlet comprises a number of individual outlet channels having a channel diameter. The outlet channel preferably has an inlet diameter that is at least equal to the outlet diameter. The die further preferably 16 comprises a die area located upstream of the channels having a diameter of at least the diameter of the outlet opening. The channel has preferably a total length of at least two times the diameter of the outlet diameter, preferably 6-4 times. These features are further in detail described in US7,776,224, herein incorporated by reference and it has 5 been found that these features contribute to an equal flow distribution over the number of outlet channels and thereby to a uniform particle diameter.

The pressure upstream of the die outlet opening is up to 250 bar, preferably up to 200 bar, more preferably up to 150 bar. The pressure is preferably at least 60 bar, more preferably at least 90 bar. As described above, the pressure may be 10 of influence to the clogging of the outlet opening of the die. A higher pressure may give less clogging.

Attractively, the particles are recovered from the solidifying medium. Many state of the art methods can be used to recover the particles, e.g. by filtration, centrifugation or by treatment in a cyclone.

15 In another embodiment the particles can be dried and sieved.

Preferably, the particles are dried after recovery, i.e. by exposure to a stream of air or an inert gas. In case water was used as solidifying medium, the particles are preferably dried by air. In case an organic liquid was used as solidifying medium it can be contemplated to wash the recovered particles with 20 water or any other liquid first. However the recovered particles can also be dried without being washed. In that case, an air stream can be used to dry the particles. It can also be advantageous to use an inert gas such as nitrogen for drying. By washing with water, the temperature of the particles may drop to an undesirably low temperature. The temperature is preferably kept as high as possible in order to 25 provide for a cost effective drying process. Said drying can e.g. take place in a vertical falling bed, wherein the particles flow downward, and wherein the drying medium such as air or nitrogen is blown in counter current upward direction. The crystallisation and drying step can preferably be efficiently combined into one unit operation.

30 If desired, e.g. when PET is used for the preparation of the proppant particles, the particles can attractively be cured by incubation of the particles at or above the crystallisation temperature Tpc thereof. Such a curing step, or crystallisation step, provides additional hardness to the particles. This curing or crystallisation can e.g. take place in a residence time crystalliser.

17

The crystallisation temperature is explained in figure 1 below. Although a separate curing step can be performed, the said curing can attractively take place during the solidifying process in the solidifying medium. In that case, the temperature of the solidifying medium must be chosen to be above the said Tpc of 5 the particles. The curing step can also attractively be combined with the drying step. In that case, the temperature of the drying medium should be above the said Tpc of the particles. Or the curing step can be performed by use of the latent heat of the particles or by external heating or by a combination thereof.

The invention also relates to a particle, obtainable by the method 10 according to the invention, having a diameter of 1.3 mm or less, preferably of 1.0 or less, more preferably of 0.8 or less, most preferably of 0.7 mm or less.

The particles according to the invention preferably have a density (i.e. specific weight) of 1.0 - 2.1 g/ml. The particles preferably have a density of above 1.05. Also, the particles preferably have a density of below 1.8 g/ml, preferably 15 below 1.35 g/ml and most preferably below 1.10 g/ml. To this end, the composition of the particle, comprising the polymer(s) and optionally filler(s) are preferably chosen to correspond with the said density.

. As described above the particles should stay in suspension during the fracturing process. This is achieved by modulating the specific weight to close 20 to that of the fracturing fluid.

Preferably, the particles obtainable by the method according to the invention have a particle size of 0.3 - 1.3 mm, more preferably of 0.3 - 0.9 mm, most preferably of 0.4 - 0.8 mm. These sizes can be conveniently be obtained by choosing a diameter of the die outlet opening to be equal to or somewhat smaller 25 or larger than the envisaged particle size, and to synchronize the cutting device with the flow rate of the melt passing the die outlet opening accordingly, preferably to allow a length of the cut melt to be 0.7 to 1.3 times the said diameter. By solidification, the cut melt solidifies into a spherical particle of optimal roundness and sphericity. There is a great demand for such particles, in particular for use as 30 proppants.

