Enhanced oil recovery method using a (co)polymer of a hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid

Abstract

A process for enhanced oil recovery comprising the following steps: a) Preparation of an injection fluid comprising at least one water-soluble (co)polymer prepared at least from 2-acrylamido-2-methylpropane sulfonic acid (ATBS) or from at least one of its salts, with water or with brine, where the 2-acrylamido-2-methylpropane sulfonic acid is a hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid having a 2-theta powder X-ray diffraction diagram comprising peaks at 10.58°, 11.2°, 12.65°, 13.66°, 16.28°, 18.45°, 20°, 20.4°, 22.5°, 25.5°, 25.88°, 26.47°, 28.52°, 30.28°, 30.8°, 34.09°, 38.19°, 40.69°, 41.82°, 43.74°, 46.04° degrees. b) Injection of injection fluid into an underground formation, c) Flushing of the underground formation using the fluid injected, d) Recovery of an aqueous and hydrocarbon mixture.

Claims

1. A process for enhanced oil recovery comprising the following steps: a. preparing an injection fluid comprising at least one water-soluble (co)polymer prepared at least from 2-acrylamido-2-methylpropane sulfonic acid (ATBS) or from at least one of its salts, with water or with brine, wherein the 2-acrylamido-2-methylpropane sulfonic acid is a hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid having a 2-theta powder X-ray diffraction diagram comprising peaks at 10.58°, 11.2°, 12.65°, 13.66°, 16.28°, 18.45°, 20°, 20.4°, 22.5°, 25.5°, 25.88°, 26.47°, 28.52°, 30.28°, 30.8°, 34.09°, 38.19°, 40.69°, 41.82°, 43.74°, and 46.04° degrees; b. injecting injection fluid into an underground formation; c. flushing of the underground formation using the fluid injected; and d. recovering an aqueous and hydrocarbon mixture.

2. The process according to claim 1, wherein the injection fluid comprises between 10 ppm and 15,000 ppm of water-soluble (co)polymer.

3. The process according to claim 2, wherein the water-soluble (co)polymer is prepared at least from 2-acrylamido-2-methylpropane sulfonic acid of which 50% to 100% is in the hydrated crystalline form.

4. The process according to claim 2, wherein the water-soluble (co)polymer is a (co)polymer obtained at least from 2-acrylamido-2-methylpropane sulfonic acid of which 50% to 100% is in the hydrated crystalline form and of at least one nonionic monomer and/or at least one anionic monomer and/or at least one cationic monomer and/or a zwitterionic monomer.

5. The process according to claim 4, wherein the nonionic monomer is chosen from acrylamide, methacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N-methylolacrylamide, N-vinylformamide, N-vinyl acetamide, N-vinylpyridine, N-vinylpyrrolidone, N-vinyl imidazole, N-vinyl succinimide, acryloyl morpholine (ACMO), acryloyl chloride, glycidyl methacrylate, glyceryl methacrylate, diacetone acrylamide and isoprenol.

6. The process according to claim 4, wherein the anionic monomer is chosen from acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, acrylamido undecanoic acid, 3-acrylamido 3-methylbutanoic acid, maleic anhydride, vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, methallylsulfonic acid, 2-methylidenepropane-1,3-disulfonic acid, 2-sulfoethylmethacrylate, sulfopropylmethacrylate, sulfopropylacrylate, allylphosphonic acid, styrene sulfonic acid, 2-acrylamido-2-methyl propane disulfonic acid; and water-soluble salts of these monomers like their alkali metal, alkaline earth metal, or ammonium salts.

7. The process according to claim 2, wherein the water-soluble (co)polymer is a (co)polymer obtained at least from 2-acrylamido-2-methylpropane sulfonic acid of which 50% to 100% is in the hydrated crystalline form and of at least one nonionic monomer and/or at least one anionic monomer, and wherein: the nonionic monomer is chosen from acrylamide, methacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N-methylolacrylamide, N-vinylformamide, N-vinyl acetamide, N-vinylpyridine, N-vinylpyrrolidone, N-vinyl imidazole, N-vinyl succinimide, acryloyl morpholine (ACMO), acryloyl chloride, glycidyl methacrylate, glyceryl methacrylate, diacetone acrylamide and isoprenol; and the anionic monomer is chosen from acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, acrylamido undecanoic acid, 3-acrylamido 3-methylbutanoic acid, maleic anhydride, vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, methallylsulfonic acid, 2-methylidenepropane-1,3-disulfonic acid, 2-sulfoethylmethacrylate, sulfopropylmethacrylate, sulfopropylacrylate, allylphosphonic acid, styrene sulfonic acid, 2-acrylamido-2-methyl propane disulfonic acid; and water-soluble salts of these monomers like their alkali metal, alkaline earth metal, or ammonium salts.

