US 20070204989 A1
Expandable and hydrophilic polymeric particles may be made in a non-emulsion system, and with controllable hardness and delay in their time to swell in a fresh or salt water environment. These particles are prepared from combining monomers, controlled monomers, stable cross-linkers, initiators, and other agents, in aqueous solution. The controlled monomers induce kinetically controllable decomposition, degrading over time, thus inducing a desired time delay in particle swelling. The delay and degree of the swelling of the particles is controlled by selection of controlled monomer, stable cross-linking agents, monomers, and process conditions. These preformed particle gels are made to an initial particle size of 0.1 micron in diameter or larger via different grinding techniques. This composition is used for modifying the permeability of subterranean formations and thereby increasing the recovery rate of hydrocarbon fluids present in the formation.
1. A method of conformance control for oil and gas production by using gel-forming materials comprising preformed particles, wherein the particles have a controlled delay time before forming a gel and expanding significantly.
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a. [CR1R2═CR3—CO-]n esters of di, tri, tetra alcohols (I);
b. [CR1R2═CR3—O-]n esters of di, tri, tetra functional acids (II);
c. [CR1R2═CR3—CR4R5—O]n esters of di, tri, tetra functional acids (III);
d. [CR1R2═CR3—CO-]m amides (IV);
e. [CR1R2═C R3—]2 of bisazo (V);
f. [CR1R2═C R3—CR4R5—]2 of bisazo (VI); and, the derivatives of (I)-(VI), wherein R1═H or Me, R2═H or Me, R3═H or Me, R4═R5═H or Me, n=2, 3, or 4, and m=2, 3, or 4.
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25. A method of using mechanical or physical processes to grind controlled particle gel to sizes ranging from about 0.1 micron to 500 micron in diameter for the purpose of conformance control in oil and gas production.
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This application claims the benefit of U.S. Provisional Application No. 60/780,950, filed on Feb. 28, 2006, which is hereby incorporated by reference in its entirety as fully set herein.
The present invention relates to the field of hydrocarbon production. Particularly, the present invention relates to the manufacture of particles with improved physical and chemical characteristics that when added to injection water will further improve the crude hydrocarbon recovery from subterranean heterogeneous reservoirs.
Many reservoirs from which oil and gas are produced are not homogeneous in their geologic properties (e.g. porosity and permeability). In fact, for many of such reservoirs, the differences in the permeability (ability to allow fluid flow) among the different geologic layers can vary as much as several orders of magnitude.
Commonly a fluid, particularly water, is injected into an injection well completed into an oil reservoir. The injected water will mobilize and push some of the oil in place to a nearby production well where the oil and injected fluid are co-produced. A wide variation in the permeability (a property that measures the ability to transmit flow) among the geologic layer of rock that contain oil within its porous spaces in the subsurface reservoir causes such water injection to be not uniform, with the larger proportion of the water entering into the higher permeability geologic layers. This condition results in a very non-uniform displacement of the oil within the reservoir, with most of the oil quickly mobilized from high permeability layers and little from the lower permeability layers. The result is the fluid exiting production wells will have quickly a high percentage of water and less and less oil. The displacement process reaches the economic limit when the level of produced water is too high, and not enough oil is recovered, at a time when a large volume of oil remains in the bypassed, and not yet swept, lower permeability regions of the reservoir.
One approach to improve the oil displacement process is to provide some means to block, or at least significantly increase the flow resistance, selectively in these very high permeability geologic zones, sometimes called “thief zones”. If a process is successful in accomplishing this objective, then the water injected thereafter is diverted to enter now preferentially other geological layers of rock with lower permeability. This then forces the water to displace oil not before contacted significantly by the injection fluid. Such a process to make the injected fluid, such as water, sweep the oil reservoir in a more uniform fashion has been called “conformance control”.