Both the polymer and filler(s) that are part of the particles of the invention should be relatively stable in the presence of typical down hole chemical environments and at the temperatures and pressures encountered in the application. For example, polyamide and polyester resins are well known for their 18 stability as engineering polymers under a variety of conditions. The stability requirements for a particular well depend e.g. on the temperature, pH, and pressure present and exposure time to these conditions that is required. To this end, the polymer comprising particle according to the invention preferably has an 5 acid solubility according to the API19C-8 standard of less than 5 %. Further, the polymer comprising particle preferably has a compressive strength of at least 45 N/mm2, preferably at least 50 N/mm2, more preferably 70 N/mm2, most preferably 100 N/mm2 as measured as indicated above and in the examples.

The compressive strength is preferably measure at a higher 10 temperature. The higher the temperature, the more the polymer tends to soften and to be less resistant to pressure. So polymers that have the required compressive strength at higher temperatures can be used at higher temperatures as well. Such temperatures are 80°C, 100°C, or 150°C or the operating temperature.

15 In a further preferred embodiment the particles according to the invention has a roundness and sphericity of 0.7 - 1, preferably of 0.8 - 1 according API19C. Such roundness can be obtained by underwater granulation techniques. Crushing elongated polymer bars into smaller particulate may also be an option, however this involves an additional processing step.

20 The particles according to the invention preferably have a pellet weight of 0.6 g or less. The term pellet weight is the weight of exactly 100 particles, and is therewith a convenient indication of the particle size.

The invention further relates to a collection of particles comprising a combination of a first plurality of particles according to the invention having a first 25 diameter, combined with a second plurality of particles according to the invention having a second diameter, the second diameter being different from the first diameter. By this a mixture of particles of different size is provided. The first plurality of particles can e.g. be produced with a first die, having another diameters for the die outlet opening as a second die, used for the preparation of the second 30 plurality of particles.

In a very attractive embodiment of the invention, the particles according to the invention or as defined by the method of the present invention are proppant particles. However the particles can be used for any desired purpose.

19

In another embodiment the polymer comprising particles according to the invention are used as component in a fracturing fluid.

The invention also relates to a fracturing fluid comprising particles according to the invention.

5 The invention also relates to a process for hydraulic fracturing of subterranean formations, comprising introducing a fluid in which particles according to the invention or as prepared according to the method according to the invention are suspended into an oil or gas well surrounded by rock, such, that fractures are created in the rock and some or all of the polymer comprising particles flow into the fractures.

10 Attractively, in the above process, particles of a first particle size are suspended in a first fluid and introduced into the oil or gas well in a first step, followed by at least a subsequent second step, wherein particles of a second particle size are suspended in a second fluid and introduced into the oil or gas well, wherein the second particle size is larger than the first. By this two-step approach, smaller particles are 15 allowed to enter the smaller regions of the fractures, where after larger particles are provided to be positioned in wider regions of the fractures.

Accordingly, the first step can preferably be followed by a plurality of subsequent steps, wherein each subsequent step differs from the preceding step in that the particle size of the subsequent step is larger than of the preceding step. This way, 20 increasingly larger particles are provided in subsequent steps,

The invention will now be further explained by the following figures and examples, wherein:

Figure 1 is a graph, showing a dsc curve according to ISO 11357-1 1997, Figure 2 shows two schematic diagrams A and B of extrusion dies for the 25 preparation of particles according to the invention, showing the principle of heating the die at the location of the die outlet opening,

Figure 3A-I show pellets of several samples as prepared according to the invention,

Figure 4 shows a reference table taken from the standard API19C for the 30 determination of particle roundness and sphericity,

Figure 5 is a schematical diagram of an apparatus for determination of the compressive strength of the particles of the invention, and

Figure 6 is a printout including a graph and a table, generated by software of the compressive strength measuring apparatus.

20

Referring to figure 1, which is taken from ISO 11357-1 1997, to which is expressly referred to herein, polymers may display specific behaviours called “glass transition”, “crystallisation” and “melting” as shown in the figure by ‘g’, ‘c’ and ‘m’, respectively. These transitions are characterised by the energy required for these 5 transitions and thereby become measurable as described in IS011357. Of relevance is Tpm, the peak indicating the melting point of the polymer, although polymers have a melting range starting at Tim and ending at T,m. Tpm, Tim, and Ttm, can easily be determined following the instructions of the IS011357 protocol.