8. The process according to claim 7, wherein the injection fluid comprises between 50 ppm and 10,000 ppm of (co)polymer.

9. The process according to claim 8, wherein the injection fluid comprises between 100 and 5,000 ppm of (co)polymer.

10. The process according to claim 7, wherein the (co)polymer is obtained at least from the 2-acrylamido-2-methylpropane sulfonic acid of which 50% to 100% is in the hydrated crystalline form and the at least one nonionic monomer.

11. The process according to claim 7, wherein the (co)polymer is obtained at least from the 2-acrylamido-2-methylpropane sulfonic acid of which 50% to 100% is in the hydrated crystalline form and the at least one anionic monomer.

12. The process according to claim 1, wherein the water-soluble (co)polymer is prepared at least from 2-acrylamido-2-methylpropane sulfonic acid of which 50% to 100% is in the hydrated crystalline form.

13. The process according to claim 1, wherein the water-soluble (co)polymer is a (co)polymer obtained at least from 2-acrylamido-2-methylpropane sulfonic acid of which 50% to 100% is in the hydrated crystalline form and of at least one nonionic monomer and/or at least one anionic monomer and/or at least one cationic monomer and/or a zwitterionic monomer.

14. The process according to claim 13, wherein the nonionic monomer is chosen from acrylamide, methacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N-methylolacrylamide, N-vinylformamide, N-vinyl acetamide, N-vinylpyridine, N-vinylpyrrolidone, N-vinyl imidazole, N-vinyl succinimide, acryloyl morpholine (ACMO), acryloyl chloride, glycidyl methacrylate, glyceryl methacrylate, diacetone acrylamide and isoprenol.

15. The process according to claim 13, wherein the anionic monomer is chosen from acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, acrylamido undecanoic acid, 3-acrylamido 3-methylbutanoic acid, maleic anhydride, vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, methallylsulfonic acid, 2-methylidenepropane-1,3-disulfonic acid, 2-sulfoethylmethacrylate, sulfopropylmethacrylate, sulfopropylacrylate, allylphosphonic acid, styrene sulfonic acid, 2-acrylamido-2-methyl propane disulfonic acid; and water-soluble salts of these monomers like their alkali metal, alkaline earth metal, or ammonium salts.

16. The process according to claim 1, wherein the injection fluid comprises between 50 ppm and 10,000 ppm of (co)polymer.

17. The process according to claim 16, wherein the injection fluid comprises between 100 and 5,000 ppm of (co)polymer.

18. The process according to claim 1, wherein the water-soluble (co)polymer is a homopolymer prepared from the hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid.

19. The process according to claim 18, wherein the injection fluid comprises between 50 ppm and 10,000 ppm of (co)polymer.

20. The process according to claim 19, wherein the injection fluid comprises between 100 and 5,000 ppm of (co)polymer.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1 illustrates the X-ray diffraction diagram of the crystals obtained according to example 1.

(2) FIG. 2 illustrates the X-ray diffraction diagram of the crystals obtained according to example 2.

(3) FIG. 3 illustrates the Fourier transform infrared spectrum of the crystals obtained in example 1.

(4) FIG. 4 illustrates the X-ray diffraction diagram of the crystals obtained according to example 2.

(5) FIG. 5 illustrates the filter ratio as a function of the form of ATBS and the molecular weight of (co)polymers.

(6) FIG. 6 illustrates the viscosity loss as a function of the form of ATBS and the iron content of (co)polymers.

(7) FIG. 7 illustrates the viscosity loss as a function of the form of ATBS and aging at 90° C. of (co)polymers.