Conformance control processes for an injection well are applied usually after offset production wells begin to experience a high fraction of water in the produced fluids. Recently, the research of injection fluid diversion has developed some chemical processes in which plugging agents are added to the injection water. These chemical systems have included bulk gel, sequential injection for in-situ gel formation, and colloidal dispersion gel (CDG). Bulk gel refers to adding polymer and a cross-linker to water and allowing some gel reaction to occur. This fluid is injected into an injection well for some period of time, followed by normal water injection. Another variation is to inject slugs of polymer solution and cross-linking solution separately, and then having a blocking gel form inside the reservoir as these chemicals mix together and react in-situ. The CDG process has a low concentration of polymer and cross-linker added together to the injection water being agitated at the surface. The process with a partial gel reaction and mixing at the surface creates fine particles before injection of this fluid. The concept is that the chemical components in this fluid will further react and create a stronger blocking gel in-situ.
However, these chemical conformance control methods have significant disadvantages: the bulk gel process described above requires high concentration of both polymer and cross-linker chemicals to make a strong gel, and the gelation time and physical properties are difficult to predict; sequential injection of polymer and crosslinker solution is questionable regarding controlling the time to create a gel in-situ and the strength of the gel that might form; colloidal dispersion gel (CDG) and the other previous methods described are unstable at more extreme reservoir conditions and therefore inadequate for reservoirs with high temperature (greater than 90 degree C.). In addition, the CDG is not suitable when the salinity exceeds 5000 ppm Total Dissolved Solids (TDS) and it is not able to block effectively the very high permeability channels.
Another, more recent, chemical approach that is gaining favor because it does not have the above disadvantages is the so-called Preformed Particle Gel (PPG) technology. The PPG particles typically are a powder product made up of a cross-linked polymer that will swell after their addition to a fresh or salt water. For their application the PPG particles are added to the injection water for some period of time, and then followed by normal water injection. These soft, swollen particles dispersed into an injected brine have the desirable property that as their suspension is injected, they will block the flow pores of the target, very high permeability geologic layers in a reservoir that have little oil remaining. Advantages of the PPG approach versus the other chemical systems described above include that the PPG product added has a known chemical composition, and that a PPG suspension in the injection water created at the surface will have predictable physical properties. In addition, these PPG suspensions can be stable and perform their desirable partial plugging action in the highest permeability zones of the reservoir at harsher reservoir conditions (up to 120 degree C. and in a brine containing up 300,000 ppm TDS).
The PPG technology, however, has two important limitations. First of all, these PPG particles swell almost immediately when exposed to water. This means that their desirable selective plugging action is confined to near the injection well, and thus these swollen particles are not able to penetrate deeply into the reservoir. This limits the volume of the reservoir that can be treated to divert the injection water into the lower permeability areas that contain high oil content. Secondly, the PPG particles commercially available have a relatively large size (hundreds of microns to millimeters in diameter). This limits their application to plugging only very high permeability (tens of Darcies) layers. In some cases the problem, highest permeability layer is not so high, perhaps less than 10 Darcies. In that case it would be advantageous to have a starting particle of a smaller size, in the range of tens of microns.
U.S. Pat. No. 5,662,168 (1997) by Smith discloses the process involves the use of a water soluble polymer in conjunction with an aluminum citrate preparation to function as a cross-linker for the polymer. However, it fails to realize the possible chromatographic separation in the subterranean formations when unreacted polymer and this cross linking agent are injected in separate solutions sequentially. This separation of components can cause the inefficient cross-linking reaction and only a very weak gel in-situ.