In figure 2, the principle of heating of the die at the location of the die outlet 10 opening is shown. Figure 2A shows an arrangement of an extrusion die 20 used for the preparation of the particles of the present invention having a die outlet opening 21 and an inlet 22. The diameter of the die outlet opening 21 is smaller than the diameter of the inlet 22, the inlet having a conical shape towards the outlet opening 21. Between the inlet and outlet a die area 23 is present, having the same diameter as the die outlet 15 opening. The die area may also comprise a region having a smaller diameter than that of the outlet opening (not shown). In the die heating elements 24 are located, in close vicinity of the die outlet opening, therewith enabling the die having an elevated temperature at the die outlet opening. For explanatory reasons and the sake of ease, the heating elements are here designed as bars that can e.g. be heated by electricity. 20 However, because of the small dimensions of the die, and the arrangement of multiple die outlet openings in a single unit such as a die plate, the heating elements can e.g. be arranged around a plurality of outlet openings instead of around a single die outlet opening. The skilled person will be aware that many arrangements of the shown principle are possible. The polymeric melt enters the die through die inlet opening 22, 25 flows through die area 23 and leaves the die through outlet opening 21. A cutting device (not shown) may be mounted at the die outlet opening 21 so as to cut the melt passing through the die into equal portions. Leaving the die outlet opening 21 the melt is received in a reservoir (not shown) comprising solidifying medium, wherein the melt solidifies. Solidification of the melt at the die outlet opening is prevented by heating of 30 the die at the die outlet opening by heating elements 24. In figure 2 B, the same reference numbers are used for the same features as in figure 2A. The heating elements are designed as channels 25 located in the vicinity of the die outlet opening 21, where through a heat exchange medium can flow by which the die at the die outlet opening can be heated in order to avoid solidification of the polymeric melt at the die 21 outlet opening. Again, such channels can be arranged around a plurality of die outlet openings. Heating of the die at the location of the die outlet opening 21 can also be effected by other heating means, such as induction of the die of a portion thereof. A combination of different heating means can also be contemplated.

5 In figures 3A-I pellets of samples RPI139, RPI147, RPI085, RPI114, RPI071, RPI103, RPI056, RPI183 and RPI196 are shown respectively. 10 Particles are encircled that we used to assess roundness and sphericity according to standard API19C, see also the below examples. Each figure 3A-I also shows a reference table that is magnified in figure 4.

10 Figure 4 shows a reference table, used in standard API19C to determine the roundness and sphericity.

Figure 5 is a schematical graph of the apparatus Instron 3366 as used in the examples to determine the compressive strength of the particles of the invention. In a heating chamber C, a lower static plate A of a larger diameter and an upper moving 15 plate B of a smaller diameter B, oriented in parallel to plate A, are accommodated. A particle D is placed on static plate A. During operation, plate B moves downward towards static plate A, therewith exerting pressure to the particle D resulting in deformation of the particle. Both pressure and distance between plates A and B are recorded by the apparatus.

20 Figure 6 shows a print out generated by Bluehill software of the Instron apparatus of figure 5. In the graph, at the X-axis, the compression is indicated in mm, whereas on the Y-axis the compression force is given in Newton (N). The diameter of the uncompressed particle can be read from the table generated by the Bluehill software. The compressive strength of 15 samples was tested, samples 1 - 5 were 25 tested at 80°C, and samples 6 - 15 at 100°C. The size of the particles was determined by measuring the diameter of the uncompressed particle by measuring the distance between plates A and B at the moment that resistance is measured. The force needed to effect 0.1 mm compression is given in the second column of the upper table, and the force needed to effect 0.2 mm compression is given in the second column of the upper 30 table. These data are also presented in the graph.

22

Examples EXAMPLE 1

Preparation of pellets

Starting materials 5 Polymers

Cristal clear polystyrene: crystal 1810 obtained from Total Petrochemicals, 64170

Lacq, France.

Medium impact polystyrene: polystyrene impact 3630 obtained from Total 10 Petrochemicals, 64170 Lacq, France.