(8) FIG. 8 illustrates the filter ratio as a function of the form of ATBS and the molecular weight of homopolymers.

(9) FIG. 9 illustrates the viscosity loss as a function of the form of ATBS and the iron content of homopolymers.

(10) FIG. 10 illustrates the filter ratio as a function of the form of ATBS and the molecular weight of post-hydrolyzed (co)polymers.

EXAMPLE EMBODIMENTS OF THE INVENTION

Example 1: Synthesis of 2-acrylamido-2-methylpropane Sulfonic Acid

(11) To a stirred 2000-mL jacketed reactor, 1522 grams of acrylonitrile was added containing 0.4% of water by weight and 180 grams of fuming sulfuric acid titrating at 104% H.sub.2SO.sub.4 (18% Oleum). The mixture was stirred for 1 hour and cooled via the reactor jacket, which held the temperature of the sulfonating mixture at −20° C.

(12) To the previous sulfonating mixture, 97 grams of isobutylene was added, at a flow rate of 1.6 grams/minute.

(13) The temperature of the mixture was controlled at 45° C. while isobutylene was added. The particles of 2-acrylamido-2-methylpropane sulfonic acid precipitate in the mixture and the solid content was about 20% by weight. The reaction mixture was filtered on a Buchner filter and dried under vacuum at 50° C. The solid obtained was 2-acrylamido-2-methylpropane sulfonic acid; it was present in the form of a very fine white powder.

Example 2: Synthesis of the Hydrated Crystalline Form of 2-acrylamido-2-methylpropane Sulfonic Acid

(14) To a 2000-mL jacketed reactor, 500 grams of 2-acrylamido-2-methylpropane sulfonic acid obtained in example 1 and 460 grams of sulfuric acid at a concentration of 10% H.sub.2SO.sub.4 were added.

(15) 250 mg of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl was added to the preceding mixture.

(16) The mixture was stirred for 10 minutes, at 20° C., to form suspension A.

(17) Suspension A was heated to a temperature of 60° C. and maintained at this temperature for 20 minutes to form solution B.

(18) Solution B was cooled to a temperature of 10° C. The cooling time between 60° C. and 10° C. was 6 hours. Suspension C of crystals of 2-acrylamido-2-methylpropane sulfonic acid was obtained. Suspension C was filtered on a vertical Robatel centrifugal dryer. A solid of composition 1 was obtained, containing 80% by weight of 2-acrylamido-2-methylpropane sulfonic acid crystals.

Example 3: X-Ray Diffraction Analysis

(19) The solids obtained in examples 1 and 2 were previously ground to form powders and were analyzed by X-ray diffraction over an angular range from 10 to 90°. The equipment used was a Rigaku miniflex II diffractometer equipped with a copper source.

(20) We observed that the solid obtained from example 2 (FIG. 2) has a 2-theta X-ray diffraction diagram with the following characteristic peaks:

(21) 10.58°, 11.2°, 12.65°, 13.66°, 16.28°, 18.45°, 20°, 20.4°, 22.5°, 25.5°, 25.88°, 26.47°, 28.52°, 30.28°, 30.8°, 34.09°, 38.19°, 40.69°, 41.82°, 43.74°, 46.04° 2-Theta degrees (+/−0.1°).

Example 4: Fourier Transform Infrared Measurement

(22) The equipment for Fourier transform infrared measurement was the Perkin Elmer Spectrum 100, whose precision is 8 cm.sup.−1.

(23) The solids obtained in examples 1 and 2 were sieved at 100 μm. The particles remaining on the sieve were dried and put in the oven at 60° C. for at least 4 hours.

(24) 10 mg of solid was weighed precisely and mixed with 500 mg of potassium bromide (KBr). The mixture was then compacted in a hydraulic press under a pressure of at least 10 bars.

(25) We observed that the following bands (FIG. 4) are characteristic of the hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid:

(26) 3280 cm.sup.−1, 3126 cm.sup.−1, 1657 cm.sup.−1, 1595 cm.sup.−1, 1453 cm.sup.−1, 1395 cm.sup.−1, 1307 cm.sup.−1, 1205 cm.sup.−1, 1164 cm.sup.−1, 1113 cm.sup.−1, 1041 cm.sup.−1, 968 cm.sup.−1, 885 cm.sup.−1, 815 cm.sup.−1, 794 cm.sup.−1.