Representative preparations of PPG, cross-linked polymeric particles using various monomers, fillers, cross-linkers and initiators are described in CN Pat. No. 1,251,856A, 1,552,793A, 1,796,484A, and 1,439,692A. Liu, Y., et al. Paper SPE 99641 (2006) describes the common particle can only be injected into and move through those porous media with permeability of about 10 Darcies or greater. The Bai, et al, paper SPE 89468 (2004) discusses about the particle gel propagation behaviors through pore throats at both microscopic and macroscopic scales. Particle gel can move through porous media only if a driving pressure gradient is larger than a threshold pressure gradient. The chemistry of the above patents only describes a single cross-linking agent in the particles, particles of very large size (as much as millimeters in diameter), and that the particle gels swell as soon as they are mixed into water. Hence their applications will be only near the injection well bore and or for reservoirs with thief zones of very high permeability zones or fractures.
U.S. Pat. No. 5,465,792 (1995) and U.S. Pat. No. 5,735,349 (1998) by Dawson et al. (BJ Services) disclose the use of swellable cross-linked superabsorbent polymeric microparticles for modifying the permeability of subterranean formation. However, swelling of the superabsorbent microparticles described therein is induced by changes of the carrier fluid from hydrocarbon to aqueous or from water of high salinity to water of low salinity. The patents are not for preformed gel. All their examples and claims are for water soluble polymer in an emulsion condition. U.S. Pat. No. 6,454,003B1 (2002), U.S. Pat. No. 6,729,402B2 (2004) and U.S. Pat. No. 6,984,705B2 (2006) by Chang et al. disclose a composition comprising expandable cross-linked polymeric microparticles to be used for modifying the permeability of subterranean formations. Large percentages of surfactant are required for preparation of the microparticles via emulsification, which increases the product cost substantially. In addition, there is an additional environmental issue by including the surfactant, not to mention the added complexity of working with an emulsion system to make the product. The unexpanded particle size can only be as large as 10 micron due to the limitation of the microemulsion system. Such small particle may get into the low permeability matrix target zones and inadvertently plug areas rich in residual oil. Again, the system disclosed in these patents involves microemulsion rather than preformed gelation. Having a microemulsion will significantly increase the cost of the treatment product. Moreover, the micron level particle size makes the product less desirable for treating very high permeability zones or fractured reservoirs.
According to the previous references, a need exists for improving the full potential of performing in-depth conformance control treatment. For in-depth conformance control, adjustable initial particle size in non-emulsion solution with low threshold pressure is favorable. Moreover, none of the references cited consider an expandable and hydrophilic polymeric particle made in a non-emulsion system that has controllable size, hardness, and swelling delay when added to fresh water or a salty brine.
The present invention is a controlled particle that has a preferred size range from 10 micron to 5 millimeters. These expandable and hydrophilic polymeric particles are made in a non-emulsion system. Furthermore they have the property of allowing their swollen state in a fresh or salt water to be delayed to a specified time under different reservoir condition and to a controllable extent.
These particles are prepared from a chemical reaction involving one or more polymerizable monomers (typically acrylamide monomer), one or more controlled monomers whose decomposition is kinetically controllable, one or more stable cross-linkers, initiators, bases, reducing promoters, regulators, stabilizers, chelating agents, thermal agents, chain-transfer agents, oxygen scavengers, pH adjusters, and gel strength modifiers in aqueous solution. The selection of the reactants, especially the controlled monomers and their concentration will control the delay time before the onset of particle swelling and also influence the extent and physical properties of the swollen particle, when such particles are added to fresh or salt water.
Several methods may be employed to reduce the size of the created particle gel to a desired size. These include, but are not limited to, mechanical methods (such as fluid energy or jet mills, stirred media mills, ball mills, colloid mills, vibrating mills, rotor mills, cutting mills, disc mills, jaw crushers, and mortar grinders), physical methods (such as spray drying), or chemical methods (polymerization in suspension). Such practices may reduce the initial size of these particles to be as small as 0.1 micron in diameter.
These gel particles may be especially advantageous when used as selective plugging agents added to fresh or salt water injected into an oil reservoir. The features of having a controllable time delay before significant onset of swelling in water, and their smaller initial size give them the desirable capability to block target higher permeability rock layers further from the injection well than otherwise.