Polyethyleneterephthalate (PET): polyclear 1101 , Invista B.Bigles CH-6301 Zug, Switzerland.

Polycarbonate : Xantar 18 UR, Mitsubishi Engineering- Plastics Corporation 40549 Düsseldorf, Germany.

15 Recycled polyethylenetherephthalate (rPET): CorePET FR80 grün, Pet

Recycling, Arnhem, Netherlands.

Fillers

Calciumcarbonate: EXH1-OM, Omya D-89537 Giengen-Burgberg,

Germany.

20 Wollastonite 95, S&B Industrial Minerals GmbH, Otavi Minerals, D-41460

Neuss, Germany.

A polymer melt of starting materials according the formulations as given in table 1 was added via standard loss-in-weight feeders to the throat of a 6 barrel long extruder (Berstorff ZE60, Germany) a twin screw length of 2400 mm equipped with a 25 standard vacuum set-up. At the outlet of the extruder, a screen pack changer with 630/315 sieve (K-SWE-121/RS, BKG, Germany) was installed to free the polymer melt from any solid particles followed by a 70/70 high pressure melt pump (BKG, Germany) to generate the required die plate pressure, as given in table 1. The polymer melt was passed through a multiple die-plate with a die-opening arrangement according to figure 30 2, the outlet opening having a diameter of 0.5 and 0.75 mm, respectively, as shown in the table below and cut to size by a cutter hub on a pelletizer (type AH2000, BKG, Germany). The solidifying medium was water or ethylene glycol having a temperature as indicated in table 1. The process parameters for each formulation can be found in table 1.

23

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Determination of the theoretical particle size

From the extruder throughput, the density of the material, the number of die outlet openings, the cutting frequency of the cutter hub (to be determined by the number of blades on the cutter hub and the speed of the cutter hub) one skilled 5 in the art can easily calculate the expected particle size. In the example of RPI139, the throughput was 300 kg/h, the number of die outlet openings was 450 and the melt passing each die outlet opening was cut by 10 knives, rotating at 3515 rpm (rotations per minute). The density of the material was 1340 kg/m3. The volume of the pellets can be calculated by the formula throughput per second per die outlet 10 opening divided by the density of the polymer melt, divided by the cutting frequency of the cutter hub per second. In this example, the particle volume is therefore 300/3600/450/1340/10/(3515/60).109=0.236 mm3. This makes an expected particle size of about 0.76 mm diameter, as the shape is assumed to be spherical (volume =TT/6*d3, i.e. 3.14/6*0.763=0.236 mm3).

15

Determination of the actual particle size.

The above expected particle size was compared with the actual particle size. The latter was obtained by weighing 100 particles (herein also ‘pellets’) on a Sartorius A200S micro-scale from Sartorius GmbH, Gottingen Germany. From the said 20 weight, one skilled in the art can now calculate the actual average pellet volume and size through the known density from the formulation. An actual pellet weight above the theoretical particle weight is an indication of die-holes being clogged, as the same volume of material is passing through a lower number of open die outlet openings, resulting in a proportionally higher volume per open die outlet opening. In this example 25 the weight of 100 pellets is 0.0395 gram, or a volume per pellet of 0.0395*10/1.34=0.295 mm3 so the number of open holes is 450 *.2367295=360. The acceptable limit for weight of 100 pellets can be calculated upfront from the maximum allowable pellet size that is specified. Production was halted approaching this limit to free the frozen die-holes.

30 Individual particle size was also measured by measuring the diameter of individual pellets as outlined above and further explained below when determination of compressive strength is discussed.

26

Separation of the pellets

The water/pellet slurry was separated in a standard centrifugal dryer. The remaining solidifying medium that passes through the centrifugal dryer 0.3 mm sieve was treated in a BKG Optigon filter drum (BKG, Germany) to remove the undersized 5 pellets and to recuperate the solidifying medium that was recycled back to the pelletizer. The undersize pellets were either wasted, or remelted and recycled into the extruder. The hot and dried pellets coming of the centrifugal dryer were subsequently sieved on a 1.0 mm sieve to remove oversized material before packaging. The oversized material was wasted, or remelted and recycled into the extruder. In case of RPI139 and RPI183, 10 the final product was then cristallised in an infra-red rotary drum cristalliser at 175°C and a residence time of 17 minutes.