(27) The infrared spectrum of the solid according to example 1 (FIG. 3) did not present the same peaks.

Example 5: Preparation of the Crystalline Hydrated Form of Acrylamide/2-acrylamido-2-methylpropane Sulfonic Acid (Co)Polymer (75/25 Mole %)

(28) To a 2000 mL beaker are added 549.5 g of deionized water, 520.5 g of 50% acrylamide solution, 97.6 g of 50% sodium hydroxide, 16.2 g of urea and 316.2 g crystals of 2-acrylamido-2-methylpropane sulfonic acid obtained in example 2.

(29) The resulting solution is cooled between 5 and 10° C. and transferred to an adiabatic polymerization reactor, then nitrogen is bubbled for 30 minutes to remove all traces of dissolved oxygen.

(30) The following are then added to the reactor: 0.45 g of 2,2′-azobisisobutyronitrile, 1.5 mL of a solution at 2.5 g/L of 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 1.5 mL of a solution at 1 g/L of sodium hypophosphite, 1.5 mL of a solution at 1 g/L of tert-butyl hydroperoxide, 1.5 mL of a solution at 1 g/L of ammonium iron (II) sulfate hexahydrate (Mohr's salt).

(31) After a few minutes, the nitrogen inlet is shut and the reactor is closed. The polymerization reaction occurs for 2 to 5 hours until a temperature peak is reached. The rubbery gel obtained is chopped and dried to obtain a coarse powder itself milled and sieved to obtain the polymer in powder form.

Example 6: Preparation of the Non-Crystalline Hydrated Form of the Acrylamide/2-acrylamido-2-methylpropane Sulfonic Acid (Co)Polymer (75/25 Mole %)

(32) The polymers are prepared as in example 5, replacing the crystalline hydrated form of 2-acrylamido-2-methylpropane sulfonic acid (example 2) with 2-acrylamido-2-methylpropane sulfonic acid that is not the crystalline hydrated form synthesized in example 1.

Example 7: Measurement of the Filter Ratio for Polymer Solutions

(33) Filtration tests were conducted on 3 polymers prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid P′1, P′2 and P′3 with respective increasing molecular weights 6.5, 9 and 11.5 million Da, prepared as described in example 6, and on 4 polymers prepared from the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid P1, P2, P3 and P4 with respective increasing molecular weights 6.5, 9, 11 and 13 million Da, prepared as described in example 5. The molecular weight grade 13 million Da is not accessible when the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid is used.

(34) The polymer solutions were prepared at an active concentration of 1,000 ppm in a brine containing water, 30,000 ppm of NaCl and 3,000 ppm of CaCl.sub.2.2H.sub.2O. The filter ratio (FR) was measured in filters having a pore size of 1.2 μm representative of deposits with low permeability. These results are shown in FIG. 5.

(35) TABLE-US-00001 TABLE 1 Polymers tested for the filter ratio Form of Molecular weight ATBS used (in million Da) Filter ratio P1 Crystalline 6.5 1.05 P2 Crystalline 9 1.03 P3 Crystalline 11 1.05 P4 Crystalline 13 1.25 P′1 Non-crystalline 6.5 1.11 P′2 Non-crystalline 9 1.15 P′3 Non-crystalline 11.5 1.38

(36) We can observe that at equivalent molecular weight, the polymers prepared from the hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P1-P3) always have a FR lower than that of the polymers prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P′1-P′3). This difference becomes bigger and bigger when the molecular weight of the polymer increases. The polymer made from the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid with molecular weight 13 million Da (P4) even has a lower FR than the polymer prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid with lower molecular weight (11.5 million Da, P′3).

Example 8: Measurement of the Resistance to Chemical Degradation of Solutions of Polymers with Equivalent Molecular Weights

(37) Resistance tests for chemical degradation of polymers P3 and P′3 were conducted in aerobic conditions in the presence of different iron(II) concentrations (2, 5, 10 and 20 ppm) in a brine composed of water, 37,000 ppm of NaCl, 5,000 ppm of Na.sub.2SO.sub.4 and 200 ppm of NaHCO.sub.3. These tests have been conducted on a polymer prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P′3) and on a polymer made from the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P3). The two polymers have the same chemical composition. The results after 24 h of contact of the polymer solution with the contaminant are shown in FIG. 6.