The present invention is to improve on the current particle and gel technology for selective plugging of high water flow channels in oil reservoirs. The Controlled Particle Gel (CPG) composition of the present invention is designed to overcome the main drawbacks of the in-situ gelation systems, which are lack of control of the timing and the extent of the gelation and flow resistance effect due to adsorption, dilution or degradation of the polymer, pH change. In addition, these chemical systems have limitations in their lack of stability to high temperature and salinity. The process disclosed here also is superior to the current Preformed Particle Gel (PPG) products that have little or no delay in their swelling behavior and are available only in sizes of hundreds of microns in diameter or larger. The CPG particles have the improved property that their delay in swelling time and extent may be controlled. This enhanced feature is due to the incorporation of a controlled monomer, which will decompose over a designed period of time that then triggers the significant expansion of the particles. Another improvement disclosed in the present invention is that any particle gel may be reduced in size to a diameter as small as 0.1 microns by mechanical, physical, or chemical methods. These improved properties allow such preformed particle gels to penetrate farther into an oil reservoir.
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The CPG particles for this invention may be made by reacting monomers, controlled monomers, stable cross-linkers, initiators, bases, reducing promoters, regulators, stabilizers, thermal agents, chain-transfer agents, oxygen scavengers, pH adjusters, gel strength modifiers, in aqueous solution under non-emulsion condition In the disclosure of CPG preparation, the term “Monomer” refers to nonionic monomer, anionic monomer, cationic monomer, zwitterionic monomer, betaine monomer, and amphoteric ion pair monomer. Nonionic monomer, anionic monomer, and cationic monomers are preferred. The representative nonionic monomers include vinyl amide, acryloylmorpholine, acrylate, maleic anhydride, N-vinylpyrrolidone, vinyl acetate, N-vinyl formamide and their derivatives, such as hydroxyethyl (methyl)acrylate CH2=CR—COO—CH2CH2OH (I) and CH2=CR—CO—N(Z1)(Z2) (2) N-substituted (methyl)acrylamide (II). R═H or Me; Z1=5-15C alkyl; 1-3C alkyl substituted by 1-3 phenyl, phenyl or 6-12C cycloalkyl (both optionally substituted) and Z2=H; or Z1 and Z2 are each 3-10C alkyl; (II) is N-tert. hexyl, tert. octyl, methylundecyl, cyclohexyl, benzyl, diphenylmethyl or triphenyl acrylamide. The vinyl amide is preferred nonionic monomer. Examples of vinyl amide include acrylamide, methacrylamide, N-methylacrylamide, N,N-dimethylacrylamide. The representative anionic monomers include polymerizable organic acids and their salts, and quaternary salts. The organic acids are preferred anionic monomer. Examples of organic acids include acrylic acid, methacrylic acid, maleic acid, itaconic acid, acrylamido methylpropane sulfonic acid, vinylphosphonic acid, styrene sulfonic acid. The representative cationic monomers include quaternary ammonium or acid salts of vinyl amide, vinyl carboxylic acid, methacrylate and their derivatives. The quaternary ammonium salt derivatives from acrylamide or acrylic acid are preferred cationic monomer.
The term “Controlled monomer” refers to kinetically controllable decomposition of monomers, wherein vinyl or allyl groups are bridged by one or more ethers, esters, azos, and amides, or other decomposable moieties. Representative controlled monomers include [CR1R2═CR3—CO-]n esters of di, tri, or tetra alcohols (I), [C R1R2═C R3—O-]n esters of di, tri, or tetra functional acids (II), [C R1R2═CR3—CR4R5—O]n esters of di, tri, or tetra functional acids (III), [CR1R2═CR3—CO-]m amides (IV), [C R1R2═C R3—]2 of bisazo (V), [C R1R2═C R3—CR4R5—]2 of bisazo (VI), and the derivatives of (I)-(VI). R1═H or Me, R2═H or Me, R3═H or Me, R4═R5═H or Me, n=2, 3, or 4, m=2, 3, or 4. Alcohols in (I) include ethyleneglycol, polyethyleneglycol, ethoxylated trimethylol, ethoxylated pentaerythritol, and their derivatives. Typical controlled monomers in class (IV) include N-tert. hexyl, tert. octyl, methylundecyl, cyclohexyl, benzyl, diphenylmethyl, triphenyl diacrylamides, diacrylamide, methacrylamide, piperazine diacrylamide, and their derivatives. Preferred controlled monomers include water soluble diacrylates and polyfunctional vinyl derivatives of a polyalcohol. More preferred controlled monomers include polyethylene glycol diacrylates. The monomers and controlled monomers may be polymerized and cross-linked in a non-emulsion aqueous solution.