Determination of pellet roundness and sphericity 10 individual pellets of different samples as encircled in figures 4A-I were visually examined for roundness and sphericity and valued in accordance with the 15 standard API19C, of which a relevant reference table is shown in figure 5. For sample for RPI139, the average sphericity was e.g. determined to be 0.8 and the average roundness to be 0.9, based on the data as presented in the table of figure 4A. These averages are also presented in the above table 1.

Calculation of the pellet density (specific gravity).

20 The specific gravity was calculated from the specific gravity of the individual components in the actual ratio of the formulation. In case of example RPI139 the specific gravity was equal to the single component PET being 1.34 g/ml.

In case of example RPI085 the formulation consists of 85 wt.% polyethylene therephthalate with a specific gravity 1.34 g/ml and 15 % wollastonite with 25 a specific gravity of 2.70 g/ml. The composition therefore has a specific gravity of 0.85*1.34+0.15*2.70=1.61 g/ml.

Determination of the compressive strength

The compressive strength was determined for individual particles at 20% uniaxial compression at an elevated temperature of 80°C. Uniaxial means that 30 compression takes place in a single axis, e.g. by moving the two parallel oriented plates towards each other.

For this measurement, a compression tester from Instron type 3366 was used, see also figure 6. This instrument comprises a dual column load frame with a 10000N load cell. This load frame is usually used in polymer testing of tensile bars and 27 has internal equipment needed for tensile bars, but in this application the apparatus is used in compressive mode. The apparatus was controlled using Bluehill software version 2.29. The operation of this frame is fully described in the operator guide M10-16281-EN Revision B and system support M10-16282-EN Revision A. Between the dual 5 columns a heated oven Instron model 3119 was installed wherein the load cell and the 2 plates were accommodated to be able to uniformly heat the sample during testing and to perform the test at a required temperature. In this example, the temperature is set at 80°C, although another temperature such as 100°C can also be set.

Individual pellets are placed between 2 flat plates. The load cell measures 10 the force to compress an individual polymeric particle of the relevant size. The load measurement is digitally recorded as a function of the compression. To calibrate the compression tester it is a standard requirement to run a compression test without a particle to measure the deformation of the equipment used and all measurements must be corrected accordingly. This compensation is a feature of the Bluehill software.

15 The compression test is fully automatic and measures the load and the compression simultaneously. The upper plate of the load cell is moved towards the lower plate, on which the particle to be tested is placed. As soon as the upper plate arrives at the particle, a pressure resistance is recorded by the apparatus. The onset of the measurement is taken when a load of 1 N is detected on the load cell, and the 20 diameter of the individual pellet is recorded at this point, based on the distance between both plates. Upon increasing the load, the load versus compression is digitally recorded by the Bluehill software, and a graph of the progressing test is generated, see figure 6 for a particle of sample RPI139. As an example specimen 1 will be used.

The pressure at the point of 20% compression is now to be calculated. In 25 the graph of figure 6, the particle size was determined by the apparatus to be 0.53 mm. This means that 20% compression is 0.106 mm. From the table, it can be seen that 0.106 mm corresponds to a compressive force of slightly above 9.49 N. The exact value can however be taken from a digital table that the Bluehill software generates (not shown). The compressive strength is the load divided by the contact surface. The 30 contact surface is calculated by dividing the volume of the original pellet by the compressed height (For calculation reasons, the shape of the particle is assumed to be cylindrical, therewith neglecting the fact that the actual contact surface will be somewhat lower than the assumed surface because upon compression, a spherical particle will take the form of a barrel rather than of a cylinder). In this case the volume of the original 28 pellets is 3.14/6*0.533=0.0779 mm3. The contact surface at 20% compression now is 0.0779/(0.53*0.8)=0.184 mm2. The compressive strength is therefore 9.49/0.184=51.6 N/mm2.