(38) We can observe that for each iron(II) concentration, polymer P3 loses less viscosity than the equivalent polymer P′3.

Example 9: Measurement of the Resistance to Thermal Degradation of Solutions of Polymers with Equivalent Molecular Weights

(39) Tests of resistance to thermal degradation for polymers P3 and P′3 were conducted in anaerobic conditions at an active concentration of 2,000 ppm in a brine composed of 30,000 ppm of NaCl and 3,000 ppm of CaCl.sub.2.2H.sub.2O. These tests have been conducted on a polymer prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P′3) and on a polymer made from the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P3). The two polymers have the same chemical composition. The polymer solutions were observed for 6 months at 90° C. The results for viscosity loss are shown in FIG. 7. We can observe that polymer P3 loses less viscosity than the equivalent polymer P′3.

Example 10: Preparation of Homopolymers from the Hydrated Crystalline Form of 2-acrylamido-2-methylpropane Sulfonic Acid

(40) To a 2000 mL beaker are added 390.5 g of deionized water, 262 g of 50% sodium hydroxide and 847.5 g crystals of 2-acrylamido-2-methylpropane sulfonic acid obtained in example 2.

(41) The resulting solution is cooled between 5 and 10° C. and transferred to an adiabatic polymerization reactor, then nitrogen is bubbled for 30 minutes to remove all traces of dissolved oxygen.

(42) The following are then added to the reactor: 0.45 g of 2,2′-azobisisobutyronitrile, 1.5 mL of a solution at 2.5 g/L of 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 1.5 mL of a solution at 1 g/L of sodium hypophosphite, 1.5 mL of a solution at 1 g/L of tert-butyl hydroperoxide, 1.5 mL of a solution at 1 g/L of ammonium iron (II) sulfate hexahydrate (Mohr's salt).

(43) After a few minutes, the nitrogen inlet is shut and the reactor is closed. The polymerization reaction occurs for 2 to 5 hours until a temperature peak is reached. The rubbery gel obtained is chopped and dried to obtain a coarse powder itself milled and sieved to obtain the polymer in powder form.

Example 11: Preparation of Homopolymers from the Non-Crystalline Hydrated Form of 2-acrylamido-2-methylpropane Sulfonic Acid

(44) The polymers are prepared as in example 10, replacing the crystalline hydrated form of 2-acrylamido-2-methylpropane sulfonic acid (example 2) with 2-acrylamido-2-methylpropane sulfonic acid that is not the crystalline hydrated form synthesized in example 1.

Example 12: Measurement of the Filter Ratio for Polymer Solutions

(45) Filtration tests were conducted on 2 polymers prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid P′5, P′6 with respective increasing molecular weights 3.1 and 5.3 million Da, prepared as described in example 11, and on 3 polymers prepared from the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid P5, P6 and P7 with respective increasing molecular weights 3.1, 5.3 and 15 million Da, prepared as described in example 10. The molecular weight grade 15 million Da is not accessible when the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid is used.

(46) The polymer solutions were prepared at an active concentration of 1,000 ppm in a brine containing water, 30,000 ppm of NaCl and 3,000 ppm of CaCl.sub.2.2H.sub.2O. The filter ratio (FR) was measured in filters having a pore size of 1.2 μm representative of deposits with low permeability. These results are shown in FIG. 8.

(47) TABLE-US-00002 TABLE 2 Polymers tested for the filter ratio Form of Molecular weight ATBS used (in million Da) Filter ratio P5 Crystalline 3.1 1.05 P6 Crystalline 5.3 1.03 P7 Crystalline 15 1.30 P′5 Non-crystalline 3.1 1.16 P′6 Non-crystalline 5.3 1.54

(48) We can observe that at equivalent molecular weight, the polymers made from the hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P5-P6) always have a FR lower than that of the polymers made from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P′5-P′6). This difference becomes bigger and bigger when the molecular weight of the polymer increases. The polymer prepared from the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid with molecular weight 15 million Da (P7) even has a lower FR than the polymer prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid with lower molecular weight (5.3 million Da, P′6).