The term “Aqueous solution” refers to water, buffer solvent, or other non-oil and non-surfactant solutions. The preferred solvent for aqueous solutions is deionized water.
The term “Stable cross-linker” refers to aluminum salt, zirconium salt, chromium salt or organic cross-linker. The organic cross-linkers, such as methylenebisacrylamide, hexamethylenetetramine, phenol aldehyde, are preferred. The stable cross-linker is optional according to specific subterranean conditions.
The initiators (e.g. ammonium persulfate, potassium persulfate, sodium persulfate, sodium bromate, sodium bisulfite, or mixture), optionally with bases (e.g. sodium carbonate, sodium bicarbonate, sodium hydroxide), reducing promoters (e.g. potassium metabisulfite, sodium sulfite, thionyl chloride, thionyl bromide), regulators (e.g. alcohols), stabilizers (e.g. phenol, m-dihydroxybenzene, hydroquinone), chelating agents (e.g. ethylene diamine tetra acetate), thermal agents (e.g. 2-acrylamido-2-methyl propane sulfonic acid), chain-transfer agents (e.g. thiols, formic acid and alkali metal formates such as sodium formate), oxygen scavengers (e.g. sodium sulfite, sodium bisulfite, sodium thiosulfate, sodium lignosulfate, ammonium bisulfite, hydroquinone, diethylhydroxyethanol, diethylhydroxylamine, methylethylketoxime, ascorbic acid, erythorbic acid, and sodium erythorbate), pH adjusters (e.g. sodium or potassium hydroxide), and gel strength modifiers (e.g. bentonite, lignocellulose, clay, montnorillonite, diatomite, kaolinoite, other fillers, or mixture), are employed to initiate the polymerization reaction.
In preparing the starting reaction mixture, the compounds to be polymerized are dissolved within an aqueous solution. The amount of aqueous solution, such as deionized water, may vary, but typically from 15 to 70% of the total weight of the initial reaction solution. The amount of monomers may vary, but typically are from 5 to 60% of the total weight of start reacting solution. The amount of controlled monomers may vary, but typically is from 0.01 to 30% of the total weight of the initial.reactinon solution. Depending upon the amount of total monomers, the stable cross-linker is typically from 0 to 5%, and the gel strength modifier is typically from 0 to 60%. The caustic component is optional to hydrolyze certain monomers, such as acrylamide, and its amount of use may vary, but typically from 0 to 10%. The pH adjusters may be necessary. The typical pH range of reacting solution is 6.5 to 11. The reducing promoters, regulators, stabilizers, chelating agent, thermal agent, chain-transfer agent, oxygen scavenger are optional according to the specific injection water and subterranean formation, and their amounts of use may vary. The order of addition for the reactants may vary; the typical order is the least polar compound first to ensure it can be dissolved completely, then followed by more polar compounds.