EXAMPLE 2 5 Samples of RPI183 and RPI196 we produced by using ethylene glycol as solidifying medium at a temperature of 115°C and 121°C respectively. This solidifying medium has a lower heat conductivity than water (water: 0.6 W/m/°C whereas most organic liquids have a substantially lower heat conductivity in the range of 0.1-0.2 W/m/°C). Therefore the relatively cold solidifying medium flowing along the die-plate is 10 causing a reduced temperature loss of the melt at the die outlet openings of the die plate. Organic fluids, in particular liquids having a boiling temperature of more than 100°C, preferably between 100 - 200°C are suitable as solidifying medium, as long as said fluids do not significantly react with the polymeric melt passing through the die outlet opening. Preferably, the solidifying medium is inert with respect of the said 15 polymeric melt. The skilled person is capable to choose a suitable fluid, and to adapt the equipment if necessary, i.e. by adapting the chamber wherein the solidifying medium is accommodated. This enables the production of smaller pellets using common pelletizing equipment. From the table it can e.g. be observed that high values are obtained for roundness and sphericity of the particles of the samples RPI183 and RPI196.

20

Claims

A method for the preparation of particles comprising at least one polymer, wherein a melt comprising the polymer is extruded by flowing the melt through at least one die into a solidification medium having a temperature lower than the melting temperature of the melt, whereby the melt in the solidifying medium solidifies to the particles, characterized in that: - the melt is selected such that, in the solidified state, it has a density of 1.0-2.1 g / ml and a pressure resistance of at least 45

A method according to claim 1, wherein controlling the temperature of the melt at the outflow opening of the mold comprises maintaining the temperature of the mold, at least at its outflow opening, at a temperature higher than 60 ° C below the melting temperature Rpm of the polymer.

The method of claim 1 or 2, wherein the melting temperature Rpm of the polymer is at least 120 ° C, preferably at least 150 ° C, more preferably at least 18 ° C, most preferably at least 200 ° C.

4. Method according to any of the preceding claims, wherein the polymer has a VICAT softening point of at least 50 ° C, preferably at least 70 ° C.

5. Method according to any of the preceding claims, wherein in step a) the temperature of the melt is at least 180 ° C, preferably at least 200 ° C, more preferably at least 230 ° C, most preferably at least 30 ° C at least 260 ° C.

A method according to any of the preceding claims, wherein in step a) the temperature of the melt is at least 20 ° C above the melting temperature Rpm of the polymer, preferably 30 ° C above the melting temperature Rpm of the polymer, with more preferably 40 ° C above the melting temperature Rpm of the polymer.

7. Method according to any of the preceding claims, wherein the temperature of the solidification medium is 50 ° C or more below the melting point of the polymer, preferably 60 ° C or more below the melting point of the polymer and / or 100 ° C or less, preferably 80 ° C or less, below the melting point of the polymer.

8. Method according to any of the preceding claims, wherein the temperature at the outlet opening of the mold is kept at a temperature that is higher than 40 ° C below the melting temperature Rpm of the polymer, preferably higher than 20 ° C below said melting temperature Rpm, more preferably at said melting temperature Rpm, more preferably at least 20 ° C, even more preferably at least 30 ° C above the melting temperature Rpm of the polymer.

A method according to any one of the preceding claims, wherein the temperature at the outflow opening of the mold is kept above the onset melting temperature Tim of the polymer.

10. Method as claimed in any of the foregoing claims, wherein the temperature of the mold, at least at its outflow opening, is maintained by supplying external heat to the mold, or by generating heat in the mold.

10 N / mm 2 at 80 ° C, - the mold has an outflow opening with a diameter of 1.2 mm at most, and - the temperature of the melt at the outflow opening is controlled such that closure of the outflow opening of the mold by solidification The melt at the outflow opening of the mold is reduced.

11. Method as claimed in any of the foregoing claims, wherein the temperature of the mold, at least at its outflow opening, is maintained by supplying external heat to the mold by placing at least one channel in the mold located in the mold. proximity to the outflow opening, and by transporting a heat exchange medium through said channel; by heat induction of the mold at the location of the outflow opening; or by the temperature of the solidifying medium, or a combination thereof.

A method according to any of the preceding claims, wherein the temperature of the solidifying medium is above 50 ° C, preferably above 90 ° C, more preferably above 120 ° C, even more preferably above 140 ° C.