Example 13: Measurement of the Resistance to Chemical Degradation of Solutions of Polymers P6 and P′6

(49) Resistance tests for chemical degradation of polymers P6 and P′6 with molecular weight 5.3 million Da were conducted in aerobic conditions in the presence of different iron(II) concentrations (2, 5, 10 and 20 ppm) in a brine composed of water, 37,000 ppm of NaCl, 5,000 ppm of Na.sub.2SO.sub.4 and 200 ppm of NaHCO.sub.3. These tests have been conducted on a polymer prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P′6) and on a polymer made from the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P6). The two polymers have the same chemical composition. The results after 24 h of contact of the polymer solution with the contaminant are shown in FIG. 9.

(50) We can observe that for each iron(II) concentration, polymer P6 loses less viscosity than the equivalent polymer P′6.

Example 14: Preparation of the Crystalline Hydrated Form of Acrylamide/2-acrylamido-2-methylpropane Sulfonic Acid (Co)Polymer (75/25 Mole %), Post-Hydrolyzed, P8

(51) To a 2000 mL beaker are added 761.9 g of deionized water, 574.2 g of 50% acrylamide solution, 35.9 g of 50% sodium hydroxide, 11.7 g of urea and 116.3 g crystals of 2-acrylamido-2-methylpropane sulfonic acid obtained in example 2.

(52) The resulting solution is cooled between 0 and 5° C. and transferred to an adiabatic polymerization reactor, then nitrogen is bubbled for 30 minutes to remove all traces of dissolved oxygen.

(53) The following are then added to the reactor: 0.45 g of 2,2′-azobisisobutyronitrile, 1.5 mL of a 5 g/L solution of 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 1.5 mL of a solution at 1 g/L of sodium hypophosphite, 2.25 mL of a solution at 1 g/L of tert-butyl hydroperoxide, 3.0 mL of a solution at 1 g/L of ammonium iron (II) sulfate hexahydrate (Mohr's salt).

(54) After a few minutes, the nitrogen inlet is shut and the reactor is closed. The polymerization reaction occurs for 2 to 5 hours until a temperature peak is reached. The rubbery gel obtained is chopped into particles with a size inclusively between 1 and 6 mm.

(55) 500.0 g of previously chopped gel is then mixed with 18.0 g of 50% sodium hydroxide, the mixture is taken and held at a temperature of 90° C. for a duration of 90 minutes.

(56) The gel is then dried and milled to obtain the polymer in powder form.

Example 15: Preparation of the Non-Crystalline Hydrated Form of Acrylamide/2-acrylamido-2-methylpropane Sulfonic Acid (Co)Polymer (75/25 Mole %), Post-Hydrolyzed, P′8

(57) The copolymer is prepared as in example 14, replacing the crystalline hydrated form of 2-acrylamido-2-methylpropane sulfonic acid (example 2) with 2-acrylamido-2-methylpropane sulfonic acid that is not the crystalline hydrated form obtained in example 1.

Example 16: Measurement of the Filter Ratio for Polymer Solutions

(58) Filtration tests were conducted on a polymer prepared from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid P′8 with molecular weight of 22 million Da, prepared as described in example 15, and on a polymer prepared from the hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid P8 with molecular weight of 26 million Da, prepared as described in example 14.

(59) The polymer solutions were prepared at an active concentration of 1,000 ppm in a brine containing water, 30,000 ppm of NaCl and 3,000 ppm of CaCl.sub.2.2H.sub.2O. The filter ratio (FR) was measured in filters having a pore size of 3 μm representative of deposits with low permeability. These results are shown in FIG. 10.

(60) TABLE-US-00003 TABLE 3 Polymers tested for the filter ratio Molecular weight Filter Form of ATBS used (in million Da) ratio P8 Hydrated crystalline 26 1.07 P′8 Non-crystalline 22 1.10

(61) We can observe that, in spite of a higher molecular weight, the (co)polymer prepared from the hydrated crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P8) presents a FR equivalent to that of the (co)polymer made from the non-crystalline form of 2-acrylamido-2-methylpropane sulfonic acid (P′8).