After the initial reaction mixture is agitated, at an ambient temperature, typically from 15 to 30 degree C., the initiator or initiators mixture is then slowly added into the dynamically mixed, sheared or oscillated reacting solution to achieve a homogenized reacting condition. The amount of initiator may vary according to the monomers concentration, but typically from 0.01% to 0.2%. Because of the exothermic nature of the reaction initiated by the addition of the initiators, evidence of the reaction is inferred by an increased temperature. Preferably the reaction is kept at the initial temperature by means of having the reactor jacketed with a cooling fluid, or having the reactor surrounded by a vessel containing a circulating fluid. A gradual increase of the temperature of the reacting system is also acceptable. The result of the reaction process will result in a fine gel ready for the post-treatment. The polymerizing and cross-linking reaction are preferably carried out in oxygen free or in a reduced oxygen environment. However, short exposure to air is also acceptable.
The reaction can be performed in either a batch process or continuous process. Due to the fast polymerization reaction, typically several minutes, the continuous process is preferred for medium to large scale production. The deoxygenated monomers and supplemental materials are continuously pumped to a reaction vessel, and the reacted gel is continuously transferred away from the vessel. The gel is squeezed through small holes and cut to small particles or lumps for stepwise baking, breaking, sieving post-treatments. The baking temperature may vary according to the specific formulation, but the typical baking temperature is 15 to 20 degree C. lower than the decomposing temperature of controlled monomers used in the formulation.
For small to medium scale production, the batch process is also preferred. The initial reacting solution, mixed monomers and supplemental materials, is deoxygenated with inert gas, such as nitrogen, for about 30 to 50 minutes. Polymerization is initiated at room temperature. The temperature typically rises to about 60 degree C. or higher by the heat released during polymerization. The polymerized mixture is typically kept at that higher temperature, usually from 65 to 80 degree C. to complete the reaction, resulting in production of soft gel lumps. The gel lumps are dried on trays in an oven, and then are ground to desirable sizes.
The size reduction of resulting polymeric gel particles can be achieved by mechanical methods (e.g. fluid energy or jet mills, stirred media mills, ball mills, colloid mills, vibrating mills, rotor mills, cutting mills, disc mills, jaw crushers, and mortar grinders), physical methods (e.g. spry drying) or chemical methods (e.g. polymerization in suspension). The mechanical grinding approaches by jet mill, ball mill and colloid mill are preferred. The reported data indicates that industrial scale ball mill can grind hard, brittle materials under 1 micron in size (e.g. Planetary ball mill by Retsch). The in-house test described in an example in this application shows the dry gel particle after being ground in a laboratory scale ball mill jar, can pass through a 400 mesh sieve, an opening of less than 37 micron diameter. An industrial scale colloid mill can grind colloid particles down to 1 micron in diameter, and an in-house test shows the gel particle suspension, after grinding in a laboratory colloid mill, can pass 200 mesh sieve, less than 74 micron diameter, under 20 psi positive pressure.
Due to the characteristics of the size of initial particle, its hydrophilic nature, and that it contains controlled monomer that will decompose to allow the particle to swell in a predictable manner, the composition of this invention can propagate far into the reservoir.
In a preferred aspect of this embodiment, this CPG composition is added to injection water as part of a secondary water recovery process, tertiary carbon dioxide injection, chemical, or air injection for recovery of hydrocarbon from subterranean sandstone or carbonate formation. This will provide controlling the near well-bore and in-depth formation conformance vertically and laterally by selectively blocking the high water channels. The composition can be added in an amount from about 50 to 20,000 ppm, preferably from about 500 to 5000 ppm and more preferably from about 1000 to 3000 ppm based on solid content, with produced water, sea water, or fresh water.
The forgoing may be better understood by reference to the following examples, which are presented for purposes of illustration and are not intended to limit the scope of this invention.
In the present example, a single aqueous phase was prepared by adding 8.25 g acrylamide, 21.75 g sodium salt of 2-acrylamido-2-methylpropane sulfonic acid, 0.386 g polyethylene glycol 200 diacrylate, and 0.0004 g methylene bisacrylamide to 30.6 g deionized water with then mixing. At an ambient temperature of 15-30° C., an initiator mixture of 400 μl 5% sodium bromate and 400 μl 5% sodium bisulfite was added slowly to the solution with strong mixing. Within about 5 minutes, the reaction of polymerization took place with heat released, resulting in a fine gel.