A method according to any of the preceding claims, wherein the temperature of the solidifying medium is below 250 ° C, preferably below 200 ° C.

A method according to any of the preceding claims, wherein the atmospheric boiling point of the solidification medium is above 90 ° C, preferably above 100 ° C, more preferably above 110 ° C, even more preferably above 120 ° C, with most preferred above 150 ° C.

15. A method according to any one of the preceding claims, wherein the coagulation medium is selected from the group consisting of water, liquid salt, an organic liquid or any mixture thereof.

A method according to any of the preceding claims, wherein the solidifying medium is inert to the melt and the particles.

17. Method according to any of the preceding claims, wherein the coagulation medium comprises an organic liquid with an atmospheric boiling point higher than 90 ° C, preferably higher than 100 ° C, even more preferably higher than 110 ° C, with even more preferably above 120 ° C, most preferably above 150 ° C.

A method according to any of the preceding claims, wherein the coagulation medium comprises a polar organic liquid.

A method according to any of the preceding claims, wherein the coagulation medium comprises an alcohol, ether, acid or ester, preferably an alcohol.

20. Method according to any of the preceding claims, wherein the coagulation medium comprises glycol, preferably ethylene glycol or propylene glycol or mixtures thereof.

A method according to any of the preceding claims, wherein the coagulation medium's thermal conductivity is less than 0.55 W / m / ° C, preferably less than 0.50 W / m / ° C, even more preferably below 0.40 W / m / ° C, most preferably below 0.30 W / m / ° C or below 0.2 W / m / ° C.

Method according to any of the preceding claims, wherein the outflow opening of the mold has a diameter of less than 1.0 mm, preferably less than 0.8 mm, even more preferably less than 0.75 mm and / or or more than 0.3 mm, preferably more than 0.4 mm.

A method according to any of the preceding claims, wherein the outflow opening of the mold cooperates with a cutting device which cuts off the melt passing through the outflow opening.

The method of claim 23, wherein the melt passes through the outflow opening 5 at a flow rate, and wherein the cutter is synchronized with this flow rate such that it cuts off equal portions of the melt passed.

25. Method according to claim 24, wherein the portions of the melt after having passed through the die and being cut by the cutter have a length corresponding to 0.7 to 5 times, preferably up to 3 times, even more preferably up to 1.3 times the diameter of the die outflow opening, said length preferably corresponding to said diameter.

26. A method according to any of the preceding claims, wherein the particles have a particle size of 1.3 mm or less, preferably 0.9 mm or less, even more preferably 0.8 mm or less and / or 0, 3 mm or more, preferably 0.4 mm or more.

27. A method according to any one of the preceding claims, wherein the melt polymer comprises a crosslinked polymer, whether or not crosslinked, a copolymer whether or not crosslinked, a terpolymer crosslinked or not crosslinked, a block of (co) polymer or not crosslinked a mixture thereof.

The method of any one of the preceding claims, wherein the polymer is thermoplastic.

29. Method according to any of the preceding claims, wherein the polymer is selected from the group consisting of polyolefins, polyethylenes, polypropylenes, polystyrenes, acetonitrile-butadiene-styrene polymers, aromatic or aliphatic partially crystallized polyesters, polycarbonates, polyamides, polyacetals, polyphenylene oxides, polyphenyl sulfides, polyether ether ketones, polyether ketone ketones, polysulfones, polyarylates or a mixture thereof.

The method of any one of the preceding claims, wherein the polymer comprises polyethylene terephthalate, preferably cross-linked, wherein the polyethylene terephthalate is preferably recycled polyethylene terephthalate, most preferably cross-linked recycled polyethylene terephthalate.

The method of any one of the preceding claims, wherein the polymer comprises polycarbonate.

The method of any one of the preceding claims, wherein the polymer melt comprises a filler.

A method according to any of the preceding claims, wherein the filler is selected from the group consisting of sand, carbon black, graphite, mica, silica, silicon carbide, alumina, quartz, nanotubes, coconut, walnut, natural fiber, fiberglass, glass beads, hollow glass beads, glass powder, glass fibers, ceramics, grit, clay (for example, kaolin), staurolite (including staurolite sand) and wollastonite or a combination thereof.