In this example, the procedure of example 1 was repeated except that 6.10 g polyethylene glycol 200 diacrylate and no methylene bisacrylamide were added to the formula. All other components and reaction conditions remained the same.
Comparison of swelling behavior of Sample 27 versus Sample 31 demonstrates the controllability the swell time and extent with our composition.
Sample 27 and Sample 31 suspensions were both prepared at a concentration of 1 wt % in distilled water and had the pH adjusted to be between 8 to 9. A portion of each suspension was aged at 40 degree C. and at 60 degree C. for 2 days. The results are shown below:
These results demonstrate that the composition of the Sample 27 is suitable for an application where the controlled monomer is designed to decompose within 2 days at 60 degree C. Furthermore, the fact that the Sample 27 remains in a fluid state over the same aging time at 40 degree C. indicate that for Sample 27 the mechanism for the loss of effectiveness of the controlled monomer, thereby causing particle expansion and a gel to form is related to its exposure to a greater extreme in temperature to 60 degree C. And Sample 31 is suitable when a longer time delay before significant particle expansion and gelation is desirable.
Grinding of Sample 27 can reduce size so that it may have a small diameter and thereby pass through smaller pore holes.
Sample 27 particles (described in Example 1 and Example 3) were added to a 0.3 wt % NaCl solution at a concentration of 1000 ppm. This particulate suspension was added to a pressure vessel. A nitrogen gas line was connected to the top of the pressure vessel and the gas pressure was adjusted to 20 psi. A valve at the bottom of the vessel was opened and the fluid exited and passed through a 200×200 mesh metal screen mounted in a sealed holder. After injection of approximately 100 ml of the particle suspension, the screen was inspected and found to have a significant coating of the particle gel on the entire surface.
Next a portion of this same Sample 27 suspension initially made to a concentration of 1000 ppm in a 0.3 wt % NaCl brine was added to a laboratory colloidal mill and exposed to 60 minutes of grinding time. This suspension of particles after grinding also were passed through a clean 200×200 mesh screen under 20 psi of driving gas pressure. In this case the screen has a much cleaner appearance with only slight evidence of any solids accumulation in or on the metal screen.
This example illustrates it is possible to reduce the size of these particles significantly by grinding. Because the hole size in a 200×200 mesh screen is about 75 microns in size, this demonstrates it is possible to grind these particles to a size less than 75 microns. Based on a rule of thumb that particles must have a diameter less than one-third the hole size to pass thoroughly successfully, it is estimated that the 60 minutes of grinding reduced the average particle size to about 25 microns in diameter. The ground particles maintain the same composition and have the same delayed swelling behavior as for the original particles.
The particles created are strongly hydrophilic and will remain primarily in the water phase.
The Sample 27 particles described in Example 1 were added as a 0.5 wt % suspension into a distilled water solution of 80 milliliters volume. Next, this 80 milliliters of particle suspension fluid was poured into a glass separatory funnel, followed by 80 milliliters of n-decane. The funnel was shaken by hand vigorously for 5 minutes, and then left standing for overnight to allow separation of the aqueous and hydrocarbon phases. Next, the bottom aqueous layer was drained off from the separatory funnel into a wide dish. This pre-weighed dish was heated until all of the liquid has evaporated. After cooling, the dish was re-weighed to determine the mass of solid particles remaining in the aqueous phase taken from the separatory funnel. By this method, over 95% of the initial mass of the particles from Sample 27 remained in the aqueous phase. This is an insignificant decrease, and is nearly the same mass of particles as the starting amount. These results confirm that the suspended particles are hydrophilic in nature and have a much stronger affinity for the aqueous phase than a hydrocarbon phase.