A method according to any of the preceding claims, wherein the melt is allowed to flow through a mold plate with a plurality of molds.

The method of claim 34, wherein the outflow openings of the molds of the plate are equal.

A method according to any of the preceding claims, wherein the mold has an inlet with an inlet opening with a diameter larger than that of the outflow opening.

37. Method according to any of the preceding claims, wherein the mold comprises a mold region which is located upstream of the outflow opening, with a diameter of at most the diameter of the outflow opening.

The method of claim 37, wherein the mold region has a length of at least twice the diameter of the outflow opening, preferably 6-4 times.

A method according to any of the preceding claims, wherein the pressure upstream of the outflow opening is up to 250 bar, preferably up to 200 bar, more preferably up to 150 bar and / or at least 60 bar, preferably at least 90 bar .

A method according to any of the preceding claims, wherein the particles are recovered from the solidification medium.

The method of claim 40, wherein the recovered particles are dried.

A method according to any of the preceding claims, wherein the particles are cured by incubating the particles at or above their crystallization temperature Tpc.

A method according to any of the preceding claims, wherein the particles are recovered from the coagulation medium and dried.

The method of claim 43, wherein the particles are dried by exposure to a stream of air or an inert gas.

A method according to any of the preceding claims, wherein the particles are cured, preferably during the solidification process, or, when it is carried out, during the drying step.

Particle obtainable by the method according to any of the preceding claims, with a particle size of 1.3 mm or lower, preferably of 1.0 mm or lower, more preferably of 0.8 mm or lower, with the most preferably 0.7 mm or lower.

47. Particles comprising at least one polymer, the particles having a density of 1.0-2.1 g / ml, a particle size of 1.3 mm or lower and a pressure resistance of at least 45 N / mm 2 at 80 ° C.

48. Particles according to claim 46 or 47, with a density above 1.05 g / ml and / or below 1.8 g / ml, preferably below 1.35 g / ml, most preferably below 1.10 g / ml.

Particles according to any of claims 46 to 48, wherein the particles have a pressure resistance of at least 45 N / mm 2, preferably of at least 50 N / mm 2, even more preferably 100 N / mm 2, most preferably of at least 150 N / mm 2 at 80 ° C.

The particles of claim 49, wherein the pressure resistance is at 100 ° C, preferably at 150 ° C.

Particles according to any of claims 46-50, wherein the polymer comprising particles have a diameter of 0.3-0.9 mm, preferably 0.4-0.8 mm.

Particles according to any of claims 46-51, with an acid solubility according to API19C-8 of less than 5%.

Particles according to any of claims 46-52, with a roundness and sphericity of 0.7-1, preferably of 0.8-1.

Particles according to any of claims 46-53, with a pellet weight of 0.6 g or lower.

A collection of particles comprising a combination of a first number of particles according to any of claims 46-55 with a first diameter, combined with a second number of particles according to any of claims 46-55 with a second diameter, wherein the second diameter differs from the first diameter.

56. Particles according to any of claims 46-55, or as defined in any one of claims 1-45, wherein the particles are 10 proppant particles.

The use of particles according to any of claims 46 to 56 as a component in a fracturing fluid.

58. Fracturing fluid comprising particles according to any of claims 46-57.

A method for hydraulically fracturing subterranean formations, including introducing a fluid, into which particles according to any of claims 46-58, or as prepared by a method according to any of claims 1-45, are suspended in an oil or gas source surrounded by rock so that fractures are formed in the rock and some or all of the particles comprising a polymer flow into the fractures.

60. Method for hydraulically fracturing subterranean formations according to claim 59, wherein in a first step, particles of a first particle size are suspended in a first fluid and introduced into the oil or gas source, followed by at least a following second step, wherein particles of a second particle size are suspended in a second fluid and introduced into the oil or gas source, wherein the second particle size is larger than the first.

A method for hydraulically fracturing subterranean formations according to claim 60, wherein the first step is followed by a plurality of subsequent steps, wherein each subsequent step differs from the previous step in that the particle size of the next step is larger than that of the previous step .